the rhodamine B-nitrite reagent, mix well and allow to stand for 30 min. Add x0.0 ml of the extraction solvent and shake the funnel for about I min. Run the organic phase into a dry centrifuge tube and centrifuge for ca. z min. Measure the optical density of the solution in a r-cm cell at 565 rnp against a compensating cell containing the extraction solvent. Carry out a blank run in the same manner using 20 ml of 6 M hydrochloric acid in place of the sample. Calibrate the method using known amounts of antimony in the range I-IS p-lg. The colour of the extracted solution is stable fo+ at least 2 11.
Under these conditions, the optimum oxidation time was found to be 30 rnin; after this period, 98 -& 1% of the antimony in the aqueous phase could be extracted with the mixed solvent. The hyclrochloric acid cohcentration is not critical between 5 M and 6 M, but on either side of this range the efficiency of extraction declines markedly. At lower acid concentrations the extraction of rhodamine I3 becomes serious.
Heer's Law is obeyed up to at least 15 pg of antimony. The sensitivity of the method is 0.0017 pg/cm2, approximately 2.5 times that found when ceric ion is used as the oxidant. Replicate determinations of 10 pg of antimony gave a coefficient of variation of 0.8% for the method.