Synthetic binary mixtures were made from 1 meq of each halide and the results recorded in Table 111. Any functional group that reacts rapidly with base in DMSO at 100 “cwill interfere in procedure 2 in which the excess base is back titrated. Esters, aldehydes, or ketones with alphahydrogens, anhydrides, and acidic compounds such as
carboxylic acids would interfere. However, if the Volhard method is used (procedures 1 and 4), there will be no interferences except with mixtures of halides. RECEIVED for review August 5, 1968, Accepted August 26, 1968. Work performed in the Ames Laboratory of the U. S. Atomic Energy Commission.
eterrnination of Trace Copper in Petroleum Middle DistiIIates C. E. Lambdin and W. V. Taylor Research and Technical Department, Texaco, Inc., P. 0. Box 1608, Port Arthur, Texas 77640
THE VERY LOW CONCENTRATIONS of copper usually present in refined petroleum middle distillates necessitate sensitive analytical methods. The amount of soluble copper in aviation turbine fuels is of considerable interest to the air transportation industry. Copper in the range of 150 ppb adversely influences the thermal stability of hydrocarbon fuels ( I ) as measured by the CFR fuel coker test (2). The sources of soluble copper in middle distillates can be traced to carry-over from primary distillation of crude and other refining processes including amounts picked up from brass valves, fittings, and heat exchanger tubes, for example. The use of cuprizone (biscyclohexanone oxaldihydrazone) as a spectrophotometric ligand for inorganic Cu(I1) was first described by Nilsson (3). Wetlesen and Gran ( 4 ) applied cuprizone to the determination of copper in pulp and paper. The high sensitivity of cuprizone was reported by Peterson and Bollier (5). High selectivity was reported by Rohde (6) who employed cuprizone for determination of copper in a variety of nonferrous metals and alloys. These publications describe formation of the cuprizone-copper complex in aqueous solutions after chemical treatment of the sample matrix to obtain inorganic Cu(I1) ions. Both inorganic and organic Cu(1I) can be successfully extracted and determined from petroleum middle distillates and other hydrocarbon solvents by complexing with solutions of cuprizone. The extraction with cuprizone is effected using methanol or other selected polar solvents. When E 00 grams of sample are extracted, copper in the range of 0.02 to 2 pg per ml can be determined. The method is applicable to a wide variety of hydrocarbon fuels and solvents. Heavier stocks can be extracted after they are diluted in a suitable light solvent. EXPERIMENTAL
Apparatus. Absorbance values were measured with a Bausch and Lomb Spectronic 20 using 1-cm cells. Reagents. Cuprizone (biscyclohexanone oxaldihydrazone) solution, 0.2z prepared in anhydrous methanol. Warm gently to effect solution. (1) “Thermal Stability of Hydrocarbon Fuels,” Progress Rept. No. 4, Air Force Contract AF-33, (616) 7241, Jan 1962, p 5. (2) “ASTM Standards,” ASTM Designation: D 1660-67, Part 17,
American Society for Testing and Materials, Philadelphia, Pa., January 1968, p 610. (3) G. Nilsson, Acta. Chem. Scand., 4, 205 (1950). (4) C. U. Wetlesen and G. Gran, Suensk Papperstidr., 55, 212 (1952). ( 5 ) R. E. Peterson and M. E. Rollier, ANAL.Cnmi., 27, 7 (1955). (6) R.K. Rohde, ibid., 38, 7 (1966).
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Figure 1. Absorption spectra of the copper-cuprizone complex in methanol 1. Copper, 1.0 p g per ml cs. reagent blank 2. Copper, 0.46 p g per ml us. reagent blank 3. Copper, 0.24 p g per ml cs. reagent blank Ammonium acetate buffer solution, 1 molar, is prepared in water and adjusted to pW 9.0 by addition of ammonium hydroxide before being diluted to volume. Standard copper solution, 100 ppm in methanol, may be prepared from either inorganic or organo copper(1H) certified reagents. Equivalent calibration curves were obtained from either class of copper reagent. Cupric acetate is recommended because of its solubility in methanol. Procedure. Samples containing 0.1 to 2.0 pg of copper are weighed into a 250-ml glass-stoppered separatory flask. Usually 100-gram samples of middle distillate are transferred into the flask, followed by addition of 8 ml of 0.2% cuprizone solution and 2 ml of ammonium acetate buffer. The flask is shaken vigorously for 1 minute with venting to relieve pressure during the first half minute. Allow 15 minutes settling time. The settling time can be shortened by centrifuging. Withdraw the aqueous methanol phase into a 25-ml volumetric flask. Repeat the extraction with another 10 ml of cuprizone-buffer solution mixture. After making the second extraction, dilute to mark with methanol. Measure absorbance of the blue solution at 606 mp against a reference solution of cuprizone reagent. Copper content is read from a straight line calibration curve. Beer’s law is obeyed for the range 0 to 2 pg of copper per ml. Sampling middle distillates for copper determination presents several difficulties. Nonreproducible analyses will often result from samples containing appreciable amounts of water. Copper-rich components have a strong tendency to
Table I. Extraction of Copper from Petroleum Middle Distillates A. 150 grams of kerosine extracted with 8 ml of cuprizone reagent in methanol and 2 ml of buffer Sample Cu content before extraction, Absorbance of Copper extracted, Copper extracted, Extraction No. PPb extract PPb
