Kinetic determination of the surfactant sodium dodecyl sulphate by use of mixed micelles

A kinetic-photometric method for the determination of the anionic surfactant sodium dodecyl sulphate (SDS) is proposed. The method is based on the accelerating effect of micelles of the surfactant on the reaction between iron(II) and 1,10-phenanthroline, which is monitored via the ferroin complex formed. While Triton X-100 micelles have no catalytic effect on the reaction, this non-ionic surfactant forms mixed micelles with SDS, thereby considerably reducing the anionic surfactant concentration at which catalysis of the complex formation reaction starts. SDS was determined over the range 50–400 μgml (detection limit 15 μgml) with a relative standard deviation of 3% for 200 μgml. The proposed method was applied to the direct determination of the surfactant in pharmaceuticals (three shampoos, a toothpaste and a rectal solution) with satisfactory results. The main advantages of using micellar catalysis instead of monomer properties for the determination of the surfactant include enhanced selectivity and experimental convenience (use of organic solvents is avoided), whereas its most serious disadvantage is the increased detection limit of the micellar catalysis method.