ANALYTICAL CHEMISTRY
664
it when an unknown is diluted. Since solutions to be diluted contain more than 50 mg. per 100 ml. equivalent, and have an absorbance greater than 0 . 5 i 5 , the error from this source is less than 27,. Below 0.5 microgram per ml. of final solution, corresponding to 10 mg. per 100 ml. of whole blood, the applicability of Beer’s law diminishes rapidly, until below 0.25 microgram per ml. the curve is quite unreliable, probably owing to the magnification of small errors. The upper limit of usefulness is determined by the amount of diacetyl monoxime present in the test solution. It is probable t h a t the range can be extended to a t least 500 mg. of urea nitrogen per 100 ml. of whole blood by proper dilution of the final solution.
If a calibration curve is set up from standard urea solutions, covering the range of 5 to 50 mg. of nitrogen per 100 ml., by steps of I mg. per 100 ml., the differences in absorbances of adjacent readings will be 0.010 or slightly more. This is more than sufficient to give a sensitivity of 1 mg. of nitrogen per 100 ml. A greater sensitivity would not be of clinical value LITERATURE CITED
(1) F e a r o n , 17.R., Biochem. J . , 33, 902 (1939). (2) K a w e r a u , E., Sci. Proc. Roy. Dublin Soc., 42, 63 (1946). (3) S a t e l s o n , S.,Scott, hI. L., and Beffe, C., .4m. J . Clin. Pathol., 21, 275 (1951). (4)Ormsby, A . A,, J . B i d . Chem., 146, 596 (1942). (5) Somogyi, XI.,J . Biol. Chem., 86, 655 (1930).
RECEIVED for review September 25%1952. Arcepted December 3, 1952.
The Determination of Copper in Mineral Oils Using an Ion-Exchange Technique HERBERT BUCHWALD
OPPER
AND
L. G. WOOD. Manchester Oil Refinery, Ltd., Manchester, England
compounds can be quantitatively removed from min-
C eral oils by adsorption on a cation-exchange resin and sub-
sequently recovered by acid washing. The copper in the acid solution, free from organic contamination, can then be determined by any of the standard methods. In the present instance copper contents as low as 0.1 p.p.m. have been determined colorimetrically. In an investigation requiring the determination of the copper content of a large number of samples of used transformer oil it \vas found that the generally accepted methods were excessively lengthy, and were not capable of application to the low concentrations of copper encountered with any degree of repeatability. T h e standard methods ( I , 5 ) involve either lwt or dry ashing of the sample, followed by the colorimetric determination of the copper content of the ash. In the former method the procedure was lengthy, requiring the oxidation of relatively large quantities of organic matter, and in the latter, loss of copper compounds by volatilization lvas a distinct possibility.
concentration of copper on a cation-exchange resin, and subsequent polarographic analysis of the solution. Frizell (4) and Wickbold (13, 14) have discussed the use of ion-exchange resins for analytical purposes, while Swanson (22) has mentioned unpublished work carried out a t Shell Thornton Research Centre on the recovery and determination of iron and copper from used engine oils by an ion-exchange technique. It was found that if a cation-exchange resin containing sulfonate groups was converted to the hydrogen form and then washed with 2-propanol to remove water, it would remove dissolved copper compounds from mineral oil quantitatively. Oil could then be washed from the resin by percolation of a further quantity of 2-propanol, which could in turn be removed by percolation of distilled xater. The copper could be recovered quantitatively by percolation of dilute sulfuric acid down the column. The copper content of the acid percolate could then be determined colorimetrically. APPARATUS
Table I.
