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Table 30-3 pH TRANSITION RANGES OF ZINC CHLORIDE SOLUTIONS AS DETERMINED BY TITRATION Molarity Endpoint ZnCl9 pH 0.101 4.90 0.499 4.42 1.010 3.45 1.996 2.62 3.UXJ£ jL*£itL 3.990 1.60 pH Transition Range 4.75 ± 1.05 4.04 ± 0.74 3.43 ± 1.25 2.72 ± 0.78 . 2.05 ; 0.54 1.82 ± 0.76 The curves in Figure 30-4 are linear from the initial starting pH. The pH change caused by salt addition is negligible in the case of sodium chloride and could be due to experimental error or a trace of acid in the sodium chloride. The results from potassium chloride testing appear to exceed experimental error and are believed real. The increase in pH, both initial and in the presence of added hydrochloric acid, may be due to a decrease in the activity of hydrochloric acid. Another possibility is that potassium ions stabilize the various zinc-chloro com plexes which would result in less acid-base hydrolysis of the zinc ion. But, it may be argued that sodium chloride should behave in a similar fashion. A third consideration is that the molarity of zinc chloride in each of the three solutions is different. The addition of either KC1 or NaCl causes volume expansion which represents a dilution of the zinc concentration on the molarity scale. On the molar scale, potassium chloride, due to its larger partial molal volume, causes a greater dilution than does sodium chloride. PARTIAL MOLAL VOLUMES OF ZINC CHLORIDE IN ZINC-CHLORINE BATTERY ELECTROLYTES Although formally defined as a limiting value, in actual practice the partial molal volume may be determined over a small change in composition. The partial molal volume of a solute generally increases as the solute concentration is increased. Therefore, the value of the partial molal volume must be referred to the concen tration at which the measurement was made. 30-8PDF Image | Development of the Zinc-Chlorine Battery for Utility
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