Sept., 19541 ABBOTT AND POLHILL 547

The Determination of Copper in Oils and Fats by Means of Dibenzyldithiocarbamic Acid and its Salts

BY D. C. ABBOTT AND R. D. A. POLHILL

A method has been developed for the rapid absorptiometric determina- tion of copper in oils and fats over the range 0.02 to 2 p.p.m. Most of the fatty matter is removed from the sample by vaporisation and the remainder is destroyed by digestion with nitric and sulphuric acids. The copper in the resulting acid solution is then determined, after dilution, by the extraction of its dibenzyldithiocarbamate with carbon tetrachloride and measurement of the optical density of this solution a t 435 m p in a suitable spectrophotometer. The method is shown to be free from interference by some other commonly occurring metals and to give a good recovery of added copper.

The preparation of the dibenzylammonium, potassium and zinc salts of dibenzyldithiocarbamic acid is also described.

A determination can be completed in 2 hours.

THE presence of traces of copper of about 1 part per million has been shown1 to have a marked effect on the development of oxidative rancidity in oils and fats. Although no toxic significance is associated with copper at this concentration,2 there is a need for a simple method, free from interference by other commonly occurring metals, for its determination. Many methods for the determination of copper in small amounts have been described, such as the use of sodium or diethylammonium diethyldithio~arbarnate,~~~,~ rubeanic acid,6,7 the phenanthroline~~ 7 9 and biscyclohexanone oxalyldihydrazone.1° * All of these methods suffer from the disadvantage that the sample requires neutralisation after the initial acid treatment and also often requires the addition of comparatively large amounts of inorganic salts, either to prevent interference by other metals or as buffers.

The need to keep the blank to as low a figure as possible indicated the desirability of using the minimum number and amounts of reagents, and the present work was undertaken to provide a method that satisfied this condition. There was an advantage in the use of salts of dibenzyldithiocarbamic acid, the copper complex of which could be extracted from acid solution, particularly if the zinc salt was used as described by Marten and Githensll and by Stone, Ettinger and Gantz.12 I t has also been that separation of copper from cobalt, nickel, iron and manganese is favoured by extraction at a low pH. The inorganic acids used may be freed from copper by distillation. (Sulphuric, nitric and perchloric acids are available commercially as “lead free” reagents and have presumably been distilled.) The use of these reagents has made possible the reduction of the blank to about 0.3 pg of copper, and the determination of copper down to a level of 0.02 p.p.m. (0-4 pg of copper). Digestion of the oils and fats was greatly facilitated by removing most of the fatty matter by vaporisa- tion before the wet combustion, and this also led to a considerable saving of time. The loss of copper during wet combustion, referred to by Wetlesen and Gran,14 was avoided by the use of the minimum amount of perchloric acid at a stage where no free hydrochloric acid could be formed by reduction.

EXPERIMEKTAL COLOUR REAGESTS IXVESTIGATED-

The preparation of zinc dibenzyldithiocarbamate involved the preparation of the dibenzyl- ammonium and potassium salts of the acid, and the suitability of all these salts as reagents for copper was investigated. As there appeared to be no adequate description in the literature of the preparation of these compounds, the methods of preparation developed by us are described below.

Standard amounts of copper were extracted from 50ml of 5 per cent. v/v sulphuric acid with and without the addition of potentially interfering metals. For the free acid and its dibenzylammonium and zinc salts, 10ml of carbon tetrachloride solution was used for

The use of solutions of the free acid was also studied.

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548 ABBOTT AND POLHILL: THE DETERMINATION OF [Vol. 79

extraction. The potassium salt was introduced i n aqueous solution into the acid solution containing copper, and the copper complex was extracted with 10 ml of carbon tetrachloride. For each sample, extraction was completed by shaking for 1 minute and, after filtering through a plug of cotton-wool, the optical density of the carbon tetrachloride solution was measured in 1.5-cm diameter cells with a Unicam SP350 spectrophotometer.

