SOAP-TEST. A solution of white curd soap in proof spirit; it is used in ascertaining the amount of hardness of waters.
The action of hard water upon soap has already been alluded to. [Cm.crum, carbonate of lime.) By using a solution of soap of known strength, and adding it to a given quantity of hard water until no more of the familiar curdy precipitate is thrown down, the amount of hard ness will obviously be at once indicated. The method of preparing and applying this soap teat is as follows : Sixteen !gains of pure carbonate of lime are dissolved in pure hydrochloric acid, the solution evaporated to dryness on a water bath, the residue re-dissolved in water, again evaporated to dryness to ensure the absence of free acid, and the residue now dissolved in one gallon of distilled water. The resulting liquid is a solution of hydrochlorate of lime, or, more correctly, chloride of calcium, but the amount of lime in it is identical with that in the sixteen grains of carbonate of lime, and inasmuch as all soluble lime salts act similarly upon soap, that is, without any influence of the acid contained in them, it follows that the gallon of chloride of calcium solution accurately represents a natural whose hardness is due to sixteen grains of carbonate of lime in a gallon. All that is now necessary is to ascertain how much of a dilute alcoholic solution of soap must be added to a given quantity of the chloride of calcium solution before a permanent lather is produced. This being done the soap-test has henceforth a value given to it, inasmuch as if it be added to any hard water, equal in volume to that of the artificial hard water previously experimented with, until a permanent lather be produced, the amount so added indicates the number of grains of carbonate of lime present in the gallon. The quantity of water tested is usually and conveniently one thousand grains, and the soap-test is used to greatest advantage when poured from a burette divided by transverse markings into measures each containing ten grains : the soap-test should, moreover, be so diluted by its proof-spirit solvent, that thirty-two measures require to be added to one thousand grains of the artificially prepared standard solution of sixteen degrees of hardness before complete precipitation of the lime is produced and a permanent lather formed.
In applying the soap-test to a normal water, the specimen of the latter should be placed in a bottle of five or six ounces' capacity. The solution of soap must then be added in small portions at a time and the mixture well shaken' in the intervals. When indications of a lather appear on the surface of the liquid, the additions of soap-test must be very small, and finally when a lather is produced that does not subside until after the bottle has remained undisturbed on its side for three minutes, the amount of soap-test added is noted ; an inspection of a table similar to the one here appended at once indicates the degree of hardness per gallon.
Excess of carbonic acid in a water is apt to decompose a lather once formed and an experiment may be thus interfered with. To avoid this source of error Professor Clark, who is the author of the process now described, recommehds that the water be violently agitated before the addition of the soap-test, the superstratum of air being two or three times renewed by suction through a glass tube. When a water is of more than sixteen degrees of hardness it should be diluted with its own bulk of distilled water before proceeding with the addition of soap-test.
Salts other than carbonate of lime confer hardness upon water, moreover an equal number of degrees of hardness is produced by very different amounts of the several salts. Thus, to produce ten degrees of hardness the annexed quantities of the following salts are necessary : Carbonate of lime . . . . . . 10'0 grains.
Sulphate of lime . . . . . 13'6 Nitrate of lime . . . . . . 16'4 Chloride of calcium . 11I Carbonate of magnesia . . . . . 8.5 Sulphate of magnesia . . . . . 12.1 Chloride of magnesium . . . . . 5.6 Inasmuch, however, as it is only the relative hardness of a water that is usually required to be known, unnecessary complication is avoided by representing that hardness in degrees, the value of which has been conventionally agreed upon by chemists. One degree is the amount of hardness that would be produced by one grain of carbonate of lime in a gallon of water : two degrees by two grains, and so on.