Abstract
A green intermediate, observed during reduction of 9,10-anthraquinone-2-sulfonate (A) to the corresponding anthrahydroquinone (AH2) in acidic aqueous solution, has been identified as a quinhydrone-like charge-transfer complex of A and AH2. Spectrophotometric and potentiometric measurements have shown that changes in the slope of the potentiometric titration curves with varying initial [A] are caused only by this complex [Formula: see text]and not by any semiquinone radical (AH·) or radical anion (A·−) intermediate. The very low semiquinone formation constant in acidic solution indicates that the pKa of AH· < 7.0. Electrochemically determined pKa values for similar radicals in the range 8.5–10.5 are invalid because of interference by charge-transfer complexes of this type.
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