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DAI Ya-ming(Material Testing Centre,Shanghai Research Institute for Materials,Shanghai 200437,China)  
It was found that the acidity of the sample solution is the essential condition for the effective generation of PbH_4 by KBH_4,and that highest efficiency of PbH_4-generation is obtained when the acidity of the sample solution after the hydride forming reaction is in the range of pH 7.8-8.6.Furthermore,it was proved that an appropriate oxidant is necessary for the oxidation of divalent lead(Pb~(2+)) to its tetravalent state.Based on these findings,an entirely new hydride-generation reagent system,composing trisodium citrate,potassium ferricyanide and potassium boronhydride,was proposed for the determination.The reagent trisodium citrate acted both as a buffer to control the pH-value of sample solution and as a complexing reagent to mask those easily hydrolyzing metal ions(including the matrix iron ion).The action of potassium ferricyanide is both as an oxidant to oxidize the lead ion to the tetravalent state,and as a masking agent to eliminate the interferences of Mn~(2+),Cu~(2+) and Ni~(2+) by forming precipitates with Fe(CN)~(3-)_6 ion through photo-catalytic action.The accuracy and precision of this method was tested by its application to analyze 5 standard samples having their lead contents ranging from w(Pb) 6.2×10~(-5)% to w(Pb) 1.3×10~(-3)%.The results obtained by this method were in good conformity with the standard values.The RSD′s found were 2.0%(n=4) for a lead content of w(Pb) 6.2×10~(-5)% and 1.5%(n=14) for a lead content of w(Pb) 1.3×10~(-3)%.The limit of determination of this method was found to be w(Pb) 5×10~(-6)%.A theoratical discussion was also made both on the acidity of sample solution for hydride-generation and on the preoxidation of lead ion by ferricyanide.
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