z
1 2 3
75 22 3 1
4
0.29 0.11 0.01 0.00
70.7 25.4 2.7 0.0 98.8
53 19 2 0 74
-
Totals
__.
B. 150 grams of kerosine extracted with 8 ml of methanol without complexing with cuprizone
Sample Cu content before extraction, Extraction No. 1 2 3
Q
ppb 75 71 66
Absorbance of extracta
Copper extracted, PPb
Copper extracted, %
0.02 0.03 0.02
4 5 4 13
5.3 6.7 .~5 . 3 17.3
Totals Absorbance developed after extraction by addition of 1 ml of 0.5% cuprizone and 1 ml of buffer.
concentrate in the water phase and are strongly surface active. Upon standing, a film of copper compounds is often adsorbed on the surface of the sample container. It is recommended that hydrocarbon samples be taken directly from the sampling point into smaller containers-Le., 4-oz bottles-and that the entire sample be extracted. Rinsing the empty sample container with the cuprizone-buffer solution used for the first extraction is recommended. To ensure against copper contamination, all glassware should be rinsed with warm 1 : 1 hydrochloric acid to remove any trace copper present prior to use. The calibration curve is prepared from dilutions in methanol of the 100 ppm standard copper solution. Solutions in the range of 0.2 to 2.0 pg of copper per ml are recommended. Use of a 10-fold molar excess of cuprizone plus sufficient buffer solution to give pH 8 to 9 is emphasized for maximum color development and repeatability. RESULTS AND DISCUSSION
Figure 2. Molar ratio plots
The absorption spectra of the copper-cuprizone complex in aqueous methanol have an absorbance maximum at approximately 606 mp. Typical curves are depicted in Figure 1. These curves closely resemble those of aqueous copper-cuprizone solutions reported by Smith (7). The colored complex after extraction from middle distillates was stable for approximately 4 hours. After this time, there was a gradual fading of about 10 over a 24-hour period. Maximum color was produced 30 minutes after addition of cuprizone. The extraction efficiency was determined by repetitive extractions of a prepared sample of oil-soluble organo copper (copper naphthenate) in kerosine previously analyzed to be copper free. Results shown in Table IA demonstrate that essentially all of the copper is removed after the second extraction. Results from Table IB show that the copper must be complexed with cuprizone to achieve effective extraction, as little copper is extracted by methanol alone. The ligand-metal molar ratio was determined, according to the method of Yoe and Jones (8), to evaluate the composition
Final concentration of copper 1. 3 X molar 2. 2 X molar
(7) G. F. Smith, “The Trace Element Determination of Copper and Mercury in Pulp and Paper,” Pamphlet No. 214, G. F. Smith Chemical Co., Columbus, Ohio, 1954.
(8) J. H. Yoe and A. L. Jones, IND.ENG.CHEM.ANAL.ED., 16, 111 (1944).
of the complex. A series of methanol solutions containing a constant amount of copper with increasing quantities of cuprizone were prepared. Both reagent cupric acetate and pure copper wire were employed as copper source, each yielding equivalent results. The results, which are plotted in Figure 2, indicate breaks corresponding to the formation of a 4 :1 complex between the ligand and the metal. Several other water-miscible solvents that possess limited solubility in petroleum middle distillates were effective replacements for methanol. These include ethanol, pyridine, and Nmethyl-2-pyrrolidone (NMP). This is by no means an exhaustive list. Solutions of cuprizone in NMP (General Aniline and Film) with the ammonium acetate buffer were especially effective for extraction and determination of trace copper in heavier petroleum stocks such as lubricating oils. Dilution of a viscous oil with a light hydrocarbon solvent prior to extraction minimizes viscosity problems.
RECEIVED for review June 10, 1968. Accepted August 16, 1968. VOL. 40, NO. 14, DECEMBER 1968
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