Preparation of Fe-free (Ni-containing) KOH. The Fe impurities in normal KOH solutions can be removed by treating with high-purity Ni(OH)2. In a clean 50 mL polypropylene centrifuge tube, 2 g of Ni(NO3)2•6H2O (99.99%) was dissolved in 5 mL of ultrapure water. 20 mL of 1 M KOH solution was added to give a Ni(OH)2 precipitate. The suspension was agitated and centrifuged, and the supernatant was decanted. The Ni(OH)2 precipitate was washed with ultrapure water for three times by centrifugation. The solid was dispersed in 10 mL of 1 M KOH by centrifugation, and the supernatant was decanted. This solid was used as the Fe-absorber. The normal KOH solutions could be cleaned by adding to this Ni(OH)2. The cleaning procedure involves dispersing Ni(OH)2 in the KOH solution, mechanically agitated over-night, followed by at least 3 h of resting.