James E. Tackett

FT-IR Characterization of Metal Acetates in Aqueous Solution

A correlation has been developed between antisymmetric and symmetric carboxylate group infrared absorption frequencies, cationic radii, and acetate group types that can be used to differentiate between ionic, unidentate, bidentate, and bridging acetate groups in aqueous metal acetate solutions. In solutions containing the alkali metal, alkaline earth, and Mn(II), Co(II), and Ni(II) ions, the acetate group is primarily ionic. The bound acetate groups in Ce(III), Pb(II), Sm(III), Y(III), and Yb(III) systems are bidentate. A solution of Cr(III) acetate has bridging acetate groups. Cu(II), Zn(II), and Cd(II) appear to form H-bonded unidentate structures in which an acetate oxygen not coordinated to the metal is hydrogen bonded to a coordinated water molecule. Hg(II) acetate has bridging acetate groups and might also have unidentate acetate groups.

Characterization of Chromium(III) Acetate in Aqueous Solution

FT-IR, 2H NMR, FAB mass spectrometry, and ion exchange chromatography were used to characterize structures in chromium acetate solutions prepared under several different conditions. Infrared and 2H NMR peaks for ionic, unidentate, bidentate, and bridging acetate groups were assigned on the basis of chemical information and appearance/disappearance correlations. FAB mass spectrometry was used to confirm the existence of the cyclic chromium trimer in solution. Ion exchange chromatography allowed separation of the complexes based on ionic charge. Replacement of acetate groups by hydroxyl groups causes ring opening and the formation of linear species. Many commercial chromium(III) acetates contain oxalate as well as acetate ligands.

Hydrolyzed Polyacrylamide/Chromium(III) Acetate Gel Chemistry

Proton NMR and visible spectroscopy are used to investigate the chemistry of hydrolyzed polyacrylamide/chromium acetate gels. Color changes in conjunction with HNMR are used to monitor chromium acetate hydrolysis, and HNMR is used to characterize Polyacrylamide in solution and to monitor increases of free acetate and decreases of polymer backbone proton and amide proton resonances during gel formation. The data indicate that chromium acetate hydrolysis is halted by polymer coordination. The polymer cross-linking reaction appears to be a simple replacement of chromium(III) coordinated acetate groups on the chromium(III) trimer by polymer carboxylate groups.

Determination of Low Amounts of Carboxylate in Polyacrylamide Using FT-IR

The concentration of polymer carboxylate groups in polyacrylamide can be critical when these polymers are used in the petroleum industry for improved oil recovery applications. Several techniques have been used to measure the relative amounts of carboxylate and amide groups in the polymer. Comparisons of results obtained with the use of titration, colorimetric analysis, elemental analysis, infrared and 13C NMR spectroscopy for this analysis have appeared in the literature. The 13C NMR technique is interesting because it gives a sequence distribution of carboxylate groups as well as their concentration.