Development of the Zinc-Chlorine Battery for Utility

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Development of the Zinc-Chlorine Battery for Utility ( development-zinc-chlorine-battery-utility )

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chloride for concentrations less than 0.2 molar. Their findings are listed in Table 30-2. Perrin unable to detect any polynuclear complexes of zinc. Our find ings indicate that the pH of a zinc chloride solution is a reliable indicator of the stoichiometry of the solution. For example, if a solution contains free alkali, OH , the equilibria presented in Table 30-2 will shift to the right and exhibit buffering characteristics. Addition of a strong acid to such a solution will produce a titration curve of which the point of inflection will indicate a stoichiometric composition. The pH at which the inflection point occurs is a function of the hydro lytic constants and the zinc-chloride concentration. As shown in Figure 30-1, an analytical end point is obtained as the solution goes from a slight excess of zinc hydroxide to a slight excess of hydrochloric acid. The pH at the end point of the titration is assumed to be the pH of a stoichiometric ZnCl^ solution at that concen tration. The assumption appears valid and the difficulty in determining the pH at this point is due to the steepness of the titration curve around the end point. The pH transition range for the solutions used in this work is given in Table 30-3. For comparison, the data points used in Figure 30-2 are also shown. Investigator/Year Biedermann/1964 Schorsch/1963 Perrin/1962 Medium 5M NaCl 2M NaCl KN°3 Hydrolysis > Zn+2 + h2o < ZnOH+ + H+ > -log K Table 30-2 LITERATURE VALUES FOR ZINC-CHLORIDE HYDROLYSIS IN SUPPORTING ELECTROLYTES 10.2 ± .2 2Zn+2+h20 Zn+2 + h2° < ZnOH+ + H+ > 2Zn+2 4- h20 < Zn2OH+3 + H+ > Zn+2 + h20 < ZnOH+ + H+ 9.01. + .02 7.2 ± .02 9.4 The effects of added sodium chloride and potassium chloride are different. The pH of the 2 molar zinc chloride solution prior to addition of either salt was 2.68. After adding sodium chloride to solution, the pH dropped slightly to 2.64. Upon addition of potassium chloride to a zinc chloride solution, the pH increased to 2.89. 30-7

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