Determination of molybdenum wear metal in lubricating oils by atomic

The United States Air Force Oil Analysis Program (OAP) makes use of atomic emission spectrophotometry (AES) and atomic absorption spectrophotometry ...
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979

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Determination of Molybdenum Wear Metal in Lubricating Oils by Atomic Absorption Spectrophotometry with a Particle Size Independent Method Costandy S. Saba University of Dayton, Research Institute, Dayton, Ohio 45469

Kent J. Eisentraut" Air Force Materials laboratory, Air Force Systems Command, United Stafes Air Force, Wright-Patterson Air Force Base, Ohio 45433

A particle size independent procedure for the quantitative determination of molybdenum wear metal in used lubricating oils using conventional flame atomic absorption spectrophotometry is described. The oil sample containing molybdenum was treated with a mixture containing hydrofluoric and nitric acids and shaken for 2 min before dilution with methyl isobutyl ketone. The effect of the type and amount of acid added, sequence of acid addition with respect to diluent, shaking time, and solubility of oxidized molybdenum species were studied. Samples containing molybdenum powder as large as 200 mesh were analyzed with an accuracy of 95.5 f 2.6%. A comparison is made between the results obtained from this procedure with those of other atomic absorption and emission techniques.

The spectrophotometric determination of trace metals in used lubricating oils provides a good indication of the degree of wear of the oil-wetted parts within an engine. An appreciable increase in the concentration of a particular wear metal being monitored will reveal abnormal wear of the component(s) constructed of that metal within the engine; this abnormal wear, if undetected, can result in engine failure. The United States Air Force Oil Analysis Program (OAP) makes use of atomic emission spectrophotometry (AES) and atomic absorption spectrophotometry (AAS) in the analysis of used aircraft lubricating oils for iron, silver, aluminum, chromium, copper, magnesium, nickel, lead, silicon, tin, titanium, and molybdenum. In excess of a million samples are analyzed each year. Of the approximately 120 Air Force OAP Laboratories located throughout the world, about one third are equipped with atomic absorption spectrophotometers. The procedure used for the determination of several wear metals involves diluting the sample with an organic solvent, methyl isobutyl ketone (MIBK) or xylene, and analyzing directly by atomic absorption. Presently, the OAP Laboratories cannot rely on AAS for the quantitative analysis of molybdenum because the direct analysis is wear particle size dependent. Because of the increasing use of molybdenum alloyed components in newer engines, such as the TF39, FlOO and TF34, the development of an accurate method for molybdenum analysis by AAS is a critical requirement of the United States Air Force t o ensure safety of flight. Examples of aircraft molybdenum alloyed components are shown in Table I. Molybdenum is one of several wear metals whose determination by AAS is particle size dependent. The smaller the particles, the more accurate is the analysis. Therefore, the development of a particle size independent method is necessary for complete analysis for molybdenum, especially for those samples containing large wear particles. 0003-2700/79/0351-1927$01 .OO/O

-

-

Table I. Examples of Aircraft Molybdenum Alloyed Engine Components engine TF34

A-10

aircraft

TF39

C-5

Pa* compressor forward shaft #7 main bearing rotating oil seal ~ 34,, 5, and 7 main bearing housings ~ 2 4,, 5, 6 and 7 main bearing rollers

and races f 2 and 5 main bearing fan shaft

FlOO

F-15A

rear compressor driveshaft assembly

T58

H-1

k 3 and 4 bearings front frame accessory driveshaft

579

F-4, A-5

turbine frame (-3 bearing housing) compressor rear frame (bearing housing) compressor front frame (.=I bearing housing) transfer gearbox rear gearbox

This work describes studies undertaken which have resulted in the development of a quantitative procedure for molybdenum in used lubricating oils by AAS. The procedure is simple, rapid, and requires minimum sample handling. This method is similar to that successfully developed for Ti in our laboratory and reported earlier ( I ) . Analytical results obtained on authentic engine oil samples using this AAS procedure are compared with the results obtained on the same samples using plasma and arc AES, direct AAS, and ashing techniques.

