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Comparing the results of different techniques to determine water harness

Annie KrichtenChem 111 Sect 103

TA: Bill Charette4/2/12

Group Members: Colin Lesnick, Erica Konston,

Matthew Leberfinger

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Introduction: Water hardness is a measure of water’s chemical and biological composition (1).

Most commonly, it is an indicator of Ca2+ and Mg2+ (1). These ions come from the dissolution of

minerals into the water supply, and with higher ion concentration, water is termed hard as

opposed to soft (1). Different techniques can be used to determine water hardness such as EDTA

titration, the use of a cation exchange resin, and measurements made during Atomic Absorption

(AA) Spectrophotometry (1). In addition, the level of the ions present in the water can be

measured several ways: using grains per gallon, milligrams per liter, or parts per million (2).

Knowing the level of water harness is important because it is considered when rating the water’s

potential interactions with our health as well as its taste, odor, color, and corrosive ability (2).

As mentioned before, EDTA and AA may be used to measure the level of ions present in the

water and thus the water hardness. EDTA uses simple titration techniques and color indicators to

show when all of the Ca2+ and Mg2+ ions have completely reacted with the EDTA solution of

known concentration (3). When the solution turns blue in color, the concentration of the Ca2+

ions may be calculated using the equation (M1 x V1) = (M2 x V2), where the 1 values correspond

to EDTA values determined by the progress through the titration, and the 2 values correspond to

the Calcium ion numbers (1). While EDTA titrations measure the presence of all divalent

cations, AA can measure for one, which can be converted into a value close (but just below) that

gained at the end of the calculations from the EDTA titrations. For AA, the calculations to find

the unknown metal concentration in the sample can be done by applying the Beer-Lambert law

(4). This is because the amount of absorbance by the sample of a monochromatic light that

corresponds to the element being analyzed is proportional to the concentration of the metal ions

in the sample (1). The two separate methods, the EDTA titrations and the AA, may be used

because the EDTA can show how much of the other divalent cations are present in the water

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supply besides the one being tested for in AA. While these methods test for water hardness,

water softening is also a concept to be understood. Water softening is performed when water is

too hard, and can be done by removing some of the calcium and magnesium (5). Water softeners

trade those minerals for something else, usually sodium (6). In our experiments with water

hardness water will be tested from different rivers across Pennsylvania and Maryland. The four

rivers to be sampled are as followed: the Delaware River near Washington Crossing, PA; the

Allegheny River; the Susquehanna River; and the South River, MD. Considering farming levels

and runoff as well as the pH of the different water supplies as an indicator of basic salts

(Magnesium and Calcium Carbonate), I hypothesize that water hardness will be highest to lowest

as follows: the Allegheny River, the Delaware River, the Susquehanna River, the South River

(7).

Procedure: In our experiments, we performed a total dissolved solids test to evaporate off the

water in our samples and compare residues left behind from salts in the water samples. The

more residue left behind, the more nonvolatile salts made up the certain volume of the water

sample (8). The amount of residue was compared in distilled water, our own water samples, and

in a calcium solution. Practice titrations were also performed before EDTA titrations with our

own water samples. If water was too soft, steps were taken to perform a different titration.

Samples were also diluted if needed. In a 1x12 well tray, solutions were scaled according to

volume of EDTA used, measured in the number of drops included in the wells. On the first well

to have the solution turned blue, the molarity and volume of the EDTA solution could be used to

calculate the molarity of the known volume of water sample used. The same titration was also

performed after our water samples were softened using baking soda. Our samples were also

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analyzed for the concentrations of calcium and magnesium using AA data and calibration graphs.

In addition, pH and concentration were tested after the divalent cations were removed by ion

exchange resins. Procedures followed Chemtrek guidelines (1).

Results: Data below is pooled from a team of four students (9-12). In the first test, the amount of

residue left behind after the evaporation of one drop of water is observed.

