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4. EFFECT OF GYPSUM AND CALCIUM CARBONATE ONPLANTS

4.1 Introduction 4.2 Winter Crops 4.3 Summer Crops 4.4 Classification of Field Crops Based on Their Tolerance to Gypsum 4.5 Fruit and Forest Trees 

4.1 Introduction

Gypsiferous soils are very variable and there are many factors that affect their properties inrelation to plant growth. Gypsiferous soils can be productive and managed profitably if they arefirst studied properly. The effect of the chemical properties of gypsiferous and calcareous soilson the growth of plants, both natural vegetation and crops, and their mineral contents have beeninvestigated by numerous authors.

Before further discussing the effects of gypsum on plants it is worth noting that measurementsof total gypsum in soils are unreliable and do not reflect the actual amount present as proved bySayegh et al. (1978). Figures quoted in the literature for the gypsum content of soils arecommonly lower than the actual amount present.

Van Alphen and de los Rios Romero (1971) conclude that up to 2 percent gypsum in the soilfavours plant growth, between 2 and 25 percent has little or no adverse effect if in powderyform, but more than 25 percent can cause substantial reduction in yields. They suggest thatreductions are due in part to imbalanced ion ratios, particularly K:Ca and Mg:Ca ratios.Hernando et al. (1963, 1965) studied the effect of gypsum on the growth of corn and wheat byvarying the gypsum level in the soil up to 75 percent. They show that high levels of gypsumcaused poor growth of corn, especially as the soil moisture was maintained at 80 percent of fieldcapacity. However, wheat showed minimum growth where the soil contained 25 percentgypsum at all soil moisture levels ranging from 15 to 100 percent of field capacity. Akhvlediani(1962) concludes in general, that agricultural production on gypsiferous chernozem andchestnut soils is not affected when the gypsum content is between 15 and 30 percent. Bureau

and Roederer (1960), report that 30 percent gypsum content in soils of Tunisia is toxic to plantgrowth. Van Alphen and de los Rios Romero (1971) state, from field observations in the EbroValley of Spain, that plant growth is reduced where the gypsum content exceeds 20 to 25percent.

Soil gypsum affects the mineral contents of plants. Hernando et al. (1965) showed, using waterculture, that increasing the concentration of SO4 mixture (K2SO4 + MgSO4) increased the uptakeof NO3, K, Mg and sulphur by corn but decreased the uptake of Ca and P. Boukhris and

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Lossaint (1970, 1972) studied the mineral contents of 52 natural species growing on gypsiferoussoils in Tunisia, and reported that various species growing under the same ecosystemresponded differently to the excess of Ca and SO4 present in the soils, depending upon theirbiogeochemical properties. In general, the chemical composition of the leaves or aerial parts ofplants is influenced by the plant family. A comparison of field observations, with water culturestudies, confirms the behaviour of plant species toward nutrient adsorption. A high level of SO 4 

in the soil can raise the SO4, level in the gypsum tolerant species (called gypsocline); but to alesser extent in the species (so-called gypsophytes) found on natural gypsum soils. However,there are some gypsophytes called thiophores which have great ability to accumulate highlevels of S in their leaves. The SO4 concentration in some thiophores is thirty times greater thanthat in other species living in the same environment (Table 4.1).

Table 4.1 CHEMICAL COMPOSITION OF WILD PLANTS ON GYPSIFEROUS SOILS

Species  % Dry matter 

Ca  Mg  Na  K  Cl  SO4 

Atriplex halimus   6.29 2.68 1.95 3.56 1.82 19.05Salsola tetrandra   1.77 1.40 15.86 3, 60 4.82 21.59

Salsola vermiculata   2-3.5 1-1.5 6.9-10 1.2-3.5 1.1-4.2 4.2-91

Anabassis oropediorum   4.8-10.9 1.5-1.6 2.0-2.2 1.1-1.8 0.6 0.6-0.7

In the literature, it is reported that the chemical as well as the physical properties of thegypsiferous soils affect plant growth and mineral composition of plants.

From intensive field observations of gypsiferous soils in Iraq, Smith and Robertson (1962) foundthat root growth was inhibited where the gypsum content of soil was over 10 percent. This is

apparently because of the poor transmission of air and water caused by poor structure. Theyalso found that soils containing more than 25 percent gypsum in the rooting zone give poorgrowth. In the spring, wheat crops wilt on shallow gypsiferous soils when other crops on deepersoils show no signs of distress. Roots do not penetrate the gypsum layer, even when it is quitewet. Kovda (1954) and other workers observe that plant roots do not penetrate a soil layercontaining 25 percent of gypsum or more. Mardoud (1980) observes that pine roots cannotpenetrate a soil layer with 60 percent of gypsum. The roots extend horizontally and the treesshow signs of poor growth compared with trees on soils with gypsum horizons at greater depth.Boyadgiev (1974) shows that the presence of well-crystallized gypsum within the first metre ofsoil affects the performance of cotton crops significantly. He found it difficult to evaluate theadverse effect of gypsum content on the cotton yields because of its link to other variableswhich also cause these soils to be less than ideal for cotton production. Boyadgiev (1974) also

noted that crops such as alfalfa could grow very well and give high yields even in soilscontaining up to 50 percent of powdery gypsum as long as no gypsic layer impeding rootelongation and extension is present in the soil profile at shallow depth. Similar effects have beennoted by Amami et al. (1967) in the oasis at Tozeur in Tunisia, where good yields of alfalfa anddate palms were obtained in the highly gypsiferous soils. Similar results were obtained in theEbro Valley of Spain with crops such as alfalfa, wheat and apricots.

It appears from the above results that the gypsum content of soils is only one of several factorswhich affect plant growth and yield of crops. The other factors are:

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i. the depth of the topsoil over a gypsic layerii. the hardness and degree of crystallization of the gypsic layeriii. the total and active calcium carbonate contentsiv. the availability of plant nutrients and moisture content in the root zonev. the type of crops grown and their relative tolerance to gypsumvi. the drainage conditions and salinity of the soil.

The performance of plants grown on shallow soils depends to a large extent on their rootsystem, the gypsum content, the fertility level of the topsoil, and the water availability during thegrowing season. In particular the presence of a hard impervious gypsic layer has a strong effecton crop production under irrigation. Percolating water dissolves gypsum and salts and stagnatesat the top of the gypsic layer creating a perched water-table, often resulting in an accumulationof gypsum and salts. The resulting high water-table may rise to the soil surface leaving salts andgypsum. Under these conditions, the performance of crops will be affected by both gypsum andsalinity. Extensive areas of gypsiferous soils in Syria, Iraq, Tunisia and elsewhere are saltaffected.