EXPERIMENTAL Apparatus. The analyses were performed on a Perkin-Elmer

Model 305B atomic absorption spectrophotometer. The output signal was fed into a Leeds and Northrup Speedomax W strip-chart recorder. Ten chart divisions are equivalent t o 1 absorbance unit; therefore, absorbance was obtained from chart divisions/ (10 X scale expansion). The relative absorbance was calculated from sample absorbance X 100/standard absorbance. Burrell Wrist-Action and Thermolyne Maxi Mix shakers were used. Samples were contained in 1-02 polyethylene bottles fitted with polyethylene screw cap lids. A plastic pipet was used t o dispense the required amount of acid solution. A Spectrametrics, Inc. Spectraspan I11 with Spectrajet I11 source (SMI-111)was used for the direct current argon plasma spectrophotometer data. A Fluid Analysis Spectrophotometer obtained from Baird-Atomic, Inc., Bedford, Mass., equipped with inductively-coupled plasma source (FAS-2PL) was used to obtain the ICP data. A BairdAtomic, Inc., atomic emission spectrophotometer (FAS-2) equipped with rotating disk arc source was also used. Reagents. The acid solution required was prepared by mixing by volume one part reagent grade concentrated (48% ) hydrofluoric acid and seven parts of reagent grade concentrated (70%) nitric @ 1979 American Chemical Society

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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979

Table 11. Instrument Parameters for Mo Analysis instrument burner control box wavelengths used slit grating molybdenum lamp nitrous oxide tank regulator burner control flowmeter setting fiow rate acetylene tank regulator burner control flowmeter setting flow rate beam height from center of burner

Perkin-Elmer 305B Perkin-Elmer Model 303-0678 313.3, 315.8, and 320.9 nm 0.7 nm

uv

30 mA, Perkin-Elmer Model 303-6045

40 psig 30 psig 5.7 (center of ball) 11.8 L/mina 1 2 psig 8 psig

4.2 (center of steel ball) 5.5 L/mina 10 mm

a Oxidant and fuel flow rates were obtained directly from the flowmeters of the burner control box and were converted into L/min using the manufacturer reported plot of flowmeter vs. L/min.

acid. Aqua regia was prepared by mixing reagent grade concentrated nitric and hydrochloric acids in the ratio of 18 to 82 by volume. A high purity 200-mesh and a submicron molybdenum powder obtained from Research Organic/Inorganic Chemical Corp. (ROC/RIC) and Atlantic Equipment Engineers (AEE), respectively, were used to prepare other standards in this work. Several stock solutions of dissolved molybdenum powder in acid/oil/MIBK (1:4:20) mixture were prepared. A typical stock solution was prepared by adding a small amount of HF/HN03 solution to 10 mg of molybdenum powder in a 4-oz polyethylene bottle. After complete solubilization,Mobil (Mil-L-7808)synthetic ester unused aircraft lubricating oil/MIBK (1:5) mixture was added to prepare one stock solution of a desired concentration in the range 0-2000 ppm. These dissolved standards were used to determine the molybdenum concentration in the simulated samples. Simulated wear samples were prepared by weighing several quantities of ROC/RIC or AEE Mo powder in 4-oz polyethylene bottles and adding the appropriate amount of unused oil. The Conostan molybdenum standard (2) is an alkyl aryl sulfonate of molybdenum obtained as a concentrate of 5000 ppm from the Conostan Division, Continental Oil Company, Ponca City, Okla. The D13 Standard is a mixture of Conostan Ag, Al, Cr, Cu, Fe, Mg, Mo, Na, Ni, Pb, Si (not a sulfonate), Sn, and Ti alkyl aryl sulfonates prepared in heavy weight hydrocarbon oil by the Technical Support Center, Joint Oil Analysis Program, Naval Air Station, Pensacola, Fla. These standards were used to determine the Mo concentration in the used oil samples. The authentic samples studied in this work were used aircraft and ground equipment lubricating oils. Procedure. Five grams of a used lubricating oil sample is mixed with 1.25 g of 1:7 by volume, HF/HN03 acid solution in a 2-02 polyethylene bottle. The mixture is shaken vigorously for 2 min on the Maxi Mix. The sample is then diluted 1:5 by weight with MIBK. The blank oil and the standards are prepared in the same manner, and the analysis is accomplished by AAS (Table 11) using a nitrous oxide/acetylene flame. The concentrations in ppm for the oil samples are obtained from the working curve. RESULTS AND DISCUSSION Effect of Acid on Molybdenum Powder i n Oil/MIBK Mixture. Aqua regia and HF/HN03 are known to react with molybdenum in ores and alloys ( 3 , 4 ) . When simulated wear molybdenum samples, diluted with MIBK, were treated with 2% by weight of either of the above acid mixtures, a complete digestion of metal did not occur. Eight 300-ppm samples were prepared as suspensions of AEE molybdenum powder in an oil/MIBK ( 1 2 by weight) mixture. One sample was treated with aqua regia and the remaining samples with 1:0.2, 1:3,

