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CHAPTER XV.

ALKALIMETRY.

^T'HE chemical principles employed in the processes of alkalimetry, basedupon the law of chemical combination, are to be found described in anyelementary work upon the science of chemistry; we shall here detail thetechnical processes of determining the value of alkalies.

The Test Solutions.

Having decided upon the form of alkalimeter or burette to be employed, itis necessary to prepare the test solutions, or solutions containing a knownproportion of the reagent to be employed. The test solutions used inalkalimetry are an acid of known density and a solution of litmus; the latterbeing coloured blue by alkalies and red by acids serves to indicate thatneutral point at which no free acid exists.

The Litmus Solution.

The litmus solution is prepared by pulverising I part of commerciallitmus, and digesting it with 8 parts of cold water for twelve hours. Thesolution should be filtered off, and divided into two portions, to one of whichsulphuric acid is added drop by drop until the litmus shows a red tinge. Thissolution should then be poured into the other. The litmus solution thus pre-pared is neutral and very sensitive.

Normal Sulphuric Acid Solution.

One volume of ordinary strong sulphuric acid is diluted with 30 volumesof water, and allowed to cool. 10 c.c. of the normal soda solution, to bepresently described, to which some litmus solution has been added, are thenemployed to calibrate the sulphuric acid, water being added to the latter until1 volume of the acid is exadtly equivalent to 1 volume of the alkalinesolution. Or very excellent results may be obtained by mixing 40 grms. ofchemically pure sulphuric acid with 1000 c.c. of water. 1 c.c. of the normalacid solution then corresponds to 0^04 grm. of sulphuric acid, from which thestrength of a solution of soda may be calculated.

Another method of preparing an acid solution of known strength is asfollows:—11276 grs. of pure neutral and anhydrous carbonate of soda aredissolved in 5 fluid ozs. of hot water; the anhydrous soda being obtained bymaintaining pure bicarbonate of soda for some hours at a dull red heat in aplatinum crucible. This quantity of neutral carbonate of soda correspondsto 100 grs. of pure potash, that is, 100 grs. of potash and 11276 grs. of neutralcarbonate of soda saturate exadtly the same quantity of sulphuric acid. This