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NITRATION WITH NITRATING MIXTURES 151

is to be prepared from:

(1) n% nitric acid

(2) s% sulphuric acid

(3) spent acid with n'% H 2 SO 4 content and s'% HNO 3 content.

To obtain 100 kg of the mixture we take

x kg of nitric acid

y kg sulphuric acid

z kg of spent acid

The following three equations with three unknowns must then be worked out:

x+y+z = 100

zn’ +xn = A

zs’+ys = B

By solving the system of equations we obtain the formulae for the calcnlation of X, y, r.

A(s-s’) + n’(B-100s)

x=

s(n-n')-ns'

y=B(n--n) + s’(A-100n)

s(n-n’)-ns’

n(100s - B) - As

z=

s(n-n’)-m’

If oleum is used it is assumed that

s = p+1.225 g

where p and g are the percentages of H 2 SO 4 and SO 3 in the oleum respectively (122.5 parts of

H 2 SO 4 are obtained from 100 parts of SO 3 ).

TEMPERATURE OF NITRATION

The nitration temperature has considerably less effect on the position of the

nitro group in an aromatic nucleus than for example the sulphonation temperature.

Nevertheless, keeping the desired temperature constant is of greatest importance

from the viewpoint of safety and of the purity of the product. The use of too high

a temperature causes the reaction to proceed violently. At high temperature by-

products, especially oxidation products, are readily formed. The oxidation reactions

are accompanied by the evolution of nitrogen dioxide. This is why the temperature

must be kept as low as possible, especially at the initial stage of reaction when

the substance is not yet or only at the initial stage of the nitration.

The safety limit of temperature depends on the chemical structure of the com-

pound being nitrated. For example, in the nitration of dinitrotoluene to trinitro-

toluene or of phenol to picric acid, temperatures near 120°C and over are consi-

dered dangerous. In the nitration of dimethylaniline to tetryl, a temperature higher

than 80°C must be considered dangerous. Esterification with nitric acid should
be carried out at a temperature close to room temperature or lower.