Both articles mention the case of CaSO₄, the solubility of which is sufficient to make practical its determination in school laboratories. The reason why the solubility product constant calculated from the observed solubility is 10 times greater than that listed in tables has received analysis and explanation in this Journal (3). To the total observed solubility of 18 mM, the zero ionic strength solubility calculated from the solubility product constant contributes only 26%, the neutral salt or activity coefficient effect makes the largest contribution, 47%, and ion pairing contributes 28%. The analysis presented is generally applicable to other salts. Two contributions in the February 1989 issue of this Journal offer a comparable treatment (4, 5).

A discussion of the very limited solubility of Fe(OH)₃ (1) fails to distinguish between two different quantities: the solubility of free Fe³⁺ ion and the total solubility of all soluble forms. The first quantity is much, much less than the latter. Revealing distribution curves and concentrations of the free ion and total soluble forms have been plotted as a function of pH (6). At pH 7 the concentration of free Fe³⁺ is only 10^-17 M, whereas that for the total of all soluble forms is much greater, 10^-9 M. From the distribution curves we learn that of the soluble forms at pH 7, 28% occurs as Fe(OH)₂⁺ and 71% as soluble Fe(OH)₃.

For hydrolysis of cations a classic reference should be consulted (7).

Literature Cited

1. Hawkes, S. J. J. Chem. Educ. 1998, 75, 1179-1181.
2. Clark, R. W.; Bonicamp, J. M. J. Chem. Educ. 1998, 75, 1182-1185.
3. Martin, R. B. J. Chem. Educ. 1986, 63, 471-472.
4. Carpenter, J. H. J. Chem. Educ. 1989, 66, 184.
5. Russo, S. O.; Hanania, G. I. H. J. Chem. Educ. 1989, 66, 148-153.
6. Martin, R. B. J. Inorg. Biochem. 1991, 44, 141-147.
7. Baes, C. F. Jr.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York, 1976.

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