Ultrasonic velocity in potassium chloride solutions in the region of their negative viscosities

Abstract

It was observed by Poiseuille, Hubener, Sprüng, Slotte and others, that when substances like ammonium iodide, potassium chloride, etc., are dissolved in water, the viscosity of water is lowered. This phenomenon has since been known as 'negative viscosity'. The compressibility of aqueous solutions of electrolytes has been studied by Gibson, Falkenhagen and Bachem6, Szalay and others, and they have derived expressions for the relationship between the concentration of electrolyte solutions and their compressibilities. The existing data show that the compressibility continuously decreases as the concentration of electrolytes is increased. But these observations are confined to the positive region of viscosity. In the present communication we record our observations on the ultrasonic velocity in potassium chloride at low concentrations at 24°. The ultrasonic energy was obtained from a quartz disk and was converted into electrical energy by a similar quartz crystal for detection. The apparatus has been described elsewhere. The slab can be rotated, resulting in a variation in the amplitude of the transmitted energy as recorded on the oscillograph screen. The minimum in the energy is due to the total reflexion of the transmitted energy, as the angle of refraction in the solid is greater than that of incidence. The refractive index at the discontinuity is: n=sinθ_l/sinθ_s=V_l/V_s, where θ_L and θ_S are the angles that the wave-train makes with the normal in the solid and the liquid respectively. V_s is the velocity in the solid, which can be determined in advance by using a liquid with known velocity. At total reflexion of the waves, due to the rotation of the slab, V_θ=V_s sin θ_l since θ_s=90°. A knowledge of θ enables us to determine the velocity in the liquid.