ALKALINITY AND HARDNESS

Chemical Variables: Total Alkalinity

Total Alkalinity: the total amount of titratable bases in water expressed as mg/L of equivalent CaCO3.

“Alkalinity” is primarily composed of the following ions: CO3

-. HCO3-. hydroxides.

ammonium. borates. silicates. phosphates. Alkalinity in ponds is determined by both the

quality of the water and bottom muds. Calcium is often added to water to increase its

alkalinity. buffer against pH changes.

Total Alkalinity Total alkalinity = 200 mg/L. Good buffering

capacity of a water source.

Freshwater 5 mg/L (soft water) to 500 mg/L (hard water).

Seawater ~ 115-120 mg/L.

Seldom see pH problems in natural seawater.

Alkalinity < 30 mg/L? Problem?

Total Alkalinity

Total Alkalinity (TA) level can be associated with several potential problems in aquaculture:

If TA< 50 mg/L: copper compounds are more toxic. avoid their use as algicides (copper sulfate)

Natural waters with less than 40 mg/L alkalinity as CaCO3 have limited biofiltration capacity. pH independent (What does this mean?)

Low alkalinity = low CO2 --> low natural productivity

Low alkalinity equals low pH.

Total Hardness Total Hardness: total concentration of metal

ions expressed in terms of mg/L of equiva- lent CaCO3.

Primary ions are Ca2+ and Mg2+. also iron and manganese.

Total Hardness approximates total alkalinity. Calcium is used for bone and exoskeleton

formation and absorbed across gills. Soft water = molt problems.

bone deformities....

or minimally...clogged pipes!

CONVERSION OF WATER HARDNESS UNITS

Water Hardness

Unit Definition

International

recommended mmol/liter

Physical

measures

mval/liter

America &

statesPPM

Englishoe

Frenchof

GermanodH

100mg CaCO3

per 1000 ml water

28 mg CaO or 50 mg CaCO3

per 1000 ml water

1 part CaCO3 per million = 1 mg CaCO3 per 1000 ml water

1 grainCaCO3

per gallon= 14.3 mg

CaCO3 per 1000 ml water

10 mg/ CaCO3

per 1000 ml water

10 mg/CaO per 1000 ml water

1 mmol/litre

1 mval/litre

1 PPM

1 Oe

1 Of

1 OdH

1

0.5

0.01

0.1429

0.10

0.1786

2

1

0.02

0.285

0.20

0.357

100

50

1

14.29

10.00

17.86

7.0

3.5

0.070

1

0.700

1.250

10.00

5.00

0.10

1.429

1

1.786

5.6

2.8

0.056

0.7999

0.5599

1

DEGREE OF HARDNESS EXPRESION SOFT: < 1.6 mmol/l = 160 PPM = 9 odH

SLIGHTLY HARD: 1.6-3.2 mmol/l = 160-320 PPM = 9-18 odH

HARD: 3.2-4.6 mmol/l = 320-460 PPM = 18-26 odH

VERY HARD: ABOVE 4.6 mmol/l = ABOVE 460 PPM = ABOVE 26 odH

Chemical Variables: pH

pH: the level or intensity of a substance’s acidic or basic character.

pH: the negative logarithm of the hydrogen ion concentration (activity) of a substance.

pH = -log(1/[H+]). Ionization of water is low (1x10-7 moles

of H+/L and 1x10-7 moles OH-/L). Neutral pH = similar levels of H+ and

OH-

Chemical Variables: pH

At acidic pH levels. the quantity of H+ predominates.

Acidic pH = pH < 7. basic = pH >7 Most natural waters: pH of 5-10. usually 6.5-9;

however. there are exceptions. Acid rain. pollution. Can change due to atm. CO2 , fish respiration. pH of ocean water is stable (carbonate buffering

system. later).

Chemical Variables: pH

Other sources of change: Decay of organic matter. Oxidation of compounds in bottom sediments. Depletion of CO2 by phytoplankton on diel

basis. Oxidation of sulfide containing minerals in

bottom soils (e.g.. oxidation of iron pyrite by sulfide oxidizing bacteria under anaerobic conditions).

Chemical Variables: Carbon Dioxide Normal component of all natural waters. Sources: atmospheric diffusion. respiration

of cultured species. biological oxidation of organic compounds.

Usually transported in the blood as HCO3-

Converted to CO2 at the gill interface. diffusion into medium.

As the level of CO2 in the medium increases. the gradient allowing diffusion is less.

Chemical Variables: Carbon Dioxide

This causes blood CO2 levels to increase. lowering blood pH.

With lower blood pH. carrying capacity of hemoglobin decreases. also binding affinity for oxygen to hemoglobin.

This phenomenon is known as the Bohr-Root effect.

CO2 also interferes with oxygen uptake by eggs and larvae.

CO2 Level Affects Hemoglobin Saturation

Chemical Variables: carbon dioxide

In the marine environment. excesses of CO2 are mitigated by the carbonate buffering system.

CO2 reacts with water to produce H2CO3. carbonic acid.

H2CO3 reacts with CaCO3 to form HCO3-

(bicarbonate) and CO32- (carbonate).

As CO2 is used for photosynthesis. the reaction shifts to the left. converting bicarbonates back to CO2.

What large-scale implications does this have?

The Effect of pH on Carbonate Buffering

Chemical Variables: carbon dioxide

Concentrations of CO2 are small. even though it is highly soluble in water

inverse relationship between [CO2] and temperature/salinity

thus. CO2 solubility depends upon many factors

Chemical Variable: Carbon Dioxide CO2 is not particularly toxic to fish or

invertebrates. given sufficient D.O. is available.

Maximum tolerance level appears to be around 50 mg/L for most species.

Good working level of around 15-20 mg/L.

Diel fluctuation opposite to that of D.O. Higher levels in warmer months of year.