Since Kuhne demonstrated the necessity for free hydrochloric acid in peptic digestion many methods have been devised for its detection and determination. This multiplicity of tests is a good indication that none of them exactly fills the requirements. The tests chiefly recommended for the detection of free hydrochloric acid may be divided into two classes, those depending on the change in color produced in some indicator, such as congo-red, dimethyl-amido-azo-benzol, benzopurpurin, tropeolin 00 and others of similar character, and those depending on the coloration produced by the acid on certain organic materials, such as the phloroglucinvanillin test of Günzberg and the sugar-resorcin test of Boaz.
Against the class of indicators as a whole we may state that, almost without exception, they give a reaction with the organic acids when present in sufficient concentration, as can be easily shown by testing the solutions of the various organic acids with them, the
WHITNEY EL. ERRORS IN THE DETERMINATION OF FREE HYDROCHLORIC ACID.. JAMA. 1905;XLV(23):1730–1731. doi:10.1001/jama.1905.52510230030001g
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