Determination of Titanium in Aircraft Lubricating Oils by Atomic Absorption Spectrophotometry Costandy S. Saba University of Dayton, Research Institute, Dayton, Ohio 45469
Kent J. Eisentraut" Air Force Materials Laboratory, Air Force Systems Command, United States Air Force, Wright-Patterson Air Force Base, Ohio 45433
A procedure for the quantitative determination of titanium wear metal in aircraft lubricating oils using conventional flame atomic absorptlon spectrophotometry Is described. The oil sample containing titanium was diluted wlth 4-methyl-2-pentanone and shaken for 10 s with a small amount of a mixture containing hydrochloric and hydrofluoric acids. The effects of the ratio and amount of acid added, fueVoxidant flow rates, shaking time, dilution, and solvent were also studled. A detection limit of 0.03 ppm titanlum was determlned. Relative standard deviations of 3.5% and 0.8% were obtained for concentrations of 3 and 50 ppm titanium, respectively. The total analysis can be accompllshed in 1-2 mln. A comparison Is made between a standard composed of an oll-soluble organotitaniumcompound wlth that prepared from tltanium metal powder.
T h e spectrophotometric determination of trace metals in used lubricating oils provides a good indication of t h e 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 constructed of that metal within the engine and, if undetected, may result in engine failure. T h e United States Air Force Spectrometric Oil Analysis Program (SOAP) 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. Of the approximately 120 U.S. Air Force SOAP Laboratories located throughout the world, about half are equipped with atomic absorption spectrophotometers. T h e procedure being used for the quantitative determination of several wear metals by AAS is similar to the recommended procedure by Perkin-Elmer Corporation ( I ) . The sample is simply diluted with an organic solvent, methyl isobutyl ketone (MIBK) or xylene, and analyzed directly by AAS. Prior to this work, the SOAP laboratories could not rely on AAS for the analysis of titanium because of the lack of a reliable, inexpensive, and rapid method. In t h e past, several turbine engine failures have resulted from SOAP undetected titanium component failure. Consequently, the development of a n accurate method for titanium analysis by AAS was a critical requirement of the United States Air Force to ensure safety of flight. This work describes t h e studies undertaken which have resulted in t h e development of a quantitative procedure for titanium in used turbine engine oils by AAS. T h e procedure is simple, rapid, and requires minimum sample handling. T h e method described herein is presently being adopted by the U.S. Air Force Spectrometric Oil Analysis Program for t h e routine analysis of titanium wear metal by AAS. Analytical results obtained on authentic turbine engine oil samples using 454
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
this AAS procedure are compared with results obtained on the same samples using various plasma techniques.
EXPERIMENTAL Apparatus. A Perkin-Elmer 305B atomic absorption spectrophotometer equipped with a corrosion resistant nebulizer was used. The output signal was fed into a Leeds and Northrup Speedomax W strip-chart recorder. In all cases a scale expansion of 30X was used for concentrations less than 10 ppm and 3X was used for 10-100 ppm with an integration time of 10 s. Ten chart divisions are equivalent t o 1 absorbance unit; therefore, absorbance was obtained from chart divisions/(lOX scale expansion). The fuel/oxidant flowmeter setting was 5.0/6.0 which is equivalent to 6.5 1. per min/12.5 1. per min. The flowmeter settings were obtained directly from the flowmeters of the burner control box; Perkin-Elmer model 303-0678. However, they were converted into flow rates in l./min using the manufacturer reported plot of flowmeter reading vs. l./min. A Burrell Wrist-Action Shaker was used for the time studies. Samples were contained in 1ounce polyethylene bottles fitted with polyethylene screw cap lids. A plastic pipet and Lab Industries Repipet Jr. were used to dispense the required amounts of acid solution. Reagents. The acid solution required was prepared by mixing by volume one part reagent grade concentrated (48%) hydrofluoric acid and three parts reagent grade concentrated (37%)hydrochloric acid. Mobil (Mil. Spec. 7808) unused aircraft lubricating oil was used as the blank and also to dilute the Conostan 5000-ppm concentrate in order to prepare a series of standards covering the range 1-100 ppm, A 325-meshtitanium powder obtained