Comparison of Two Colorimetric Methods for Determination of Copper in Mineral Oils J. M. HOWARD and H. 0.SPAUSCHUS Major Appliance Laboratories, General Elecfdc
b The carbamate method has been adapted to the determination of dissolved copper in mineral oils. The copper is extracted with acetic acid and determined colorimetrically as the carbamate complex which has an absorption maximum at 440 mp in carbon tetrachloride-acetic acid solution. This method is compared with the neocuproine method of Zall, McMichael, and Fisher. Both methods obey Beer’s law over the concentration range of 2 to 200 pg, of copper. per 50 ml. of finat solution. Absorbance of the copper-carbamate complex is 1.6 times that of the neocuproine complex, thus providing greater sensitivity for the former. Iron does not interfere with copper determination by the neocuproine method, but a slight modification of the carbamate method is required if iron is present.
P
PRODUCTS, in Contact with copper or copper alloys, gradually dissolve small quantities of the metal. I n certain applications the presence of dissolved copper in petroleum products initiates or accelerates undesirable chemical effects. For example, it has been reported that dissolved copper (a) causes heavy deposits in the precombustion chamber of some Diesel engines (11)’ ( b ) accelerates the oxidation of transformer oils (5, IO), and ( c ) produces copper plating on steel bearings in refrigeration conipressors (7-9). I n the course of investigation of the mechanism of ( c ) , a precise analytical method for determination of small quantities of dissolved copper in mineral oil was adapted and tested extensively in this laboratory. I n this method, copper is extracted with acetic acid, complexed as the carbamate (6), and the absorbance a t 440 mp is determined in carbon tetrachlorideacetic acid solution. Various methods for the determination of copper in oil are reported in the literature. The standard method (1) involves ashing the sample and then determining the copper content of the ash. Other workers have used sulfuric (4), alcoholic hydrochloric (S), and dilute nitric (9) acids to estract the copper before determination. More ETROLEUM
1016
ANALYTICAL CHEMISTRY
Co., Louisville,
Ky.
recent work has utilized ion exchange ( 2 ) to concentrate the copper. In 1957, Zall, McMichael, and Fisher (11) reported the development of an improved method requiring no separation or concentration of the copper, which is complexed with neocuproine and determined spectrophotometrically in chloroform solution. This paper describes the carbamate method and presents a comparison with the neocuproine method. EXPERIMENTAL
Apparatus and Reagents. All absorbance measurements were made on a Cary recording spectrophotometer using fused silica cells of 20-mm. light path. Reagents included : 2,g-dimethyl1,lO-phenanthroline hemihydrate (neocuproine), sodium diethyldithiocarbamate (0.2% aqueous solution), and metallo organic copper (copper content determined electrolytically after wet ashing) and iron compounds (h’ational Spectrographic Laboratories, Inc., Cleveland, Ohio). All other reagents were analytical reagent grade. P r o c e d u r e s . N Eo c U P R O I N E METHOD. Reference (11). CARBAMATE METHOD. Weighed quantities of standard copper solution (47.9 pg. of Cu per gram of mineral oil) were transferred t o 125-ml. separatory funnels containing 10 ml. of mineral oil to give a satisfactory working volume. Ten milliliters of glacial acetic acid were added and each separatory funnel mas vigorously shaken for 3 minutes. The solution was allowed to separate into two phases and the acid phase was transferred to a 250-ml. separatory funnel. The stem of the 125-ml. funnel was washed with 1 to 2 ml. of acetic acid after transfer and the washings mcrc added to the 250-ml. funnel. This extraction procedure was repeated with two additional portions of acetic acid. Ten milliliters of the carbamate solution were then added to the combined acid extracts and the two solutions were thoroughly mixed by shaking. The brown copper-carbamate complex was extracted with three IO-ml. portions of carbon tetrachloride following the same procedure as for the original acid extraction. The carbon tetrachloride extracts were transfcrred to 50-ml. volumetric flasks, 10 ml. of acetic acid (to remove any turbidity) mere added, and the volumes of solutions were made to 50 ml. with ad-
Table I.
