water hardness
/ WAW-ter HARD-ness /
Think of why soap lathers richly in some places and barely foams in others, or why a kettle furs up with scale. That difference comes down to water hardness — a measure of how much dissolved calcium and magnesium the water carries.
Formally, water hardness is the total concentration of calcium and magnesium ions in water, usually reported as an equivalent amount of calcium carbonate. It is the property that consumes soap, forms scum, and deposits scale in pipes and boilers when the water is heated.
It matters for households, agriculture, and industry, and is one of the textbook uses of complexometric titration. The standard caveat: 'hardness' lumps calcium and magnesium together into one number, so a single titration value does not tell you the split between the two unless you run the analysis in separate steps.
A water utility measures hardness by titrating a sample with EDTA using Eriochrome Black T at pH 10; the EDTA volume converts to milligrams of calcium carbonate per litre, the standard hardness figure.
An EDTA titration gives total hardness as calcium carbonate equivalent.
To split the result, calcium is titrated separately at high pH (around 12), where magnesium drops out as hydroxide and is not counted; the difference from total hardness then gives the magnesium.