The Salt Error of Indicators Caused by Standard Alkaline Buffers Themselves

1. It is found that with many indicators there is a big discrepancy between the true alkalinities or pH values of solutions of pure sodium hydroxide and of standard alkaline buffers which give the same actual colour. This discrepancy must be ascribed to salt error caused by the buffer itself and exceeds in the most extreme case two whole units of pH; that is, an error of 100 fold in determining alkalinity. In only three cases, namely alizarin yellow G, tropeolin O, and thymol violet, was this error inappreciable. Most of the thirty indicators tested were found for various reasons unsatisfactory in the alkaline range studied. 2. Several indicators show a maximum depth of colour after sufficient indicator has been added, but above a certain concentration further addition of indicator diminishes the colour again.