LaMotte AM-21 Water Pollution Detection Outfit - Manual (Page 12)

Calcium, Magnesium, & Total Hardness Test
Calcium, magnesium and total hardness factors of a water should be
considered as a group since the total hardness of a water generally represents
the total concentration of calcium and magnesium ions expressed as calcium
carbonate. Other ions may contribute to the hardness of water, but in natural
waters all but calcium and magnesium are present in insignificant quantities.
When the hardness of a water is greater than the sum of the carbonate and
bicarbonate alkalinity, the amount in excess is called “noncarbonate
hardness” and such waters may contain considerable amounts of chloride
and sulfate ions. This is an important factor to consider when treating
potable water by ion exchange methods. The hardness of water may range
from zero to hundreds of milligrams per liter, (or parts per million),
depending on the source or the treatment to which it has been subjected.
A knowledge of the hardness of water is of great importance in the industrial
uses since it is the chief source of scale in heat exchange equipment, boilers,
pipe lines, etc. From the domestic standpoint, hard water consumes
excessive quantities of soap, forming curds and depositing a film on hair,
fabrics and glassware.
Total Hardness of water is determined by titration with a EDTA solution,
using Calmagite as the endpoint indicator. The total hardness minus the
calcium hardness equals the magnesium hardness. Calcium is determined by
EDTA titration in a manner similar to the total hardness determination.
Drinking water quality standards, as determined by the US Public Health
Service, set limits of calcium hardness at 200 ppm and magnesium hardness
at 150 ppm. Waters with a total hardness in the range of 0-60 ppm are
termed soft; from 60-120 ppm medium hard, from 120-180 ppm hard and
above 180 ppm very hard.
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