The Polarizing Power of Metal Cations in (Hydr)Oxides

✍ Scribed by Gamal M.S. El Shafei

• Book ID: 102575530
• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 103 KB
• Volume: 182
• Category: Article
• ISSN: 0021-9797
• DOI: 10.1006/jcis.1996.0457

✦ Synopsis

The proton binding isotherms of simple and mixed hydroxides are associated with amphoteric character.

were measured by potentiometric titration. The hydroxides were

The parameter Z/r 2 determines the acidic or basic characselected so that the metal cations (Al, Ni, Bi, and Ca) were of ter of inorganic (hydr)oxides (8). The present article fodifferent polarizing power, expressed as Z/r 2 , where Z is the cation cuses on the acid-base properties of simple and mixed hycharge and r its radius. The results show that the acidity of the droxides precipitated by addition of base. The hydroxides simple hydroxides increases as Z/r 2 increases. The basicity follows were selected on the basis of differences in polarizing power, a reverse order and is not completely congruent with Z/r 2 . The Z/r 2 , of the metal cations. The cations selected were Al /3 , experimental PZC was found to increase with basicity. A correla-Ni /2 , Bi /3 , and Ca /2 , for which Z/r 2 Å 10.68, 4.2, 3.25, tion between basicity (or PZC) and Z/r 2 has been noticed when the latter was normalized to (CN) 0 , the coordination number of and 2.04, respectively. Mixed hydroxides containing Al were oxygen in each hydroxide, and it has been shown that for simple precipitated and their acid-base properties were examined. and mixed hydroxides the PZC increases as (Z/r 2 )/(CN) 0 de-The acid-base character was investigated by measuring the creases. Such dependence is interpreted in terms of hydrolysis of proton binding isotherms using a potentiometric titration surface metal cations and protonation of surface O 02 /OH 0 ions. method ( 9). An empirical equation is proposed which predicts the point of zero charge of many metal (hydr)oxides, confirming the suggested role EXPERIMENTAL of the parameter Z/r 2 of the metal cation in controlling the charge at the metal (hydr)oxide interface.