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was ground with corundum No. 1500, and polished with fine alumina. The surface was rinsed with deionized water prior to each dissolution experiment. The rinsed surface of galena was then photographed. Surface area was determined by using a planimeter on the photographic enlargement. All chemicals used were of reagent grade, and de-ionized water was used in the preparation of all the solutions. Experimental Procedures A 500 ml glass separable flask with a fitted lid having five necks was used as a reaction vessel for all dissolution experiments. The center hole of the lid was used for the accomodation of a stirring rod. The other four necks were used for the introduction of nitrogen gas into the dissolution medium, the discharge of the hydrogen sulfide gas together with nitrogen gas, the fixation of the galena specimen at a predetermined place against a stirrer, and the accomodation of a sampling tube, respectively. A water bath provided with a temperature controller was used as a thermostat ( • 0.5 ~ Each dissolution medium was deaerated with nitrogen gas for 30 min prior to each experiment. A galena sample was placed facing downwards in a dissolution medium, then the stirrer was activated to begin a dissolution experiment. Nitrogen gas was bubbled into the dissolution medium throughout each run. By so doing, the hydrogen sulfide produced was discharged from the dissolution medium and exhausted from the reaction vessel, and at the same time, a back flow of atmospheric gas into the reaction vessel was avoided. Aliquots of 10 ml each were withdrawn from the solution at appropriate time intervals and were subjected to quantitative analysis for dissolved lead by using an atomic absorption spectrometer.

YASUHIRO AWAKURA is Instructor, Department of Metallurgy, Kyoto University, Kyoto, Japan 606, SHINYA K A M E I Measurement of Activity of Hydrogen Ion is Research Engineer, The Furukawa Electric Co., Ltd., Nikko City, Japan 321-14, HIROSHI MAJIMA is Professor, Department The acid solutions and acidic chloride solutions used of Metallurgy, Kyoto University, Kyoto, Japan 606. in this study were fairly concentrated, and therefore Manuscript submitted November 13, 1979. hydrogen ion activity rather than hydrogen ion con1SSN 0360~2141/80/0911-0377500.75/0 METALLURGICALTRANSACTIONSB 9 1980 AMERICAN SOCIETY FOR METALS AND VOLUMEliB, SEPTEMBER 1980--377 THE METALLURGICALSOCIETY OF AIME

centration, was thought to be an appropriate rate-law parameter. Approximate values of activities of hydrogen ions in solutions containing HC1, HC1-NaC1, HC104 and HC104-NaC1 were determined at 25 ~ by measuring the electromotive force of the following cell: Pt-Pt black (H2, latm) I test solution ] salt bridge ] 3.3N KC1 ] Ag-Ag chloride

15t Temp oJ

55"C 1600 rpm

The electromotive forces of these cell are expressed by Eq. [1] RT

emfH. = - 0 . 2 0 6 + ~

In an+ + Vjp

[1]

where Vjp is a liquid junction potential occurring in the boundary region between the test solution and the salt bridge. The value of the liquid junction potential were ev