Table 111. Effect of HF/HNO, and HCl/HNO, Addition o n the AAS Analysis of ROC/RIC M o Powder in Oil before Dilution Mo amount powder of acid concn, per gram of oil,g ppm 600 0.15 600

600 600 2000 2000 2000 3000 4000

0.20

0.25 0.30 0.25 0.50 0.25 0.40 0.50

recovery, acid (ratio) HFIHNO, ( 1 : 7 ) HFIHNO, (1:7) HFIHNO, ( 1 : 7 ) HF/HNO, ( 1 : 7 ) HF/HNO, (1:3) HF/HNO, (1:3) HCl/HNO, ( 4 . 6 : l ) HCl/HNO, ( 4 . 6 : l ) HCl/HNO, ( 4 . 6 : l )

%a

93 98 99 97 98 100

4 11 41

2000 and 600 ppm dissolved Mo powder calibration standard was used.

1:4.6, 1:6, 1:7, 1:8,and 1:lO by volume HF/HN03. The AAS absorbances of these samples were compared to the absorbance of the 300-ppm standard solution. The analyses yielded a relative absorbance of 6 for the aqua regia sample and 27-66 for the H F / H N 0 3 treated samples, indicating that neither type of acid solution reacted completely with Mo. Even though the 1:7 ratio of H F / H N 0 3 solution gave the highest absorbance, larger amounts of H N 0 3 did not improve the recovery of Mo. Also, when the quantity of H F / H N 0 3 was increased to 18% by weight, the absorbance of Mo increased but digestion was still incomplete. No improvement was observed when a 245 base hydrocarbon oil was used or when Mobil ester oil was diluted with xylene. Effect of Acid on Molybdenum Powder i n Oil before Dilution. In a separate experiment, it was discovered that adding the acid directly to the oil sample prior to dilution with a solvent was much more effective than adding the acid to the already diluted sample. Aqua regia and H F / H N 0 3 were added to the oil sample in order to determine which acid mixture yields the highest absorbance signal. Table I11 shows HF/HN03 to be more effective than aqua regia. Quantitative recoveries of Mo were obtained with either 1:3 or 1:7 by volume HF/HNOB. The minimum quantity required for the complete recovery of Mo was 0.2 g acid per gram of oil. The relative absorbance obtained from using the 0.15 g H F / H N 0 3 (1:7) was 93 while the average relative absorbance of all other amounts from using the same acid was 98. I t was concluded that an amount of 0.2 g of acid per gram of oil was sufficient for complete reaction. An experiment was performed to ensure that the acid-treated oil sample forms a homogeneous mixture with MIBK before the analysis by AAS. An oil/MIBK (15) mixture was saturated with H F / H N 0 3 (1:3), After centrifuging and separating the phases, it was discovered that up to 1.1 g of acid per gram of oil dissolved, but the quantity required to dissolve Mo in the same mixture was only 0.2 g of acid per gram of oil. Hence, there is no possible separation of phases during the analysis when this method is applied. Effect of Time on the Recovery of Molybdenum after Acid Addition. Two solubilized Mo standards, seven simulated molybdenum samples and one used oil sample were selected to study the effect of shaking time. T o each gram of sample 0.25 g of HF/HN03 (1:7) was added. After shaking for the desired time, the sample was diluted with MIBK ( 2 g MIBK/g oil). The results in Table IV indicate that 30-60 s should yield complete digestion of Mo particulates in oil samples. An experiment was performed to determine the solubility and stability of the dissolved Mo powder in the oil-MIBK-acid solution. Several samples of dissolved Mo powder and Conostan Mo standard were left undisturbed for three days in the oil-MII3K-acid solution. At the end of three

ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979

sample is below 30 ppm and that the two curves are approximately the same below this value, and since Conostan standards were used to calibrate the other spectrophotometers used in this study, it was decided for the AAS-acid procedure that Conostan standards be used to determine the concentration of Mo in the used oil samples. The slope of the Conostan curve was 8% lower than that of the molybdenum powder. This small difference could not be explained on the basis of viscosity and surface tension ( 5 , 6 )since both standard solutions have the same matrix composition. However, the analyte behavior in the flame, due to differences in chemical composition, may account for this difference. Also, there should be no viscosity and surface tension effect between samples and standards since the oil matrix, dilution, and chemical form of the molybdenum powder standard are more similar to that of the sample. For the purpose of testing the reliability of the absorbances obtained for the molybdenum powder, two different gravimetric amounts of AEE molybdenum powder and two different amounts of the ROC/RIC powder were suspended in oil. The samples were treated with HF/HNOe, diluted with MIBK, and analyzed according to the previously stated procedure. Each sample was analyzed a t least six times and the concentration was obtained from the respective Mo powder standard working curve. The average percent recoveries and standard deviations were 98.0 f 3.8 and 101.3 f 3.870 for the two AEE powder samples compared with 96.0 4.2 and 94.6 f 3.0% for the ROC/RIC powders. Simple application of the theory of propagation of errors to these values yields an accuracy of 99.6 f 2.7 and 95.5 f 2.6% for the AEE and ROC/RIC powders, respectively, indicating that the submicron and the 200 mesh (74 pm) particle size molybdenum powders were quantitatively analyzed. Comparison of Results Obtained from This Procedure w i t h Those of O t h e r AAS a n d AES Procedures for t h e S a m e Samples. The results of the direct analysis for molybdenum in used lubricating oil by flame AAS, argon plasma and rotating disk electrode AES are recorded in Table VI. The results obtained by adding HF/HN03 and dry ashing are also listed. Hofstader et al. (7) demonstrated that dry ash-AAS is a reliable technique for determining molybdenum in crude oil samples. The dry ashing was performed prior to AAS analysis to establish the reliability of the proposed method. Our results indicate that using only flame AAY without dry ashing is ineffective and does not yield quantitative measurements for Mo. Similarly low results were cibtained using the IFAS-2. However, good agreement is observed between the recommended procedure and both the argon plasma and

Table IV. Effect of Shaking Time on the Concentration of Mo by the Particle Size Independent Method

shaking time

30 s 60 s 5 min 30 min l h 2h 24 h

600 ppm Mo powder sample no. recovered 579-17 concn, ppmQ ppmb 543 577 561

579 579 573 5 20

1.7 1.4 1.2 1.4 1.2 1.4 1.4

a Wavelength at 316 nm and 600 ppm dissolved Mo Wavelength at 313 nm and powder standard were used. 24 ppm dissolved Mo powder standard were used.

