Abstract:

Lithium iron phosphate, LiFePO4 (LFP), is widely used as a cathode material due to excellent safety and electrochemical performance. To selectively precipitate LFP directly from solution, iron ions must be in their ferrous redox state. Therefore, accurate ferrous iron concentration measurement is crucial to calculate LFP supersaturation.

Ferrous ions at low concentrations can be quantified using spectrophotometry, where chromogenic reagents form stable-colored complexes with ferrous ions. Ferrozine is a chromogenic reagent that forms a magenta ferrous-ferrozine complex. The intensity of this complex can be correlated with ferrous ion concentration over a linear range through the Beer-Lambert law. The ferrozine assay was developed in 1970 by Stookey, who measured dissolved iron in potable water. Stookey’s protocol has since been adapted for biochemistry, geochemistry, soil science, and food science applications. This poster first reviews the history and discusses the impacts of the divergences of the adapted ferrozine assay protocols from 1970 to 2026. Reported methods were compared with respect to final pH, temperature, colour development time, ladder iron source, ladder concentration range, reducing agent, reagent concentration, reagent order, final redox potential, and in some cases, necessary pretreatment steps. Protocol varied within disciplines, which affects assay comparability and highlights the need for protocol standardization.

Next, this poster experimentally investigates the effect of the iron counter anion on the ferrozine-ferrous absorbance and linearity. Calibration ladders from 0-5 ppm Fe were created using ferrous ammonium sulphate, ferrous sulphate, ferric nitrate (iron standard for ICP), and concentrated ferric chloride. Absorbance magnitudes and linearity were compared. Statistically significant differences resulting from anion type were identified. Related to LFP precipitation, the linear range of the iron phosphate ladder was smaller than that of the other anion types, and the iron chloride absorbance was higher across all concentrations. This highlights the importance of the matching to ladder anion type to the sample predominant anion.

Overall, this works strives to consolidate more than 50 years of ferrozine protocol development and identifies the effect of a range of anion types on the results of this protocol.

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Prof. Philippe Ouzilleau