FORMATION OF METAL CHELATES WITH SULPHO-SUBSTITUTED AROMATIC HYDROXY CARBOXYLIC ACIDS PART 3, MIXED LIGAND COPPER(II) CHELATES OF 3,5-DISULPHOSALICYLIC, 3-HYDROXY-5,7-DISULPHO-2-NAPHTHOIC AND CATECHOL-3,5-DISULPHONIC ACIDS