Determination of Sodium Sulfate in Detergents by Anion-Exchange Chromatography in Nitric Acid-Iron(III) Nitrate Media

Abstract

Sodium sulfate in detergents was determined by high speed liquid chromatography on a porous micro-spherical strong anion exchanger (TSK-Gel LS-222) column with 0.1 mol/l nitric acid-56times;10-3 mol/l iron(III) nitrate as an eluent. Iron(III) is very effective as the eluting agent since it forms the complexes with sulfate, pyrophosphate, and triphosphate in detergents. It also serves as a color-developing agent for sulfate ions in the column efflue, nt. The capacity factor of sulfate decreased with increasing iron(III) nitrate concentration in the eluent, but the peak area of sulfate obtained from the chromatogrin increased, as shown in Fig. 1.Anionic surface-active agents, were strongly held by the anion- exchanger and those of sulfate ester type were not hydrolyzed in the eluent. Inorganic and organic builders such as condensed phosphates and citrate were not sorbed to any significant extent on the column. By the postcolumn method described previously, pyrophosphate, sulfate, and triphosphate were eluted in that order as shown in Fig. 2. The distribntion coefficient of pyrophosphate decreased remarkably in the presence of iron (III) in the eluent, because pyrophosphate forms a stable iron (III) pyrophosphate complex and was not sorbed by the anion exchanger as predicted by Eqs. 6, 7, and 8. Iron (III) and triphosphate also react to form the stable complex as is the case for iron (III) pyrophosphate complex. Ingredients in detergents did not interfere with the analysis. The calibration curve was linear over a range of 40∼420 mg/100 ml (Fig. 3). Analysis of a standard solution and a standard detergent yielded satisfactory results (Tables 1 and 2). Sodium sulfate in several commercial detergents was determined by the proposed method without any interference and the results agreed with those of conventional chelatometry, as shown in Table 3.