4.2 Winter Crops

Wheat  

Wheat grows least well when the gypsum content in soils is around 25 percent (Hernando et al. 1963). Smith and Robertson (1962) observe that wheat grown on shallow gypsiferous soilsshows signs of wilting in the spring and the roots were unable to utilize moisture from the wetgypsic layer below. Van Alphen and de los Rios Romero (1971) record high yields of wheat ongypsiferous soils in the Ebro Valley, Spain. Mardoud (1980) found that Mexican wheat cultivarsyielded an average of 4 tonnes per hectare on the shallow gypsiferous soils of the EuphratesValley (gypsum content less than 25 percent in the 0-15 cm layer and 25-35 percent in the 15-30 cm layer). This yield is considered very satisfactory under the conditions in the Valley.

Barley  

Barley (H. vulgare) is an important crop well adapted on gypsiferous soils under dry farmingagriculture, and is widely used by farmers in Northern Syria and Iraq. A two-course rotation ispractised. Fallow is extensively used after wheat or barley, in localities where the averageannual rainfall is 200-300 mm. All the evidence suggests that barley like wheat is tolerant togypsum. Yields of 1.5 to 3 tonnes per hectare were obtained under rainfed agriculturedepending upon total seasonal rainfall and distribution. However, Mardoud (1980) obtained upto 4 tonnes per hectare in the Euphrates Valley with supplementary irrigation.

Vetches  

American vetch (Vicia dassicarpa) was tested under irrigation on various types of gypsiferoussoils of the Euphrates Valley. The active nodules were very limited in number in the firstcropping season and yield, on the very shallow gypsiferous soils, ranged between 1 to 2 tonnesof seeds or 8.4 tonnes of green fodder per hectare. The yield during the second seasonimproved significantly averaging 2.5 tonnes of seeds or 25 tonnes of green fodder per hectare.Lower yields were obtained on sandy gypsiferous soils. Under rainfed farming agriculture andwith rainfall exceeding 250 to 300 mm, vetches could be a good crop to follow wheat or barley inthe rotation.

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Lentils  

Lentils (Lens esculenta) is a dry farming crop, which is usually alternated with wheat or barley ina two-course rotation. Lentils are used as well as vetches on the rainfed, gypsum-affected soilsof Iraq and Syria as a break crop. Lentils under irrigation gave, in shallow gypsiferous soils, amoderate yield of 1.3 to 1.5 tonnes per hectare in the first year. An average yield of 0.5 to 1

tonnes per hectare was obtained on the sandy gypsiferous soils of the Euphrates Valley. As inthe case of vetches, it was noticed that few nodules were present on the root system in the firstyear of cropping with obvious signs of nitrogen deficiency on the plots receiving no nitrogenfertilizer (Mardoud 1980).

Broad beans  

Broad beans (Vicia faba) are usually planted, under dry farming conditions, in the higher rainfallzone (350 to 500 mm as annual rainfall). Supplementary irrigation is required to obtain higheryields under lower rainfall conditions. Broad beans grown in irrigated fields of shallow and veryshallow gypsiferous soils of the Euphrates Valley gave a poor yield of 0.3 to 0.5 tonnes perhectare at first. Data on the performance of broad beans on deep gypsiferous soils are notavailable.

Trifolium  

Trifolium (Trifolium alexandrinum) is an important fodder crop under irrigated agriculture. Matar(private communication) tested its tolerance to gypsum in pots, under greenhouse conditionswith ample K and P fertilization. Four successive cuts were harvested at the flowering stage.The effect of gypsum content was significant in the first cut, reducing the yield by more than 50percent where the gypsum content exceeded 20 percent (Table 4.2). The effect of gypsumbecomes much less in the following cuts. This significant reduction in yield could reflect thedelay in the seed emergence and stand establishment caused by the gypsum and calciumcarbonate contents. The total fresh weights of the four successive cuts demonstrates thattrifolium is quite tolerant to gypsum content in soils. Data on its performance on a field scale arenot available.

Table 4.2 EFFECT OF GYPSUM CONTENT OF SOILS ON YIELDS (FRESH WEIGHT INGRAMS) OF FOUR CONSECUTIVE CUTS OF TRIFOLIUM GROWN IN POTS (Matar,unpublished work)

Gypsum (%)  No. of cuts  Total weight 

1  2  3  4 

0 281.3 133.2 363.0 409.8 1187.2

5 281.6 161.8 316.1 414.5 1174.0

10 260.0 154.9 322.2 432.5 1169.7

20 233.8 143.9 292.6 375.6 1045.9

40 129.6 123.2 282.4 389.1 924.3

Alfalfa  

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The performance of alfalfa (Medicago sativa) as a major crop on gypsiferous soils is quiteimpressive. High yields of irrigated alfalfa are recorded on the highly gypsiferous soils of theEbro Valley of Spain (Van Alphen and de los Rios Romero 1971). Amami et al. (1967) reportedyields of 10 tonnes per hectare of alfalfa on light textured gypsiferous soils containing 20 to 25percent gypsum in the root zone. Mardoud (1980) harvested 20 to 60 tonnes per hectare offresh fodder in the first and the second year of cropping. The yields were obtained where alfalfa

was grown in medium deep soils (less than 25 percent gypsum content in the first 45 cm of thesoil profile); and similar yields, 14 and 61 tonnes per hectare were also obtained in shallowergypsiferous soils (15 to 20 cm depth with 25 percent gypsum content). The performance ofvarious alfalfa cultivars tested on the shallow gypsiferous soils of the Euphrates Valley wassignificantly different. The result of variety trials is reported in Table 4.3.

Table 4.3 YIELDS OF ALFALFA GROWN IN SHALLOW GYPSIFEROUS SOILS1 

1 15-30 cm deep with less than 25 percent gypsum

Variety  Green production (tonnes per hectare)  Number of replicates  Sensitivity to frost 

1st year  2nd year 

ASR 13 32.320 66.820 3 Sensitive

ASR 11 32.250 51.130 3 Resistant

Selection 28.00 67.100 2 Resistant

Colient 26.260 65.940 3 Resistant

Heiroprovian 24.540 60.740 2 Resistant

Siwa 23.490 56.200 2 Resistant

Provience 22.950 59.320 2 Sensitive

Moaba 19.640 45.110 4 Resistant

Local 17.600 44.800 4 Sensitive

Europe 17.500 39.650 1 Sensitive

Lahonta 17.160 44.380 2 Resistant

Good performance of alfalfa is also reported by Vieillefon (1976) in Tunisia on soils containingmoderate amounts of calcium sulphate with little surface soil incrustation. Akramov (1981) whorecently studied the effect of alfalfa residues on reclaiming gypsiferous solonchaks, reports thatploughing-under of alfalfa and the addition of manure converted unproductive gypsiferous soils(with >40 percent gypsum) into productive ones. As well as performing well, alfalfa improvesstructure in gypsiferous soils and increases their productivity.