Copper Content of Solutions of Copper Yaphthenate in Mineral Oil
Copper, P.P.M. Added Found 57.50 56.90 11.80 11.30 11.80 11.30 5.90 5.40 1.65 1.10 1.60 1.10
Blank, P.P.hI. 0.55 0.55 0.55 0.55 0.55 0.55
Copper Found less Blank, P.P.M. 56,95 11.25 1?,!25 a . 35 1.10 LO5
Massey (8) has shown that copper compounds can be quantitatively removed from mineral oil by sulfuric acid washing, and the concentration of copper can be determined colorimetrically by the sodium diethyldithiocarbamate method a t concentrations as low BS, 0.05 p.p.m. Kreulen (7) used an acid washing technique involving refluxing the sample with 10% sulfuric acid followed by removal of the acid layer. Both methods suffer from the disadvantage that a small amount of organic matter is always present i n the acid layer after separation, and has to be removed by vigorous oxidation or wet ashing. Various authors have proposed methods depending upon w-et ashing or acid extraction ( I O ) , followed by titration with dithizone ( 2 ) or amperometric titration (9). Cranston and Thompson (3) have shown that traces of copper in milk products can be determined by removal of organic matter,
A chromatographic column approximately 35 cm. long and 1 cm. in diameter with an upper reservoir of 100 ml. capacity was used. The lower end of the column was closed by a stopcock, and the resin was held in place by a plug of glass wool. .4 “Spekker” photoelectric absorptiometer was used for the colorimetric determination of the recovered copper. REAGENTS
Sulfuric acid, 10% v/v, il.R. grade Ammonium hydroxide, 20% v/v, A.R. grade iimmonium hydroxide, “880”, A.R. grade Ammonium citrate solution. Five-hundred milliliters of A.R. grade “880” ammonium hydroxide and 500 grams of -4.R. grade citric acid were diluted to 1 liter with distilled water. Gum acacia solution, 0.5%, in distilled water, filtered before use Sodium diethyldithiocarbamate solution, 0.2% in distilled water, freshly prepared 2-Propanol, absolute, redistilled Indicator paper, B.D.H. “Wide range” Ion-exchange resin, “Zeo-Karb” 215 PROCEDURE
The column was filled with a slurry of the resin in distilled water to give a bed approximately 30 cm. deep. The level of liquid was never permitted, a t any time, to fall below that of the resin. Percolation of 50 ml. of 10% sulfuric acid through the column over a period of 15 minutes ensured that the resin was in the hydrogen form. Before the acid level had fallen below that of the resin, the column was washed with successive 10-ml. ortions of distilled water until the effluent had a pH of about 4 ($etermined
665
V O L U M E 2 5 , NO. 4, A P R I L 1 9 5 3 by the indicator paper). The column % a s then prepared for the oil sample by percolation of 60 ml. of 2-propanol, m-hich removed water from the resin. -410-gram sample of the oil under test was diluted with an equal volume of 2-propanol and percolated through the column, the beaker was washed thoroughly with 2-propanol, and the washings were added to the column. A total volume of 80 ml. of 2-propanol \vas used. It should be noted here that, although the particular oil samples used during this investigation were completely miscible with 2propanol a t room temperature, this may not always be the case. If the oil and 2-propanol are not miscible the addition of a small amount of A.R. grade benzene to the mixture ensures complete miscibility and does not affect the results in any way. After percolation of the oil and 2-propanol through the column all the copper originally present in the oil A as held on the resin, which was then ~ a s h e da i t h 60 ml. of distilled water in order to remove 2-propanol. All the effluent was discarded, and a 400 ml. beaker was used to collect the further washings. The copper was recovered by percolating 60 ml. of 10% sulsulfuric acid through the column followed by 20 ml. of distilled water, the rate of efflux being so arranged that this operation required 15 minutes. The copper content of the effluent was determined colorimetrically as follows. The solution R-as evaporated to a volume of approximately 20 ml. and transferred to a 100-ml. volumetric flask. The beaker mas washed with 20 ml. of ammonium citrate solution followed by 20 ml. of 20y0 ammonium hydroxide. The vr-ashingswere added to the flask, which was cooled a t this point. A sufficient volume of “880” ammonium hydroxide was then added to raise the pH of the solution above 9, followed by 10 ml. of gum acacia solution and an equal volume of 0.2% sodium diethylthiocarbamate solution. ilfter adjusting the volume to 100 ml. with distilled water the color intensity of the solution was determined on the “Spekker” photoelectric absorptiometer which had been previously calibrated with solutions of knon n copper content. On application of the method to oils of known copper content high results were obtained. This was traced to the presence of copper in the sulfuric acid. Accordingly a blank colorimetric determination was carried out on 60 ml. of the acid used in the percolation.
