The electrical conductivity of the water-sulphur trioxide system in the region of the composition H,SO, has been reinvestigated. The values of the minimum conductivity and the composition at which it occurs have been determined at lo", 25", and 40" from experiments in which the electrical conductivity and the freezing point of the same solution in a combined conductivity cell and cryoscope were measured over a range of compositions in the neighbourhood of the composition H,SO,. New values are given for the conductivities of solutions of water and sulphur trioxide in sulphuric acid.IN a previous investigation of the electrical conductivity of the water-sulphur trioxide system in the neighbourhood of the composition H2S04 it was found that at 25" the minimum conductivity occurs at the composition H,SO, to within &0+003m (H,O or H,S20,). At the freezing point of sulphuric acid (10*37"), Kunzler and Giauque found that the minimum conductivity did not occur exactly at the composition H2S04 but at 99.996 & 0.001 weight yo (Le., 0.002 It was subsequently stated by Kunzler that the composition of minimum conductivity changes by less than 0403 weight yo ( k , 0.0015 mole per kg. of solution) over the range 5-20'. It w i l l be shown in a later paper that the exact position of minimum conductivity is of considerable importance in connection with the determination of the mobilities of the HsSO4+ and HS0,ions in sulphuric acid, and a knowledge of these mobilities is essential for an understanding of the mechanism of chain-conductance in sulphuric acid. Since neither Kunzler and Giauque nor Kunzler made any measurements at 25", or gave actual conductivity values t at any temperature, we considered it worth while to repeat our earlier measurements at 25' with greater accuracy and to extend them to other temperatures. In order to achieve the accuracy necessary for this work and other work on the conductivities of solutions in sulphuric acid (see, e.g., refs. 27 and 28), the conductivity bridge and cells used previously were replaced by apparatus of considerably improved design. 0.0005 mole of H,O per kg. of solution). EXPERIMENTAL Cmtductance Bridge.-This consisted of an A.C. Wheatstone bridge network with a resistantcapacitance Wagner earth, the design being based on Luder's The resistance boxes used as standards were non-inductive &decade boxes (Doran Instrument Co. and Croydon Precision Instrument Co.) reading to 11,111 ohms in 0.1 ohm steps. They were calibrated by the method of self-comparison with an accuracy of 0.02y0. For most measure-* Part XXT, preceding paper. f These authors measured the variation of the resistance of sulphuric acid with change of composition in a cell of unknown cell constant.precision bridge circuit.