Other Winter Crops  

Other cereal crops can be grown on gypsiferous soils with success. Loomis (1944) shows thatthe yields of oats (Avena sativa) improved slightly in the presence of gypsum in a potexperiment.

4.3 Summer Crops

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Many summer crops can be grown on gypsiferous soils with various degrees of success. Theseinclude cotton, sugar beet, potato, groundnut, soybean, sesame, tomato and sunflower.

Cotton  

Cotton is an important cash crop. Boyadgiev (1974) notes that crystallized gypsum particles insoils depressed the yield of cotton in the Euphrates Valley. Mardoud (1980) obtained goodyields of cotton (4 tonnes per hectare) in moderately deep gypsiferous soils (with 45 cm of soil,with less than 25 percent gypsum content), 1.94 tonnes per hectare in medium deep sandy soilswith 25 to 50 percent gypsum content in the root zone. Vieillefon (1976) found little effect ofgypsum on the performance of cotton in soils with medium calcium sulphate content at AinZerig. Minashina et al. (1983) observes that yields of cotton grown on grey-brown gypsiferoussoils fell by 16 percent in soils with 10 percent gypsum. A larger decrease in yield was observedin soils with higher gypsum levels. They also observe that the quality of the cotton fibredeteriorates as the gypsum content in the soil increases. Although the field observations in theEuphrates Valley gave promising yields of cotton, greenhouse and recent field studies byMinashina suggest that cotton is not always sufficiently tolerant to gypsum. More research isneeded to study the effect of gypsum on cotton cultivars in the various gypsiferous soil types.

Sugar beet  

Limited work has been done on the suitability and tolerance of sugar beet (Beta vulgaris) ingypsiferous soils. Mardoud (1980) obtained 17.5 to 22.5 tonnes per hectare of autumn-grownsugar beet and 35 tonnes per hectare of spring-cultivated sugar beet in moderately deepgypsiferous soils in the Euphrates Valley. These yield levels are considered moderate.

Corn  

Corn (Zea mays) is one of the main crops in the irrigated areas of the arid zone. The effects of

soil gypsum content on corn growth and nutrient composition has been studied by severalworkers, for example

Hernando et al. (1963, 1965). Growth of corn was reduced with the high gypsum levels. Theinteraction between gypsum content and soil moisture stress was found significant in its effecton corn growth and performance. Mardoud (1980) obtained 2 tonnes per hectare of corn seedsgrown in moderately deep gypsiferous soils of the Euphrates Valley. Some foliar symptoms ofmicro-element deficiences were noticed, however.

Soybean  

Soybean (Glycine max) is strongly affected by the gypsum content of soils. On the sandy

gypsum soils of the Euphrates Valley (25 to 50 percent gypsum content) it gave very low yieldsof about 0.4 tonnes per hectare of grain seeds (Mardoud 1980). Soybean, grown in pots,however, showed a marked tolerance to gypsum. The effects of Rhizobium innoculation, offertilization and of the gypsum content in different gypsiferous soils on seed production were notstudied.

Groundnut  

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Groundnut (Arachis hypogaea) is a good leguminous crop for light to medium textured soils.Yields of 1.25 to 1.42 tonnes per hectare of seeds have been obtained in the Euphrates Valleyon sandy gypsiferous soils, with 25 to 50 percent gypsum content (Mardoud 1980). In spite ofadequate P and K fertilization, groundnut grown in pots with 6 kg soil showed a gradualdecrease in fresh weight of tops as the gypsum content increased. The average yield of freshtops dropped by 35 percent at a gypsum content of 40 percent. The effect of gypsum content on

the yield of seed was more pronounced (Figure 4.1). There were no visual signs of nutrientdeficiencies on the plants. There were no nodules on the roots of plants grown in the gypsumtreatments.

Walker et al. (1976) found that application of gypsum to soils low in calcium increased thepercentage of oil in all peanut cultivars; while the nitrogen content of the seed was reduced.Davidson et al. (1983) reports that application of gypsum to groundnuts grown in Georgiaincreased germination and reduced aflatoxin contents by 40 percent.

Figure 4.1 Effect of gypsum content on yield of groundnut tops and seed in a potexperiment 

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Tomato  

Field tomatoes (Lycopersicum esculentum) showed significant tolerance to gypsum. A yield of17 tonnes per hectare of fruit was obtained when tomato was grown in moderately deepgypsiferous soils in the Euphrates Valley (Mardoud 1980). In recent years, irrigation withsulphate-rich water has been of interest to many scientists studying the growth and yield of

tomato crops. Papadopulos (1984) found that the weight of fresh tomato fruit was decreased byabout 50 percent when irrigated with sulphate-rich water. Similarly, Russo (1983) found thattomatoes grown in gypsiferous-sodic soils gave low yields because they had a small fruitweight. Martinez et al. (1984) report that the total sulphur content in leaves and roots of tomatoplants is significantly increased as the S04 levels in the substrate increase. Increasing the levelo f NO3 in the growing medium increases tomato yields and the total sulphur accumulated in theleaves and roots decreases. However, at high levels of SO4 in the growing medium the additionof NO3 decreases the yield. That leads to the reasonable conclusion that the effect of SO4 is notion specific but is mainly an osmotic effect.

Potato  

Potato (Solarium tuberosum) planted in the moderately deep gypsiferous soils of the EuphratesValley with less than 25 percent gypsum content in the top 45 cm gave a poor yield of 7.3tonnes per hectare of tuber. More research is needed to determine the tolerance of potatoes togypsum.

Sesame  

Sesame (Sesasum orientale) crop grown in the gypsiferous soils of the Euphrates Valley gavean average yield of 1.8 tonnes per hectare (Mardoud 1980).

Sunflower  

The only published work on the effect of gypsum content on sunflower (Helianthus annuus) performance is that of Mardoud (1980) who obtained 1.27 to 1.7 tonnes per hectare of seedgrains on sandy gypsiferous soils, containing 25 to 50 percent gypsum.

Other Summer Crops  

Other summer crops for which data are available include:

Sorghum, an important crop for animal feed and human consumption yielded between 2.25 and3.9 tonnes per hectare in the medium deep gypsiferous soils of the Euphrates Valley. However,the yield dropped to less than 0.65 to 1.85 tonnes per hectare when grown in shallower soils (15

to 30 cm thickness with less than 25 percent gypsum content).

Onion, a sulphur-loving plant, yields about 24 tonnes/ha in the sandy gypsiferous soils of theEuphrates Valley. The cooking quality of the bulbs is, however, unsatisfactory and the taste isvery strong.

Figure 4.2 Effect of soil gypsum content on relative yields of leaves and roots of Burleytobacco 

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American Burley, the irrigated broad-leaved tobacco, grown in pots with gypsum contentranging between 0 and 40 percent, is extremely sensitive to gypsum. The yield of leavesdropped by more than 95 percent when grown in soil containing 5 percent gypsum or more(Figure 4.2). Complex symptoms of various nutrient deficiencies appeared on the tobacco

leaves. Ryding (1978) found in greenhouse trials, that application of gypsum decreased theyield of flue-cured tobacco although the Ca content of leaves was increased.

