The Internal Pressure of Pure and Mixed Liquids

Measurement of internal pressures. Defining internal pressure as T(∂p∂T)ᵥ, designated Tγ, it can be measured on the principle of a constant volume thermometer. This was done for 8 pure liquids and 12 mixtures of 50 mole percent composition. Values for the former are given at 15^∘{}, 20^∘{}, 25^∘{} and 35^∘{}, and for the latter at 20^∘{}, 25^∘{} and 35^∘{}. The values of γ in atmospheres per degree at 20^∘{} are as follows: 1 heptane, 8.66; 2 acetone, 11.22; 3 carbon tetrachloride, 11.47; 4 benzene, 12.58; 5 carbon disulfide, 12.67; 6 ethylene chloride, 14.17; 7 ethylene bromide, 15.20; 8 bromoform, 15.32. The values of γ for the mixtures at 20^∘{} are, designating the components by the foregoing numbers, 1-2, 9.27; 1-3, 9.68; 1-4, 9.86; 1-5, 9.84; 1-7, 10.66; 1-8, 10.86; 3-4, 12.19; 4-5, 12.32; 4-6, 12.92; 4-8, 14.05; 2-5, 11.77; 5-7, 13.98.Relations satisfied by (∂p∂T)ᵥ.---It was found, further, (1) that γ is a function of the specific or molal volume only; (2) that for each pure liquid v²T₁ γ is a constant, a, v being the molal volume, and T₁ the temperature at which the pressure is 1 atmosphere (a function of molal volume); (3) that the values for the mixtures are less than additive, less than those calculated from the equation of Biron, γ=γ₁γ₂(γ₁N₁+γ₂N₂) where N₁ and N₂ are the mole fractions of the components, here 0.5; (4) that they are given within 1 or 2 percent by the relation a=(a₁a₂)1/2 except in some of the mixtures of carbon disulfide, acetone and ethylene chloride---the first of these is in other respects irregular, and the last two are polar; and (5) that a still better agreement is given by considering that γv is additive.Indirect determination of compressibility.---The compressibility, β, can be calculated by combining γ with the coefficient of expansion, α=βγ. In most instances, the agreement with the directly determined values is satisfactory.