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Study of redox flow battery systems for residential applications MnO4- + 5VO2+ + H2O Mn2+ + 5VO2+ + 2H+ 𝐸° = 0.510V It is important to notice that throughout the titration the initial V2+ ions will be oxidized into V3+ which then, the same mole number of oxidized ions from V2+ plus originally existing V3+ ions, will be oxidized into VO2+ and then into VO2+. Not taking this into account will lead into miss calculation of the ions concentration. Since the equivalence point is where the amount of vanadium ions is equal to the amount of titrator ions and also is the point where there are no more vanadium ions of such oxidation state to be oxidized, the vanadium ions concentration can be determined, by knowing the volume of titrator used, with the following equations: [V2+] = KMnO4 𝑉Electrolyte (𝑉EP2 −2∙𝑉EP1 [V3+]= KMnO4 KMnO4 𝑛 2+ ∙ V 𝑛KMnO4 )∙[KMnO ] 𝑛 3+ 4 ∙ V 𝑉EP1 ∙[KMnO ] 4 (4.2) (4.3) (4.4) 𝑉Electrolyte 𝑛KMnO4 (𝑉EP3 −2∙𝑉EP2 +𝑉EP1 )∙[KMnO ] 𝑛 2+ [VO2+] = KMnO4 KMnO4 KMnO4 4 ∙ VO 𝑉Electrolyte 𝑛KMnO4 where V is the volume and ni is the number of moles of species i. The V5+ ions concentration was determined by using 0.10 M ammonium iron (II) sulfate (Sigma- Aldrich, 99 % purity), (NH4)2Fe(SO4)2.6H2O, in 0.10 M sulfuric acid to reduce VO2+ to VO2+ [69, 70]: Fe2+ + VO2+ + 2H+ Fe3+ + VO2+ + H2O E° = 0.229 V 𝑉EP1 ∙[Fe2+] 𝑛 + Fe2+ 2 𝑉Electrolyte ∙ VO2 (4.5) 𝑛Fe2+ [VO+] = The titration procedure was further optimized for VisBlue 8 system by stirring the samples for 10 seconds before being pipetted to minimize measuring error. Each experiment was repeated at least two times and the repeatability and precision of the three best essays was assessed by calculating the variation coefficient with Equation (4.6) [71]. VC(%)= ∙100 (4.6) 𝑥̅ Where standsforstandarddeviation(Equation(4.7))and𝑥̅standsforaverageoftheresults obtained experimentally [71]. =√∑𝑥2−(∑𝑥)2 𝑖𝑖 𝑖𝑖 𝑛−1 𝑛(𝑛−1) (4.7) Due to the lack of sample volume, 1 mL of electrolyte sample was used to analyse the electrolyte on positive half-cell of VisBlue 8 and the V5+ ions concentration on positive half-cell Chapter 4: Methods and materials 24PDF Image | Tubular Vanadium Air Redox‐flow battery
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