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

Filtration with a membrane filter was studied to remove cationic surfactant s (CS) which interfered with the spectrophotometric determination of nonion ic surfactants; (NS) with thiocyanate-iron(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 s ample solution, the sample solution was filtered with a polyvinylidenediflu oride membrane filter (pore size 0.1 mum). NS in the sample solution was fi ltered through the filter, while CS associated with AS remained on the filt er. The NS in the filtrate was extracted into toluene with thiocyanate-iron (III), and was measured spectrophotometrically. The procedure was as follow s: 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 w ith water, 5 ml of a sodium chloride solution (2 M) and 10 ml Of toluene we re 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 thiocy anate solution (10 M), 5 ml of an iron(III) chloride solution (1 M) and 1 m l of a sodium chloride solution (2 M) were added to the toluene solution an d the mixture was shaken for 2 min. NS associated with thiocyanate-iron (II I) was formed in the toluene layer, and its absorbance was measured at 510 nm. Good results were obtained in recovery tests using sample solutions con taining NS in the range of 0 similar to0.1 mg/50ml as heptaoxyethylenedodec ylether. This filtration method is easier than the usual ion exchange metho d.