Results showed that solutions of the free acid and the zinc and dibenzylammonium salts (in equivalent concentrations of 0.04, 0.05 and 0.075 per cent. w/v, respectively) completely extracted up to 40 pg of copper, with no interference from 200 pg of cobalt, nickel, iron or manganese. The potassium salt, used as 5 m l of a 0.01 per cent. w/v aqueous solution, gave complete extraction of copper and the interference from cobalt was slight (200 pg of cobalt G 0.4 pg of copper). Nickel, iron and manganese did not interfere at this con- centration (200 pg). Solutions of the zinc and potassium salts and of the free acid retained their power to form a complex with copper for at least 24 hours. The dibenzylammonium salt appeared to be less stable in solution. PREPARATION OF COLOUR REAGENTS-

The dibenzylammonium, potassium and zinc salts of dibenzyldithiocarbamic acid were prepared as described below by methods developed from those used by Losanitschl5 for the preparation of analogous compounds. 0

Dibenzylammoaium dibenzyldithiocarbamate--Add 1 a 5 ml of carbon disulphide to a solution of 5.0 ml of dibenzylamine in 5 ml of light petroleum, boiling range 40" to 60" C. Stir the mixture well until the oil that separates becomes solid (5 to 10 minutes). Add 20 ml of light petroleum and grind the solid with the flattened end of a glass rod so as to reduce it to a fine powder. Separate the dibenzylammonium dibenzyldithiocarbamate by filtration, wash it with small quantities of light petroleum -until it is free from carbon disulphide, and allow it to dry on the filter.

Potassium dibenzyldithiocarbamate-To each gram of dibenzylammonium dibenzyl- dithiocarbamate prepared as described above, add 0-7 ml of a 3 N solution of potassium hydroxide in 75 per cent. v/v aqueous ethanol, and warm the mixture gently until solution is complete. Allow it to cool and add an excess of ether (10 ml for each gram of dibenzyl- ammonium salt taken) to precipitate the potassium salt. Stir the resulting slurry well, separate the solid by filtration, wash it with ether and allow it to dry a t room temperature. Potassium dibenzyldithiocarbamate is thus obtained as white silky needles of the trihydrate ; the yield is 90 per cent. of the theoretical. The trihydrate possesses no sharp melting point; it loses water of crystallisation over the range 95" to 125" C (found: K, 10.9 per cent. ; calculated for C,,H,,NS,K.3H20, 10.7 per cent.). The anhydrous material, which is formed by drying the trihydrate to constant weight in vacuo over sulphuric acid, has a m.p. 205" to 206" c (loss of weight on drying, 14.4 per cent.; calculated for C,H,,NS2K.3H20, 14.8 per cent.). Cornpin16 describes potassium dibenzyldithiocarbamate trihydrate as white silky needles that melt over the range 102" to 120" C.

A quantity of the dibenzylammonium salt can be recovered from the preparation of the potassium salt. Wash the ethereal filtrate well with water, separate the ether layer and allow the ether to evaporate. The dibenzylamine recovered in this way can then be treated with carbon disulphide as described aborre.

Zinc dibenzyldithiocarbamate-To a solution of 0.2 g of the potassium salt prepared as above, add 4 ml of a 4 per cent. W/V aqueous solution of zinc sulphate, ZnS04.7H,0. Separate the resultant curdy precipitate by filtration, wash it with water and dry it over sulphuric acid in vacuo. Zinc dibenzyldithiocarbamate is thus prepared as a white powder having a m.p. 187" to 188" C; the yield is almost quantitative. To obtain the salt in a more crystalline form, add 95 per cent. v/v aqueous ethanol to a concentrated solution of the salt in carbon tetrachloride to the stage of incipient precipitation and set the mixture aside, when the zinc salt separates as fine white needles, m.p. 187" to 188" C (found: Zn, 10.6 per cent,; calculated for C,H2,N,S4Zn, 10-7 per cent .) .

