A Varian DMS-100 spectrophotometer equipped with a Hellma ¯ow cell, (178-010-OS) setting at 498 nm was used coupled to an Intralab x±y recorder. An Ismatec (Reglo) and Milan peristaltic pumps, furnished with Tygon tubes, were employed to propel all solutions. A home made proportional commutator [25] was used to choose the separation/loading and measurement/cleaning steps. All connections were made using Perspex materials. The manifold was built up with polypropylene tubes of 0.7 mm bore.