1 января, 2016 
Pokraskin A solution of 75 grams of technical sodium nitrite in a small volume of water is filtered and made up to 1 liter at 17.5°C. This solution is used to titrate 50 cc. 1N sulfanilic acid solution, as follows:
The sulfanilic acid solution is pipetted into a 500-cc. beaker, diluted with 200 cc. ice water, and acidified with 25 cc. concentrated hydrochloric acid. The nitrite solution is added from a burette whose tip extends beneath the surface of the liquid. After 45 cc. has been added, the addition is continued dropwise until a drop of the mixture on starch — iodide paper produces an immediate, very weak, but permanent blue coloration. This test must be made by spotting (not rubbing) the starch — iodide paper. The whole diazotization takes about 10 minutes. From the volume of nitrite solution used, it can be calculated how much water must be added to make the solution exactly 1 N. The solution should always be diluted to 1N strength instead of using it as it comes out, since the use of a factor in all subsequent calculations involves too much work.
After the sulfanilic acid and nitrite solutions have been standardized, a 1 N aniline solution is prepared. Pure aniline (200 cc.) is distilled from a small distillation flask (Fig. 7, page 71) at such a rate that the distillation is complete in 45 minutes. The aniline which comes over within a range of one-half degree between 184 and 185°C. is used for preparing the solution. The specific gravity should be 1.0260 to 1.0265 at 17.5°.
93 grams of the pure aniline is dissolved in 150 cc. pure 30 per cent hydrochloric acid and the solution is made up to 1 liter at 17.5°C.
If the nitrite and sulfanilic acid solutions have been prepared correctly, 100 cc. of either the sulfanilic acid or the aniline solution should require exactly 100 cc. of the nitrite solution.
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