Zinc dibenzyldithiocarbamate is available commercially as a greyish-white powder having a m.p. 176" to 178" C. This may be purified by the ethanol - carbon tetrachloride procedure outlined above, when it forms fine needles, m.p. 187" to 188" C.

Dibenzyldithiocarbamic acid-Solutions of th.e free acid in carbon tetrachloride (0-04 per cent. w/v) were prepared by extraction with a known volume of the solvent from an acidified solution of the requisite amount of the potassium salt.

The yield is 5.8 g of solid of m.p. 82" to 83" C.

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Sept., 19541 COPPER IN OILS AND FATS 549

METHOD All measurements of optical density are made at a wavelength of 435mp with a

spectrophotometer.

REAGENTS- Sulphuric acid, concentrated-Distilled or “lead free.” Nitric acid, concentrated-Distilled or “lead free. ” Perchloric acid, 60 per cent. w/w-‘‘Lead free.” Sodium sulphite solution-A 5 per cent. w/v aqueous solution. Carbon tetrachloride.

Zinc dibenxylditlziocarbamate-A 0.05 per cent. w/v solution in carbon tetrachloride. Dibenxylammonium dibenzyldithiocarbamate-A 0.075 per cent. w/v solution in carbon

Dibenzyldithiocarbamic acid-A 0-4 per cent. w/v solution in carbon tetrachloride. Potassium dibenxyldithiocarbamate-A 0.01 per cent. w/v aqueous solution.

And one of the following colour reagents-

tetrachloride .

DIGESTION OF SAMPLE-

Introduce about 20 g of the oil or fat, accurately weighed, into a dry 200-ml silica digestion flask. (The 200-ml long-necked boiling flasks that can be purchased from The Thermal Syndicate Ltd., are suitable. Silica flasks are preferred, as they stand up to the vigorous heating conditions better than would flasks‘imade of the usual resistance glasses.) Heat the flask until the oil begins to fume, allowing the vapours to be swept away by a good draught. Continue heating until only about 1 to 2 ml of the oil remain. Cool the flask, add 3 to 3.5 ml of sulphuric acid and then 2 to 3 ml of nitric acid. After the initial vigorous reaction has subsided, complete the digestion by heating the mixture, with the addition of further small amounts of nitric acid, until the solution is colourless when cold. (The addition of a few drops of perchloric acid is often useful in the last stages of the digestion.) When the solution is cool, add 10 ml of water to i t and again heat it until fumes are evolved.

EXTRACTION OF COPPER-

Transfer the liquid remaining in the flask to a 100-ml separating funnel, dilute i t to 50 ml with water and add 1 ml of 5 per cent. w/v sodium sulphite solution to remove traces of nitrous fumes. Add 10ml of the carbon tetrachloride solution of the colour reagent (or 5 ml of aqueous solution of the potassium salt together with 10 ml of carbon tetrachloride), stopper the funnel and shake it for 1 minute. Filter the lower layer through a plug of cotton- wool introduced into the stem of the funnel, and then measure the optical density of the extract a t 435mp. Determine the blank on the reagents under the same conditions. Determine the amount of copper present by reference to a standard curve prepared as described below. The complete determination can be made in about 2 hours.

PREPARATION OF A STANDARD CURVE-

Prepare a standard solution of copper by dissolving 0.157 g of copper sulphate penta- hydrate in water containing 5 ml of 5 per cent. v/v sulphuric acid and dilute it to 200 ml. This solution contains 200 pg of copper per millilitre. With suitable dilutions of this stock solution, extract the copper from a solution in 50 ml of 5 per cent. v/v sulphuric acid with 10 ml of reagent as described above, and determine values of optical density. (Measurements of optical density at 435 m p by means of a Hilger Uvispek spectrophotometer show a linear relationship to the copper content of the standard.)

