in hydrogen fluoride HF (liquid); (Ar); K2NbF7 dissolved in HF; pptd. by adding soln. of KCl; recrystd. from HF soln.; filtered; washed with ethanol; dried in vac. at110-120°C;

sulfuric acid in sulfuric acid Electrolysis; dissoln. of freshly pptd. Nb(V) hydroxide in 12 M H2SO4 (heating), then treatment with Zn; not isolated, detd. by electrochemical measurements;