4.4 Classification of Field Crops Based on Their Tolerance to Gypsum

The tolerance, yield and product qualities of many agricultural crops grown on gypsiferous soilsare not yet well known. As a first approximation, the main agricultural crops are classified below

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into four main groups in relation to gypsum tolerance based on the available data. Thispreliminary classification should be considered a guideline only.

Group I: Tolerant to gypsum - agricultural crops which show tolerance to 40 percent of gypsumin soil without a significant decrease in yield: alfalfa, trifolium, wheat, barley, lentil, oats, tomatoand onions.

Group II: Semi-tolerant to gypsum - agricultural crops which show tolerance to 20 percent ofgypsum in soil without a significant decrease in yield. The yield may drop by about 50 percent athigher levels of gypsum (say 40 percent gypsum). This group includes: broad beans, sugarbeet, sorghum, corn, soybean and sesame.

Group III: Semi-sensitive to gypsum - agricultural crops which show tolerance of up to 10percent of gypsum without a significant drop in yield. Yields fall at higher levels of gypsum. Thisgroup includes: cotton, groundnut, potato and sunflower.

Group IV: Sensitive to gypsum - among the test crops, tobacco was sensitive to gypsum.

4.5 Fruit and Forest Trees

In addition to the gypsum content of the surface layer and its distribution in the profile, manyworkers have found that the hardness and degree of cementation of the gypsic layer is of greatimportance to the success of fruit orchards and forest trees. A cemented layer at shallow depthimpedes the extension of the root system by mechanical resistance and consequently limits thegrowth and production. Kalashnikov and Romanov (1949) found, from some afforestationexperiments conducted on dark chestnut soils overlying gypsum, that the greater the soil depthabove the gypsum layer the more suitable the soils are for afforestation. They also found thatoak (Quercus) and pear trees, 13 to 15 years old, grew 3.5 to 5.0 metres high when the gypsumlayer was at 170 cm from the surface and 2 to 4 metres only when the gypsum layer was at 1.10

to 1.25 metres depth. Mardoud (1980) noticed that the roots of forest trees, such as pines(Pinus halepensis) and eucalyptus, grown in the Euphrates Valley of Syria could not penetrateany gypsic horizon containing more than 60 percent of gypsum. Their roots extend horizontallyand the trees showed signs of weak growth compared with trees grown in soils with no gypsichorizon. Other researchers report that several species were found resistant to gypsum and gavegood yields in highly gypsiferous soils. Van Alphen and de los Rios Romero (1971) cite that highyields of apricots (Amermeniaca vulgaris) were obtained in the Ebro Valley of Spain whengrown on gypsiferous soils with a gypsic layer at a depth of 30 to 60 cm. The average apricotyield obtained in the El Burgo de Ebro was in the order of 8 tonnes per hectare.

A good yield of palms was obtained in gypsiferous soils containing 50 percent gypsum in theoasis of Tozeur, Tunisia (Amami et al. 1967).

Minashina (1956) reported excellent growth of grapevines grown on gypsiferous soils of theKirovabad Massif, even in soils with a gypsic layer at shallow depth. But work carried out in theEuphrates Valley in Syria, has shown that grapevine tolerance to gypsum depends on thevariety grown. Except Muscat and Cardinal, all varieties tested gave a poor growth on themoderately deep gypsiferous soils of the Euphrates Valley (Mardoud 1980). More research isneeded to determine grapevine varieties and rootstocks tolerant to gypsum. In Spain, wineproduced from grapes grown on gypsiferous soils is of poor to medium quality, but table grapesare of good quality (Prof. Roquero, personal communication). In the Murcia area of Spain,

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where the soils are highly gypsiferous with a marked petrogypsic horizon, apricots, peaches,pears, olives and grapes are extensively planted. The farmers are very satisfied with the highyields obtained. The peach varieties are grafted onto rootstocks tolerant to soils with a largecalcium content.

Observations on other fruit trees grown in the moderately deep gypsiferous soils of the

Euphrates Valley show that many species tolerate gypsum, including pomegranate (Punica granatum), peaches (Amygdalis persica), plums and figs (Ficus carica) and apricots. Pistachio(Pistacia vera) however shows poor adaptability to gypsiferous soil conditions. Only three of 46trees of local pistachio cultivars remained alive after four years.

Several papers have been published recently on the effect of high sulphate-rich waters on ionadsorption and fruit quality of lemon trees. Cerdá et al. (1982) and Fernandez et al. (1983)found that sulphate ions are not taken up as readily as other soluble ions such as chloride orboron; and never exceeded 240 mmole/kg of plant. These authors believe that some of thesulphate effects reported in the literature might not be the result of sulphate uptake but due tothe salinity of the soil solution. Rind thickness and rugosity were the only fruit qualitycharacteristics affected by the sulphate application. The effect of the gypsum content in soils on

the yield of lemon trees and other citrus varieties is not yet known.

Boukhris and Lossaint (1970, 1972) in a study conducted on the gypsiferous soils of Tunisia,found that resinous trees including Pinus halepensis and other oligophore trees absorb few ofthe ions present in gypsiferous soils and control their uptake of ions very efficiently. They arepoor in all mineral nutrients especially potassium and calcium. Constant Mg, S, N and Pcontents were observed in pines and other trees grown under the various climatic conditions ofthe gypsiferous soils of Tunisia. The low levels of nutrients in pines make them quite successfuland adaptable to all types of soil environments including gypsiferous ones. Wild (1974) founddwarfing of woody species, particularly of Colophospermum mopane, grown on a gypsumdeposit in Botswana. He attributed the dwarfing partly to the poor physical properties of thesecompacted and clayey soils.

The available information on the tolerance of various species of fruit trees and their rootstocksto the gypsum content of soils is still inadequate. Much more information is needed on theperformance and adaptation of various tree species to the gypsiferous soils, and on the effect ofgypsum on fruit yield and quality.

