How can we measure (or detect) the ferrous ions in an aqueous acidic solution after immersing a steel sample in it to corrode for a long period of time?
I would try a variation of the Zimmermann-Reinhardt method to determine the ferrous ion concentration, in which the ferrous ions are oxidized by permanganate. In order to follow a long lasting corrosion I would suggest developing a flow-injection (FIA) method, in which a dilute permanganate concentration in the Zimmermann-Reinhardt reagent serves as carrier solution. By injecting microliters of the sample solution in this carrier solution and after a sufficient reaction coil the carrier solution passes a photometric cell using a wavelength of maximal absorption of the permanganate ion. If the sample matrix does not interfere, increasing ferrous ion concentrations will results in corresponding smaller absorption signal peaks. After calibration with standard ferrous solutions a calibration curve will be used to calculate the ferrous ion concentration of the sample solutions. Since only microliters are used for the injection, the volume of the primary sample is not significantly being reduced. The concentration of the permanganate in the carrier solution of the FIA method depends on the expected concentration of the ferrous ions. It might by that a very dilute permanganate solution will need a longer reaction time; means a longer reaction coil will be needed.
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