from Metal Hydrides Co. (now Ventron Corp.) was used to prepare other standards used in this work. A quantity of 53.14 mg titanium powder, suspended in blank oil, was used to prepare 265.7 g of a 200-ppm stock solution which was contained in a polyethylene bottle. One ml of the acid solution was added to the stock solution and the mixture was shaken vigorously for 5 min to quantitatively dissolve the titanium powder. Aliquots were taken to prepare a series of standards in the range 1-100 ppm. The samples studied in this work were used aircraft lubricating oils, obtained from Dover, McConnell, and Kelly Air Force Bases, respectively, which were also analyzed by AES in the SOAP laboratories and found to contain titanium. The Conostan titanium standard (2) is an alkyl aryl sulfonate of titanium obtained as a concentrate of 5000 ppm from the Conostan Division, Continental Oil Company, Ponca City, Okla. The Special D12 standard is a mixture of Conostan Ag, Al, Cr, Cu, Fe, Mg, Mo, Ni, Pb, Si (not a sulfonate), Sn, and Ti alkyl aryl sulfonates Table I. Instrumental Parameters Instrument Burner control box Wavelength Slit Grating Titanium lamp Nitrous oxide Tank regulator Burner control Flowmeter setting Acetylene Tank regulator Burner control Flowmeter setting
Perkin-Elmer 305B Perkin-Elmer Model 303-0678 365.6 nm 0.2 mm
uv
40 mA 40 psig 30 psig
6.0 (center of ball) 12 psig 8 psig
5.0 (center of steel ball)
Table 11. Effect of C2H2 Flowmeter Setting on the Absorbance of Sample and Standard at NzO Flowmeter Setting 6.0 (12.5 l./min) Dover No. 5 Without acid, With acid, A X lo3 A x 103
CzHz Flowmeter Setting
Flow rate, l./min
4.5 4.7 5.0 5.2 5.5
5.9 6.2 6.5 6.9 7.2
4.8 5.6 4.2
0:o 0:4 1:3 1:l 3:1 4:O 1:3 1:l 3:1
Amount of HF/HCl Dover No. 5, mixture, ml A x 103 0.0 0.15 0.15 0.15
0.15 0.15 0.30 0.30 0.30
5 PPm Conostan, A x 103
125.7 178.4 201.8 184.2 168.1
4.1 7.0 8.0 7.7 5.1
3.8 6.7 8.0 7.3 4.4
Table 111. Effect of HF/HCl Added to the Oil Sample on the Absorbance; 15 Seconds Shaking Time HF/HCl
100 ppm Ti powder, A x 103
95t
Kelly No. 1, A X lo3
5.7 5.8 10.5 10.7 10.5 7.2 10.7 10.7
10.5
22.5 23.0 30.6 31.3 31.6 27.5 32.0 31.6 31.4
.
90-
" -
75..
; Q, p 6090-
75-,-
4
Effect of Acetylene/Nitrous Oxide Flow R a t e s on Absorbance. Table I1 shows the effect of N20 and CzH2 flow rates on titanium absorbance. From these results it is evident that the fuel/oxidant flowmeter settings were optimum at 5.0/6.0 (6.5 1. per mid12.5 1. per min) for the sample treated with acid, the Ti powder standard, and the Conostan T i standard. However, 4.7/6.0 (6.2 1. per min/12.5 1. per min) were the optimum settings for the sample without the addition of acid. Titanium in used turbine engine oil is considered to be present as wear metal particles which are oxidized in the presence of hydrofluoric and hydrochloric acids. Therefore, the optimum flame condition is not the same for the same sample with and without the acid because of the two different chemical forms of titanium. I t should be noted that the same amount of acid stock solution was also added to the blank and there was no apparent difference in the signal, Le., the same baseline was obtained as that of the blank oil without the acid under the same settings of the instrument. Reigle and coworkers ( 3 )indicated that free-atom fractions of Ti, Al, Mo, and W vary with fuel/oxidant ratio in atomic absorption flame analysis. Effect of Hydrofluoric a n d Hydrochloric Acid Addition on the Sample. The results shown in Table I11 indicate that 1:3 HF/HCI was an effective ratio for maximum titanium absorbance. There was no appreciable difference in absorbance when different ratios of HF/HCl were added or when the quantity was doubled. It took the action of both H F and HCl to completely digest the titanium particles suspended in the used lubricating oi1-(4). Effect of Time on the Absorbance a f t e r Acid Addition. This experiment was performed to determine the solubility of titanium species in the oil-MIBK-acid solution. Two ali-
0
9075-
R E S U L T S A N D DISCUSSION
0
0
'
60-
prepared in lightweight hydrocarbon oil by the Naval Air Rework Facility, Naval Air Station, Pensacola, Fla. Procedure. Five grams of a used turbine engine oil sample were diluted with 10 g MIBK and 0.15 ml of 1:3 by volume, HF/HCI acid solution in a 1-ounce polyethylene bottle. The mixture was shaken vigorously by hand for 10 s. The blank oil and the standards were prepared in the same manner, and the analysis was accomplished by AAS (Table I) using a nitrous oxide burner head. The concentrations in ppm for the oil samples were obtained from the working curve.
