Determination of Wear Metals in Used Lubricating Oils by Atomic Absorption Spectrometry J. A. BURROWS Division o f Chemical Physics, Commonwealth Scientific and Industrial Research Organization, Melbourne, Australia
J. C. HEERDT Commonwealth Railways, Port Augusta, South Australia
J. B. WlLLlS Division o f Chemical Physics, Commonwealth Scientific and Industrial Research Organization,, Melbourne, Australia
b Traces (0 to 100 p.p.m.) of copper, chromium, iron, lead, and silver in used lubricating oil can b e determined by atomic absorption measurement of solutions of the oil in 2-methyl-4pentanone. Calibrating solutions are prepared by dissolving commercially available organo-metallic compounds in 2-methyl-4-pentanone containing the appropriate concentration of unused oil. The results agree satisfactorily with those obtained by atomic absorption or colorimetric measurement of solutions of the ashed specimens, and the technique is in routine use for the determination of these metals in the used lubricating oil from railway diesel engines.
A
of used lubricating oils for traces of wear metals has long been recognized as essential for the prevention of major breakdowns in diesel engines, but the methods hitherto employed have either been excessively tedious or have required the use of expensive and complicated equipment. Standard chemical methods for the determination of trace metals in oils, such as those of the -i.S.T.M. ( I ) , require ashing of the specimen followed by chemical treatment and colorimetric measurement of a solution of the ash. Such methods are time-consuming and are subject to interference between one metal and another. Emission spectrometry has been widely employed, since the use of techniques such as the porous cup ( 7 ) , rotating disk ( 2 , 6), rotating platform (IO), or vacuum cup (9) electrode allows the introduction of the oil into the arc or spark without the need for prior ashing. The relative advantages and disadvantages of most of these techniques have been discussed by Fry ( 5 ) . All these techniques suffer, however, from the usual disadvantages of emission methods, such as interelement interference, and require the use of expensive direct-reading equipXALYSIS
ment, if large numbers of routine measurements are to be made. The present work investigates use of atomic absorption spectrometry ( I 2 ) to determine trace quantities of the wear metals copper, chromium, iron, lead, and silver in used lubricating oil without prior ashing of the sample. The atomic absorption technique is generally free of interference effects, uses samples in liquid or solution form, and requires relatively inexpensive equipment. Atomic absorption spectrometry in the determination of chromium, iron, and nickel in gas oils before cracking has been discussed by Barras ( 3 ) . Sprague and Slavin ( I f ) showed that used lubricating oils diluted with p xylene can be sprayed into the flame of an atomic absorption instrument to give measurable absorption for barium, copper, iron, lead, silver, and sodium, though they did not check the results of their measurements against those of standard analytical methods. EXPERIMENTAL
Apparatus. T h e atomic absorption spectrophotometers were of the single-beam type described by Box and Walsh ( 4 ) , with 10-cm. stainless steel burners using premixed air-acetylene. The instrument a t the C.S.I.R.O. laboratory was a Techtron AA-3 spectrophotometer (Techtron Pty. Ltd., 271 Huntingdale Road, East Oakleigh, Victoria, Australia) employing a grating monochromator, while the Commonwealth Railways laboratory used a Techtron AA-2 instrument with a Zeiss
Table I.
M4QIII silica prism monochromator Argon-filled hollow-cathode lamps (Atomic Spectral Lamps Pty. Ltd., 25 Islington Street, Collingwood, Victoria, Australia) were used as light sources. Table I shows the instrumental conditions. Sensitivities of the two instruments were almost identical. Materials. Heptane (B. D. H. Laboratory Reagent, British D r u g Houses Ltd., Poole, Dorset, England), 2-methyl-4-pentanone (Hopkin and William Ltd., Chadwell Heath, Essex, England), and commercial kerosine did not need further purification before use. Calibrating solutions of oils containing known concentrations of metal were prepared a t the C.S.I.R.O. laboratory from stock solutions in 2-methyl-4pentanone of copper butyl phthalate, chromium acetylacetonate, and ferric acetylacetonate; these stock solutions contained 100 pg./ml. of the metal concerned. For lead, a stock solution containing 100 pg./ml. of the metal in 2-methyl-4-pentanone was prepared by dilution of a standard oil containing 1000 p.p.m. of lead as naphthenate (verified by atomic absorption analysis of the ash from both wet- and dry-ashed samples). For silver, silver nitrate was dissolved in a small quantity of 1 : 5 nitric acid, which was then diluted with about five times its volume of ethanol, and the resultant solution was diluted to volume with 2-methyl-4pentanone to give a stock solution containing 100 pg./ml. silver. The results obtained with this were checked using a solution of silver stearate whose silver concentration had been measured by ashing and atomic absorption measurement.
