Standard acid and alkali are solutions of an acid or alkali the exact strengths of which are known. The usual standard solutions are the "normal" and the "deci-normal." The normal solution of hydrochloric acid contains 36.5 gram, hydrochloric acid in 1 litre; the deei-normal contains one-tenth of this amount. The strength of a solution of an acid or an alkali is determined by measuring, say, 10 c.c, and titrating with either alkali or acid, as the case may be, and using some indicator, such as litmus, which changes colour when the point of neutrality is reached: the standard solution is dropped in from a burette, and when the titration is finished, the amount of standard solution used is read off, and from this it is easy to calculate the amount of acid or alkali present in solution. A " factor " is sometimes used for calculation when the strength of the standard solution is not exactly correct ; for instance, suppose we have an absolutely correct standard acid, and we then make a standard soda solution which is rather too strong, instead of diluting it to the correct strength, we may use it as it is, and multiply the results by a " factor." Suppose 10 c.c. of the standard acid requires 9 c.c. of the soda solution, then the latter is 10/9 = 1.11 times too strong; the figures 1.ll constitute the " factor."