0
60-
A
n
45-
I
^
n
A
C
o
o
5
10
3 0 - 8 ~
5I
IO 20 30 40 50 60
Figure 1. Effect of
shaking time on absorbance using Dover No. 5 oil
sample Five grams of oil mixed with 10 g MIBK and the following amounts of acid: (0) 0.05rnl; (A)0.10 ml; (0) 0.15 ml; ( 0 )0.20 ml; (A)0.25 ml;).( 0.30rnl 1 1 -
I
P-
z
-
13-
i P
1519-
1
I
I
2
3
4
5
6
ASPIRATION FLOW RATE ( m l l m i n l
Figure 2. Dilution effect on aspiration flow rate using Kelly No. 1 oil
sample quots of the same sample were shaken after being treated with the acid solution. One was continuously shaken and the other remained undisturbed for a specified period of time. At the end of this period, the analysis was accomplished by AAS and the absorbances were compared. The relative absorbance was calculated as the ratio of the absorbance of the unshaken sample to that of the shaken sample. A relative absorbance of less than one would indicate a decrease in the solubility of the titanium species in the mixture. Table IV shows the relative absorbances for three samples: namely, Dover No. 5, McConnell No. 1,and Kelly No. 1. The aliquots were 15 g used oil sample diluted with 30 g MIBK and treated with the amount of acid indicated. The mean and standard deviation of the relative absorbances were calculated ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
455
Table IV. Relative Absorbance, the Ratio of the Absorbances of the Unshaken,to the Shaken Sample Acid added, ml
Sample= Dover No. 5
0.30 0.375 0.45 0.30 0.375 0.45 0.30 0.375 0.45
McConnell No. 1 Kelly No. 1
a
10min
20min
1h
2h
1.00
1.00
0.98
0.96
0.98 1.02
1.00 0.99 1.02 0.96 0.78 0.98
0.97 1.00 0.96 0.85 0.99 0.98 0.98
1.00 1.00 1.01 0.98 1.03
1.03 1.00
1.00 0.98 0.98 0.98 0.99
1.00
1.00 0.98
18h
2days
3days
7days
1.02
1.00
1.00
0.97 0.90 1.01 0.99 0.89
0.98 1.00 0.98 0.98 0.99 1.00 0.99 0.99 1.00
1.00 0.99
1.00 1.00 1.00
1.00 1.00 1.00
1.00 1.00
Mean f std dev 1.00 f 0.01 0.99 f 0.02 0.98 f 0.04 1.00 f 0.01 0.98 f 0.02 0.94 f 0.08
1.00 0.97 1.00 1.00 0.99 0.98
0.98 1.00 0.99 0.99
1.00
1.00 f 0.02 1.00 i 0.02 0.99 f 0.01
Each sample contained 15 g used oil, 30 g MIBK, and specified amount of acid.
