CH. VIII.] FREE HYDROCHLORIC ACID. IQ7 



reagent. This is an alcoholic solution of phloroglucin 

 (symmetrical tri-hydroxy benzene, C 6 H 3 (OH) 3 and vanillin 



-OCH 3 

 CH -OH 



When evaporated with hydrochloric acid on the water 

 bath these condense to form a brilliant red compound, 



(-20** 18^8 



2C 6 H 6 3 + C 8 H 8 3 = C 20 H 18 8 + H 2 0. 



In the absence of free hydrochloric acid only a brown 

 colour is obtained. A brown colour also develops in the 

 presence of hydrochloric acid if the mixture be over- 

 heated. 



There are two methods of applying the test. One is to 

 dilute the solution until it fails to give a positive reaction, 

 assuming that it is just positive with 0-0004 N.HC1 

 (0*00146 per cent.). The other, and better, method is to 

 titrate a measured sample with soda until a drop of the 

 titrated mixture just fails to give the reaction. 



From comparisons with the electrical method of 

 measuring hydrogen-ion concentration (p. 19) it would 

 seem that the estimation by Gunzberg's reagent is accurate 

 when the acidity is high or moderate, but that it gives too 

 low a result with gastric contents relatively deficient in 

 hydrochloric acid. 



At one time it was claimed that Toepfer's reagent 

 (p. 22) only reacted with free hydrochloric acid. It has 

 now been shown that both this indicator and also congo red 

 react with lactic acid if this be sufficiently concentrated, 

 and that the amount of free hydrochloric acid as deter- 

 mined by their use may be far in excess of the amount 

 actually present. 



Results of analysis. In normal cases the following may 

 be taken as the limits, the results being expressed in grams, 

 of HC1 per cent. 



