電位差滴定法による四酢酸鉛を用いるEDTAの定量

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Potentiometric titration of ethylenediaminetetraacetic acid (EDTA) with lead tetraacetate was investigated. The titration procedures are as follows. An aqueous solution (100 ml) containing 40 ml of glacial acetic acid, 2 ml of 4 M sodium acetate, and 10.00 ml of 0.05 M lead tetraacetate in glacial acetic acid, was titrated with a standard EDTA solution by using a platinized platinum wire electrode as indicator electrode. The potential of the electrode was measured against a saturated calomel electrode, which was connected to the solution through an agar bridge of 30% KNO3. The measurement of the potential was made at 2 minutes after each addition of the titrant. The end point indicated that Pb(IV) ion reacts with EDTA in molar ratio of 1: 1. The potential change in the vicinity of the end point was about 50 mV per 0.10 ml of the titrant. EDTA could be determined over the concentration range of 0.020.05 MM within the error of ± 1.0% and within the relative standard deviation of ± 0.1%. The addition of Pb(II) ion into the titrated solution up to 2% of the concentration of Pb(IV) ion did not interfere with the determination. Interference of Pb(II) ion with the determination was observed, as shown in the two-stepped titration curves in Fig. 7 when its concentration was larger than 2% of that of Pb(IV) ion in the titrated solution. The conditional formation constant of the chelate of Pb(II) ion with EDTA was calculated to be about 104 M-1 under the experimental condition. Effects of the change in the concentration of acetic acid or sodium acetate and temperature of the titrated solution were examined. When the concentration of acetic acid in the titrated solution increased to 90 vol. %, the potential break became too indistinct to be used for the determination of EDTA, as shown in Fig. 4. When the concentration of sodium acetate in the titrated solution decreased to less than 0.08 M, a negative error was remarkable as shown in Table III. When the temperature of the titrated solution was higher than 25°C, a negative error was noted as shown in Fig. 6. It was considered that the potential of the indicator electrode beyond the end point of the titration curve would be determined predominantly by the platinum ion-platinum system, which was produced by the dissolution reaction of the platinum oxide layer in excess EDTA (Fie. 9). The present potentiometric method is as satisfactory for the standardization of EDTA as the indicator method.
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電位差滴定法による四酢酸鉛を用いるEDTAの定量

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