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NITRATION AND NITRATING AGENTS 17

to a minimum at concentrations of 96-97% HNO 3 , and then on further dilution

it rises again.

This is explained by the fact that anhydrous nitric acid is dissociated, but in

the presence of small quantities of water is converted into an undissociated hydrate

HNO 3 .H 2 O. It is only on further dilution that the hydrate dissociates.

In conductometric measurements of two-component system where HNO 3 was

one of the components, Usanovich and his co-workers [50-50c] confirmed the

view that nitric acid has an amphoteric character-it can be an acid or a base de-

pending on the other component. Thus,it acts as a base towards sulphuric acid

and as a strong acid towards water. It also acts as an acid towards CH 3 OOH and

CCl 3 COOH and combines with these compounds, but behaves as a base with

CH 2 ClCOOH and CHCl 2 COOH. I

Addition of compounds such as H 2 SO 4 , H 3 PO 4 , towards which nitric acid acts

as a base, increases the nitrating properties of HNO 3. On the contrary, the addition

of compounds such as water or nitrobenzene, towards which nitric acid behaves

as an acid, decreases the nitrating action of HNO 3.

Hantzsch’s argument, that with strong acids like sulphuric or phosphoric, the

ions H 2 NO 3 + and H 3 NO 3 2+ are formed, led Usanovich to the inference that it was

just these cations that were the nitrating agents for aromatic compounds.

On the other hand, the addition of water results in the formation of NO 3

• ions,
  which therefore seem to be the nitrating agents for aliphatic hydrocarbons. Swi-
  narski and Dembiriski [51] and Swinarski and Piotrowski [52] have examined
  the electrical conductivity of nitric and sulphuric acid mixtures with and without
  water. The results of these measurements are shown in the diagram, presenting

the relationship between conductivity and HNO 3 concentrations.

The principal conclusions drawn by the authors are:

1. The high conductivity of anhydrous H 2 SO 4 is probably due to dissociation

2H 2 SO 4 <-> H 3 SO 4 + + HSO 4 - (30)

2. A sharp increase in the conductivity of the mixture, on adding water in
   quantities below 9% H 2 O, indicates the complete ionization of nitric acid. Prob-

ably the dissociation may be expressed by eqn. (18) which has already been

referred to

HNO 3 + 2H 2 SO 4 -> NO 2 + + 2HSO 4 - + H 3 O+ (18)

3. The conductivity diminishes with further increase in concentration of HNO 3.

The reason for this may be the formation of less mobile ions, for example

Hantzsch’s hydrated nitronium ion:

HNO 3 + 2H 2 SO 4 -> H 3 NO 3 2+ + 2HSO 4 - (31)

The authors pointed out the similarity of the conductivity curves (Fig. 4) and
viscosity curves (Fig. 14) which they observed. The shapes of both curves may

be explained in the same way, namely by the formation and disappearance of

(H 3 NO 3 )2+ and NO 2 + ions.