Calcium carbonate

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Calcium carbonate

IUPAC name[hide] 

Calcium carbonate

Other names[hide] 

Limestone; calcite; aragonite; chalk; marble; pearl 

Identifiers

CAS number 471-34-1 

ChemSpider 9708 

UNII H0G9379FGK 

KEGG D00932 

ChEBI CHEBI:3311 

RTECS number FF9335000

Jmol-3D imagesImage 1 

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Image 2 

SMILES 

[show] 

InChI 

[show] 

Properties

Molecular formula CaCO3 

Exact mass 100.0869 g/mol

Appearance Fine white powder

Density 

2.71 g/cm3 (calcite) 

2.83 g/cm3 (aragonite) 

Melting point 825 °C (aragonite)

1339 °C (calcite)[2]

 

Boiling pointdecomposes

Solubility in water  0.00015 mol/L (25°C)

Solubility product, K sp  4.8×10−9[1]

 

Solubility in dilute acids soluble

Acidity (pK a) 9.0

Refractive index (nD) 1.59

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Structure

Crystal structure Trigonal

Space group 32/m

Hazards

MSDS ICSC 1193

EU Index Not listed

NFPA 704

 

0

0

0

Flash point   Non-flammable

Related compounds

Other anions  Calcium bicarbonate

Other cations 

Magnesium carbonate 

Strontium carbonate 

Barium carbonate 

Related compounds Calcium sulfate

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Except where noted otherwise, data are given for materials

in their standard state (at 25 °C, 100 kPa) 

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Infobox references

 

Crystal structure of calcite

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance

found in rocks in all parts of the world, and is the main component of  shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural

lime, and is usually the principal cause of  hard water. It is commonly used medicinally as a

calcium supplement or as an antacid, but excessive consumption can be hazardous.

Contents

[hide] 

  1 Chemical properties 

  2 Preparation 

  3 Occurrence 

  4 Geology 

o  4.1 Carbonate compensation depth 

o  4.2 Taphonomy 

  5 Uses o  5.1 Industrial applications 

o  5.2 Health and dietary applications 

o  5.3 Environmental applications 

  6 Calcination equilibrium 

  7 Solubility 

o  7.1 With varying CO2 pressure 

o  7.2 With varying pH 

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o  7.3 Solubility in a strong or weak acid solution 

  8 See also 

  9 References 

  10 External links 

[edit] Chemical properties

See also: Carbonate 

Calcium carbonate shares the typical properties of other carbonates. Notably:

  it reacts with strong acids, releasing carbon dioxide:

CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) 

  it releases carbon dioxide on heating (to above 840 °C in the case of CaCO3), to form calcium

oxide, commonly called quicklime, with reaction enthalpy 178 kJ / mole:

CaCO3 → CaO + CO2 

Calcium carbonate will react with water that is saturated with carbon dioxide to form the

soluble calcium bicarbonate. 

CaCO3 + CO2 + H2O → Ca(HCO3)2 

This reaction is important in the erosion of  carbonate rocks, forming caverns, andleads to hard water in many regions.

[edit] Preparation

The vast majority of calcium carbonate used in industry is extracted by mining or

quarrying. Pure calcium carbonate (e.g. for food or pharmaceutical use), can beproduced from a pure quarried source (usually marble).

Alternatively, calcium carbonate is prepared by calcining crude calcium oxide. 

Water is added to give calcium hydroxide, and carbon dioxide is passed through this

solution to precipitate the desired calcium carbonate, referred to in the industry asprecipitated calcium carbonate (PCC):[3]

 

CaCO3 → CaO + CO2 

CaO + H2O → Ca(OH)2 

Ca(OH)2 + CO2 → CaCO3 + H2O

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[edit] Occurrence

Calcium carbonate is found naturally as the following minerals in the

form of  polymorphs: 

  Aragonite   Calcite 

  Vaterite or (μ-CaCO3)

The trigonal crystal structure of calcite is most common.

The calcium carbonate minerals occur in the following rocks: 

  Chalk 

  Limestone 

  Marble 

  Travertine 

Calcite

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Aragonite

Marble

Travertine

[edit] Geology

Carbonate is found frequently in geologic settings and constitute an

enormous carbon reservoir. Calcium carbonate occurs as the polymorphs 

aragonite and calcite. A polymorph is a mineral with the same chemicalformula but different chemical structure. The carbonate minerals form

the rock types: limestone, chalk , marble, travertine, tufa, and others.Calcite commonly occurs as sediments in marine settings. Calcite is

typically found around the warm tropic environments. Calcite

precipitates in warmer shallow environments more than it does under

colder environments because warmer environments do not favor thedissolution of CO2. This is analogous to CO2 being dissolved in soda.

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When you take the cap off of a soda bottle, the CO2 rushes out. As the

soda warms up, carbon dioxide is released. This same principle can beapplied to calcite in the ocean. Cold-water carbonates do exist at higher

latitudes but have a very slow growth rate.

In tropic settings, the waters are warm and clear. Corals are moreabundant in this environment than towards the poles where the waters

are cold. Calcium carbonate contributors, including plankton (such ascoccoliths and planktic foraminifera), coralline algae, sponges, 

brachiopods, echinoderms, bryozoa and mollusks, are typically found in

shallow water environments where sunlight and filterable food are moreabundant. The calcification processes are changed by the ocean

acidification. 

Where the oceanic crust is subducted under a continental plate sedimentswill be carried down to warmer zones in the astenosphere and

mesosphere where the calcium carbonate is decomposed to carbondioxide which will give rise to explosive vulcanic eruptions. 

[edit] Carbonate compensation depth

The carbonate compensation depth (CCD) is the point in the ocean

where the rate of precipitation of calcium carbonate is balanced by the

rate of dissolution due to the conditions present. Deep in the ocean, thetemperature drops and pressure increases. Calcium carbonate is unusual

in that its solubility increases with decreasing temperature. Increasing

pressure also increases the solubility of calcium carbonate. The CCD can

range from 4 – 

6 km below sea level.

[edit] Taphonomy

Calcium carbonate can preserve fossils through permineralization. Mostof the vertebrate fossils of the Two Medicine Formation, known for its

duck-billed dinosaur eggs, are preserved by CaCO3 permineralization.[4]

 

This type of preservation preserves high levels of detail, even down tothe microscopic level.[4] However, it also leaves specimens vulnerable to

weathering when exposed to the surface.[4]

 

[edit] Uses

[edit] Industrial applications

The main use of calcium carbonate is in the construction industry, either

as a building material or limestone aggregate for roadbuilding or as aningredient of cement or as the starting material for the preparation of 

builder's lime by burning in a kiln. However, due to weathering mainly

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caused by acid rain, calcium carbonate (in limestone form) is no longer

used for building purposes on its own, and only as a raw/primarysubstance for building materials.

Calcium carbonate is also used in the purification of  iron from iron ore in

a blast furnace. The carbonate is calcined in situ to give calcium oxide,which forms a slag with various impurities present, and separates from

the purified iron.[5]

 

In the oil industry, calcium carbonate is added to drilling fluids as a

formation-bridging and filtercake-sealing agent; it is also a weightingmaterial which increases the density of drilling fluids to control the

downhole pressure. Calcium carbonate is added to swimming pools, as a

pH corrector for maintaining alkalinity and offsetting the acidic

properties of the disinfectant agent.