Table V. Effect of Solvent on the Absorbance of Ti for Dover No. 5 Sample Solvent MIBK Ethanol Acetone p-Xylene Isopropyl alcohol Acetylacetone
/
L
/
9.6
1.9 1.9 7.7 3.4 7.2
> 1:5 > 1:3 > 1:2 > 1:l
Figure 3 shows that the 1:l dilution with MIBK for sample Kelly No. 1 gave the highest signal; however, it deviates from linearity. A similar experiment was performed using the Dover No. 5 oil sample and the 1:ldilution gave the same absorbance as the 1:2 dilution. For this reason a dilution of 1:2 was chosen because the signal is greater than using the 1:3, 15,and 1:9 dilutions and no deviation from linearity occurs. Effect of Solvent o n the Absorbance. Some ketones, alcohols, and aromatic compounds were used as solvents to find the best medium for maximum absorbance. The sample and the blank oil were diluted 1:2 by volume with the solvent and 456
0 040
A x 103
a t the end of seven days. There was no significant decrease in the relative absorbances for the three samples considered. Effect of Shaking T i m e o n t h e Acid T r e a t e d Sample. A stock solution of HF/HCl, 1:3 by volume was prepared and different quantities were added to the same sample. Six 5-g aliquots of Dover No. 5 , diluted 1:2 by weight with MIBK, were shaken for different times and the absorbance of each vs. time was plotted (Figure 1).The samples containing 0.10, 0.15,0.20,0.25, and 0.30 ml of acid mixture reached constant absorbance within five seconds of shaking. However, the sample containing 0.05 ml acid stock solution did not attain constant absorbance for ten minutes. In another experiment, it was determined that a sample containing 0.15 ml of only hydrofluoric acid attained maximum absorbance in 20 minutes. Even though 0.1 ml of acid solution was sufficient to reach maximum absorbance in five seconds, 0.15 ml of acid and 10 seconds of shaking time were subsequently chosen as optimum conditions. Similar results were obtained when 325-mesh titanium powder was suspended in oil and treated in the same manner. Effect of Dilution on Aspiration Rate a n d Absorbance. The time in seconds to aspirate a 1-ml diluted sample was measured. From these data, the aspiration flow rate in ml/min was calculated. The aspiration rates of sample and standard were found to be practically the same for all dilutions with MIBK studied. Figure 2 indicates that the rate of aspiration decreased in the order 1:9
I
/ /
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
t
0.005
1
DILUTION
Figure 3. Dilution effect on absorbance using Kelly No. 1 oil sample
the results are recorded in Table V. It is shown that T i absorbance decreased in the order of the following solvents; MIBK
> p-xylene > acetylacetone > isopropyl alcohol > acetone ethanol
-
As a result of this experiment, MIBK was chosen as the diluent for this work. Relative S t a n d a r d Deviation. An experiment was conducted on a Dover sample diluted 1:2 with MIBK to check the reproducibility of the procedure and the instrument. The results show that the relative standard deviation for the reproducibility of the procedure was 3% for 5 trials and that of the instrument 2% for 10 trials. The Air Force Standard, Special D12-3 (3 ppm) and D12-50 (50 ppm) gave relative standard deviations of 3.5% and OB%, respectively, for 5 different samples of each. Peterson (5) obtained similar results for the 2 ppm Air Force Standard diluted 1:2 with MIBK. The experimentally measured detection limit for T i was 0.03 ppm at 30X scale expansion. The procedure used for measuring the detection limit was the direct measurement procedure reported by Slavin et al. (6). T h e Working Curve. Conostan titanium standard as well as Ti powder in oil were used as the calibration standards to construct two different working curves. The concentration ranged from 0 to 100 ppm Ti in the original oil. The two curves were established by plotting absorbance vs. T i concentration before dilution. The correlation coefficient, the slope, and intercept were obtained by the method of least squares. The slope and intercept of the Conostan curve were 2.13 X and 6.74 x respectively. Those of the titanium powder
Table VI. Comparative Results for Titanium Analysis by AAS and AES Sample Dover No. 1 Dover No. 2 Dover No. 3 Dover No. 4 Dover No. 5 Dover No. 6 McConnell No. 1 Kelly No. 1
Solvent extractiona 1.6b 1.7 3.0 1.5
5.9 4.4 5.0 -
Direct analysis' 1.0
0.9 1.2
0.9 2.9 2.2 1.9 11.7
This methodd 1.4 1.6 2.8
This methode 1.6 1.8 3.2
1.1
1.4
4.6 4.4 4.2 19.5
5.5 5.1
4.9 20.2
APESf
ICP-AESg
1.2 1.9 2.9 2.0 5.9 4.7 4.2 20.9
1.5 2.7 1.4 5.7 4.9 4.3 17.0
1.5
a Average of 4 aliquots (from reference 7). b Average of 2 aliquots. The sample was diluted 1:2 w/w with MIBK. Conostan was used as the standard. d The sample was diluted 1:2 w/w with MIBK and treated with 0.15 ml of acid solution. Conostan was used as the standard. e Same as ( d ) except Ti powder in oil was used as the standard. f These analyses were performed by E. P. Williams, using Spectrametrics Spectra-Span I11 Argon Plasma Emission Spectrophotometer,Spectrametrics, Inc. Andover, Mass. g These analyses were performed by C. A.,Peterson using Inductively-Coupled Plasma Atomic Emission Spectroscopy, Ames Laboratory-ERDA and Department of Chemistry, Iowa State University, Ames, Iowa.