Table V. Relative Absorbance, the Ratio of the Absorbance of the Unshaken to the Shaken Sample Three Days after Acid Addition sample, PPm

10

30 50 75 100

Mo Conostan

Mo powder

0.95 1.00 0.96 0.97 0.98

0.96 0.96 0.98 0.98 0.99

*

days, the absorbances were measured before and after shaking for 10 min. The results in Table V indicate that no Mo species precipitated from solution; the analyses before and after shaking were identical. T h e Working Curve. Conostan molybdenum standard as well as Mo powder in oil were used as the calibration standards to construct two different working curves. The concentrations ranged from 0-150 ppm Mo in the original oil. The two curves were established by plotting absorbance vs. Mo concentration before dilution. The line of best fit was determined by the method of least squares. The slope, intercept, and correlation coefficient of the Conostan curve were 3.28 X 7.44 X and 0.9998, respectively, those of the molybdenum powder curve were 3.56 X 6.78 X and 0.9995. The standard working curve of Mo metal powder should give the most reliable concentration of an actual used oil sample in any range, because the powder is highly pure and it should approximate the molybdenum alloyed-wear debris. Considering that the expected concentration of a used oil

Table VI. Comparative Results for Molybdenum Analyses b y AAS and AES PPm sample

direct

M60-1 M60-2 M60-3 M60-4 M60-5 M60-6 579-17 T58 T56 585 AEE Mo

2.7 6.5 18.4 3.8 7.8

8.1

0.2 0.1

0.1 0.1

AAS HFIHNO, 7.9, 8.7b 29.7, 28.5 37.6, 38.0 32.2, 28.5 25.6, 24.0 37.1, 41.5 1.4, 1.5 3.4 1.5

ashing

SMI-I11

7.5 28.6

8.5, 9.7d 28.2, 28.7 35.0, 40.4 28.0, 25.8 21.5, 26.2 28.0, -2.3, 2.3 0.1 0.1 0.6 15, 104f

__

30.0 23.1

__ 1.2 __

AESC FAS-2FLd

FAS-2e

8.8

28.0 40.2 31.2 27.6

__

3.0 1.1. 0.8 2.6 14.6

22 29 22 18

26 0.1 -:0.1

12.5 __ 2.0 152.7 .. 61 Samples were diluted 1 : 5 with MIBK after acid addition. D13 Conostan Samples were analyzed o n e d a y later. standard was used. Samples were diluted 1:4 with kerosene. e M60 samples were analyzed at Fort Hood SOAP Laboratory, Killeen, Texas. Turbine engine samples were analyzed at TSC, Pensacola, F1. f Instrument was optimized for Mo powder maximum emission. 1

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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979

ashing methods for the M60 samples (Table VI). The particle size distribution of metallic particulates in oil plays a very important role in the effective analyses of used oil samples. As particle size increases, the absorbance signal decreases (8-11). Using the spectrophotometers equipped with nebulizer-spray chamber sample introduction systems, poor efficiency of nebulization was obtained for samples which contained considerable amounts of "large" molybdenum alloyed particulates, and hence only those particles "small" enough will reach the source and be analyzed effectively (12). The remainder will collect on the walls of the spray chamber and be washed into the drain solution. The recoveries of molybdenum in a simulated sample with 1-Krn maximum particle size were 100, 60, 80% obtained using SMI-111, FAS-BPL, and FAS-2, respectively (11). Direct analysis by flame AAS was ineffective in analyzing any significant amount of the same sample. The higher temperature of the argon plasma (13)is expected to determine particles in this size range more effectively than the flame atomic absorption technique. Direct analysis of the 579-17, T58, T56, and 585 aircraft turbine engine oil samples was not quantitative by any of the instruments employed because of the presence of "large" molybdenum alloyed particles. Upon adding the required amount of H F / H N 0 3 to these samples, higher amounts of molybdenum were recovered. The capability of this procedure was also demonstrated when a simulated Mo sample was analyzed by all the instruments considered. Examination by an optical microscope revealed that this sample contained particles up to 40 Km in size. The results of the analyses (Table VI) using the direct flame atomic absorption, plasma emission, and rotating disk arc emission spectrophotometers were approximately 1, 15, and 40% of the acid method, respectively. This particular experiment does not by any means show that the plasma instruments are incapable of analyzing particles of large diameters, because previous results (12) point out that the state of the art sample introduction systems in these instruments restrict the path of the large particles and hence prevent them from reaching the source. Until better sample introduction is found and the plasma capability is clearly defined, the simple procedure described in this paper for flame atomic absorption spectrophotometry provides a rapid, quantitative, and easy method of analyzing for molybdenum wear metal in oil. It is a particle size independent procedure which utilizes the relatively inexpensive and generally available technique of conventional flame AAS.