Instrumental Conditions for Determination of W e a r Metals in Oil Lamp Spectral Spectral slit width, A.
Metal
current, ma.
line, A.
cu
6-12 10 6-10 4-6 4
3247 3579 2483 2833 3281
Cr Fe
Pb Ag
Grating
Prism
VOL. 37, NO. 4, APRIL 1965
579
LU
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5
c:s
P
0
5
I0
/5
20
ME TAL CONCENTRATION ,uy/ml Figure 2. Typical calibration curves for metals in 2-methyl4-pentanone solution containing 20% w./v, of oil
-I0
I
I
I
10
15
20
I
5
.
PERCENT
w/, O F
OIL IN SOLUTION
Figure 1 Absorbance of metals in 2-methyl-4-pentanone solutions containing different concentrations of oil
At the Commonwealth Railways Laboratory calibrating solutions were prepared by dilution of Nuodex standards (Nuodex Products Co., Elizabeth, N. J.) containing copper, chromium, iron, or lead with the appropriate amount of 2-methyl-4-pentanone.
Atomic Absorption Analysis of Ashed Samples. After t h e oil was thoroughly shaken, 10 to 20 grams were weighed into a crucible (platinum, silica, or porcelain) and the oil was heated, ignited, a n d burned. T h e residue was heated in a muffle furnace a t 550-600° C. until oxidation of the carbon was complete and then dissolved in a few drops of concentrated hydrochloric acid or aqua regia and made up to a suitable volume (5 to 100 ml.). Atomic absorption measurements were made using metal standards made up in the appropriate dilute acid.
Colorimetric Analysis of Ashed Samples for Copper and Iron. After ashing 10 to 20 grams of oil in the above manner the ash was dissolved in a little concentrated hydrochloric acid and the solution was made u p to 100 ml. T h e iron was determined by colorimetry with o-phenanthroline, following ASTM method D-810, and the copper with sodium diethyldithiocarbamate. DEVELOPMENT OF METHOD
Choice of Flame. An air-acetylene flame was used throughout the work as this flame is normally required for the determination of chromium and 580
ANALYTICAL CHEMISTRY
iron (8). Moreover, a lower temperature flame, such as that of air-coal gas, produced carbonaceous deposits in the slit of the burner. Choice of Solvent. Heptane, kerosine, and 2-methyl-4-pentanone were investigated as possible solvents for lubricating oils, All three were found to be miscible in all proportions with the oils, but their behavior when sprayed into the flame differed considerably. Heptane was aspirated very rapidly by the atomizer and caused the flame to become highly luminous and unsteady. A satisfactory flame for absorption measurements could be obtained either by reducing the air pressure to a very low value, which had the effect of reducing the sensitivity, or by introducing an auxiliary supply of air directly into the spray chamber. Since the sensitivity with heptane solutions was somewhat less than with solutions in 2-methyl-4-pentanone, and since commercial instruments of the type used here do not normally have provision for the supply of auxiliary air, the use of heptane was not further investigated. Kerosine and 2-methyl-4-pentanone did not unduly disturb the flame when aspirated providing the leanest possible air-acetylene mixture was used. The sensitivity when using kerosine was somewhat less than with 2-methyl-4pentanone, so that attention was concentrated on the latter solvent. For metals such as copper and iron, where the highest sensitivity is not required,
kerosine as solvent would, however, avoid exposure of the operator to the penetrating and somewhat unpleasant smell of 2-methyl-4-pentanone. Sprague and Slavin ( I f ) have shown that p xylene is also suitable. Effect of Oil Concentration on Absorption. Solutions containing more than about 20y0 w./v. of oil were too viscous to be sprayed efficiently through the atomizer, and work was restricted to oil concentrations not greater than this. Figure 1 shows typical curves for the absorbances of metals in 2-methyl-4pentanone containing different quantities of oil. I n general, the absorbance reaches a peak a t a n oil concentration of about 5y0 w./v. and for copper, iron, and silver, a t least, this seems to be due to two opposing effects: slight enhancement of the metal absorption by small concentrations of oil, and depression of the absorption of the metal a t higher oil concentrations due presumably to the effect of increasing viscosity on the efficiency of the atomizer. Varying the viscosity of the oil does not seem to affect the absorbance values, so that while it is clearly necessary to add oil to the solutions used for calibration, it is not necessary to match the viscosity closely to that of the oil being analyzed. Figure 2 shows typical calibration curves for the five metals in 2-methyl4-pentanone solution containing 20y0 w./v. of oil. RECOMMENDED PROCEDURE
This is essentially the same for the five metals. After thorough mixing of the oil sample, 5 grams are weighed into a 25-ml. volumetric flask and diluted to volume with 2-methyl-4pentanone.
