Near-infrared spectroscopic study of basic aluminum sulfate and nitrate

The tridecameric Al-polymer [AlOAl(OH)(HO)] was prepared by forced hydrolysis of Al up to an OH/Al molar ratio of 2.2. Under slow evaporation crystals were formed of Al-nitrate. Upon addition of sulfate the tridecamer crystallized as the monoclinic Al-sulfate. These crystals have been studied using near-infrared spectroscopy and compared to Al(SO).16HO. Although the near-infrared spectra of the Al-sulfate and nitrate are very similar indicating similar crystal structures, there are minor differences related to the strength with which the crystal water molecules are bonded to the salt groups. The interaction between crystal water and nitrate is stronger than with the sulfate as reflected by the shift of the crystal water band positions from 6213, 4874 and 4553 cm for the Al sulfate towards 5925, 4848 and 4532 cm for the nitrate. A reversed shift from 5079 and 5037 cm for the sulfate towards 5238 and 5040 cm for the nitrate for the water molecules in the Al indicate that the nitrate-Al bond is weakened due to the influence of the crystal water on the nitrate. The Al-OH bond in the Al complex is not influenced by changing the salt group due to the shielding by the water molecules of the Al complex.