Calcium carbonate has traditionally been a major component of blackboard chalk. However, modern manufactured chalk is mostly

gypsum, hydrated calcium sulfate CaSO4·2H2O. Calcium carbonate is a

main source for growing Seacrete, or Biorock . Precipitated calcium

carbonate (PCC), pre-dispersed in slurry form, is a common fillermaterial for latex gloves with the aim of achieving maximum saving in

material and production costs.[6]

 

Fine ground calcium carbonate (GCC) is an essential ingredient in the

microporous film used in babies' diapers and some building films as the

pores are nucleated around the calcium carbonate particles during the

manufacture of the film by biaxial stretching. GCC or PCC is used as afiller in paper because they are cheaper than wood fiber. Printing and

writing paper can contain 10 – 20% calcium carbonate. In North America,

calcium carbonate has begun to replace kaolin in the production of glossy paper. Europe has been practicing this as alkaline papermaking or

acid-free papermaking for some decades. PCC has a very fine and

controlled particle size, on the order of 2 micrometres in diameter, usefulin coatings for paper. 

Calcium carbonate is widely used as an extender in paints,[7] in particular

matte emulsion paint where typically 30% by weight of the paint is either

chalk or marble. It is also a popular filler in plastics.[7]

 Some typical

examples include around 15 to 20% loading of chalk in unplasticizedpolyvinyl chloride (uPVC) drain pipe, 5 to 15% loading of stearate

coated chalk or marble in uPVC window profile. PVC cables can use

calcium carbonate at loadings of up to 70 phr (parts per hundred parts of 

resin) to improve mechanical properties (tensile strength and elongation)and electrical properties (volume resistivity). Polypropylene compounds

are often filled with calcium carbonate to increase rigidity, a requirement

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that becomes important at high use temperatures.[8]

 It also routinely used

as a filler in thermosetting resins (sheet and bulk molding compounds)[8]

 and has also been mixed with ABS, and other ingredients, to form some

types of compression molded "clay" poker chips. Precipitated calcium

carbonate, made by dropping calcium oxide into water, is used by itself 

or with additives as a white paint, known as whitewashing. 

Calcium carbonate is added to a wide range of trade and do it yourself  adhesives, sealants, and decorating fillers.[7] Ceramic tile adhesives

typically contain 70 to 80% limestone. Decorating crack fillers contain

similar levels of marble or dolomite. It is also mixed with putty in settingstained glass windows, and as a resist to prevent glass from sticking to

kiln shelves when firing glazes and paints at high temperature.

In ceramics / glazing applications, calcium carbonate is known as

whiting,[7] and is a common ingredient for many glazes in its white

powdered form. When a glaze containing this material is fired in a kiln,the whiting acts as a flux material in the glaze. Ground calciumcarbonate is an abrasive (both as scouring powder and as an ingredient of 

household scouring creams), in particular in its calcite form, which has

the relatively low hardness level of 3 on the Mohs scale of mineralhardness, and will therefore not scratch glass and most other ceramics, 

enamel, bronze, iron, and steel, and have a moderate effect on softer

metals like aluminium and copper. A paste made from calcium carbonate

and deionized water can be used to clean tarnish on silver.[9]

 

[edit] Health and dietary applications

500-milligram calcium supplements made from calcium carbonate

Calcium carbonate is widely used medicinally as an inexpensive dietarycalcium supplement or gastric antacid.[10] It may be used as a phosphate

binder for the treatment of  hyperphosphatemia (primarily in patients with

chronic renal failure). It is also used in the pharmaceutical industry as aninert filler for tablets and other pharmaceuticals.[11] 

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Calcium carbonate is known among IBS sufferers to help reduce

diarrhea. Some individuals report being symptom-free since startingsupplementation. The process in which calcium carbonate reduces

diarrhea is by binding water in the bowel, which creates a stool that is

firmer and better formed. Calcium carbonate supplements are often

combined with magnesium in various proportions. This should be takeninto account as magnesium is known to cause diarrhea.

Calcium carbonate is used in the production of toothpaste and has seen a

resurgence as a food preservative and color retainer, when used in or

with products such as organic apples or food.[12]

 

Excess calcium from supplements, fortified food and high-calcium diets,

can cause the milk-alkali syndrome, which has serious toxicity and can

be fatal. In 1915, Bertram Sippy introduced the "Sippy regimen" of hourly ingestion of milk and cream, and the gradual addition of eggs and

cooked cereal, for 10 days, combined with alkaline powders, whichprovided symptomatic relief for peptic ulcer disease. Over the nextseveral decades, the Sippy regimen resulted in renal failure, alkalosis,

and hypercalcemia, mostly in men with peptic ulcer disease. These

adverse effects were reversed when the regimen stopped, but it was fatalin some patients with protracted vomiting. Milk alkali syndrome

declined in men after effective treatments for peptic ulcer disease arose.

During the past 15 years, it has been reported in women taking calcium

supplements above the recommended range of 1.2 to 1.5 g daily, forprevention and treatment of osteoporosis, and is exacerbated by

dehydration. Calcium has been added to over-the-counter products,

which contributes to inadvertent excessive intake. Excessive calciumintake can lead to hypercalcemia, complications of which include

vomiting, abdominal pain and altered mental status.[13] 

As a food additive it is designated E170[14]; INS number 170. Used as an

acidity regulator, anticaking agent, stabiliser or colour it is approved for

usage in the EU,[15]

 USA[16]

 and Australia and New Zealand.[17]

 It is usedin some soy milk  products as a source of dietary calcium; one study

suggests that calcium carbonate might be as bioavailable as the calcium

in cow's milk .[18]

 Calcium carbonate is also used as a firming agent in

many canned or bottled vegetable products.

[edit] Environmental applications

In 1989, a researcher, Ken Simmons, introduced CaCO3 into theWhetstone Brook  in Massachusetts.[19] His hope was that the calcium

carbonate would counter the acid in the stream from acid rain and save

the trout that had ceased to spawn. Although his experiment was asuccess, it did increase the amounts of aluminium ions in the area of the

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brook that was not treated with the limestone. This shows that CaCO3 

can be added to neutralize the effects of acid rain in river ecosystems.Currently calcium carbonate is used to neutralize acidic conditions in

both soil and water.[20][21]

 Since the 1970s, such liming has been

practiced on a large scale in Sweden to mitigate acidification and several

thousand lakes and streams are limed repeatedly.

[22]

 

[edit] Calcination equilibrium

Calcination of limestone using charcoal fires to produce quicklime hasbeen practiced since antiquity by cultures all over the world. The

temperature at which limestone yields calcium oxide is usually given as

825 °C, but stating an absolute threshold is misleading. Calcium

carbonate exists in equilibrium with calcium oxide and carbon dioxide atany temperature. At each temperature there is a partial pressure of carbon

dioxide that is in equilibrium with calcium carbonate. At room

temperature the equilibrium overwhelmingly favors calcium carbonate,because the equilibrium CO2 pressure is only a tiny fraction of the partial

CO2 pressure in air, which is about 0.035 kPa.

