Sodium dihydrogen citrate

All solutions were prepared with ultrapure Millipore deionized water obtained via a Milli-Q^® Advantage A10^® Direct water purification system, with a resistivity of 18.2 MΩ cm at 25.0 °C. The following chemicals were used as received without further purification: potassium chromate (Alfa Aesar, 99.0%), sodium dihydrogen citrate (Alfa Aesar, 99.0%), citric acid-disodium salt sesquihydrate (Alfa Aesar, 99.0%), glacial acetic acid (Millipore, ACS grade), sodium acetate anhydrous (Millipore, ACS grade), glycolic acid (Acros, 99.0%), succinic acid (TCI America, ≥99.0%), disodium succinate anhydrous (Acros, 99.0%), dl-malic acid (TCI America, ≥99.0%), propanoic acid (TCI America, ≥99.0%), potassium propionate (TCI America, ≥98.0%), potassium nitrate (Avantor Performance Materials US, 99.0–100.5%), potassium hydroxide (BDH Chemicals, 85.0%), and hydrochloric acid (BDH Chemicals, 36.5–38%). Potassium chloride (BDH Chemicals, 99.0–100.5%) was recrystallized by slow diffusion of ethanol into a supersaturated KCl solution in water and dried under vacuum before use.
Concentrations of Cr(vi) stated throughout correspond to the value calculated based on the added K₂CrO₄. As reported previously,^{7,21 (link)} dichromate is likely present at the higher 7.00 mM Cr(vi) concentrations used.