75 Gms. commercial sodium nitrite are dissolved in a little water, filtered, and made up to 1 litre at 17.5°. 50 C.cs. of the normal sulphanilic acid solution are then titrated with it in the following manner:

The solution is measured out into a half-litre beaker by means of a pipette, and is then diluted with 200 c.cs. ice water and acidified with 25 c.cs. crude conc, hydrochloric acid. The nitrite solution is then run in under the surface of the liquid from a burette, and as soon as 45 c.cs. have been added, the remainder is run in drop by drop until the starch-iodide paper when touched with a drop of the liquid (not rubbed across) causes a faint but permanent blue coloration. The diazotization occupies 10 minutes. From the number of c.cs. of nitrite solution used up it is easy to calculate how much water must be added to render the solution exactly normal. It is then made up exactly to the requisite strength, as the use of a factor causes too much unnecessary labour. The little extra work involved in standardizing the solution is more than made up by the subsequent saving in time.

When the sulphanilic and nitrite solutions have been made up as described, a Normal aniline solution is also prepared: 200 c.cs. pure aniline are distilled from a small distilling flask as shown in Fig. 431 at such a rate that the distillation occupies about three-quarters of an hour. The fraction of aniline distilling within half a degree and between 184 and 1850 is used for the preparation of the solution. In passing it may be noted that almost chemically pure aniline is obtainable commercially. The specific gravity should be between 1.0260-1.0265 at 17.5°.

Exactly 93 gms. pure aniline are dissolved in 150 c.cs. of pure

1 High-boiling liquids are generally distilled with this simple type of apparatus i.e. without the use of a Liebig condenser, the receiver being sometimes cooled with a stream of water.

30 % hydrochloric acid, and the solution is made up to 1 litre at

If the sodium nitrite solution and the sulphanilic acid solution have been correctly prepared 100 c.cs. sulphanilic acid solution and 100 c.cs. aniline solution should each require exactly 100 c.cs. nitrite.