The use of soda reagent does not avoid entirely the error due to
solubility of the salts of calcium and magnesium; consequently, if much
depends on the results, as in water softening, gravimetric
determinations of the calcium and magnesium that remain in solution
should be made and a correction should be applied for those amounts.
_Reagent._—Prepare soda reagent from equal parts of sodium hydroxide and
sodium carbonate. It should be approximately tenth normal.
_Procedure._—Measure 200 cc. of the sample and 200 cc. of distilled
water into 500 cc. Jena or similar glass Erlenmeyer flasks. Treat the
contents of each flask in the following manner. Boil 15 minutes to expel
free carbon dioxide. Add 25 cc. of soda reagent. Boil 10 minutes, cool,
rinse into 200 cc. graduated flasks, and dilute to 200 cc. with boiled
distilled water. Filter, rejecting the first 50 cc., and titrate 50 cc.
of each filtrate with N/50 sulfuric acid in the presence of methyl
orange or erythrosine indicator. The non-carbonate hardness in parts per
million of calcium carbonate is equal to 20 times the difference between
the number of cubic centimeters of sulfuric acid required for the soda
reagent in distilled water and the number of cubic centimeters of N/50
sulfuric acid required for the soda reagent in the sample.
Water naturally containing bicarbonate and carbonate in excess of
calcium and magnesium requires a larger amount of acid to neutralize the
sample after it has been treated than is required to neutralize the
volume of soda reagent originally added. (See p. 39.)