n contains acetic acid and
no free mineral acid. However, the diazotisation is better carried out
with hydrochloric acid, and the presence of the latter is necessary to
give stability to the solution. If before the diazo solution is used a
quantity of acetate of soda be added to it, the free hydrochloric acid
liberates acetic acid from the acetate, and the chloride of the diazo
body changes into its acetate. It is better to add an excess above the
two molecules of acetate of soda which are required.

The combination when aniline and beta-naphthol are used, as the amine
and phenol respectively, is shown in the following equations:--

C{6}H{5}N:NCl + C{10}H{7}OH +
Diazo-benzene chloride, B. naphthol,

NaOH = 2NaCl + C{6}H{5}N:NC{10}H{6}OH + H{2}O
Caustic soda, Benzene- azo-naphthol, Water.

Or, with naphthylamine and naphthol, thus:--

C{10}H{7}N:NCl + C{10}H{7}OH + NaOH =

NaCl + C{10}H{7}N:NC{10}H{6}OH + H{2}O
Naphthalene azo-naphthol.

By the action of nitrous acid upon amido-azo bodies a group of bodies
called diazo-azo compounds are obtained which contain the group N:N
twice over, thus:--

C{6}H{5}N:NC{6}H{4}NH{2}HCl + NaNO{2} + 2HCl =
Benzene-azo-aniline-hydrochloride,

NaCl + C{6}H{5}N:NC{6}H{4}N:NCl + 2H{2}O.
Diazo-azo-benzene-chloride.

When this compound is combined with naphthol diazo-azo dyes are
produced.

C{6}H{5}N:NC{6}H{4}N:NC{10}H{6}OH.
Benzene-azo-benzene-azo-naphthol.

The molecular weights of the bases, phenols and chemicals employed are
the following:--

1. Hydrochloric acid, HCl--36.5. 2. Caustic soda, NaOH--40. 3. Nitrite
of soda, NaNO{2}--69. 4. Acetate of soda, NaC{2}H{3}O{2}3H{2}O--136.

1. Commercial hydrochloric acid at 32 deg. Tw. contains about 365 grams of
HCl in a litre, or 3-1/2 lb. in a gallon.

2. The commercial 77 per cent. soda must always be used, and for
practical purposes it may be taken as pure. It is best to make a
solution which contains 160 grams NaOH in a litre of water.

3. The nitrite supplied is almost chemically pure, and is easily soluble
in water. In order to make a solution 140 or 290 grams are dissolved per
litre.

4. Crystallised acetate of soda contains 3 molecules of water of
crystallisation, and is usually somewhat moist. Instead of 136 grams 140
are taken to allow for moisture. The amount is dissolved in about 500
cubi