A novel Redox Flow Battery (RFB) was designed and developed at the University of Manchester.  This system eliminated the ion-exchange membrane altogether by employing porous flow through electrodes.  This system was characterised [1] using the ferrocyanide/ferricyanide [hexacyanoferrate (II)/hexacyanoferrate (III)] redox couple which had relatively fast electrochemical kinetics at several electrode surfaces.  In this study an accurate method had been developed to determine the concentrations of the ferrocyanide/ferricyanide species from samples obtained from the battery.  UV/Visible spectroscopy was chosen as the analytical tool for this purpose.  Analysis of a range of standard solutions showed that by measuring the absorbance 260 nm and 420 nm it was possible to determine the concentration of both ferrocyanide and ferricyanide ions in a mixture of the two active species.  Using the correlations developed, it was possible to determine the concentrations of ferrocyanide and ferricyanide ions to an accuracy of ± 4% in mixtures with concentrations in the range 0.01 to 0.1 mM.  In work reported elsewhere [1] this analytical method was used to characterise the Redox Flow Battery and it was found that high conversion (>90%) could be achieved at relatively high concentrations (0.1 M) of the active species.