Separation of micro quantities of bismuth and iron in lead by EDTA masking-coprecipitation method

Study of micro-analysis by EDTA masking-coprecipitation method. VI

Abstract

“EDTA masking-coprecipitation method” is a separation method in which a sample element and interfering ones are masked with EDTA and analysing trace elements are coprecipitated as hydroxides with a collector. This method is applicable to many metallic samples and excellent in simplicity, quickness and ease of subsequent treatments. This will be useful as an analytical method for trace elements in metals by using it together with atomic absorption, s. w. polarography and so on.
The similar separation methods will be able to be developed by changing the masking agent and the precipitant or by using more than two chelating agents together.
The theoretical consideration of EDTA masking-coprecipitation method has been performed. An equation for the pH-shift for hydroxides precipitation in the presence of EDTA is derived from the equation of equilibrium of the chelate formation and that of solubility product. The measures for the selection of analysing trace elements, matrix ones and collectors are given by this calculations.
This time, bismuth(III) and iron(III) have been selected as analysing trace elements, lead as the matrix element and magnesium as the collector. Various analytical conditions were examined and EDTA masking-coprecipitation of bismuth and iron was established as follows.
The sample solutions are prepared by dissolving 00.5g of lead sample in 5ml of nitric acid and water and transferred to a 200ml centrifuge tube. 1ml of magnesium solution(10mglml) and 010mlof 10% EDTA which is slightly in excess of that equivalent to the sample taken are added. The volume is made up to about 100ml with water and 4F sodium hydroxide is added until the precipitating point and 23ml in excess. The solution is centrifuged at about 2700 rpm for 5 minutes and the precipitate is washed once with 100 ml of 0.1 F sodium hydroxide.
In addition, in the case of bismuth(III) coprecipitation, the sample taken is 0.10.5 g, magnesium solution is 3ml and 10% EDTA is 210 ml.
Determination method of bismuth is s. w. polarographic one with 1N hydrochloric acid supporting electrolyte and that of iron is ο-phenanthroline extraction spectrophotometric method.
In the absence of the matrix element, the percentage of coprecipitation of bismuth decreases. This effect is explained by the facts that bismuth has the tendency of precipitating as a basic salt and that the possibility of forming colloidal precipitate.