496 PHENOLPHTHALEIN, CONGO-RED, TROP^EOLIN, ETC. [BOOK II. 



Phenolphthalein is a colourless crystalline powder, which is 

 sparingly soluble in water but abundantly soluble in alcohol, the 

 solution in the latter possessing a pale yellow colour. The addition 

 of alkalies causes this yellow solution to assume an intense red colour, 

 which is bleached by the addition of acids and by the action of C0 2 . 



Phenolphthalein is invariably added in alcoholic solution to the 

 liquids of which the reaction is to be determined. As previously 

 stated, acid phosphates behave towards it as acids, i.e. in their 

 presence the addition of alkali must be continued until they are 

 converted into perfectly neutral salts before the red colouration 

 makes its appearance. Acid compounds of albuminous substances 

 behave towards phenolphthalein as free acids. 



Inasmuch as tincture of litmus cannot, with expediency, be added 

 directly to the gastric contents and the use of litmus paper is 

 inconvenient, phenolphthalein is to be preferred to litmus in deter- 

 mining the total acidity of the gastric juice. 



To summarize: in litmus and phenolphthalein we possess indicators 

 which merely inform us of the acid reaction of the gastric juice, but 

 furnish us with no information as to whether a free acid be present or 

 not. 



Congo-red AS was previously said (p. 94), these two reagents 



and tropseoiin. p ossess special value in the investigation of the gastric 

 juice and of the contents of the stomach, inasmuch as they are not 

 affected by salts with acid reactions or by compounds of albuminous 

 substances with hydrochloric acid. 



According to Martius and Liittke, however, no reliance can be 

 placed on these reagents as indicating whether the free acid of the 

 gastric juice is organic or mineral. 



Paper stained with congo-red is to be employed as the readiest 

 and most delicate test of the presence of free acids in the gastric 

 juice, whilst in determining the total amount of free acid (as distin- 

 guished from the total acidity) by the aid of a standard alkaline 

 solution, a solution of 00 tropseoiin (1 part dissolved in 10 of weak 

 spirit) should be employed. 



Pnioro-giu- Whilst congo-red and tropseoiin indicate the 'presence 



cin-vaniilin of free acids in the gastric juice, they cannot be relied 

 and 'resorcin,' upon to tell us whether the acid be organic or mineral. 

 Gunzbur g' s reagent 1 (Phloro-glucin- vanillin), described 

 at p. 94, is totally unaffected by organic acids, 

 but indicates the existence of free mineral acids. If, 

 therefore, the contents of the stomach furnish a positive reaction 

 with it, we can assert that they contain free hydrochloric acid. To 

 the directions given at p. 94 for the employment of this reagent, 

 should be added that a few drops of the gastric juice are sufficient to 



1 Giinzburg, 'Neue Methods zum Nachweise freier Salzsaure im Mageninhalt,' 

 Centralblatt f. klin. Med. 1887, No. 40. 



