Sodium dichromate dihydrate

5 mg of Pt/C (40 wt.%, Johnson Matthey Co.) was sonicated with DI water, 28.6 μL of Nafion solution (5 wt.%, Aldrich), and 400 μL of 2-propanol (99.9%, Aldrich). After dispersing the ink, 3.5 μL of ink was dropped onto a glassy carbon electrode. All electrochemical measurements were carried out using an Autolab potentiostat (PGSTAT 302N). A standard three electrochemical cell with Pt wire and saturated calomel electrode (SCE) was utilized as a counter and reference electrode, respectively. All potentials were reported with respect to the reversible hydrogen electrode (RHE) using hydrogen oxidation method. In order to get a reproducible data, several potential sweeps between 0.05 V and 1.2 V (vs. RHE) were applied to the electrode under Ar. For CO oxidation experiments, CO was introduced into Ar-purged 0.1 M HClO₄ electrolyte for 12 min at a constant voltage (0.05 V vs. RHE). After saturation of the Pt with CO, excess CO was eliminated by purging with Ar for 30 min and then the electrode was immersed into the HClO₄ (in reference) and HClO₄ + hexavalent chromium ion (Sodium dichromate dihydrate, Cr₂Na₂O₇·2H₂O, Sigma-Aldrich). The CO oxidation was obtained from 0.05 V to 1.2 V vs. RHE. For control experiment with trivalent chromium, chromium (III) chloride hexahydrate (CrCl₃·6H₂O, Sigma-Aldrich) was used.

Sodium dichromate dihydrate (SDD; 99.95% pure; CAS No. 7789-12-0) was obtained from Sigma-Aldrich Inc. (Milwaukee, WI) and was stored at room temperature and protected from light. Captan (98.3% purity; CAS No. 133-060-2) was obtained from AK Scientific, Inc. (Union City, CA), and folpet (98% purity; CAS No. 133-07-3) was obtained from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA). The dose formulations of Cr(VI) in drinking water were prepared weekly at concentrations of 514 mg/L SDD (∼180 ppm Cr(VI)) in tap water. All dose formulations were verified weekly at Eurofins Lancaster Laboratories Environmental (Lancaster, PA). Cr(VI) concentrations were verified analytically by method SW-846 7196A (Online Supplemental Table S1). The detection limit is 0.04 ppm Cr(VI). Dose formulations for both captan and folpet were prepared weekly in diet at concentrations of 6,000 and 16,000 ppm.