To avoid mistaking the false or magnesium end-point for the true one[35] when adding the soap solution to waters containing magnesium salts, read the burette after the titration is apparently finished, and add about 0.5 cc. more of soap solution. If the end-point was due to magnesium the lather will disappear. Soap solution must then be added until the true end-point is reached. Usually the false lather persists for less than five minutes.
If more than 7 cc. of soap solution is required for 50 cc. of the water take less of the sample and dilute it to 50 cc. with distilled water which has been recently boiled and cooled. This step reduces somewhat the disturbing influence of magnesium,[107a] which consumes more soap than an equivalent weight of calcium.
At best the soap method is not a precise test on account of the different relative amounts of calcium and magnesium in different waters. For hard waters, especially in connection with processes for purification and softening, it is advised that this method be not exclusively used. If the same water is frequently analyzed it may be of assistance to standardize the soap solution against a mixture of calcium and magnesium salts, the relative proportions of which approximate those found in the water.
The strength of the soap solution should be determined from time to time, to make sure that it has not materially changed. Record all results in parts per million of calcium carbonate.
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.
───────────────────────────┬─────────────────────────────────────────── Unit. │ Equivalent. ───────────────────────────┼──────────┬──────────┬──────────┬────────── │Parts per │ Clark │ French │ German │ million. │ degrees. │ degrees. │ 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 ───────────────────────────┴──────────┴──────────┴──────────┴──────────