5 января, 2016 
Pokraskin a-Naphthol couples much more easily than /З-naphthol and would give too high values in alkaline solution. Hence, the coupling is carried out in acetic acid solution in the following manner:
The a-naphthol is dissolved as described for /З-naphthol, and the solution is diluted and then precipitated with dilute acetic acid in the presence of 25 cc. 25 per cent sodium acetate solution. The coupling is carried out as with уЗ-naphthol, except that when the reaction for a-naphthol disappears, the solution is made alkaline with sodium hydroxide, reprecipitated with acetic acid, and the titration continued until the a-naphthol reaction again disappears. Frequently, as much as 30 per cent of the total volume of the diazonium solution is added in the second part of the titration, since so much of the naphthol is carried down by the dye.
Only a-naphthol can be determined in this way since /3-naphthol does not couple in acetic acid solution. If it is desired to determine the /?-naphthol subsequently, 0.1 N p-nitrophenyldiazonium solution is added until all of the /J-naphthol is reacted. Thus, it is easy to determine both a — and (3 naphthols in impure a-naphthol samples.
Dihydroxynaphthalenes (Molecular Weight 160)
These compounds are determined in exactly the same way as a-naphthol. They couple very readily and the “after-coupling” is usually strong and very impure, so the endpoint is easily determined.
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