Introduction
Water hardness is caused by the presence of calcium and magnesium salts. High levels of
hardness prevent the formation of lather with soap, and can cause scaling in water systems,
particularly steam boilers, engine cooling systems and fresh water pipework. Hardness is an
important control test in a wide variety of applications.
The Total Hardness Test provides a simple method of checking water hardness over the
range 5 - 500 mg/l CaCO3.
Method
Calcium and magnesium ions are complexed by reaction with ethylenediaminetetraacetic
acid (EDTA). Excess calcium and magnesium ions react with a specific indicator to produce
a distinctive coloration. The Hardness test uses a tablet reagent containing a standardized
amount of EDTA with eriochrome black as indicator. The test is carried out by adding tablets
one at a time to a sample of water until the colour changes from plum red to blue. The Total
Hardness result is calculated from the number of tablets used in relation to the volume of
water sample taken.
Test Range
The test is normally carried out on a 50ml sample although a larger sample may be used if a
lower test range is required. The table below indicates the sample size appropriate to various
calcium hardness test ranges.
Test Range Sample Size
0 - 100 mg/l CaCO3 200 ml
0 - 250 mg/l CaCO3 100 ml
0 - 500 mg/l CaCO3 50 ml
For 200 ml samples uses the container provided to measure 2 x 100 ml samples into a
suitable clear container which is thoroughly clean.