Speciation of the products resulted from the reaction of51Cr(VI) with concentrated perchloric and trifluoromethanesulfonic acids

Abstract

The speciation of the products from the reaction of 51Cr(VI) with concentrated perchloric and trifluoromethanesulfonic acids was carried out by ion chromatography, using HC104 and Ca(C104) 2 as eluents. The reaction (1 h) of high specific activity 51Cr(VI) with 99% CF3SO3H produces 47.6% of the 51Cr as a 3+ species (hexaaquochromium(III)), 27.6% as a 2+ species and 1.7% as a 1+ species. The reaction with 70% HC104 gave 82.5% of the 51Cr as a 3+ species, 14.0% as a 2+ species and 2.9% as a 1+ species. The 1+ and 2+ species undergo slow reactions to give the hexaaquo form.

A radiochromatograpic study of the reaction of 51CrO3 with hydrochloric acid

“…However, the results from previous studies on the reduction of Cr(VI) in solutions of several oxidizing acids [10][11][12][13][14], not involving a conventional reducing species such as chloride, suggest that acid, i.e., the hydrated proton, may be a defining reagent in reduction pathways. Thus, in the present case, the oxidation of chloride to chlorine by Cr(VI) may be the dominant reaction but other paths, such as "acid reduction" [11][12][13] may also contribute. The role of the different anions present, which depend on the acid used, is also important as they form complexes whose stability influences the overall kinetics of the acid-reduction process.…”

“…The reduction of Cr(VI) in many other acids, including the so-called "oxidizing acids", has also been reported [4][5][6][7][8][9][10][11][12][13][14]. Detailed studies of the reduction of low concentrations (up to 10 −3 mol L −1 ) of Cr(VI) with several different acids, including HNO 3 [9,10], HClO 4 [12], CF 3 SO 3 H [12], H 2 SO 4 [13], and HCl [13], have shown that the rate of reduction increases as the concentration of the *Author for correspondence (E-mail: chc@iqm.unicamp.br).…”

“…Detailed studies of the reduction of low concentrations (up to 10 −3 mol L −1 ) of Cr(VI) with several different acids, including HNO 3 [9,10], HClO 4 [12], CF 3 SO 3 H [12], H 2 SO 4 [13], and HCl [13], have shown that the rate of reduction increases as the concentration of the *Author for correspondence (E-mail: chc@iqm.unicamp.br). acid increases or as the concentration of the Cr(VI) decreases.…”

Reduction of trace quantities of chromium(VI) by strong acids

“…The reduction of Cr(VI) by many acids, including the so-called "oxidizing acids", has also been reported. [9][10][11][12][13][14][15][16][17][18][19] Detailed studies of the reduction of low concentrations (less than 10 -3 mol L -1 ) of Cr(VI) with several different acids, including HNO 3 , 13,14 HClO 4 , 17 CF 3 SO 3 H, 17 H 2 SO 4 , 18 and HCl, 18 have shown that the relative rate of reduction increases as the concentration of the acid increases or as the concentration of the Cr(VI) decreases. Similar reductions have been reported for Cr(VI) present in natural waters, after acidification of the sample for storage.…”

“…3 Haight et al 26 attributed the reduction in perchloric acid to the action of water. However, this is not likely to be the case as the rate of reduction increases with a decrease in the activity of water, as seen in the extreme cases of 98% H 2 SO 4 18 and 99% CF 3 SO 3 H. 17 Stollenwerk and Grove 21 suggest that the electrons for the Cr(VI) reduction could be provided by the oxygen in HCrO 4 while Espenson 27 suggests the addition of protons, two for the first step (Cr(VI) to Cr(V)) and one for the second (Cr(V) to Cr(IV)). However, no satisfactory overall reaction mechanism for the acid reduction of Cr(VI) has yet been proposed.…”