“…[5][6][7][8][9] The complex formation with azo derivatives of resorcinol via phenolic oxygen, azo nitrogen and thiazolyl or pyridylazo nitrogen, gives colored chelates with metal cations. [10][11][12][13][14][15][16][17][18][19] Tetrazolium salts are used as reagents for the extraction-spectrophotometric determination of metals, e.g. Mo(VI), W(VI), Ge(IV), Tl(III), Nb(V), V(IV), V(V), Ga(III).…”