A pH buffer may be used to achieve a more stable operation. Luckily, the ligand Bhmbpy itself is a weak base. Bhmbpy and its protonated form HBhmbpy are a buffer pair with pKa around 2 -4. Therefore, we use a ligand ratio of 1:3.5 (Fe:Bhmbpy) for later tests. Extra ligands function as buffer solution that can keep the solution pH stable in optimized region for longer time. The optimized operation pH for this iron complex is 2 -4.