12
Stock solutions cmtaining 100 fig.,'nil.
lb
of the respective metals in 2-methyl-
, ,
4-pcntanone are made up from suitable standards as described in the preceding sections. From thew, by dilution wit'h 2-meth?.1-4-l,entanone and incorporation of 207, ~ v .v. , of an unused oil which has been checked for freedom from the metals of intrrest, t,he following five calibrating solutions are prepared:
. . . . . . . .
(1) Cu 0.0, Cr 0.0, Fe 0.0, P b 0.0,
x g 0.00 pg. '1111. (2) Cu 2.5, Cr 2.5, Fe 2 . j l P b 5.0, .ig 0.25, pg.,'ml. (3) Cu 5.0, Cr 5.0, Fe 5.0, P b 10.0, -ig 0.50 p g l m l . (4) Cu 7.5, Cr 7 . 5 ) Fe 7.5, P b 15.0, A l g0.75 pg.,'nil. (5) Cu 10.0, Cr 10.0, Fe 10.0, P b 20.0, Xg 1.00 pg./ml. Tl'hen the leanest possible air-acetylene flame and the experimental conditions of Table I are used, the transmissions of the solutions for each metal are measured in the order: standards, samples, standards, samples, standards. Care should be taken to shake the solutions immediately before measurement. The transmission values of each standard are averaged, converted to absorbance, and plot'ted against concentration; the met'al concentrations of the sample solutions are read from this graph. The range of calibrating solutions described above should cover the metal concentrations in most of the oil samples encountered in practice. Specimens containing abnormally high concentrations of metal may require further dilution and the use of calibrating solut,ions containing a corresponding concentration of unused oil. For the accurate measurement of the iron content of an oil containing a heavy sludge deposit, 10 grams of oil are ashed as described above; the ash is dissolved in a little hydrochloric acid or aqua regia and made up to 25 ml. The solution is measured relative to standards made up to contain 0, 5, 10, 15, and 20 pg./ml. of iron as ferric alum.
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RESULTS A N D DISCUSSION
Table I1 shows the metal contents of a number of used lubricating oils determined by atomic absorption measurement of solutions of the oils in 2methyl-4-pentanone and of solutions of their ash, and also by colorimetric measurement after ashing. When one considers the difficulties of sampling used oil specimens, which frequently contain heavy sludge deposits, the agreement between the different techniques is surprisingly good. With copper and iron there appear to be small systematic differences between the values obtained by measurement of solutions of an oil and solutions of its ash, and it has been noticed that oils containing much sludge tend to give a lower iron content when measured directly than when ashed. Since some of the wear metal may be present as VOL. 37, NO. 4, APRIL 1965
581
Table 111. Investigation of lnterelement Interference in Determination of Wear Metals in Oil
(All solutions were in 2-methyl-4-pentanone containing 20Y0 w./v. unused oil) Other metals AbMetal Concn., present, sorbpg./ml. ance detd. pg./ml. cu 6 0,594 cu 6 Cr 20, Fe'lO, P b 40, Ag 3 0.577 Cr 20 0.757 Cr 20 Cu 6, Fe'lO, Pb 40, Ag 3 0.742 Fe 10 0.658 Fe 10 Cu 6, Cr'20, Pb 40, Ag 3 0,658 Pb 40 0.668 Pb 40 Cu 6, Cr'20, Fe 10, Ag 3 0.658 0.553 3 3 Cu 6, Cr'20, Fe Ag 10, Pb 40 0.553 Table IV. Reproducibility of Direct Dilution Method on Replicate Samples Measured at Same TimeQ
Concn. Relative Metal in oil, std. dev., detd. p.p.m. % cu 14.1 0.8 Cr 21.2 1.0 Fe 14.3 1.2 Pb 13.2 3.3 Ag 1.3 2.1 a Measurements carried out at the C.S.I.R.O. laboratory.
