Comparison of EPR Absorption Intensities of Manganese(II), Copper(II), and Peroxylamine Disulfonate in Solution. — The Semiquinone Formation Constant of the System Duroquinone‐Durohydroquinone in Methanol‐Water.

Relative EPR absorption intensities of CuCl2 · 2 H2O, MnSO4 · H2O, MnCl2 · 4 H2O, and potassium peroxylamine disulfonate in aqueous solution and of CuCl2 · 2 H2O and MnCl2 · 4 H2O in methanol‐water solution (equal parts by weight) were measured by analog double integration of the output signal of a field modulation spectrometer. The absorption intensity was normalized to the signal exhibited by a secondary ruby standard located in the microwave cavity. Except for CuCl2 at concentrations below 0.1 M, all substances were found to be appropriate primary standards for the determination of spin concentration in aqueous and in aqueous‐methanolic solutions. The semiquinone formation constant of the system duroquinone‐durohydroquinone in methanol‐water (equal parts by weight) is (1.4 ± 0.1) · 10−5 at apH* of 10.3 and 25 °C.