GASTRIC TESTS. 83 



digestion. Hence it is important to be able to detect them. 

 They can be distinguished from free acids from the fact 

 that they are not neutralized by calcium carbonate in the 

 cold as the free acids are. When, therefore, a gastric juice 

 is neutralized by adding to it finely powdered calcium car- 

 bonate, like precipitated chalk (which must itself be neu- 

 tral), no acid phosphates are present, but the reaction was 

 due to free acids. If the color of the litmus-paper is ob- 

 scured by the excess of calcium carbonate, this may be rinsed 

 off with distilled water. If the reaction remains acid after 

 calcium carbonate has been added, acid phosphates are pres- 

 ent. Their amount can be determined by finding the total 

 acidity of 10 cubic centimeters, then adding calcium car- 

 bonate to 15 cubic centimeters, filtering and determining 

 the acidity of 10 cubic centimeters of the filtrate. This 

 latter is due to the phosphates. The difference between 

 the two is the amount of the free acids. 



195. Test the reaction of the acid phosphates 1 to 

 litmus-paper. 



196. Show that they are not neutralized by calcium 

 carbonate in the cold. 



197. Show that the dilute free acids (both HC1 and 

 lactic) can be so neutralized. 



For a short time after food has been taken hydro- 

 chloric acid may be wanting, or present only in traces in 

 the gastric juice, without any pathological significance, 

 but in one to three hours after a meal it should be found 

 in larger amounts. It is then, under normal conditions, 

 about 0.1 to 0.3 per cent, of the weight of the juice. 



*Acid sodium phosphate, NaH 2 PO 4 , can be prepared by adding 

 carefully orthophosphoric acid to common sodium phosphate until 

 it does not precipitate barium chlorid. An excess of the acid must 

 be avoided. 



