2. Standard soap solution. Dissolve 100 grams of dry white Castile soap in 1 liter of 80 per cent alcohol, and allow this solution to stand several days before standardizing. Pure potassium oleate made from lead plaster and potassium carbonate may be used in place of Castile soap.
First method of standardization.—Dilute 20 cc. of the calcium chloride solution in a 250 cc. glass-stoppered bottle to 50 cc. with distilled water which has been recently boiled and cooled. Add soap solution from a burette, 0.2 or 0.3 cc. at a time, shaking the bottle vigorously after each addition until a lather remains unbroken for five minutes over the entire surface of the water while the bottle lies on its side. Then adjust the strength of the stock solution with 70 per cent alcohol so that the resulting diluted soap solution will give a permanent lather when 6.40 cc. of it is properly added to 20 cc. of standard calcium chloride solution diluted to 50 cc. Usually 75 to 100 cc. of the stock soap solution is required to make 1 liter of the standard soap solution. The quantity of calcium carbonate equivalent to each cubic centimeter of the standard soap solution consumed in the titration is indicated in Table 6.
| Table 6.—Total hardness in parts per million of CaCO3 for each tenth of a cubic centimeter of soap solution when 50 cc. of the sample is titrated. | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Cubic centimeters of soap solution. | 0.0. | 0.1. | 0.2. | 0.3. | 0.4. | 0.5. | 0.6. | 0.7. | 0.8. | 0.9. |
| 0.0 | 0.0 | 1.6 | 3.2 | |||||||
| 1.0 | 4.8 | 6.3 | 7.9 | 9.5 | 11.1 | 12.7 | 14.3 | 15.6 | 16.9 | 18.2 |
| 2.0 | 19.5 | 20.8 | 22.1 | 23.4 | 24.7 | 26.0 | 27.3 | 28.6 | 29.9 | 31.2 |
| 3.0 | 32.5 | 33.8 | 35.1 | 36.4 | 37.7 | 38.0 | 40.3 | 41.6 | 42.9 | 44.3 |
| 4.0 | 45.7 | 47.1 | 48.6 | 50.0 | 51.4 | 52.9 | 54.3 | 55.7 | 57.1 | 58.6 |
| 5.0 | 60.0 | 61.4 | 62.9 | 64.3 | 65.7 | 67.1 | 68.6 | 70.0 | 71.4 | 72.9 |
| 6.0 | 74.3 | 75.7 | 77.1 | 78.6 | 80.0 | 81.4 | 82.9 | 84.3 | 85.7 | 87.1 |
| 7.0 | 88.6 | 90.0 | 91.4 | 92.9 | 94.3 | 95.7 | 97.1 | 98.6 | 100.0 | 101.5 |
This table does not provide for the use of so large volume of soap solution for a single determination as former ones because the end-point becomes somewhat obscured in the presence of magnesium if more than 7 cc. is used.
Second method of standardization.—Dilute 100 cc. of the stock soap solution to 1 liter with 70 per cent alcohol. This dilute solution should be of such strength that approximately 6.4 cc. of it will give a permanent lather when 20 cc. of standard calcium chloride solution diluted to 50 cc. with distilled water is titrated with it. Determine the amount of soap solution required to give a permanent lather with 50 cc. of distilled water and with 5, 10, 15, and 20 cc. of standard calcium chloride solution diluted to 50 cc. with distilled water. Finally plot on cross-section paper a curve showing the relation of various quantities of soap solution to corresponding quantities of standard calcium carbonate solution and therefore to parts per million of hardness.
Procedure.—Measure 50 cc. of the water into a 250 cc. bottle and add to it soap solution in small quantities in precisely the same manner as described under the standardization of the soap solution. From the number of cubic centimeters of soap solution used obtain from Table 6 or from the plotted curve the total hardness of the water in parts per million of calcium carbonate.
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.