RESULTS The method was tested for the recovery of added copper and was compared with a

direct acid extraction method. An oil-soluble copper standard was prepared by dissolving 9.4 mg of copper carbonate in 5 ml of oleic acid and diluting with olive oil to give a solution containing 10 pg of copper in 89 mg of oil. Results for the recovery of copper are shown in Table I.

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550 ABBOTT AND POLHILL [Vol. 79 TABLE I

DETERMINATION OF KKOWK AMOUNTS OF COPPER IS OILS

Copper Copper Copper Copper

CLg PP P6‘ Pg

Olive A . . .. . . .. .. 1.1 10-6 12.0 10.9 1.2 10-7 11.8 10.6

Olive B . . .. .. .. .. 0.3 Sunflower .. .. .. I . 0.3 10.4 10-5 10.2

Oil used in oil, added, found, recovered,

W i t h aqueous potassiunr dibenzyldithiocarbamate as reagent f o r copper-

By the direct acid exfvartion method- Olive A . . . . .. .. .. 1.0 10.9 11.7 10.7 Olive B . . .. .. .. .. 0-36 Sunflower .. .. .. .. 0-4 10.3 10.4 10.0

COSCLUSIONS- Comparisons have been made between the method described in this paper and one

involving direct extraction by vigorous shaking with a mixture of hydrochloric acid and dilute nitric acid at 100” C. Extraction by diluted nitric acid, as is used in the determination of copper in butter,17 has been found to result in explosive ebullition with some oils.lS The recovery of copper added as the oleate to the two oils was complete, either by direct acid extraction or by the method described. For critical work, however, wet combustion is preferable as there is the possibility that copper may be present in a form that makes direct extraction by acid difficult or incomplete.

We do not wish to express any preference for any of the four copper reagents used in this work, as the results were identical whichever complexing agent was used. The choice of reagent may be governed by its availability.

The authors wish to thank the Government Chemist for permission to publish this paper.

1. 2. 3. 4. 5. 6. 7. 8. 9.

10.

11. 12. 13. 14. 15. 16. 17 . 18.

REFERENCES King, A. E., Roschen, H. L., and Irwin, W. H , Oil avd Soap, 1933, 10, 204. Ministry of Food Food Standards Committee’s Report on Copper, Analyst , 1931, 76, 554. Callan, T., and Henderson, J . A. R., Ibid., 1929, 54, 650. Sylvester, N. D., and Lampitt, L. H., Ibid., 1935, 60, 377. Ovenston, T. C. J., and Parker, C. A., A n a l . Ch im. Acta, 1950, 4, 136. Allport, N. L., and Skrimshire, G. H., Phavrpz. J . , 1932, 129, 248. West, P. W., and Compere, M., A n a l . Chew. , 1949, 21, 628. Moss, M. L., and Mellon, DL G., Ibid., 1942, 14., 931. McCurdy, W. H., and Smith, G. F., Ibid. , 1952, 24, 371. Riddett, K. J., “Organic Reagents for Metals, Monograph KO. 12,” Hopkin and M‘illiams Ltd.,

Martens, R. I., and Githens, R. E., Axal. Chew., 1952, 24, 991. Stone, I. , Ettinger, R., and Gantz, C., Ibid., 1’953, 25, 893. Irving, H. M., and Williams, R. J . P., Analys t , 1952, 77, 813. Wetlesen, C.-U., and Gran, G., Svensk Papperstidning, 1952, 55, 212. Losanitsch, S. M., Ber., 1891, 24, 3021; 1907, 40, 2970. Compin, L., Bul l . SOC. Chivn. France, 1920, 27, 468. British Standard 769 : 1952, pp. 19-21. Williams, P. IS., Private communication.

London, 1953.

DEPARTMENT OF THE GOVERNMENT CHEMIST GOVERNMENT LABORATORY

CLEMENT’S INN PASSAGE STRAND, LONDON, W.C.2 Apri l Zzd, 1934

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