The degree of acidity is examined by adding a drop of a one-per-cent. alcoholic solution of phenolphthalein to 10 c.c. of the filtrate and adding again as many cubic centimetres of a one-tenth normal sodium-hydrate solution1 until a slightly red color arises. The amount of cubic centimetres of the one-tenth sodium-hydrate solution required for that purpose is multiplied by ten and expressed with this figure - i.e., the degree of acidity is expressed by the number of cubic centimetres of a one-tenth normal sodium-hydrate solution required to saturate or make slightly alkaline 100 c.c. of the filtrate. Thus if we find that 10 c.c. of the filtrate require 6 c.c. of the one-tenth normal sodium-hydrate solution in order to bring on the red color after the addition of phenolphthalein, we say the acidity is 60. The figure of acidity multiplied by 0.00365 gives the percentage amount of hydrochloric acid. If, for instance, the acidity is 60, then the percentage of hydrochloric acid will be 60 X 0.00365 = 0.219 per cent.

1 The normal or standard solution of sodium hydrate contains 40 parts of sodium hydrate (chemically pure) to 1,000 parts of distilled water.

The different elements comprising the acidity, and their quantitative determination, we shall describe later on.