in water addn. of concd. hydrochloric acid to ferrocene deriv., reflux for 1 h, cooling, satn. with H2S; filtering-off ppt. (H2S), washing in succession with water, acetone, and ethanol, neutralization of filtrate with 10percent aq. NaOH, extn. with Et2O, drying exts. over Na2SO4, evapn. to isolate ferrocene deriv.;