5 января, 2016 
Pokraskin A solution of 0.01 mole of the acid in the required amount of soda solution is diluted to about 250 cc., acidified with 25 cc. concentrated hydrochloric acid, and titrated with 1 N nitrite solution. The per cent purity is given by no. of cc. x 10. The endpoint must be determined by
spotting the starch-iodide paper, since accurate results cannot be obtained by streaking.
It should be noted that many highly reactive diazonium salts (especially when in strong mineral acid solution) rapidly turn starch-iodide paper blue, and it is essential, therefore, to know the sensitivity of the paper used. Sulfanilic acid, metanilic acid, and naphthylaminesulfonic acids are diazotized at 15°C. Cleve acids cannot be determined so easily because they couple with themselves immediately. In this case, it is best to add the bulk of the nitrite to the neutral soluion and acidify the mixture with good stirring. The diazotization can also be carried out directly at 0°C., adding nitrite until the original violet color gives way to a pure brown. The indirect method is preferred, however, because it is more rapid.
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