Effect of cationic surfactant on the formation of ferron complexes

In the spectrophotometric determination of aluminium and iron with ferron (7-iodo-8-quinolinol-5-sulphonic acid, H(2)L), 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.