The S2O5(2-) ion in theory is capable of reducing 4 copper ions. Just use a fairly strong excess amount of this, to assure that the reaction
completes. I would take 1 mole of copper sulfate for half a mole of S2O5(2-), which is a two-times as much amount as theoretical. Bisulfate is dirt
cheap, so do not save on that too much.
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