it is more than simple. You must titrate the Fe2+/Fe3+ mixture with KMnO4 in acidic solution.The mixture will use the permangamante during the oxydation of Fe2+ to Fe3+. When all Fe2+ dissapeared the solution turns violet (KMnO4 in excess), then stop the titration.
The amount of permanganate used is to find the amount of Fe2+. The rest was Fe3+ from the beginning.
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