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NITRATION WITH NITRATING MIXTURES 143

nitration of mononitro- to dinitro-toluene while the spent acid from the latter is
used for the nitration of toluene to mononitrotoluene. The portion of the spent
acid re-used is sometimes called re-cycle acid.
There are two ways of utilizing the spent acid in the nitration process:
(1) An earlier method, which consists in blending the spent acid with con-
centrated nitric acid and oleum in such proportion as to obtain a mixture of the
exact composition required for the manufacture of the nitro compound to be
produced. A drawback of this method is that the compound being nitrated is mixed
with concentrated acid. Therefore the nitration may sometimes proceed too
vigorously. Taking this into consideration, the reaction must be very carefully
controlled, avoiding any rise of temperature, especially in the initial period of
nitration.
The rise of temperature is brought about not only by the heat generated by
the reaction, but also by the heat of dilution of the nitrating acid by the water
formed during the nitration.
The more concentrated the acid, the higher the heat of dilution (Table 16).
Therefore, the temperature jump caused by the formation of a given amount of
water is higher in the initial period of nitration than at a later stage after dilution
has taken place.
Heat capacity also affects the rise of temperature during nitration. Since the
heat capacity of a concentrated acid is much lower than that of a dilute acid (Table
17), the same amount of heat generated brings about a higher rise of temperature
at the beginning of the nitration than at the end. So this is yet another factor which
makes for difficulties when carrying out nitration with concentrated acid.
The considerable effect of accumulated heat and a big rise of temperature may
be avoided by applying a more modern method such as that described below.
(2) The substance to be nitrated is mixed with the spent acid and then nitric
acid is introduced slowly, the concentration of the acid being increased gradually
so that nitration proceeds slowly.
As soon as the substance being nitrated is mixed with the spent acid, it uses
up the HNO 3 present in it. Further addition of nitric and sulphuric acids does
not result in such a considerable rise of temperature as that occurring in the first
method. Two reasons account for this:

(a) The water formed during the nitration process, when evolved in the al-

ready dilute acid, does not produce such a large quantity of heat as in the first

method (heat of dilution of sulphuric and nitric acids and their mixtures are given

in Table 16 (Rhodes and Nelson’s data [4]). See also [16].

(b) As mentioned above the heat capacity of dilute acid is higher than that

of concentrated acid. For this reason, during the initial period of the reaction when

the acid is more dilute the rise of temperature is lower than when, on adding con-

centrated nitric and sulphuric acids or oleum, an increase in concentration takes

place.
It follows from the above that nitration by the second method is safer. It may