Table 11. Comparison of Results Obtained by .4cid Washing and Ion-Exchange Techniques Copper, P.P.M. By Ion-Exchange By acid wash
Determined
Blank=
Determined less blank
In practice it was necessary that the method give results repeatable to 0.05 p.p.m. on oils containing as low as 0.1 p.p.m. of copper. Thus a series of oils with copper contents between 0.1 and 1.5 p.p.m., determined by the acid washing technique of Massey, was analyzed by the present method. A 20-gram sample was used in the case of the oil of lowest copper content. The results are summarized in Table 11. The figures in Table I1 show that the results obtained by the ion-exchange method are directly comparahle with those obtained by the more laborious acid washing technique. A series of tests was carried out in which the 2-propanol-oil percolate and the water washings were examined for the presence of copper. I n no case was a measurable quantity of copper detected in either percolate. A comparison of the results obtained using the wet and dry ashing techniques (5) and the present method for two oils of known copper content is shown in Table 111.
Table 111. Results of Analysis for Copper by Wet and Dry Ashing and Ion-Exchange Techniques Actual content 1.35 22.1
Copper, P.P.M. BY wet ashing By dry ashing 1.46 0.87 20.0
By ion-exchange 1.37 22.6
The dry ashing technique gives low results, the error becoming more serious a t low copper contents such as are usually encountered in used transformer oils, whereas the wet ashing technique gives more accurate results. However, the latter method is laborious and requires a relatively long period to ensure complete oxidation of all organic matter. On the other hand the ionexchange method is rapid and gives accurate and repeatable results. The data obtained show that the present method is applicable to the determination of dissolved copper in mineral oil down t o a concentration of 0.1 p.p.m. The method is simple in operation and requires only a short time (about 2 hours) per determination. The results are repeatable to 0.05 p.p.m. and accurate to the same limit over the range 0.1 to 57 p.p.m. ACKNOWLEDGMENT
The authors would like to thank Leonard Massey of Metropolitan Vickers Electrical Co., Ltd., for discussion and advice on the acid washing technique and the Directors of Manchester Oil Refinery, Ltd., for permission to publish this paper. LITERATURE CITED The variation of the blanks is due t o the fact that these determinations were carried out over a n extended period, several batches of acid being used.
If iron is present in the solution it is complexed with the citrate and, at a pH greater than 9, will not interfere with the determination of copper using sodium diethyldithiocarbamate (6, 11). Sickel, cobalt, and bismuth in quantities greater than that of the copper will interfere in the colorimetric estimation. Alternatively the percolate from the column after concentration may be treated as in (1, 5 ) for the determination of copper and iron. APPLICATION
-1 series of solutions of copper naphthenate in mineral oil was prepared, and the copper contents were determined by the method. The results are summarized in Table I. The method is applicable to solutions containing 1 to 57 p.p.m. of copper, and the results are accurate and repeatable to 0.05 p.p.m.
(1) A?;erican Society for Testing Materials, Philadelphia, Pa., -4.S.TM. Standards on Petroleum Products and Lubricants,” 1951, A.S.T.M. 810-48. (2) Assaf, A. G., and Hollibaugh, W.C., IND.ENG.C H m r . , ANAL. ED., 14, 806 (1942). (3) Cranston, H. A . , and Thompson, J. B., Ibid., 18, 323 (1946). (4) Frizzel, L. D., I b i d . , 16, 615 (1944). (5) Institute of Petroleum, 26 Portland Place, London I T 1, England, ”Standard Methods for Testing Petroleum and Its Products,” 1952, IP 120/48. (6) Karsten, P., Rademaker, 8. C., and Walraren, J.‘J., Anal. Chim. Acta, 2, 705 (1948). ( 7 ) Kreulen, D. J. W., J . Inst. Petroleum, 38, 449 (1952). (8) Massey, L., Metropolitan Vickers Electrical Co., Ltd., private communication. (9) Parks, T. D., and Lykken, Louis, - ~ N A L CHEW, . 22, 1503 (1950). (10) Ritterhausen, E. P., and De Gray, R. J., IND.ENQ.CHEM., AXAL.ED.,14, 806 (1942). (11) Snell, F. D., and Snell, C. T., “Colorimetric Methods of Analysis,” 3rd ed., Vol. 2, p. 108, New York, D. Van Nostrand Co., 1949. (12) Swanson, B. S., Inst. Petroleum Rev., 6, 7 3 (1952). (13) Wickbold, R., 2. anal. Chem., 132, 241 (1951). (14) Ibid.,p. 321. RECEIVED for review September 17, 1952. Accepted December 17, 1952.
