672 GAGE: THE DETERMINATION OF TRACES OF LEAD [Vol. 83 The Determination of Traces of Lead and Bismuth in Organic Material BY J. C. GAGE (Imfierial Chemical Industries Ltd., Industrial Hygiene Research Laboratories, The Frythe, Welwyn, Herts.) It has been shown that the method previously described for the deter- mination of lead in organic material ma.y be subject to interference from bismuth, but this can be avoided by reducing to 0.1 N the concentration of hydrochloric acid used to extract lead from its diethyldithiocarbamate complex in organic solution. The determination of both lead and bismuth can be made on the same solution by a second extraction of the organic layer with 1.75 N hydrochloric acid. IN two previous papers112 a method has been described for determining traces of lead in organic material.It was claimed that the dieth yldithiocarbamate separation of lead used in this method holds back any bismuth that may b'e present and that relatively large amounts of this metal do not interfere in the determination of lead. Experience in other laboratories has not entirely supported this claim, and a further investigation has indicated that the conditions of separating the lead must be more clearly defined in order to remove interference from bismuth. The possibility of adapting the rnethod so that both lead and bismuth can be determined when present together in trace a.mounts has been investigated. EXPERIMENTAL DETERMINATION OF LEAD IN THE PRESENCE OF BISMUTH- In the original method, lead was extracted, after destruction of organic matter by dry ashing, as its diethyldithiocarbamate complex into a mixture of equal volumes of pentanol and toluene.The lead was extracted from the olrganic layer with dilute hydrochloric acid, which was then added to an alkaline solution of dlithizone, and the lead dithizonate complex was extracted with carbon tetrachloride for the colorimetric determination of lead. The dilute hydrochloric acid used to extract lead from its diethyldithiocarbamate complex solutionDec., 19581 AND BISMUTH IN ORGANIC MATERIAL 673 in pentanol - toluene mixture was prepared by diluting constant-boiling acid (1 + 9) with distilled water; this dilute acid is approximately 0.6 N, but different batches show considerable variation. When the concentration of this acid is varied, all other conditions in the method remaining unchanged, there is no effect on the extraction of lead; this is shown by the results in Table I over a range of acid concentrations from 0.05 t o 0.65 N.The extraction of bismuth from the diethyldithiocarbamate complex solution has, however, been found to be greatly influenced by the concentration of acid used. The results in Table I1 show that, when 1 mg of bismuth, added as a standard solution of bismuth nitrate in diluted nitric acid (1 + 9), is subjected to the analytical procedure, negligible amounts of bismuth are extracted with acids below 0.5 N, but, above this concentration, the amount extracted rises sharply. It can also be seen from Table I1 that sub-maximum extraction of bismuth is also influenced by the composition of the solvent; the higher the proportion of pentanol in the mixture the greater the extraction.TABLE I EFFECT OF ACID CONCENTRATION ON THE EXTRACTION OF LEAD FROM A SOLUTION The optical densities of the lead dithizonate complex from 20 yg of lead in carbon tetrachloride were measured at 515 mp in a 1-cm cell OF ITS DIETHYLDITHIOCARBAMATE COMPLEX IN PENTANOL - TOLUENE MIXTURE Concentration of hydrochloric acid, N . . 0.05 0.1 0.25 0.5 0.65 Optical density . . . . . . . . 0.362 0.341 0.384 0.370 0.361 TABLE I1 EFFECT OF ACID CONCENTRATION AND COMPOSITION OF SOLVENT MIXTURE ON THE The optical densities of the bismuth dithizonate complex from 1 mg of bismuth in carbon tetrachloride were measured at 495 mp in a 1-cm cell EXTRACTION OF BISMUTH FROM A SOLUTION OF ITS DIETHYLDITHIOCARBAMATE COMPLEX Composition of solvent Optical density a t different concentrations of mixture hydrochloric acid P r h > Pentanol, Toluene, % v/v % v/v 0.1 N 0.25 0.5 N 0.65 N 0.8 N - - 0.062 - 40 60 0 50 50 0 0.006 0-025 0.124 0.5 13 60 40 0.005 - - 0.20 - The results in Tables I and I1 indicate that, when 0.1 N hydrochloric acid is used to extract the organic layer, extraction of lead will be complete, but no bismuth will be removed, even when present in large excess.DETERMINATION OF BISMUTH- A series of solutions containing 20 pg of bismuth has been submitted to the complete procedure, hydrochloric acid solutions ranging from 