fi-Naphthol

1.42 Gms. (=1/100 mol.) naphthol are dissolved in 2 c.cs. caustic soda-lye of 30 % strength, and 25 c.cs. of 10 % sodium carbonate solution are added. Ice-cold diazobenzene solution is then run in from a measuring cylinder or an ice-jacketed burette until a drop tested on filter-paper no longer forms any orange-red dye on its edge when tested with diazonium solution. Owing to the presence of impurities a coloured line sometimes forms after a few seconds where the paper has been touched, but the colour is always muddy, and with a little practice can always be distinguished from the pure naphthol colour. The number of c.cs. used gives directly the percentage composition of the β-naphthol. A good quality product should be at least 99.5 %.

a-Naphthol.

α-Naphthol couples much more readily than β-naphthol, and would give too high values in alkaline solution. For this reason the coupling is effected in acetic acid solution in the following manner: the a-naphthol is dissolved up in the same way as the β-naphthol, and is then precipitated with dilute acetic acid in presence of 25 c.cs. of 25 % sodium acetate solution. The coupling is then effected as described for β-naphthol, and as soon as the reaction for a-naphthol has disappeared the whole is dissolved up by adding caustic soda solution, after which it is reprecipitated with acetic acid; "aniline solution" is added, and so on until the α-naphthol reaction has really completely gone. Frequently it is necessary to add as much as 30 % of the diazo solution subsequently owing to the naphthol being carried down with the precipitated dye.

Only α-naphthol can be estimated in this manner, as β-naphthol does not couple in acetic acid solution. If it is subsequently desired to estimate the β-naphthol content as well, diazotized nitraniline solution is added until all the β-naphthol has been coupled up. In this way it is possible to estimate both naphthols in an impure specimen of α-naphthol.

Dihydroxynaphthalenes (M.w. 160).

These are determined in the same way as for α-naphthol. They couple up very quickly and the "after-coupling" is usually very pronounced and impure, so that it is easy to determine the end-point.