Carbamate Method
copper,
Predicted copper,
fig.
rg.
Known
2.2 4.8 7.6
1.8
Difference, rg.
-0.4 $0.7 -0.9 -0.5 $1.4 +0.9 +0.3 i0.3
5.5 6.7 10.2 9.7 23.9 25.3 29.4 30.3 37.2 37.5 43.6 43.9 49.6 51.4 $1.8 -2.7 88.1 85.4 121.0 117.0 -4.0 141.4 +0.8 140.6 170.7 171.8 $1.1 201.9 203.3 $1.4 The standard deviation is f1.03. Table
Known
copper, fig.
II.
Neocuproine Method
Predicted copper, /rg.
Difference, Pg.
3.3 +1.1 M.6 4.8 +0.2 9.6 10.5 t0.9 $1.4 ii.8 13.2 21.0 -3.6 24.6 $1.2 51.7 52.9 +2.2 73.5 75.7 95.9 -3.9 99.8 123.8 123.0 -0.8 -0.5 Eo.3 i49.8 175.7 -0.4 176.1 +2.6 200.4 203.0 The standard deviation is f l .24. 2.2
ditional carbon tetrachloride. A blank consisting of all reagents, with the exception of the copper solution, was carried through the procedure and used as the reference solution during a b sorbance measurements. All absorbance measurements were made at a wavelength of 440 mp. RESULTS AND DISCUSSION
Precision and Accuracy. The d a t a from 14 analyses by the carbamate method and 12 analyses by the neocuproinc method show that the two methods obey Beer’s law over t h e range of 2 to 200 pg. of copper. The least squares lines for the standard curves are: Carbamate method: Absorbance = 0.0082 ( p g . of copper) 0.0002 Neocuproine method: Absorbance = 0.0051 ( p g . of copper) - 0.0068
+
Over these concen1,ration ranges the intensity of the copper-carbamate complex is 1.6 times that (of the neocuproine complex. The least squares lines were used to predict the copper values for each standard solution. These predicted values were then compared to the known values and the standard deviations were calculated (Tables I and 11). Effect of Iron. I n studies of the mechanism of solution and transfer of copper in mineral oil (?), the oil samples contained disso.ved iron as well as copper. Consequently, experiments rrere conducted t o determine whether dissolved iron interfered with the copper determination. A nbmber of standard copper solutions containing known concentrations of iron (added as metallo organic iron) were run by each method. No modification of the neocuproine method was necessary to correct for the presence of iron, but the following addition was used with the carbamate method. The combined ac>tic acid extractions were brought to pH 9 or greater with ammonium hydroxide (1:l) and 1.0 gram of citric acid was added
before the addition of the carbamate solution (6). The effect of iron is shown in Table
Table HI. Effect of Dissolved Iron on Copper Determination
111.
CARBAMATE METHOD SUMMARY
Dissolved copper in the range of 2 to 200 pg. per gram of mineral oil can be determined with high accuracy (u e 1 pg. of Cu) by either the neocuproine or carbamate method. The procedures for these methods are much less complex than those required by earlier methods. The neocuproine method is the simpler and quicker of the two. The carbamate method offers an advantage a t low concentrations since the copper-carbamate complex has a greater absorbance than the copper-neocuproine complex. LITERATURE CITED
(1) American Society for Testing and Materials, Philadelphia, Pa., “Part 5,
Fuels, Petroleum, Aromatlfc Hydrocarbons, Engine Antifreezes, A.S.T.M.
D-810-48. 1952. (2) Buchwald, H.9 wood, G., A N A L . CHEM. 25, 664 (1953). (3) Hackett, C. E. S., Anal. Chim. Acta 12, 358 (1955). (41 . , Kreulen. D. J. W.. J. Inst. Petroleum 38, 449 (1952).
