ELEMENTARY CHEMICAL PHYSIOLOGY 231 



or three drops of a solution of phenol -phthalein are added, and the solution is 

 titrated with caustic soda solution until a faint pink colouris just obtained. 



The number of c.c. of alkali required is noted. 



2. Free Hydrochloric Acid. Titrate another 10 c.c. sample as above using 

 4 drops of a 0-5 per cent, alcoholic solution of dimethylaminoazo -benzol as 

 indicator. The end point is reached when the red colour changes to yellow. 



3. Total Hydrochloric Acid. Titrate still another 10 c.c. sample of the 

 filtered gastric contents using 3 drops of a 1 per cent, aqueous solution of 

 alizarin red as indicator. The end point is the change of the yellow colour 

 to red violet. 



Phenolphthalein titration (1) gives the total acidity of the juice. 

 Dimethylaminoazo -benzol titration (2) gives the free hydrochloric acid. 

 Alizarin red (3) reacts with all but the combined acidity. 

 Therefore : 



1 = total acidity. 



2 = free hydrochloric acid. 



3 = all acidity less combined. 

 (1-3) = combined hydrochloric acid. 



(1-2) = combined hydrochloric acid, organic acids and acid salts. 

 ( l-2)-(l-3) = organic acids and acid salts (probably chiefly acid phosphates). 



The number of c.c. - NaOH required for each 10 c.c. of gastric juice 



jg 

 for neutralisation multiplied by 0-0365 (as 1 c.c. alkali equals 0-00365 



grm. HC1), gives the acidity for 100 c.c. gastric juice in grms. HC1. 



Total hydrochloric acid may also be determined as follows : 10 c.c. of 

 filtered gastric contents are placed in a platinum dish and evaporated to 

 dryness on the water bath. The dish is then heated to a low red heat, so that 

 charring is complete, but the resulting carbonaceous material is not burnt up. 

 The mineral chlorides alone now remain in the dish. The contents of the dish 

 are rinsed with hot distilled water through a funnel into a 100 c.c. measuring 



N 

 flask. The flask is cooled, 5 c.c. nitric acid and 20 c.c. - silver nitrate solution 



are added, and the contents made up to 100 c.c. The amount of silver nitrate 

 used in precipitating the chloride present is then determined by Volhard's 

 method (see p. 288). A similar experiment is performed with the same 

 volume of the gastric contents, to which slight excess of sodium carbonate 

 solution is added before evaporation, and again the amount of silver nitrate 

 used in precipitating the chloride determined. The first experiment gives the 



mineral chloride present, equivalent, say, to 5 c.c. - silver nitrate. The 

 second experiment gives the total chloride, equivalent, say, to 10 c.c. 

 silver nitrate. The difference gives the [volatile chloride, that is, the 

 hydrochloric acid free and combined with protein. In the hypothetical case 

 this is 10 5 = 5 c.c. silver nitrate. The gastric contents, therefore, 



Q OgK y K 



contain - - per cent, total hydrochloric acid. 



Normal human gastric contents obtained after a meal containing very little 

 protein usually contain about 0-2 per cent, total hydrochloric acid. This 

 hydrochloric acid determination is of value, as it is the best measure of the 

 secretory activity of the gastric mucous membrane in pathological conditions. 



The Organic Matter. If pure gastric juice be cooled to C., a precipitate 

 falls down. On analysis, this precipitate is found to have nearly the same 

 percentage composition as protein ; and on testing its action on a solution of 

 protein, it is found to contain pepsin. Pepsin of similar composition can also 



