Improved theoretical and experimental investigation of the transient hot-wire method for measuring the thermal conductivity of liquids, especially of its applicability to electrolytic solutions

The transient hot-wire method is to-day widely used as a method for measuring the thermal conductivity of liquids. Its "classical" field of application is pure, dielectric liquids. The method is often claimed to be "elegant", fast, and convenient. It is therefore amazing that only a few attempts have till now been made at expanding it for measurements on electrolytic solutions. That expansion is always carried out • or proposed carried out - in the literature by surrounding the hot wire with an electrically insulating layer. However, the layer gives rise to both practical and theoretical difficulties. E.g. Alas [l ] (IMT) used for theoretical reasons an extremely thin layer but he did not succeed in finding a material that is not dissolved by bases. 11 ] Is therefore confined to aqueous salt solutions. In [3] the present author presented a modification of the method in
which the layer is avoided by altering the device so that the counter electromotive force from the electrolytical polarisation is quickly made to balance with the voltage applied to the measuring cell. A naked hot wire can therefore be used, the electrolytic solution being barred from carrying electric current. However, mutters are not quite so simple as indicated, and an analysis is necessary to ensure that indeterminable sources of error do not occur. That analysis is to a certain degree given in { 3 ], which builds upon a former work [2 ] by the same author in which certain sources of error of the method when it is applied to dielectric liquids were treated. [2] and [3] contain the main points of the author's Ph. D. (lie.scient.) thesis, which appeared in Danish. The present work is - together with [ 4 ] - an improvement of the investigations indicated above. In [4 ]tbe theory of the modification - which was in [3] rather limited - is discussed in every detail. In the present paper the experimental part of the work has been widened and [2 1 [3], and [4 ] are abstracted together with additions for making them quite up-to-date. The set-up was in [ 3 ] only developed to a semiautomatic stage and only 5 electrolytes were measured on in aqueous solutions of different concentrations in order to give experimental support to the modification. The accuracy was 1-1.5%. Now full automation has been introduced and 9 new electrolytes in different concentrations have been measured on - some of which are not found in the literature on X measurements -. The accuracy turned out to be improved to 0.2-0.5% by the introduction of full automation. In total 76 individual electrolytic solutions have now been measured on at room temperature with the modified device, and every measurement on an individual solution is, in its turn, the mean of IG or 5 determinations. The results are compared with values given in the literature and the agreement is found very satisfactory.