Semi micro ph electrode

Ferrihydrite reduction was monitored by measuring the concentration of aqueous Fe(II) and total Fe(II) (total Fe(II) is comprised of aqueous Fe(II) and Fe(II) extracted from the solids by 0.75 M HCl) with the ferrozine assay [24 ]. Briefly, 1 mL of HEPES (50 mM)-buffered ferrozine reagent [25 (link)] was added to 0.05 mL of sample, and the Fe(II) concentration was measured at 562 nm with a spectrophotometer. The concentrations of sulfate and phosphate in samples were determined by using an ion chromatograph (Dionex 3000) with an IonPac AS11 analytical column (250 x 2 mm, Dionex) and 1–20 mM KOH at a flow rate of 0.5 mL min^-1 (Dionex Corporation, Sunnyvale, CA, USA). The concentrations of glucose and acetate, lactate, and other organic acids were measured by high-performance liquid chromatography (HPLC) with an Agilent 1100 series HPLC equipped with refractive index and ultraviolet absorbance detectors (Aglient Technologies, Santa Clara, CA, USA). The samples were diluted with an equal volume of 10 mM H₂SO₄, and 50 μL of the diluted sample was injected on a Bio-Rad Aminex HPX-87H ion-exchange column (7.8 × 300 mm; Bio-Rad Laboratories, Hercules, CA, USA). The column was eluted isocratically with 5 mM H₂SO₄ at a flow rate of 0.6 mL min^-1 at 50°C, with analyte detection at 210 nm. The aqueous-phase pH was measured by Semi Micro pH electrode (Thermo Scientific, Waltham, MA, USA).