Table V.
Reproducibility of Direct Dilution Method on Samples Taken and Measured over a Period of Four Weeksa
I
TI
Rel. Metal Concn., s.d., Concn., % p.p.m. detd. p.p.m. Cu 15.5 3 . 5 13.5 Cr 4.0 11.0 0.6 Fe 20.8 6.6 9.0 Pb 1.7 6 . 7 120 a Measurements carried out at
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fairly large particles and may be incompletely vaporized on passage through the flame such differences are not unexpected. The results of Table TI also show that the various additive compounds in lubricating oil do not affect the determination of wear meta!s by the atomic absorption technique, E d the experiments summarized in Table I11 indicate that the influence of one wear metal on the determination of another is also negligible. The reproducibility of the method is shown in Tables IV and V. Table TV shows the standard deviation of measurements made on replicate samples of an oil taken at the one time, while Table V shows the standard deviation of measurements on an oil sampled a t intervals over a period of four weeks. The reproducibility of the measurements in Table V is considerably worse than that of those in Table 1V. This is attributed chiefly to the difficulty of accurately sampling oils containing sludge deposits. The atomic absorption method has been in use since mid-1963 at the laboratory of the Commonwealth Railways and some 600 oil analyses are carried out per month using this technique. The results of recovery tests, in which known concentrations of standard metal compounds were added to the oil before the determination of wear metals, are summarized in Table VI.
ANALYTICAL CHEMISTRY
Oil I11
IV V Rel. Rel. Rel. Rel. s.d., Concn., s.d., Concn., s.d., Concn., s.d., % p.p.m. % p.p.m. % p.p.m. % 4.0 5 . 0 6.9 1 . 7 20 38.2 2.3 3 . 6 10.9 30.4 5.6 49 0 . 8 25 6.5 5.4 2.7 6.3 5 . 5 55 2 2 . 5 1 . 7 26 1 . 2 2 3 . 5 70.8 2.9 the Commonwealth Railways laboratory.
Table VI.
Metal detd. cu Cr Fe Pb Ag
Summary of Recovery Tests
No. of oils measured 9 8
6
8
9
Range
of Av. recovery, recovery,
70
Yo
95-103 97-100 100-105 95-103 91-109
99 98 103 100 102
ACKNOWLEDGMENT
The authors are indebted to J. A. Bate of the Victorian Railways for supplying specimens of used oil and for measuring their metal contents colorimetrically; to A. W. H. Pryce, British Petroleum Australia Ltd., for supplying lead naphthenate and silver stearate standards; and to E. S. Pilkington, C.S.I.R.O. Division of Mineral Chemistry, for much useful discussion. LITERATURE CITED
(1) A.S.T.M. Standards on Petroleum Products and Lubricants Method D 810
(1960). (2) Barney, J. E., Kimball, W. A,, ANAL. CHEM.24, 1548 (1952). (3) Barras, R. C., Jarrell-Ash Sewsletter No. 13, June 1962. (4) Box, G. F., Walsh, A., Spectrochim. Acta 16, 255 (1960). (5) Fry, D. L., A p p l . Spectry. 10, 65 (1956). --, (6) P m b r i l l , C. M., Gassmann, A. G., 0 Neill, W. R., ANAL. CHEM.23, 1365 (1951). ( 7 ) Gassmsnn, A. G., O'Xeill, W. R., ANAL.CHEM.21, 417 (1949). (8) Gatehouse, B. AI., Willis, J. B., Spectrochim. Acta. 17, 710 (1961). (9) McGowan, R. J., A p p l . Spectry. 15, 179 (1961). (10) Rozsa, J. T., Zeeb, L. E., Petrol. Proc. 8 , 1708 (1953). (11) Sprague, S., Slavin, W., Perkin\
-
Elmer Atomic Absorption Sewsletter,
No. 12, April 1963. (12) Walsh, A., Spectrochim. Acta 7, 108 (1955). RECEIVEDfor review October 22, 1964. Accepted December 28, 1964.