Copper added.
Iron added.
39.0
40.0 110.0
a.
54.4
rg.
Copper
ReDifcovered. ference. rg. 38.1 57.2
rg.
-0.9 +2.8
NEOCUPROINE METHOD
57.1 102.1
58.6 104.8
56.2 101.3
+0.9 -0.8
( 5 ) Massey, L., Ibid., 38, 281 (1952). (6) Sandell, E. B., “Colorimetric,, Determination of Traces of Metals, 3rd ed., Vol. 3, p. 444, Interscience, New York, 1959. (7) Spauschue, H. O., ASHRAE J., in
press.
(8) Steinle, H., Kaltetechnik 7, No. 4, 101 (1955). (9) Steinle, H., Seeman, W., Ibid., 3, KO. 8. 194 (19511. (10) Thompson, C. N., J . Inst. Petroleum 44, 295 (1958). (11). Zall, D. M., McMichael, R. E., Fisher, D. W., ANAL. CHEM. 29, 88 (1957).
RECEIVED for review December 13, 1962. Accepted March 28, 1963.
Quantitative Radiochemical Procedure for Analysis of Polonium-210 and Lead-212 in Minerals HUGH T. MILLARD, Jr.l Division of Geological Sciences, California Institute of Technology, Pasadena, Calif.
b A method for the analysis of polonium-2 10 (1 38.4-day half life) and lead-21 2 ( 1 0.6-ho~’r half life) in zircons and other natural systems has been developed. The procedure employs spontaneous elcctrodeposition on silver for the isolatior of polonium-2 10 and controlled-potential electrogravimetric separation of lead-2 1 2 plus added lead carrier. A diethyldithiocarbamate extraction procedure for the lead was developed to be used prior to plating in the! presence of ions which precipitate at the pH used for lead deposition. Th,? amounts of the deposited nuclides arl? then determined by alpha-counting. The effects of temperature, volume, and inhibiting ions on the yield and rate of deposition of polonium wore also studied. The procedure was calibrated using minerals whose lead-21 0 and thorium2 3 2 contents had been determined by other methods; it was tested on a composite uraninitle-thorite mixture. Finally, it was applied in the analysis of four zircon samples and one uranothorite sample.
A
of the degree of disequilibrium in the radioactive decay chains in natural materials is of value in explaining geochronological inconsistencies and in more general studies of the geochemistry of the members of these chains. Rosholt (20-22, 94, 26) has reported methods for the detailed study and classification of the various types of radioactive disequilibria found in geological samples. He divides the chains into groups, each having a relatively long-lived parent which can be assumed to be in radioactive equilibrium with its immediate daughters. The uranium group contains two parent nuclides: 4.5 X 109-year uranium-238 and 7.1 X 108-year uranium-235. The parents of the other groups in the uranium-238 series are: 8.0 X 104-year thorium-230, 1622-year radium-226, 3.8dag emanation-222 (radon group), and 22-year lead-210 (lead group). The parent of the only other group in the uranium-235 series is 34,300-year protactinium-231 (protactinium group), All of the daughters in the thorium series have sufficiently short half lives KNOWLEDGE
to provide only one group in this series, the thorium group. The objective of this study was to find alternative procedures for analyses of the lead and thorium groups and to apply these procedures to mineral separates with particular attention t o uranium and thorium systems in nature. Assuming that the thorium series is ordinarily in radioactive equilibrium in geological samples, quantitative analysis for any member of the thorium group allows us to find the thorium-232 content. In the uranium-238 series the loss of lead group members from a sample can only produce short-term disequilibrium due to the short half life of lead-210 (22 years). However, because this group comes a t the end of the chain, the knowledge of ita amount relative to uranium is helpful in evaluating the state of radioactive equilibrium farther up the chain. This state of equilibrium may be affected by the gain or loss of Present address, Department of Cheniistry, University of California, San Diego, La Jolla, Calif.
VOL 35, NO. 8, JULY 1963
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