Spectrophotometric determination of nonionic surfactants after removing of cationic surfactants by filtration with a membrane filter.

Abstract

Filtration with a membrane filter was studied to remove cationic surfactants (CS) which interfered with the spectrophotometric determination of nonionic surfactants (NS) with thiocyanateiron (III) or thiocyanate-cobalt (II). Materials of the filter, filter pore size and suction rate were studied to separate NS from CS. After the addition of anionic surfactants (AS) to a sample solution, the sample solution was filtered with a polyvinylidenedifluoride membrane filter (pore size 0.1 μm). NS in the sample solution was filtered through the filter, while CS associated with AS remained on the filter. The NS in the filtrate was extracted into toluene with thiocyanateiron (III), and was measured spectrophotometrically. The procedure was as follows: one milliliter of LAS solution (0.5 mg/ml) was added to a 50 ml sample, which was filtered at a suction rate 3-6 ml/min. After washing the filter with water, 5 ml of a sodium chloride solution (2 M) and 10 ml of toluene were added to the filtrate and NS in the filtrate was extracted into toluene by shaking for 2 min. After standing for 30 min, 5 ml of a potassium thiocyanate solution (10 M), 5 ml of an iron (III) chloride solution (1 M) and 1 ml of a sodium chloride solution (2 M) were added to the toluene solution and the mixture was shaken for 2 min. NS associated with thiocyanate-iron (III) was formed in the toluene layer, and its absorbance was measured at 510 nm. Good results were obtained in recovery tests using sample solutions containing NS in the range of 0∼0.1 mg/50 ml as heptaoxyethylenedodecylether. This filtration method is easier than the usual ion exchange method.