PHYSIOLOGICAL CHEMISTRY 381 



1. Total Acidity. A measured quantity (10 c.c.) of filtered gastric contents 

 is mixed in an Erlenmeyer flask with ten times its bulk of distilled water. Two 

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



titrated with ^ caustic soda solution until a faint pink colour is just obtained. 

 The number of c.c. of alkali required is read off, and the result expressed as the 

 amount of JQ alkali required to neutralise the acids in 100 c.c. filtered gastric 



contents. Thus, an acidity of 40 would mean that 40 c.c. of ^ alkali had been 



required to neutralise the acids of 100 c.c. gastric contents. The result may also 

 be expressed in terms of HC1, and this is the method most useful in physiology. 



1 c.c. JQ alkali equals '00365 gr. HC1. If, for example, 100 c.c. of gastric juice 



require 50 c.c. y^ alkali to neutralise it, the acidity in terms of HC1 will be 



0-1825. In other words, the percentage of acid will be equivalent to 0'1825 HC1. 



2. Total Hydrochloric Acid (i.e. the free HCl + the HC1 combined with 

 protein). 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 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. 434). 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 pre- 

 cipitating the chloride determined. The first experiment gives the mineral 



chloride present, equivalent, say, to 5 c.c. y^ silver nitrate. The second experi- 

 ment 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 com- 

 bined with protein. In the hypothetical case this is 10-5 = 5 c.c. ^ silver nitrate. 



The gastric contents, therefore, 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 hydro- 

 chloric acid determination is of value, as it is the best measure of the secretory 

 activity of the gastric mucous membrane in pathological conditions. 



3. Free Hydrochloric Acid. 1 This can be approximately determined by 



N 

 titrating 10 c.c. gastric juice with -^ caustic soda, using Giinzberg's reagent as 



1 In testing for free HC1 in solutions containing protein, it is important to bear 

 in mind that the HC1 will gradually become combined with the protein. The 

 solutions should therefore not be allowed to stand for long before testing them. 



