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Potentiometric Titration of Water Hardness, Calcium, and Magnesium

S. Virojanavat and C. O. Huber
Journal (Water Pollution Control Federation)
Vol. 51, No. 12 (Dec., 1979), pp. 2941-2947
Stable URL: http://www.jstor.org/stable/25040519
Page Count: 7
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Potentiometric Titration of Water Hardness, Calcium, and Magnesium
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Abstract

The chelometric titration of calcium and magnesium with ethylenediamine tetraacetic acid using a lead dioxide and lead sulfide electrode pair is described. The titration endpoint is located by a simple sensing comparator circuit, using one operational amplifier. The proposed endpoint detection gives results in agreement with the standard method for total hardness. Samples of calcium carbonate down to 20 mg/l can be titrated. Precision for hardness titrations is 0.9% for 20-mg/l samples. Calcium can be titrated in the presence of magnesium at pH 12.4 to a first endpoint, then the titration is continued at pH 10.4 to the second endpoint for magnesium. The estimated standard deviation for a calcium concentration of 16 mg/l is 1.5%, a magnesium concentration of 8 mg/l is 3.4%, and for total calcium plus magnesium is 0.9%. Heavy metals (for example, ${\rm Cu}^{2+}$, ${\rm Pb}^{2+}$, and ${\rm Ni}^{+}$) can also be titrated by the technique using either acetate (pH 5) or ammonia buffers.

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