Cu-PAN指示薬を用いたEDTA滴定における鉄(III)の影響
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概要
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Effect of a small amount of iron(III) on the EDTA titration of lead in the acetate buffer solution of pH 5 using Cu-PAN indicator has been previously shown, by the authors, not to be a blocking phenomenon, but to give definite but mostly unstoichiometric possitive errors for lead values obtained. In this paper, the reaction mechanism of EDTA titration of iron(III) under the same conditions is investigated for its unstoichiometric nature with relation to the hydrolysis of iron(III) in the titration procedure. <BR>Hydrolysis of iron(III) in the acetate buffersolution of pH 5 was observed photometrically by measuring absorbances of hydrolyzed and thorough stirred solution at 420 mμ, as these absorbances, which are supposed to be based on the formation of basic acetate (mono-acetic dihydroxide) of iron(III), showed a linear relation with the amount of iron(III) present in the titrated solution. <BR>Such measurements agreed well with the unstoichiometric nature of the titration, showing the following mechanism as the conclusion : <BR>(1) When the end point of the titration is taken at the first instantaneous color change of the indicator, EDTA is consumed for the titration of freed Cu(II) mainly from the replacement reaction of iron(III) with Cu-EDTA in the indicator. <BR>(2) When the end point of the titration is taken at the continuing color change for 30 seconds of the indicator, the more EDTA is consumed for the titration of the more freed Cu(II) from further dissolution of hydrolyzed iron(III) by Cu-EDTA. <BR>(3) Complete isolation of Cu(II) from Cu-EDTA in the indicator is, in general, disturbed by the hydrolysis of iron(III), though promoted a little by the presence of PAN as a sub-chelate formation reagent, resulting in the unstoichiometric nature of the titration.
- 社団法人 日本分析化学会の論文
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