At temperatures above 550 °C the equilibrium CO2 pressure begins to

exceed the CO2 pressure in air. So above 550 °C, calcium carbonate

begins to outgas CO2 into air. However, in a charcoal fired kiln, theconcentration of CO2 will be much higher than it is in air. Indeed if all

the oxygen in the kiln is consumed in the fire, then the partial pressure of 

CO2 in the kiln can be as high as 20 kPa.

The table shows that this equilibrium pressure is not achieved until thetemperature is nearly 800 °C. For the outgassing of CO2 from calcium

carbonate to happen at an economically useful rate, the equilibriumpressure must significantly exceed the ambient pressure of CO2. And for

it to happen rapidly, the equilibrium pressure must exceed total

atmospheric pressure of 101 kPa, which happens at 898 °C.

Equilibrium pressure of CO2 over CaCO3 (P) vs. temperature (T).[23] 

P

(kP

a) 

0.055 

0.13 

0.31 

1.80 

5.9 

9.3 

14  24  34  51  72  80  91  101 

179 

901  3961 

T

(°C) 550 

58

7 

60

5 

68

0 

72

7 

74

8 

77

7 

80

0 

83

0 

85

2 

87

1 

88

1 

89

1 

89

8 

93

7 

108

2 

124

1 

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[edit] Solubility

[edit

]

Wit

hvary

ing

CO2 

pres

sure

Calci

umcarbo

nateispoorl

y

solub

le inpure

water

(47mg/L

at

normalatmo

spher

icCO2 

partia

l pressure as shown below).

The equilibrium of its solution is given by the equation (with dissolved

calcium carbonate on the right):

CaCO3 Ca2+

+ CO32 –  K sp = 3.7×10

−9to 8.7×10

−9at 25 °C 

where the solubility product for [Ca2+

][CO32 – 

] is given as anywherefrom K sp = 3.7×10−9 to K sp = 8.7×10−9 at 25 °C, depending upon the

data source.[23][24] What the equation means is that the product of 

molar concentration of calcium ions (moles of dissolved Ca2+

per

Calcium ion solubility as a function of 

CO2 partial pressure at 25 °C (K sp = 4.47×10−9) 

(atm)  pH [Ca2+] (mol/L) 

10−12  12.0  5.19 × 10−3 

10−10  11.3  1.12 × 10−3 

10−8  10.7  2.55 × 10−4 

10−6  9.83  1.20 × 10−4 

10−4  8.62  3.16 × 10−4 

3.5 × 10−4  8.27  4.70 × 10−4 

10−3  7.96  6.62 × 10−4 

10−2  7.30  1.42 × 10−3 

10−1

  6.63  3.05 × 10−3

 

1  5.96  6.58 × 10−3 

10  5.30  1.42 × 10−2 

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liter of solution) with the molar concentration of dissolved CO32 – 

 

cannot exceed the value of K sp. This seemingly simple solubilityequation, however, must be taken along with the more complicated

equilibrium of  carbon dioxide with water (see carbonic acid). Some

of the CO32 – 

combines with H+

in the solution according to:

HCO3 –

H+

+ CO32 –

  K a2 = 5.61×10−11

at 25 °C 

HCO3 – 

is known as the bicarbonate ion. Calcium bicarbonate ismany times more soluble in water than calcium carbonate — 

indeed it exists only in solution.

Some of the HCO3 –  combines with H+ in solution according to:

H2CO3 H+ + HCO3 –  K a1 = 2.5×10−4 at 25 °C 

Some of the H2CO3 breaks up into water and dissolvedcarbon dioxide according to:

H2O + CO2(dissolved) H2CO3  K h = 1.70×10−3

at 25 °C 

And dissolved carbon dioxide is in equilibrium with

atmospheric carbon dioxide according to:

where k H = 29.76 atm/(mol/L) at 25 °C

(Henry constant), being the CO2 partial

pressure. 

For ambient air, is around 3.5×10−4

 

atmospheres (or equivalently 35 Pa). The last

equation above fixes the concentration of dissolvedCO2 as a function of , independent of the

concentration of dissolved CaCO3. At atmospheric

partial pressure of CO2, dissolved CO

2concentration

is 1.2×10−5

moles/liter. The equation before thatfixes the concentration of H2CO3 as a function of 

[CO2]. For [CO2]=1.2×10−5

, it results in

[H2CO3]=2.0×10−8

moles per liter. When [H2CO3] isknown, the remaining three equations together with

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H2O H+ + OH –  K = 10−14 at 25 °C 

(which is true for all aqueous solutions), and thefact that the solution must be electrically neutral,

2[Ca2+] + [H+] = [HCO3 –] + 2[CO3

2 –] + [OH –]

make it possible to solve simultaneously forthe remaining five unknown concentrations

(note that the above form of the neutralityequation is valid only if calcium carbonate

has been put in contact with pure water or

with a neutral pH solution; in the case where

the origin water solvent pH is not neutral, theequation is modified).

Travertine calcium carbonate deposits from a

hot spring 

The table on the right shows the result for[Ca2+] and [H+] (in the form of pH) as a

function of ambient partial pressure of CO2 

(K sp = 4.47×10−9

has been taken for thecalculation).

  At atmospheric levels of ambient CO2 the

table indicates the solution will be slightly

alkaline with a maximum CaCO3 solubility of 

47 mg/L.

  As ambient CO2 partial pressure is reduced

below atmospheric levels, the solution

becomes more and more alkaline. At

extremely low , dissolved CO2,

bicarbonate ion, and carbonate ion largely

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evaporate from the solution, leaving a

highly alkaline solution of  calcium

hydroxide, which is more soluble than

CaCO3. Note that for = 10−12 atm, the

[Ca2+

][OH−]

2product is still below the

solubility product of Ca(OH)2 (8×10−6

). For

still lower CO2 pressure, Ca(OH)2 

precipitation will occur before CaCO3 

precipitation.

  As ambient CO2 partial pressure increases to

levels above atmospheric, pH drops, and

much of the carbonate ion is converted to

bicarbonate ion, which results in higher

solubility of Ca2+

.

The effect of the latter is especially evident

in day-to-day life of people who have hardwater. Water in aquifers underground can be

exposed to levels of CO2 much higher thanatmospheric. As such water percolates

through calcium carbonate rock, the CaCO3 

dissolves according to the second trend.When that same water then emerges from the

tap, in time it comes into equilibrium with

CO2 levels in the air by outgassing its excess

CO2. The calcium carbonate becomes lesssoluble as a result and the excess precipitates

as lime scale. This same process isresponsible for the formation of  stalactites and stalagmites in limestone caves.

Two hydrated phases of calcium carbonate,monohydrocalcite, CaCO3·H2O and ikaite, 

CaCO3·6H2O, may precipitate from water at

ambient conditions and persist as metastablephases.

