The apparent hydratian of ions. Part III. The densities and viscosities of saturated solutions of ammonium chloride in hydrochloric acid

The solubility of ammonium chloride in aqueous solutions of hydrochloric acid of concentration up to 13N has been determined at 25°. The densities and viscosities of the saturated solutions have also been measured. The results are compared with those previously obtained by the author for sodium and potassium chlorides and lithium chloride. The same type of formula, d = K + k₁a + k₂b, is found to apply for the relation between densities and the concentrations a and 6 of acid and salt. The solution volume of the acid is substantially the same as in the solutions containing the other salts. The apparent density of the water is highest, and hence its solution volume lowest, in the solutions containing ammonium chloride. This salt gives a solution volume comparable with that of rubidium chloride, the ammonium ion having a relatively high volume compared with sodium and potassium, in accordance with the evidence from the measurements of radii in crystals. The principal factors governing the viscosities are the numbers of particles present and their sizes relative to that of the water molecule, which is the predominant species. The general treatment of the viscosity data is in agreement with these ideas if the relative sizes of the particles are based upon measurements of the radii of ions and atoms in crystals. The removal of an ammonium ion results in an increase in viscosity which is almost exactly equal to that caused by the addition of a chlorine ion. The effects of both of these ions remain constant over a large part of the series of solutions examined. This constancy is taken as an indication of the absence of hydration for these ions. The ammonium ion has an effect upon viscosity relatively less than that of either the water molecule or the ion H₃O^.. Its effective size as regards viscosity is somewhat larger than that of a nonhydrated potassium ion, but smaller than that of the partially hydrated potassium ions in a saturated potassium chloride solution.