Effect of cationic surfactant on the formation of ferron complexes

Abstract

In the spectrophotometric determination of aluminium and iron with ferron (7-iodo-8-quinolinol-5-sulphonic acid, H 2L), the addition of cationic surfactants greatly improves the linearity of the calibration curve and widens the useful pH range. The effect of cetyltrimethylammonium chloride (CTMAC) on the stepwise stability constants ( K 1, K 2 and K 3) of the ferron complexes of aluminium and iron (ML +, ML − 2 and ML 3− 3) and on the acid-dissociation constants ( K a1 and K a2) of ferron has been studied in connection with the role of the surfactant. CTMAC greatly increases the value of K 3 while exerting little effect on K 1 and K 2, thus rendering ML 3− 3 the predominant species even at very low concentration of free L 2−. It also has some effect on the acid-dissociation constants of ferron, but sometimes it acts to decrease the free L 2− concentration. At is therefore concluded that the improvements due to addition of surfactant should be attributed to the increased K 3 value. The presence of surfactant micelles is not essential, because the surfactant has a favourable effect when present at well below its critical micelle concentration, and because the continuous variations plots show a peak at a point corresponding to the composition M: L: Q (Q = cationic surfactant) = 1:3:3.