Drinking Water Monitoring - The Drinking Water - page 203

natural freshwaters. Calcium is an important component of aquatic plant cell
walls and the shells and bones of many aquatic organisms. Magnesium is an
essential nutrient for plants and is a component of the chlorophyll molecule.
Hardness test kits express test results in ppm of CaCO 3 , but these results can
be converted directly to calcium or magnesium concentrations:

Calcium Hardness as ppm CaCO 3 × 0.40 = ppm Ca (10.1)

Magnesium Hardness as ppm CaCO 3 × 0.24 = ppm Mg (10.2)

Note: Because of less contact with soil minerals and more contact with
rain, surface raw water is usually softer than groundwater.

As a general rule of thumb, when hardness is greater than 150 mg/L, soft-
ening treatment may be required for public water systems. Hardness deter-
mination via testing is required to monitor the efficiency of treatment.

DID YoU KNoW?
Household ion-exchange water softeners may identify hardness in
terms of grains per gallon, so it is useful to know that 1 grain per gallon
= 17.1 mg/L.

Measuring Hardness

A hardness test follows a procedure similar to an alkalinity test, but the reac-
tions involved are different. After the sample is carefully measured, a buffer
is added to the sample to correct pH for the test and an indicator to signal
the titration endpoint. The indicator reagent is normally blue in a sample of
pure water, but if calcium or magnesium ions are present in the sample the
indicator combines with them to form a red complex. The titrant in this test
is ethylenediaminetetraacetic acid (EDTA), used with its salts in the titration
method; EDTA serves as a chelant that pulls the calcium and magnesium
ions away from the red complex. The EDTA is added dropwise to the sample
until all the calcium and magnesium ions have been chelated away from the
complex and the indicator returns to its normal blue color. The amount of
EDTA required to cause the color change is a direct indication of the amount
of calcium and magnesium ions in the sample. Some hardness kits include
an additional indicator that is specific for calcium. This type of kit will pro-
vide three readings: total hardness, calcium hardness, and magnesium hard-
ness. For more information on interference, precision, and accuracy, consult
the latest edition of Standard Methods .