Radiometric and Spectrophotometric Studies of the Behavior of Chromium(VI) Oxide in Concentrated Perchloric Acid

Abstract

Integration of the peak areas was taken as an indication of mass. In order to study the stability of the parent compound, a solution of 50 micrograms of both radiolabeled and stable fluoro ascorbic acid in 4-5 mL of water for injection was analyzed by HPLC at 7-15 minute intervals.In the absence of fluorine-18, the FAA appears to undergo decomposition in aqueous solution at a relatively slow rate, however, the radiolabelled FAA decomposition rate is significantly accelerated as shown in Fig. 2. Further, the decomposition of the carrier-added FAA results in a compound without ultraviolet absorbance as indicated in the figure. Based upon both the decrease in mass of the carrier product and the radioactivity analyses of the 18 F-FAA and the I8 F-dehydro compound as summarized in Table 1, the rate of decomposition of the parent compound appears to follow first-order decomposition kinetics. AcknowledgementA preliminary report of this work was presented orally at the Eleventh International Symposium on Radiopharmaceutical Chemistry, held August 13-17, 1995 in Vancouver, B. C. Canada. The manuscript is dedicated in memory of Professor Edward Rack. The authors gratefully acknowledge the cyclotron staff for their assistance in the preparation of the fluorine-18 radionuclide. Chromium(VI) oxide / Perchloric acid / Acid reduction / Chromium determination SummaryA study of the behavior of 5, Cr0 3 in 70% HC10 4 over the temperature range from 20 to 194°C by means of Cr-51 labelling, UV-VIS spectrophotometry and ion exchange chromatography, shows that the solubility of 5 'CrO, depends on a competition between the dissolution process and the acid reduction of solution phase Cr(VI). These processes occur simultaneously and are dependent on both the temperature and the concentration of Cr(VI), as shown by comparison between radiometric measurements (where total chromium can be accurately determined) and spectrophotometric measurements (where only the Cr(VI) is detectable at the wavelengths studied). These conclusions are confirmed by PbCr0 4 precipitation of 51 Cr(VI), where at 194 °C, 97% of the total chromium appears as Pb"Cr0 4 while at 86 °C only 5% does. Cation exchange chromatography of the solution after brief contact of 51 CrO, with concentrated HC10 4 at 20 °C shows only traces of 5l Cr(VI), most of the radioactivity eluting as "Cr(H 2 0)6 + , with smaller amounts of species with +2 and +1 charges. These results imply serious limitations to the spectrophotometric determination of low concentrations of total chromium in alloys or in biological material which use dissolution in 70% HC10 4 as a primary analytical step.

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 rate of reduction in the CrO 3 -37% HCl system is very rapid, when compared to the rates for the CrO 3 -70% HClO 4 [11] and CrO 3 -65% HNO 3 systems [14], probably due to the presence of the oxidizable chloride ion.…”

“…The specific activity was 400 GBq mol −1 . All samples were submitted to radionuclidic and radiochemical analysis [11], to show that 51 Cr was the only radionuclide present and that approximately 98.8% of the 51 Cr was initially present as Cr(VI). ( 51 Cr)CrO 3 was prepared by adding high specific activity 51 Cr(VI) to a hot, nearly saturated aqueous solution of inactive CrO 3 (Baker), to produce, by vacuum drying, a doped solid with a specific activity of 4.65 MBq mol −1 .…”

“…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).…”

“…On the other hand, studies of the reaction of solid CrO 3 with concentrated HClO 4 [11] or HNO 3 [14] show that the acid-reduction process depends on the solubilization of the Cr(VI). Low concentrations of Cr(VI) in the solution phase lead to a very rapid reduction, but Cr(VI) concentrations above 10 −2 mol L −1 make the process much slower.…”