TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Method: The testing method of oxidation of mixed electrolytic solution, comprises placing standard electrolytic solution with intermediate oxidation state at one pole of unit cell of vanadium redox flow battery and mixed electrolytic solution whose oxidation state to be tested in another pole, and measuring open circuit voltage of mixed electrolytic solution is 0 V or not, and determining whether oxidation numbers are same or different. The method further comprises verifying the oxidation states of first and second electrolytic solutions, by placing standard electrolytic solution at one pole of unit cell of vanadium redox flow battery and first or second electrolytic solution mixed electrolytic solution whose oxidation state to be tested in another pole, and measuring open circuit voltage of electrolytic solution is 0 V or not, and determining oxidation numbers using the difference between standard and sample electrolytic solution. The mixing step comprises determining mixing ratio and mixing the first and second electrolytic solutions according to the ratio. The mixing ratio is first electrolytic solution: second electrolytic solution is equal to |X2-A|:|X1-A|, where X1 is oxidation state of first electrolytic solution, X2 is oxidation state of second electrolytic solution and A is intermediate oxidation state of electrolyte used in cathode and anode. Preferred Components: In the testing method, the first electrolytic solution has vanadium oxidation state of +4. The second electrolytic solution has vanadium oxidation state of +3. The mixed electrolytic solution and standard electrolytic solution have vanadium oxidation state of +3.5.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: In the first electrolytic solution, vanadium(V) oxide or vanadyl sulfate is utilized as vanadium oxide source. The second electrolytic solution comprises vanadyl sulfate and/or vanadium sesquisulfate as vanadium oxide source and a reducing agent. The acid solution is sulfuric acid, hydrochloric acid, phosphoric acid and/or nitric acid.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The reducing agent is ethanol, methanol and/or oxalic acid. EXAMPLE - No suitable example given.