0.1 to 2.0 AT being used to extract the diethyldithiocarbamate complex. The optical density reached a maximum value with 1.5 N acid, and no increase was observed at higher acid concentrations.This maximum optical density has been found to be unaffected by minor variations in the proportions of the solvent mixture. In all subsequent experiments, 1.75 N hydrochloric acid was used to extract the bismuth from the first stage of the method. A calibration graph prepared by plotting the optical densities of a series of standard bismuth solutions, measured at 495 mp in 1-cm cells, against their bismuth contents, gave a straight line with a slope of 0.018 units per pg. The slope of this line is almost identical with that of a calibration graph for lead prepared in a similar manner at 515 mp. The results are shown in Table 111. TABLE I11 EFFECT OF ACID COSCENTRATIOK ON THE EXTRACTION OF BISMUTH The optical densities of the bismuth dithizonate complex from 20 yg of bismuth in carbon tetrachloride were measured at 495 my in a 1-cm cell Concentration of hydrochloric acid, N .. 0.1 0.5 1.0 1.6 1.55 2.0 Optical density . . .. . . . . 0.031 0.073 0,188 0.329 0.330 0.333674 GAGE [Vol. 83 DETERMINATION OF LEAD AND BISMUTH TOGETHER- To determine traces of lead and bismuth in organic material without including the error due to destruction of organic matter, a sample of tinned spinach was ashed in a silica basin at 450" C, a little magnesium nitrate solution being used to remove final traces of carbon. The ash was dissolved in constant-boiling hydrochloric acid, the solution was evaporated to dryness and the residue was dissolved in the constant-boiling acid diluted (1 + 9) with distilled water to make a solution equivalent to 40 per cent.w/v of spinach. Ten millilitres of this solution were subjected to the analytical procedure, two 10-ml portions of 0.1 N hydrochloric acid being used to extract the lead, and then two 10-ml portions of 1.75 N hydrochloric acid to extract the bismuth. Each of these extracts was submitted to the final colour-development stage, and the optical density of the dithizone complex in carbon tetrachloride was measured in 4-cm cells. The sample solution was found to contain 1.3 pg of lead in 10 ml, after the reagent blank equivalent to 1.5 to 2 pg of lead had been subtracted; no bismuth was found in the sample or in the reagent blank solution. To 10-ml portions of the sample solution were added known amounts of lead and bismuth, and then the analytical procedure was carried out; with a sample containing 400 pg of lead, it was found necessary to extract three times with 0.1 N hydrochloric acid to remove final traces before proceeding to the determination of bismuth.To confirm that iron does not interfere in this deter- mination, an experiment was made in which an excess of iron was present; the results are shown in Table IV. TABLE IV RECOVERY OF ADDED LEAD AND BISMUTH FROM SPINACH Determinations were carried out on 10-ml portions of the spinach extract (equivalent to 4 g of spinach), and each result is the mean of two determinations Lead added, r g 4 4 400 4 0 4 Bismuth added, Pg 0 400 4 4 0 4 Iron added, PLg 0 0 0 0 400 400 Lead found, Bismuth found, Pg CLg 3.6 - 4.4 - 3.6 4.0 1.2 0 4.4 4.0 3.25 - DISCGSSIOK OF RESULTS The investigation has shown that the concentration of hydrochloric acid used to extract lead from its solution as diethyldithiocarbamate in pentanol - toluene mixture is not critical, but that it must be controlled for the determin,xtion of lead in the presence of bismuth and for the subsequent separation of bismuth for colorimetric determination as its dithizone complex.The experiments with ashed spinach extract, fortified with known amounts of lead and bismuth, have shown that both metals a t concentrations equivalent to 1 p.p.m. in the fresh spinach can be determined with adequate accuracy in the presence of 100 p.p.m. of the other metal. Lead and bismuth together, both at a concentration of 1 p.p.m., can be deter- mined on the same solution by successive extraction with 0.1 N and 1-75 N hydrochloric acid, recoveries being within 10 per cent. of the expected values. The presence of 100 p.p.m. of iron causes no interference in the determination of bismuth, but a slight reduction has been found in the recovery of lead. Technical assistance in this investigation was provided by Miss Sylvia Morrissey and RIr. T. V. H. Chalker. REFERENCES 1. 2. Gage, J. C., A?talyst, 1955, 80, 789. __ , Ibid., 1957, 82, 453. Received July 8th, 1958