Rationale: Electrospray mass spectrometry (ESI-MS) is one of the most effective methods for assessing the state of metals in solution. For ions with a redox potential close to ~0.55 V, such as Cr6+ , reduction of the metal in solution occurs in the ESI-MS system. In our studies, it was observed that [HCrO4 ]- undergoes reduction, resulting in the formation of [CrO3 ]- . The precise mechanism remains ambiguous. The reduction of hexavalent chromium to pentavalent chromium is supported by Frost diagrams, reinforcing our confidence in the validity of the ESI-MS measurement method. The reduction mechanism in ESI-MS was clarified, and a system was devised to eliminate electron donation during the reduction of Cr6+ in solution.
Methods: To determine the state of Cr6+ by ESI-MS, CrO3 in solid form was dissolved in ultrapure water to prepare a solution of 500 × 10-6 mol/L (μM) concentration. The pH was adjusted to 4.0, 5.3, 6.3, 8.2 and 9.1 and subsequently measured. CrO3 solutions with various concentrations of 10, 100 and 500 μM were prepared and adjusted to a pH of ~7 using tetramethylammonium hydroxide to measure Cr6+ under different conditions.
Results: Cr6+ in solution was soluble and existed as an oxoacid with a negative charge independent of pH. Cr6+ was stable over a wide pH range at various concentrations. The ESI-MS method determined the negative ion [HCrO4 ]- as the stable ion, but [CrO3 ]- was also present as a byproduct. Therefore, we were interested in the presence of other species, such as [CrO3 ]- , which could have formed owing to the reduction of Cr6+ .
Conclusions: In ESI-MS system, it undergoes reduction to form [CrO3 ]- . The high flow rate of ultrapure water in pump insulated the acceptance of electrons by Cr6+ preventing its reduction. Further in-depth ESI-MS studies could explain the complex formation and behavior of Cr6+ in aqueous solution.
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