[edit] With varying pH

Consider the problem of the maximum

solubility of calcium carbonate in normal

atmospheric conditions ( = 3.5 × 10−4

 atm) when the pH of the solution is adjusted.

This is for example the case in a swimming

pool where the pH is maintained between 7

and 8 (by addition of  sodium bisulfate 

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NaHSO4 to decrease the pH or of  sodium

bicarbonate NaHCO3 to increase it). Fromthe above equations for the solubility

product, the hydration reaction and the two

acid reactions, the following expression for

the maximum [Ca

2+

] can be easily deduced:

showing a quadratic dependence in [H+].

The numerical application with theabove values of the constants gives[citation

needed ] 

pH  7.0 

7.2 

7.4 

7.6 

7.8 

8.0  8.2  8.27 

8.4 

[Ca2+]

max 

(10−6

mol/

L) 

1

8

0 

7

1.

7 

2

8.

5 

11

.4 

4.

52 

1.8

0 

0.7

17 

0.5

19 

0.2

85 

[Ca2+

]

max (mg/

L) 

7.

2

1 

2.

8

7 

1.

1

4 

0.

45

5 

0.

18

1 

0.0

72

1 

0.0

28

7 

0.0

20

8 

0.0

11

4 

Comments:

  decreasing the pH from 8 to 7 increases

the maximum Ca2+ concentration by a

factor 100. Water with a pH maintained

to 7 can dissolve up to 15.9 mg/L of 

CaCO3. This explains the high Ca2+ 

concentration in some mineral waters

with pH close to 7.

  note that the Ca2+

concentration of the

previous table is recovered for pH =

8.27

  keeping the pH to 7.4 in a swimming

pool (which gives optimum HClO/ClO− 

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ratio in the case of "chlorine"

maintenance) results in a maximum

Ca2+

concentration of 1010 mg/L. This

means that successive cycles of water

evaporation and partial renewing may

result in a very hard water before

CaCO3 precipitates (water with a Ca2+ 

concentration above 120 mg/L is

considered very hard). Addition of a

calcium sequestering agent or complete

renewing of the water will solve the

problem.

[edit] Solubility in a strong or

weak acid solution

Solutions of  strong (HCl), moderatelystrong (sulfamic) or weak  (acetic, citric, sorbic, lactic, phosphoric) acids are

commercially available. They are

commonly used as descaling agents toremove limescale deposits. The

maximum amount of CaCO3 that can be

"dissolved" by one liter of an acidsolution can be calculated using the

above equilibrium equations.

  In the case of a strong monoacid with

decreasing acid concentration [A] = [A−],

we obtain (with CaCO3 molar mass =

100 g):

[A]

(mo

l/L) 

1 

1

0−

1 

10−2 

10−

3 

10−

4 

10−

5 

10−

6 

10−

7 

10−

10 

Initi

al

pH 

0.

0

0 

1.

0

0 

2.

00 

3.0

0 

4.0

0 

5.0

0 

6.0

0 

6.7

9 

7.0

0 

Fina

l pH 

6.

7

5 

7.

2

5 

7.

75 

8.1

4 

8.2

5 

8.2

6 

8.2

6 

8.2

6 

8.2

7 

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Diss

olve

d

CaC

O3 

(g

per

liter

of 

acid

) 

5

0.

0 

5.

0

0 

0.

51

4 

0.0

84

9 

0.0

50

4 

0.0

47

4 

0.0

47

1 

0.0

47

0 

0.0

47

0 

where the initial state is the acid solution

with no Ca2+

(not taking into account

possible CO2 dissolution) and the finalstate is the solution with saturated Ca2+.

For strong acid concentrations, all

species have a negligible concentrationin the final state with respect to Ca2+ and

A−

so that the neutrality equation reduces

approximately to 2[Ca2+

] = [A−] yielding

. When the concentrationdecreases, [HCO3

−] becomes non-

negligible so that the preceding

expression is no longer valid. For

vanishing acid concentrations, one canrecover the final pH and the solubility of 

CaCO3 in pure water.

  In the case of a weak monoacid (here

we take acetic acid with pK A = 4.76)

with decreasing total acid concentration

[A] = [A−]+[AH], we obtain:

[A]

(mol/L) 

1 

1

0

−

1 

10

−2 

10−

3 

10−

4 

10−

5 

10−

6 

10−

7 

10−

10 

Initi

al

pH 

2.

3

8 

2.

8

8 

3.

39 

3.9

1 

4.4

7 

5.1

5 

6.0

2 

6.7

9 

7.0

0 

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Fina

l pH 

6.

7

5 

7.

2

5 

7.

75 

8.1

4 

8.2

5 

8.2

6 

8.2

6 

8.2

6 

8.2

7 

Diss

olve

d

CaC

O3 

(g

per

liter

of 

acid

) 

4

9.

5 

4.

9

9 

0.

51

3 

0.0

84

8 

0.0

50

4 

0.0

47

4 

0.0

47

1 

0.0

47

0 

0.0

47

0 

For the same total acid concentration, the

initial pH of the weak acid is less acid

than the one of the strong acid; however,the maximum amount of CaCO3 which

can be dissolved is approximately the

same. This is because in the final state,

the pH is larger than the pK A, so that theweak acid is almost completely

dissociated, yielding in the end as manyH+

ions as the strong acid to "dissolve"the calcium carbonate.

  The calculation in the case of 

phosphoric acid (which is the most

widely used for domestic applications)

is more complicated since the

concentrations of the four dissociation

states corresponding to this acid must

be calculated together with [HCO3−],

[CO32−], [Ca2+], [H+] and [OH−]. The

system may be reduced to a seventh

degree equation for [H+] the numerical

solution of which gives

[A]

(mol

1  1

0−

10−2 

10−3 

10−

4 

10−

5 

10−

6 

10−

7 

10−

10 

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/L)  1 

Initi

al

pH 

1.

0

8 

1.

6

2 

2.

25 

3.

05 

4.0

1 

5.0

0 

5.9

7 

6.7

4 

7.0

0 

Fina

l pH 

6.

7

1 

7.

1

7 

7.

63 

8.

06 

8.2

4 

8.2

6 

8.2

6 

8.2

6 

8.2

7 

Diss

olve

d

CaC

O3 (g

per

liter

of 

acid

) 

62.

0 

7.3

9 

0.87

4 

0.12

3 

0.053

6 

0.047

7 

0.047

1 

0.047

1 

0.047

0 

where [A] = [H3PO4] + [H2PO4−] +

[HPO4

2−

] + [PO4

3−

] is the total acidconcentration. Thus phosphoric acid is

more efficient than a monoacid since atthe final almost neutral pH, the second

dissociated state concentration [HPO42−

]

is not negligible (see phosphoric acid).