April, 19501 ALLO\-ING ELEMENTS I N COPPER-BASE ALLOYS 201 The Micro-Estima tion of Cadmium BY F. P. DWYEK AXD N. A. GIBSON SYNOPSIS-A new reagent, triphenylmethylarsonium iodide, is recommended for the nephelometric and gravimetric estimation of cadmium in the presence of zinc. The nephelometric procedure is suitable in the range 1 to 100 pg. of cadmium per ml., with an accuracy of approximately 5 per cent., and the gravimetric procedure for the estimation of quantities of the order of 10 mg., with an accuracy of 1. per cent. The accuracy of the nephelometric method, however, decreases markedly when the comparative standard and the unknown solution are far apart in concentration. THE usual method for the quantitative estimation of cadmium depends upon the formation of sparingly soluble salts of the type R,CdX, (X = I or CNS), in which an organic base of high molecular weight is used as the cation.Many reagents based on this principle, however, fail to separate cadmium from zinc, whilst others are unsatisfactory because of the tendency of excess of reagent to be occluded, lack of definite composition, or unsuitable physical properties of the precipitate. The most valuable reagent appears to be trimethylphenyl ammonium iodide,l but the solubility of the salt (Me,PhN),CdI, limits its applicability for much less than 1 mg. of the metal. The colorimetric method using dithizone, developed by Fischer and Leopoldi,2 is claimed to be suitable in the range 1 to 100 pg. of cadmium, but as this reagent forms coloured complexes with many metals, considerable care must be exercised in its use.Micro-quantities of the metal can be estimated by spectrographic and polarogfaphic methods, and it is claimed by Hammond3 that these are the only completely satisfactory procedures, since a specific quantitative reagent for cadmium has not yet been found. The quaternary arsonium salts of the tetraiodocadmate ion (CdI,)” have been described in a previous paper,4 in which it was shown that with several of the arsonium iodides, cadmium could be detected at concentrations below 1 pg. per ml. The salts (R,As),CdI, had very high molecular weights, and separated usually as almost colloidal suspensions, which appeared to be suitable for the nephelometric estimation of cadmium in the presence of zinc. This observation has now been confirmed, and the reagent triphenylmethylarsonium iodide is put forward as suitable for the nephelometric estimation of cadmium in the concentration range of 1 to 100 pg.per ml., and for the gravimetric estimation of 1 to 100 mg. THE NEPHELOMETRIC ESTIMATIOK OF CADMIUM REAGEKTS AND APPARATUS-Pure recrystallised triphenylmethylarsonium iodide4 was dissolved in 0-5 per cent. potassium iodide solution so as to obtain a 0.5 per cent. solution. This solution was almost saturated with respect to the arsonium salt. The protective colloid solution, a 1 per cent. solution of gelatin, was prepared freshly each week. The standard cadmium salt solution (1000 pg. of cadmium per ml.) was prepared by dissolving A.R. cadmium oxide in a slight excess of dilute sulphuric acid, and diluted as required.The opacity of the solutions was determined with a Spekker photo-absorptiometer, with neutral filters H508. The cells were 10 mm. thick (capacity, 9 ml.) for solutions containing from 1 to 10 pg. of cadmium per ml., and 2-5mm. thick (capacity, 2ml.) for solutions containing from 10 to 1OOpg. of cadmium per ml.202 DWYER AND GIBSON THE MICRO-ESTIMATION OF CADMIUM [Vol. 75 PRocEDuRE-(a) Solutions containing 1 to 8 pg. of cadmium per m1.-Reference solution : 10 pg./ml. The test and the reference solutions (10 ml.) in separate test tubes were each treated with gelatin solution (2 drops) and mixed thoroughly. To both, simultaneously, the reagent solution (5 ml.) was then added, and after stirring, each was transferred at once t o the absorptiometer tubes. (b) Solutions containing 10 to 100 pg.of cadmium per m1.