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148 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

FIG. 22. Specific heat of nitric acid, sulphuric acid, and water mixtures (McKinley

and Brown [6]).

at a temperature of 32°C. The total acid content in the mixture is 50%, and the HNO 3 content in

the anhydrous mixture is 30/50 x 100 = 60%.

The value 50 corresponding to the total acid in the mixture is found on the x-axis (Fig.

21) and then the point of intersection of the coordinate with the curve, that represents the 60%

content of HNO 3 in the mixture of acids, is sought. The ordinate of this point gives the enthalpy

value at 0°C equal -58.8 kcal/kg. In the diagram shown in Fig. 22, the specific heat of the mix-

ture is found in an analogous way. It equals 0.66 kcal/kg°C.

Hence the relative enthalpy of the acid at 32°C is:

H 32 = -58.8 + 6.66 (32-O) = - 37.7 kcal/kg

Example 2. A mixture of acids having a temperature of 35°C and the composition:

HNO 3 21%

H 2 SO 4 62%

H 2 O 17%

is to be prepared.

To obtain 100 kg of the mixture, it is necessary to mix:

30.0 kg of 70% nitric acid

65.3 kg of 95% sulphuric acid

4.7 kg of water

The temperature of all the compounds should be 24°C

The relative enthalpy of the mixture of acids is calculated in the same way as in example

1, taking into account that the total acid content (HNO 3 +H 2 SO 4 ) is 83% and the HNO 3

content in the anhydrous mixture amounts to 25.3%. The relative enthalpies of the mixture are

I determined from the tables as in example 1.