Table 1: Total Dissolved Solids TestSample, Reference dH2O Calcium solution Specific sample

Delaware River, PA (9) none Less defined lining than Del. R sample

More solid lining than Ca soln sample

Results indicate that there is a more defined residue mark left behind from the Delaware River

water than there is from the drop of 1 x 10-3 M Ca2+. Next, water hardness was calculated for the

four water samples involved by various methods including AA and EDTA. The EDTA method

was used for the original water samples and for the same samples after they were softened by

baking soda and resin beads.

Table 3: EDTA titration and AA calculations of water hardnessSample,

ReferenceAA absorbance

valuesEDTA

# of dropsDiluted for AA?

Diluted for

EDTA?

Hadness values (grains/gal)

Ca Mg unsoftened Baking soda

softened

Resin softened

EDTA unsoft.

EDTA Baking

soda softened

EDTA Resin

softened

AA

Delaware River, PA

(9)

0.1984 0.1213 4 4 < 1 no no 4.7 4.7 < 1.2 4.595

Allegheny River, PA

(10)

0.2058 0.0937 3 5 4 no no 3.5 5.8 4.7 4.3

South River, MD

(11)

0.4438 0.6668 7 6 1 Ca: ½Mg: ¼

Ca: ½Mg: ¼

8.2 7.0 1.2 16.9

Sesquehanna River, PA

(12)

0.2011 0.0724 4 9 4 no no 4.7 10.5 4.7 3.9

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Sample calculations are below of how hardness values can be calculated from the data gathered:

the absorbance levels for Ca2+ and Mg2+ and the wells in which the titrated solution turned blue in

color.

Sample Calculations: EDTA hardness (9).(M1 x V1) = (M2 x V2),

where M = Molarity, V = Volume, the 1 values correspond to EDTA values determined by the progress through the titration, and the 2 values correspond to the Calcium ion numbers

(2x10-4M)(4 drops) = (M2)(1 drop)(M2) = 0.008M

0.008M x 100,000 = 80ppm

80ppm x (1 grain/gal/17.1ppm) = 4.7 grains/gal (CaCO3)

Sample Calculations: AA hardness (9).Fig. 1,

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Figure 1: Calibration Curve (Ca)

0 10 20 30 40 50 600

0.050.1

0.150.2

0.250.3

0.350.4

0.450.5

f(x) = 0.00938669220489978 x + 0.00793850155902007

Calcium Concentration versus Absorbance

Concentration (ppm)

Abso

rban

ce Calibration Line

*The Beer-Lambert Law states that the amount of absorbance by the sample of a monochromatic light that corresponds to the element being analyzed is proportional to the concentration of the metal ions in the sample (1).

Figure2: Calibration Curve (Mg)

0 5 10 15 20 25 30 350

0.050.1

0.150.2

0.250.3

0.350.4

0.450.5

f(x) = 0.0154663884892086 x + 0.0161744028776978

Magnesium Concentration versus Absorbance

Concentration (ppm)

Abso

rban

ce

Once calculations are made, data gathered in Table 3 can be visually expressed in Figure 3.

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Figure 3: A Comparison of Techniques and Water Hardness Values in Four Samples

1 2 3 40

2

4

6

8

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Water Hardness Values Across Samples

Delaware River, PA (8)

Allegheny River, PA (9)

South River, MD (10)

Sesquehanna River, PA (11)

1. EDTA unsoftened 2. EDTA softened with baking soda 3. EDTA softened with resin 4. AA

Hard

ness

(gra

ins/

gal)

to 16.9

*The AA water hardness value from the South River is cut off to better see the other data

Discussion: From Figure 3, it can be noted that by EDTA testing, there is sometimes an increase

in water hardness from the unsoftened water to the water softened with baking soda, though it is

not so for all water samples. In addition, the water hardness does decrease or stay about the

same in all samples from the original sample to the water softened by resin. In all samples,

hardness is lower from water softened by baking soda to water softened by resin. In the

softening process, the baking soda exchanged the Ca2+ and Mg2+ cations with sodium and the

resin made the exchange with protons (13). Perhaps the protons had more of an impact because

they would lower the pH of the sample and because there is more of a difference between the

two ions mainly tested for and H+, than between the two ions and Na+. While the AA hardness

values should be lower than EDTA hardness levels, since AA only accounts for two of all of the

divalent actions, it is not always so. This could be due to human errors or misunderstanding of

what to do with the dilution factors. Analyzing in a different manner (across the four samples),

