This can be done by any one of the following methods:

1. Mintz's Method

To 10 c.c. of the filtrate de-cinormal sodium hydrate is added in such a quantity that a drop of the mixture no longer responds to Giinz-burg's phloroglucin-vanillin test. The amount of the decinormal soda solution used multiplied by ten gives the figure of the free hydrochloric acid. The percentage of free hydrochloric acid can be obtained from this figure in the same manner as above stated for the total acidity, by multiplying it by 0.00365.

2. Method Of Moerner And Boas

3 - The degree of acidity of free hydrochloric acid is here determined either by Congo paper or by a one-per-cent solution of Congo red as an indicator which turns blue in the presence of the acid. The decinormal soda solution is then added until the blue color begins to turn red. Boas takes 5 c.c. of the filtrate and 5 c.c. of the watery Congo-red solution (one per cent). I myself add only one or two drops of the same solution to the filtrate. The estimation is done in the same way as before.

1S. Mintz: "Eine einfache Metliode zur quantitativen Bestim-mung der freien Salzsaure im Mageninhalt." Wiener klin. Woch-enachr., 1889, No. 20.

2 Moerner: Maly's Jahresbuch f. Thierchemie, vol. 19, p. 253.

3Boas: Centralbl. f. klin. Medicin. 1891, No. 2.

3. Toepfer's Method

Toepfer makes use of dime-thylamido-azobenzol in a half-per-cent alcoholic solution for the recognition and the estimation of the amount of free hydrochloric acid. Hydrochloric acid even in small quantities gives a red color with this indicator. The decinormal solution of sodium hydrate is added until the red color disappears; a faint yellow color arises.2 This method has been thoroughly studied in this country by J. Friedenwald 3 and highly recommended.

Prom my own experience I would recommend this method for the quantitative determination of free hydrochloric acid, when the presence of the latter has been first demonstrated by Gunzburg's test; for lactic acid, if present in considerable quantity, may also give a positive reaction with Toepfer's solution.

In a paper on this subject4 which I published very recently it can be easily seen that lactic acid, if alone present, responds to Toepfer's test even if it exists in a percentage above 0.1, and in gastric contents if present in a percentage of 0.2.

1G. Toepfer: Zeitschr. f. physiolog. Chemie, Bd. 19, Heft i., 1894.

2 If desirable, both indicators (pbenolphthalein and Toepfer's solution) can be added at once and the determination of free hydrochloric acid and total acidity computed simultaneously. An alcoholic solution containing one per cent phenolphthalein and one-half per cent of di-methylainido-azo-benzol will still better serve this purpose.

3 J. Friedenwald: Medical Record, April 6th, 1895.

4Max Einhorn: "The Dimethylamido-azobenzol or Toepfer's Test for Free Hydrochloric Acid." New York Medical Journal, May 9th, 1896.