Electron paramagnetic resonance study of iron oxalate in calcium oxalate renal stones.

Weak electron paramagnetic resonance (EPR) signals from a number of calcium oxalate renal stones are attributed to an iron oxalate component. The g-value of the resonance is 2.0036 and its width is approximately 9 gauss. The EPR resonance from stones has the same characteristics as resonances from iron introduced into calcium oxalate and oxalic acid as an impurity. A sharp increase in EPR signal when calcium oxalate renal stones are exposed to intense light is attributable to the reduction of Fe3+ and the formation of the oxalate radical ion (C2O4)-.