A method for preparing a redox flow battery electrolyte is provided. In some embodiments, the method includes the processing of raw materials containing sources of chromium ions and/or iron ions. The method further comprises the removal of impurities such as metal ions from those raw materials. In some embodiments, a reductant may be used to remove metal impurities from an aqueous electrolyte containing chromium ions and/or nickel ions. In some embodiments, the reductant is an amalgam. In some embodiments, the reductant is a zinc amalgam. Also provided is a method for removing ionic impurities from an aqueous acid solution. Further provided a redox flow battery comprising at least one electrolyte prepared from the above-identified methods.