One English degree of hardness, Clark’s scale, is equivalent to 1 grain per Imperial gallon of calcium carbonate. One French degree of hardness is equivalent to 1 part per 100,000 of calcium carbonate. One German degree of hardness is equivalent to 1 part per 100,000 of calcium oxide, and multiplied by 17.9 gives parts per million of calcium carbonate. The relations of these various scales are indicated in Table 7.
| Table 7.—Conversion table for hardness. | ||||
|---|---|---|---|---|
| Unit. | Equivalent. | |||
| Parts per million. | Clark degrees. | French degrees. | German degrees. | |
| One part per million | 1.00 | 0.07 | 0.10 | 0.056 |
| One Clark degree | 14.3 | 1.00 | 1.43 | .80 |
| One French degree | 10.0 | .70 | 1.00 | .56 |
| One German degree | 17.9 | 1.24 | 1.78 | 1.00 |
TOTAL HARDNESS BY SODA REAGENT METHOD.[[47]][[74]][[81]][[94d]]
Add standard sulfuric acid to 200 cc. of the sample until the alkalinity is neutralized. (See Procedure with methyl orange, p. [37].) Then apply the non-carbonate hardness method (pp. [34]–35). This method gives fairly satisfactory estimates of total hardness of hard waters.
TEMPORARY HARDNESS BY TITRATION WITH ACID.
Determine the alkalinity in presence of methyl orange (see p. [37]) in the original sample and also in the sample after boiling, cooling, restoring to the original volume with boiled distilled water, and filtering. The difference between the two, if any, is the temporary hardness. This is the most accurate method of determining the temporary hardness of ordinary waters. Iron bicarbonate is included as a part of the temporary hardness.
NON-CARBONATE HARDNESS BY SODA REAGENT METHOD.[[47]][[74]][[81]][[94d]]
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.