CONCLUSIONS Direct analyses for determining molybdenum wear metal in used oil samples is ineffective using atomic absorption and atomic emission. The addition of HF/HNO, to the oil sample prior to dilution minimizes the effect of particles on the analyses and hence permits quantitation by AAS. Oil samples containing 200-mesh molybdenum powder were quantitatively analyzed using this acid/AAS method. This procedure is of wide use and is also applicable to molybdenum determinations by other techniques, such as plasma emission and rotating disk arc emission spectrophotometry, etc.

ACKNOWLEDGMENT The authors thank the personnel of the Joint Oil Analysis Program, Technical Support Center, Naval Air Station, Pensacola, Fla., for their cooperation in obtaining the used oil samples reported in this work.

LITERATURE CITED C. S. Saba and K. J. Eisentraut, Anal. Chem.. 49, 454 (1977). T. P. Matson, At. Absorp. News/., 9(6),132 (1970). A. I. Busev, "Analytical Chemistry of Molybdenum", Daniel Davey and Co., New York, 1964,p 72. F. A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry, a Comprehensive Text", 3rd ed, Interscience Publishers, New York, 1972, p 946. Roland Herrmann in "Flame Emission and Atomic Absorption Spectrometry", Voi. 11, J. A. Dean and T. C. Rains, Eds., Marcel Dekker, New York, 1971,pp 59-60. 8. V. L'vov, "Atomic Absorption Spectrochemical Analysis", American Elsevier Publishing Company, New York, 1970,p 175. Hofstader et ai., "Analysis of Petroleum for Trace Metals", Adv. Chem. Ser., 156, American Chemical Society, Washington, D.C., 1976,pp

149- 159. J. H.Taylor, T. T. Bartels, and N. L. Crump, Anal. C b m . , 43,1780 (1971). C. S.Saba, P. S.Fair, J. R. Brown, W. E. Rhine, and K. J. Eisentraut, Abstracts, 30th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1979,No. 62. P. S. Fair, J. R. Brown, W. E. Rhine, and K. J. Eisentraut, Ref. 9,No.

357. W. E. Rhine, P. S.Fair, C. S.Saba, J. R. Brown, and K. J. Eisentraut, Ref. 9,No. 439. K. J. Eisentraut, T. J. Thornton, W. E. Rhine, C. S. Saba, J. R. Brown, and P. S. Fair, Symposium on Oil Analysis, Materialpruefstelle der Bundeswehr und Bundesakademie fur Wehrverwattung und Wehrtechnik Erding, West Germany, 4-6 July 1978,pp 95a.09.02-95a.09.BI0. V . A. Fassel, Science, 202, 183 (1978).

RECEIVED for review May 3, 1979. Accepted July 12, 1979. Presented in part at the 176th National Meeting, American Chemical Society, Division of Analytical Chemistry, Miami Beach, Fla., September 13, 1978.