-Reference solution: 100 pg./ml. In this concentration range, 2 ml. each of the reference and test solutions were used, with 2 drops of gelatin solution as before, and 1 ml. of the reagent. The reference solution increased in optical density for 10 to 15 minutes after the addition of the reagent, remained steady at this maximum for a further 10 to 15 minutes, after which the white precipitate of the complex salt began to separate. It was usually an hour before the separation was visible. The reading for the test solution was the mean of the readings taken while the reference solution gave a steady reading, one reading being taken each minute for 6 minutes. A greater volume of the gelatin solution unnecessarily prolonged the time required for the estimation, without improving the reproducibility of the readings.The effect of raising the temperature was to decrease the time required to reach the maximum optical density, which was itself reduced, but in the range 15" to 25" C. these effects were not significant. The acidity of the solutions was not critical, and usually lay between 0.01 and 0401 N . Calibration curves were constructed for each range of concentrations, using the mean value of six determinations for each point on the curve. The results obtained by the use of these curves are shown in Tables I and 11. TABLE I REFEREXCE SOLUTION COXTAINING 10 ~G./ML. SAMPLES CONTAINING 1 TO 8 p G . OF CADMIUM PER ML. COMPARED WITH Found Cd used, -2 Max.Pi5 /ml. 1 1.25 0.85 2 2.40 1.55 4 4-36 3.60 6 6.70 5.55 8 8.35 7.80 Mean Mean deviation, deviation, 0.15 15 0-35 18 0.35 9 0.40 7 0.20 3 Clg./ml. % TABLE I1 SAMPLES CONTAINING 10 TO 8 0 ~ ~ . OF CADMIUM PER ML. COMPARED WITH REFEREKCE SOLUTIOS CONTAINING 100 ~G./ML. r Cd used, Max. 10 12-0 20 21.0 40 41.6 60 62.6 80 81.5 Pg./ml* Found --7 Mean 31 in. deviation, CLg./ml. 7.5 1.0 18.5 1.0 3'3.0 0.5 68.5 1.0 78.5 1.0 Mean deviation, 10 5 1.3 1.7 1.3 % TABLE I11 CADMIUM IN PRESENCE OF A HUNDREDFOLD QUAKTITY OF ZINC Reference solution : 10 pg./ml. or 100 pg./ml. as appropriate Mean Cd Cd used, additional KI) deviation, 1.0 1.2 20 8.0 6.9 14 10.0 7.0 30 80.0 72.5 9 found (without Mean Pg*/ml. 94 Mean Cd found (with Mean additional KI) deviation, % 1.35 36 7.9 1.3 11-3 13 81.2 1.5April, 19501 203 In order to test the suitability of the reagent in presence of large amounts of zinc, solutions were prepared containing 100 times as much of this metal as cadmium.Employing the procedures above, only part of the complex cadmium salt was obtained, owing to the reduction in iodide ion concentration by the formation of the ZnI," ion. This difficulty was overcome by addition of 1 or 2 drops of 10 per cent. potassium iodide solution. The results of a series of tests are shown in Table 111. DWYER AXD GIBSON : THE MICRO-ESTIMATION OF CADMIUM THE MICRO-GRAVIMETRIC ESTIMATION OF CADMIUM A solution of cadmium sulphate (50 ml.), containing 10 mg. of cadmium, was heated t o boiling-point, and the triphenylmethylarsonium iodide reagent (50 ml., three equivalents), was added slowly, with stirring.The mixture was allowed to stand for 1 hour to cool, and then cooled in running water for a further half-hour. The precipitate was collected in a sintered-glass filter (porosity 4), washed with 0.5 per cent. potassium iodide solution, and dried at 105" to 110" C. The compound (Ph,MeAs),CdI, contains 8.903 per cent. of cadmium. The results of typical determinations are shown in Table IV. TABLE I V Cd used, mg. 10 10 10 10 11* 11* 12* 12* Cd found, Percentage error mg- 9-94 - 0.6 9.99 - 0.1 10.04 + 0.4 10.05 + 0-5 11-06 + 0-6 11.19 + 1.7 11-78 - 1.8 12.19 + 1.6 * Determinations made by students. In the presence of zinc acetate (3.36 g., equivalent to 1 g. of zinc, or 100 times the amount of cadmium) it was necessary to add 10 ml. of 10 per cent. potassium iodide solution in order to precipitate the cadmium quantitatively, but as shown in Table V, the reagent is equally satisfactory in such large concentrations of zinc. TABLE V Cd used, Cd found, Percentage error mg. mg- 10.00 10-06 0.6 10.00 10.06 0.6 10.00 10.09 0.9 In both the nephelometric and gravimetric methods using this reagent, interference is caused by silver, lead, mercury, copper, bismuth, antimony and arsenic, which either form insoluble iodides or similar complex compounds. As has already been shown by Pass and Ward,' these elements are easily eliminated by boiling the acid solution with iron wire and filtering before addition of the reagent. REFERENCES 1. 2. 3. 4. Pass, A., and Ward, A. M., Analyst, 1933, 58, 667. Fischer, H., and Leopoldi, G., Micvochim. Acta, 1937, 1, 30. Hammond, W. H., Trans. Electrochem. Soc., 1945, 88, 393. Dwyer, F. P., Gibson, N. A., and Nyholm, R. S., J . BOG. Roy. Soc., N.S.W., 1944, 78, 226. DEPARTMENT OF CHEMISTRY UNIVERSITY OF SYDNEY N.S.W., AUSTRALIA J d y , 1049