A continuous extraction method was investigated to remove Mn
II, Fe
III, Co
II and Cu
II from sodium borate as a pH buffer reagent. The removal of impurities in a reagent is very important to decrease the reagent blank value, which would determine mainly the limit of detection in analyses of metals. 5,7-Diiodooxine (HR) was used as an extraction reagent because it has a high distribution constant between the aqueous phase and chloroform. The extractabilities of metal ions by the proposed method {aqueous phase (100 cm
3) and chloroform (40 cm
3)} were estimated based on the metal concentration in a sodium borate solution determined by graphite-furnace AAS. In the present method, the HR concentration in chloroform was 1.0 × 10
-3 M and the pH of the borate solution as a sample was 9.2. The organic phase containing the reagent and its metal chelates was transferred to a distillation vessel at a rate of 2.0 cm
3 min
-1 after separation from the aqueous phase by a Teflon phase separator in an extraction vessel under high-speed stirring. The concentration of HR in the extraction vessel was then maintained constant by the addition of a fresh HR chloroform solution at a rate of 2.0 cm
3 min
-1. Above 99% of the metals as impurity could be removed after 40 min of extraction. The experimental behavior of the removal of the impurity by the present method agreed with theoretical curves calculated by using an equation derived in this study. The present method was applied to sodium borate to be reagent grade. The sodium borate obtained by the purification was used as a pH buffer reagent to determine zinc by a chemiluminescence method. Furthermore, the effect of purified sodium borate on the blank value was examined by a catalytic analysis. As a result, very low values of the reagent blank were observed by each method.
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