The hardness of the water in the natural state is the ‘total hardness.’ By boiling for an hour and making up loss by evaporation with boiled distilled water and again determining the hardness, the ‘permanent hardness’ is found. That which has been removed by the boiling is the temporary hardness.
Table A.
Soap test measures corresponding to one thousand measures of water of each degree of hardness.
| Degree of hardness. | Soap test measures. | Difference. |
| 0 | 14 | 18 |
| 1 | 32 | 22 |
| 2 | 54 | 22 |
| 3 | 76 | 20 |
| 4 | 96 | 20 |
| 5 | 116 | 20 |
| 6 | 136 | 20 |
| 7 | 156 | 19 |
| 8 | 175 | 19 |
| 9 | 194 | 19 |
| 10 | 213 | 18 |
| 11 | 231 | 18 |
| 12 | 249 | 18 |
| 13 | 267 | 18 |
| 14 | 285 | 18 |
| 15 | 303 | 17 |
| 16 | 320 | — |
The standard water of 16° of hardness is thus made:—Pure carbonate of calcium (Iceland spar) is weighed out into a porcelain or platinum dish in the proportion of 16 grains for a gallon of solution. It is dissolved in weak hydrochloric acid, and the whole cautiously evaporated to dryness over a water bath, then re-dissolved in water and again evaporated to drive off any excess of acid. The dish is covered with a glass during the operation to prevent loss by spirting. The resulting neutral chloride of calcium is dissolved in a gallon of pure distilled water if 16 grains were weighed out, or a proportionate quantity in other cases. The soap solution can be made by dissolving good curd soap in weak methylated spirit in the proportion of one ounce of soap to the gallon. A potash soap made as follows is, however, less liable to change: 150 grains of lead plaster (Emplastrum plumbi, B. P.) and 40 grains of dry potassic carbonate are rubbed together in a mortar and repeatedly extracted with small portions of methylated spirit, triturating the mass meanwhile, till about a pint of spirit has been used; filter and add an equal bulk of recently boiled distilled water. Whichever method is followed the clear solution has now to be standardised by the ‘water of 16° of hardness.’ 1000 grains of the water of 16° of hardness are placed into a bottle, and this soap solution is run in from a burette until a permanent lather is formed. The soap solution must be fortified by strong soap solution or diluted with alcohol till 320 measures produce a lather permanent for five minutes in 1000 grain-measures of water of 16° of hardness.
Chlorine. To 1000 grains of the water add a drop or two of neutral chromate of sodium, so as to tinge the water yellow; run in standard nitrate of silver till the liquid acquires a very faint red tinge, showing that all the chlorine has been precipitated and that red silver chromate is beginning to be formed. The number of grains of standard solution divided by 100 will give the grains of chlorine in one gallon of the water.
The standard solution is prepared by dissolving pure nitrate of silver in the proportion of 47·90 grains to one gallon of distilled water.
Ammonia is always carried out as described in the account of Messrs Wanklyn and Chapman’s process.
Nitrate and Nitrites.—These substances can be most expeditiously estimated by the indigo process as follows: 200 grain-measures of the water are placed in a flask and a little of a standard solution of indigo added thereto; twice the volume of pure sulphuric acid is then suddenly poured in from a measuring cylinder, and the whole shaken. The temperature rises immediately to about 270° Fahr., and the blue colour will probably be immediately discharged; more indigo, therefore, must be rapidly run in till a brown-green tint shows itself. This gives the trial estimation,
but the maximum amount of indigo is only used up when all the indigo is added previous to the addition of acid; hence a second experiment is now started, and an amount equal to that previously used run in at once, and on it is poured exactly twice as much sulphuric acid as there is water and indigo in solution. The second result will be somewhat higher than the first. If the solutions below mentioned be used, the amount of indigo required by the 200 grains of water divided by the number of grains of indigo required to bleach 200 c.c. of standard nitre represents the grains per gallon of nitrogen as nitrates and nitrites. The standardising of the indigo with the nitrate solution is performed exactly as for an actual water. The requisites are a solution of pure potassium nitrate of known strength, say 14·442 gr. of nitre (equivalent to 2 gr. of nitrogen or 9 gr. of nitric acid) in a gallon of distilled water. 2. A solution of indigo made by dissolving soluble indigo carmine in distilled water in such a proportion that 200 gr. is about equal to 200 gr. of nitre solution. 3. Strong pure oil of vitriol; it must be free from nitrous compounds, not become turbid when diluted, and its specific gravity not be less than 1·84. It is important to maintain the same proportion of acid, and not to allow the temperature to fall below 250° F. throughout the experiment.