Electrochemical method to determine the local composition of a liquid phase within a solidifying NH₄Cl-H₂O system

Knowledge of liquid composition distribution is needed for improved understanding of energy and species transports occurring during mixture solidification. This paper describes an experimental diagnostic technique for monitoring the solute concentration of the ammonium chloride aqueous solution undergoing a phase change without resort to a sampling procedure. The method is based on the principles of an electrochemical cell and can be extended to any binary salt solution having chloride anions. The successive chloriding and dechloriding of the working electrode is employed to eliminate the influence of the drift caused by the dissolution of the silver chloride layer in a high concentration environment. The diagnostic technique was applied to the diffusion controlled solidification to measure the transient distribution of the liquid composition with the uncertainty of ±2.5%. The simultaneously measured temperature and liquid concentration data confirmed the nonequilibrium undercooling of the interdentritic liquid.