NOVELTY - Method of adjusting a concentration of a cathode redox-active element and/or a concentration of an anode redox-active element in a redox flow battery (10) involves providing a catholyte solution (20) and an anolyte solution (30) comprising redox-active element(s), converting redox-active element(s) at each valence state to a predetermined valence state (s1), determining a concentration of the redox-active element(s) and adjusting the concentrations of the cathode redox-active element and anode redox-active element to meet a predetermined redox flow battery operating performance parameter.    USE - Method of adjusting concentration of cathode redox-active element and/or concentration of anode redox-active element in redox flow battery used for generating electrical power.    ADVANTAGE - The method enables rapid, reliable, consistent, and accurate adjustment of concentration of redox-active elements in redox flow battery with improved battery efficiency, energy utilization, and repetitive charge/discharge cycles.    DETAILED DESCRIPTION - Method of adjusting a concentration of a cathode redox-active element and/or a concentration of an anode redox-active element in a redox flow battery involves providing a catholyte solution comprising a cathode redox-active element having valence state(s), converting the cathode redox-active element at each valence state to a predetermined valence state (s1), determining a concentration of the cathode redox-active element at each valence state, providing an anolyte solution comprising an anode redox-active element having valence state(s), converting anode redox-active element at each valence state to a predetermined valence state (s2), determining a concentration of the anode redox-active element at each valence state, and adjusting the concentration of the cathode redox-active element at each valence state in the catholyte solution and the concentration of the anode redox-active element at each valence state in the anolyte solution to meet a predetermined redox flow battery operating performance parameter. INDEPENDENT CLAIMS are included for the following:    (1) method of determining amount of a redox-active element in a catholyte and an anolyte of a redox flow battery;    (2) method of determining vanadium ion concentration in a vanadium redox flow battery; and    (3) method of determining vanadium concentration in an electrolyte solution in a vanadium redox flow battery.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic view of a redox flow battery.    Redox-flow battery (10)    Catholyte solution (20)    Anolyte solution (30)    Positive electrode (40)    Negative electrode (42)