than SDS itself, with a consequent ''dip'' in the surface Three methods for determining the concentration of sodium dotension isotherm (e.g., Figure 1 and Ref. ( 3)). This renders decyl sulfate (SDS ) in aqueous solutions have been investigated. surface tension an unreliable assessment of SDS concentra-The first was to titrate SDS by cetyltrimethylammonium bromide tion. Conductivity is a convenient method to measure con-(CTAB) , where the endpoint (corresponding to 1:1 neutralization ) centration in a given matrix solution; however, below 5 1 was determined by measuring the potential drop between a surfac-10 05 M, the error in measurement becomes too high. The tant ion selective electrode and a silver/ silver chloride reference other main drawback is that a low background electrolyte electrode. The second was to measure the endpoint of an analogous concentration is required, precluding measurements at high titration by following the formation of the insoluble CTAB/SDS ionic strength. complex with turbidimetry. The third was to measure the turbidity of a solution as a function of SDS concentration in the presence of The desire for a more specific analysis has driven the myristyltrimethylammonium bromide (MTAB). Of these methods, considerable research effort on optimization of electrodes which all provide a reasonable assessment of the SDS concentration, selective for surface active species (e.g., Refs. (4-6). With the last was certainly the fastest and most convenient, particularly the benefit of a large number of electrode samples, good for large numbers of samples. α§ 1998 Academic Press direct determination of SDS has been achieved. This work Key Words: sodium dodecyl sulfate; cetyltrimethylammonium has culminated in the recent appearance on the market of a bromide; myristyltrimethylammonium bromide; surfactant selecnumber of surfactant sensitive electrodes (7, 8), with varitive electrode; turbidity; mixed aggregate formation; determinaable accuracy and lifetimes. These have provided a possible tion; titration.
avenue for the determination of SDS, with the principle advantage of selectivity for the analyte. Another phenomenon which can be exploited is the formation of an insoluble