In the quantitative determination of the acidity of gastric contents a decinormal solution of sodic hydrate is usually employed. Each cubic centimeter neutralizes an acidity equivalent to .00365 gm. of hydrochloric acid. Six cubic centimeters is ordinarily considered an average amount to neutralize the total acidity of 10 cubic centimeters of normal contents. The acidity of gastric contents is most commonly expressed by multiplying by 10 the total number of cubic centimeters of N/10 NaOH required to neutralize 10 cubic centimeters of gastric contents. A normal total acidity is, therefore, about 60. The solution of sodic hydrate, however, is not permanent; should it be accurate when received from the chemist it will not be accurate after a few days, and the variations may be considerable to one using it infrequently.
To minimize the difficulties imposed by constant change in the concentration of sodic hydrate solutions some workers determine at frequent intervals