THE SPECTROSCOPIC EXAMINATION OF ALUMINUM SALT SOLUTIONS

Aluminum salts solutions activate metallic aluminum surfaces at different rates. The objective of this research was to attempt to establish whether or not structural differences among the four salts could be observed. The infrared, ultraviolet, and visible spectra of various concentrations of aluminum chloride, nitrate, perchlorate, and sulfate were measured. The infrared spectra included several general features: (1) A broadening of the OH stretching band of water; (2) A splitting of the OH stretch of water into several absorption bands; (3) The emergence of a new absorption band at 2400 cm('-1); (4) Occasional splitting of the OH bending mode of water; (5) The splitting of the 1370 cm('-1) absorption band of NO(,3)('-) by Al(III) interaction. In general, the infrared results indicated that no difference at the molecular level existed between aluminum chloride, sulfate, and perchlorate. Nitrate was the exception, an interaction between the aluminum cation and the nitrate anion was experimentally observed. These aluminum salts solutions have relatively low pH's. The literature indicates that the predominant structure should be monomeric species; however, several infrared bands suggested the existence of dimeric species or structures that would include Al-O-Al or Al-OH-Al stretchings. The wavelength of maximum absorption for aluminum chloride, perchlorate, and sulfate, in the ultraviolet region, was located at 240 nm. Aluminum nitrate exhibited a maximum absorption at approximately 303 nm; it also, absorbed in the 240 nm range but no maximum was observed because absorbance values were above 2.5. The results of ultraviolet spectroscopy of these aluminum salts suggested the following: (1) A charge transfer reaction scheme has been proposed; (2) Sulfate anion formed a stable complex with aluminum, possibly a strong ion-pair, at high sulfate concentrations; (3) Chloride and perchlorate did not replace an inner sphere group of aluminum; (4) Aluminum nitrate spectra suggested that the nitrate anion was interacting with aluminum. Visible spectra of the aluminum salts revealed little or no valuable information. Since the conditions under which complexing occurred (dilute solutions and high pH values) were quite different from the actual aqueous environment little information could be ascertained. In addition, it was hoped that complexing would be affected by the nature of the anion present; this behavior was not experimentally observed.