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Determination Of The Hardness Of Water Water

Determination of the Hardness of Water

One of the factors that establishes the quality of a water supply is its degree of hardness.

Hardness is defined as calcium and magnesium ion content. Since most analyses do not

distinguish between Ca

and Mg

, and since most hardness is caused by carbonate mineral

deposits, hardness is usually reported as parts per million (ppm) of calcium carbonate (by

weight). A water supply with a hardness of 100 ppm contains the equivalent of 100 g of CaCO

in 1 million g of water or 0.1 g in 1 L of water (or 1000 g of water since the density of water is

about 1 g/mL).

Water Hardness

calcium carbonate (ppm)

designation

0-43

Soft

43-150

Slightly Hard

150-300

Moderately Hard

300-450

Hard

450

Very Hard

Water hardness is usually noticed because of difficulty in lathering soap and the formation of a

scum in the bathtub. Ca

and Mg

form insoluble salts with soaps causing precipitation of the

soap scum. Another effect of hard water is “boiler scale”. When hard water comes into contact

with dissolved carbonates, a precipitate of insoluble calcium carbonate forms. This “scale” can

build up on the inside of water pipes to such a degree that the pipes become almost completely

blocked.

Water hardness can be readily determined by titration with the chelating agent (Greek χηλή,

chelè, meaning claw) EDTA (ethylenediaminetetraacetic acid). This reagent is a weak acid that

can lose four H (in bold) on complete neutralization; its structural formula is:

HOC

COH

HOC

COH

The four acid oxygen sites and the two nitrogen atoms have unshared electron pairs, which can

form bonds to a metal ion forming a complex ion or coordination compound. The complex is

quite stable, and the conditions of its formation can ordinarily be controlled so that it is selective

for a particular metal ion.

Determination Of The Hardness Of Water Water