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NITRATION AND NITRATING AGENTS^33

H 3 NO 3 2+ -> NO 2 + + H 3 O+ (16)

Krejci, Novotny and Ruzicka [79] have also recently examined the viscosities

of mixtures of nitric and sulphuric acids with water from the point of view of their

practical importance.

In 1961 Hackel, T. Urbanski, Kutkiewicz and Sterninski [80] have examined

the viscosities of these mixtures and presented the results as the curves of a tri-

angular diagram (Fig. 15). It can be seen that a maximum viscosity exists for the

solution containing 20-25 mole % of sulphuric acid, i.e. for the compound

(HNO 3 ) 3 .H 2 SO 4.

APPLICATION OF KINETIC STUDIES TO ELUCIDATE THE

STRUCTURE OF NITRATING MIXTURES

The first attempts to study nitration kinetics were made by L. Meyer and co-

workers [81,82]. It was in their publication that for the first time a statement

appeared that the nitration reaction is irreversible and that its rate decreases with

time.

Martinsen [83] observed already in 1905 that the rate of nitration of nitrobenzene

with a mixture of nitric and sulphuric acids depends on the concentration of

sulphuric acid, viz. it increases with the concentration up to 88.7% H 2 SO 4 ,

after which further increase in sulphuric acid concentration results in a de-

creasing rate of nitration.

In 1924, Klemenc and Schöller [34] stated that m- xylene-2,6-disulphonic acid

is nitrated with nitric anhydride in sulphuric acid twice as rapidly as when it is

nitrated with nitric acid in sulphuric acid, the molar concentration being the same.

This was illustrated in the following scheme, where the nitrating agent was denoted

by X:

(42)

It is clear, that the scheme corresponds to the eqn. (5). It follows that the

concentration of NO 2 + ions from N 2 O 5 is twice as high as that of NO 2 + ions

from HNO 3 (see eqn. 19).

J. A. Hetherington and Masson [84], basing their views on the studies of benzene

nitration kinetics, stated that the reaction of nitrobenzene nitrated with a mixture

of HNO 3 , H 2 SO 4 and H 2 O would stop before all the nitric acid was used if

the amount of sulphuric acid in the mixture was insufficient to form the H 2 SO 4 .H 2 O