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NITRATION AND NITRATING AGENTS 29

The triangular diagram of Sapozhnikov was modified by Vandoni, on the
basis of his own experimental data, to the diagram of HNO 3 activities (Fig. 13).
Thus instead of isobars of HNO 3 vapour pressures he constructed curves of equal

activities. The advantage of such a diagram is, among other things, that unlike

vapour pressure, activity is independent of temperature.

FIG. 12. Vapour pressure and activity H/H 0 of HNO 3 in the solution

HNO 3 -H 2 SO 4 -H 2 O containing 50% HNO 3 (Vandoni [72]).

Activity as a function was introduced by Lewis in 1908, and a full description was given by
Lewis and Randall [74] in 1923. The activity ai of a substance “i” can be defined [75.76] as
a value corresponding to the mole fraction of the substance “i” in the given phase. This value
is in agreement with the thermodynamic potential of the ideal mixture and gives the real value
of this potential.
Lewis and Randall give an example of calculating the activity of a solute from its vapour
pressure. When a solution is in equilibrium with the vapour of the solute x 2 , we may measure
the vapour pressure of x 2 over a range of concentrations, and by knowing the fugacity of the
vapour at each pressure we may obtain the activity of the solute in the solution. When we may
assume that the vapour is a perfect gas, the activity a 2 in the solution may be taken as pro-
portional to p 2 , the vapour pressure of the solute. Hence. as we pass from the mole fraction N 2
to an infinitely dilute solution of mole fraction Nx 2

Now by Henry’s law, 2 approaches a constant value, at infinite dilution which may be