Report – CHEM120 – Determination of Water Hardness using EDTA

CHEMESTRY 120-02                                                   2009-11-17
DETERMINATION OF WATER HARDNESS USING E.D.T.A.

Stefan Martensson C0347318
Lab Partner: Danielle

Procedure: Please refer to handout ‘Experiment #7’ and page 35-38, Chemistry 120 Lab Manual, 2009 Edition, Camosun College. Plus additional handout: Report and Calculation Guide.

Theory:
Hard water is water that has high mineral content (mainly calcium and magnesium ions). Hard water minerals primarily consist of calcium (Ca2+), and magnesium (Mg2+) cations, and sometimes other dissolved compounds such as bicarbonates and sulfates. Calcium usually enters the water as either calcium carbonate (CaCO3), in the form of limestone and chalk, or calcium sulfate (CaSO4), in the form of other mineral deposits. Hardness in water can cause water to form scales and a resistance to soap. It can also be defined as water that doesn’t produce lather with soap solutions, but produces white precipitate (scum) (Wikipedia, 2009).

EDTA is an organic acid chemists use in many applications. Its usefulness arises because of its role as a chelating agent, i.e. its ability to “sequester” metal ions such as Ca2+ and Fe3+. After being bound by EDTA, metal ions remain in solution but exhibit diminished reactivity (Wikipedia, 2009).

Data and Results:
Raw Data:

Used Unknown #172.

2.00g EDTA

0.2649g CaCO3


Table A – Standardization of EDTA.

Trial Start Vol. (ml) End Vol. (ml) Amount EDTA (ml) Correcting for added Mg (-4.12ml)
1 2.45 30.92 28.47 28.47-4.12 = 24.35
2 1.50 30.11 28.61 28.61-4.12 = 24.49
3 1.61 30.10 28.49 28.49-4.12 = 24.37

Margin of Error on Standardization:

((Highest-Lowest)/Average)*100 should be within +/- 0.25%

28.61-28.47=0.14          Average: 85.57

Margin of error in EDTA: (0.14/85.57)*100 = 0.16% <0.25% = OK!

Table B – Analysis of Hard Water

Trial Start Vol. (ml) End Vol. (ml) Amount EDTA (ml) Correcting for added Mg (-4.12ml)
1 2.52 41,71 39.19 39.19-4-12 = 35.07
2 2.40 42.39 39.99 39.99-4.12 = 35.87
BLANK 0.50 4.62 4.12

Molar mass of CaCO3 = 100.09g/mol

Mass of CaCO3 weighed out: 0.2649g

1A) 0.2649g/100.09g/mol = 0.002647mol CaCO3

1B) 0.002647mol/0.2500l = 0.1059mol/l [CaCO3]

1C) [CaCO3] = [Ca2+] = 0.1059mol/l

2A) 0.1059mol/l*0.02500l = 2.6475*10-4 mol Ca2+ in the 25ml sample.

2B) 2.6475*10-4mol Ca2+ = 2.6475*10-4mol EDTA.

2C) 2.6475*10-4 / ((0.02435l+0.02449l+0.02437l)/3) = 0.01085mol/l EDTA = [EDTA]

3A) 0.01085mol/l * ((0.03507l+0.03587l)/2) = 3.848*10-4mol Ca2+Unknown

3B) 3.848*10-4mol Ca2+Unknown / 0.02500l = 0.01539mol/l Ca2+ = [Ca2+]

3C) 0.01539mol/l * 40.08g/mol = 0.6168g/l

0.6168g/l * 1000 = 616.8mg/l = 616.8ppm Ca2+ in the Unknown.

Discussion

Two major sources of the experiment were the evaluation of what was purple and what was blue on the indicators. My personal opinion was not at all the same as others. Perhaps there are more exact indicators that shift colour more defined. This error would make us pour too much titrate into the solution, thus making the standard concentration lower than it actually were.

Second source could be weighing of the salts to make up the standard, which also would influence the concentration and bias the results up or down, depending on if we weighed too little or too much.

Conclusion

The concentration was concluded to be 616.8ppm Ca2+ in the unknown, about three times as high as you would expect the prairie provinces water hardness to be, and 200 times as high as the water around Vancouver.

One Response to “Report – CHEM120 – Determination of Water Hardness using EDTA”

  • matt:

    1B) VL should be .025 L

    thanks for this by the way, much more simple than my TA’s explanation.

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