SENSITIVITY ENHANCEMENT IN THE SPECTROPHOTOMETRIC DETERMINATION OF SILICATE BY A FLOTATION-EXCHANGE-EXTRACTION METHOD USING AN ION ASSOCIATE

A flotation - exchange - extraction method for sensitivity enhancement in spectrophotometry is proposed. The concept of the proposed method can be applied to various system in which an ion associate can float on the interface between an aqueous and an organic phase. The method consists of three main steps: the flotation of an ion associate formed between an analyte and a dye ion (D1), the exchange of the analyte ion for another dyew ion (D2) and the extraction of the ion associate (D1.D2), where D1 and D2 are counter ions to each other. The proposed concept has been applied to the determination of silicate. The molar absorptivity and the absorbance of the reagent blank were 7.6 x 10(5) l mol-1 cm-1 and 0.023 at 620 nm, respectively, when D1 and D2 were Malachite Green and tetrabromophenolphthalein ethyl ester. The standard deviation (SD) and relative standard deviation (RSD) for ten measurements of 4 x 10(-7) M silcate solution were 0.004A and 1.1%, respectively. The SD and RSD for the reagent blank were 0.003A and 10.3%, respectively. The detection limit for silicate is about 5 x 10(-9) M or 0.15 p.p.b.