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the Allegheny River seems to have the softest water followed by the Delaware River. The South

River then has the hardest water followed by the Susquehanna River. While these results do not

match my predictions exactly, they do provide insight into the varying levels of water hardness

in rivers in Pennsylvania and into Maryland. Results compared to standard values also suggest

that the most accurate method of finding water hardness is likely EDTA (14). This is likely

because EDTA tests for the presence of all divalent cations and because hardness values

calculated from the EDTA data are more consistent and precise (9-12). While the determination

of which well the color changed in can lead to slight sources of error because picking the wrong

well or having different drop sizes would affect the molarity of the water sample, I trust the

EDTA’s resultant water hardness values more than those from the AA tests. AA only looks at

two cations, there can be issues with machine calibration, and there are many more points where

human error can accumulate in the mathematical calculations of the hardness values.

Conclusions: In sum, different than my hypothesis, the Allegheny River seems to have the softest

water followed by the Delaware River, and the South River has the hardest water followed by the

Susquehanna River. The EDTA results and water hardness values seem to be more trustworthy,

and trends can be noted between EDTA tests such that hardness values lower from water

softened by baking soda to water softened by resin. More variation occurred in other aspects of

the EDTA data such that there is sometimes an increase in water hardness from the unsoftened

water to the water softened with baking soda, though it is not so for all water samples. In

addition, the water hardness decreases or stays about the same in all samples from the original

sample to the water softened by resin. In most of the data, the AA hardness values were lower

than or very close to the EDTA hardness values.

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Works Cited

1. Thompson, Stephen. "The Chemistry of Natural Waters." PSU Chemtrek. 18th ed. Plymouth,

MI: Hayden-McNeil, 2012. 10-2-0-22. Print.

2. "Explanation of Water Hardness." Fairfax Water. 5 Jan. 2011. Web. 1 Apr. 2012.

3. Flaschka, H. A. EDTA Titrations; an Introduction to Theory and Practice. New York:

Pergamon, 1964. Print.

4. Tissue, Brian M. "Atomic-Absorption Spectroscopy (AA)." Atomic-Absorption Spectroscopy.

21 Aug. 1996. Web. 31 Mar. 2012.

5. Gabrielli, C. "Electrochemical Water Softening: Principle and Application." Desalination

201.1-3 (2006): 150-63. ScienceDirect. Web. 31 Mar. 2012.

6. Klenck, Thomas. "How It Works: Water Softener." Popular Mechanics. 2012. Web. 1 Apr.

2012.

7. "Rivergages.com: Providing River Gage Data for Rivers, Streams and Tributaries." Web. 1

Apr. 2012.

8. Mallock, A. "Hardness." Nature 117 (1926): 117. Print.

9. Krichten, Annie. “Experiment 10 – Water.” Lab Notebook: 27-30. 13 Mar. 2012.

10. Lesnick, Colin. Lab Notebook: 36-41. 13 Mar. 2012.

11. Konston, Erica. Lab Notebook: 38-41. 13 Mar. 2012.

12. Leberfinger, Matthew. Lab Notebook: 27-28. 13 Mar. 2012.

13. Blanning, Harry K., and Albert D. Rich. Boiler Feed and Boiler Water Softening. 3rd ed.

Chicago, Nickerson & Collins, 1942. Print.

14. Williams, Samuel R. Hardness and Hardness Measurements. American Society for Metals,

1942. Print.

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*May also see:

Brown, Theodore L., Eugene H. LeMay, Bruce E. Bursten, and Catherine J. Murphy. Chemistry:

The Central Science. 11th ed. Upper Saddle River, NJ: Pearson Education, 2009. Print.