curve were 2.22 X and -1.96 X The correlation coefficient was 0.9995 for both curves. In order to test the reliability of the absorbances obtained from the titanium powder standard, seven different gravimetric amounts of 325mesh Ti powder were each placed in a 500-ml polyethylene bottle and diluted with the blank oil to produce a certain concentration in the range 10 to 100 ppm. The slope of this working curve was 2.16 X loF3 compared with 2.08 X which is the slope of the Conostan standard in the same concentration range. At the end of a one-year period an experiment was performed to determine the extent of air oxidation on the surface of the Ti powder used in this study. Mesh sizes of 100 (99.9%) and 325 (99%]Ti powders obtained from Ventron Corp. and Research Organic/Inorganic Chemical Corp., respectively, were dissolved in oil and compared with the 325-mesh T i powder obtained from Metal Hydrides Inc. The absorbances obtained from the three powders when plotted vs. T i concentration gave a linear curve with a slope of 1.93 X This slope compared favorably with that obtained from Conostan Ti standard and that of the Ti powder working curve. Therefore, the extent of air oxidation on the surface of Ti powder was not appreciable within a one-year period. Oil calibration standards of T i metal powder should give the most reliable concentration of an actual used oil sample since the oil matrix, dilution, and most importantly the chemical form of T i are more similar to that of the sample. Comparison of Results Obtained f r o m T h i s Method w i t h O t h e r AAS a n d Plasma Methods f o r the S a m e Samples. The results recorded in Table VI indicate that the values obtained for samples analyzed by AAS for T i gave results comparable to that of the solvent extraction procedure developed in this laboratory (7). The solvent extraction technique is used to extract titanium from the organic phase into the aqueous phase. The organic phase is a used lubricating oil diluted with MIBK, and the aqueous phase is a mixture of hydrochloric and hydrofluoric acids. The direct analysis of titanium in used lubricating oil did not yield quantitative results. This could be due to the fact that titanium particles do exist in different sizes and only those "small" enough atomize by the acetylene-nitrous oxide flame; and as the size of the particles increases, the absorption signal decreases (8). There was no absorption signal observed when 325-mesh (44 pm in size) titanium powder was suspended in unused oil. The addition of the acid stock solution to the oil sample digests the metal. No effect on the absorption of Ti resulted from varying the concentration of H F or HC1 (Table 111). A comparison of the results of this work with two independent methods, namely Inductively-Coupled Plasma
Atomic Emission Spectrophotometry (ICP-AES) (9) and Argon Plasma Emission Spectrophotometry (APES) is also shown in Table VI. Both methods utilize the Air Force Special D12 standards described previously. The data reported in Table VI indicate good agreement of the analytical results obtained for titanium in the same samples between this novel flame atomic absorption procedure and that based on solvent extraction. Furthermore, good agreement is also demonstrated with results obtained using independent techniques, namely plasma emission spectrophotometry, wherein the higher temperature of the plasma source would be expected to reduce particle size effects which are shown to seriously influence titanium wear metal analysis using direct flame atomic absorption procedures. The simple flame atomic absorption procedure described in this paper is both rapid and quantitative and offers an easy way to analyze for titanium wear metal in oil. It is a particle size independent procedure which utilizes the relatively inexpensive and generally available technique of conventional flame atomic absorption spectrophotometry. ACKNOWLEDGMENT The authors thank Gerald Jenkins of the 301FMS SOAP Lab, Rickenbacker AFB, Ohio, for his assistance with atomic emission analyses, and the personnel of the SOAP Management Office, Kelly AFB, Texas, for their cooperation in providing the used oil samples. The authors are grateful to Ernest P. Williams, Spectrametrics, Inc., Houston, Texas, and Charlie A. Peterson, Ames Laboratory-ERDA and Department of Chemistry, Iowa State University, Ames, Iowa, for their plasma emission analyses. LITERATURE CITED (1) "Analytical Methods for Atomic Absorption Spectrophotometry", PC-1 Perkin-Elmer Corp., Norwalk, Conn., March 1973. (2) T. P. Matson, At. Absorp. News/., 9 (6), 132 (1970). (3) L. L. Reigle et al., J. Chem. Eng. Data, 18 ( l ) ,79 (1973). (4) M. Codell, "Analytical Chemistry of Titanium Metals and Compounds", interscience Publishers, inc., New York, N.Y., 1959, p 14. (5) G. E. Peterson, "Atomic Absorption Application Study No. 542", The Perkin-Elmer Corporation Instrument Division, Norwalk, Conn., July 1973. (6) S. Slavin, W. B. Barnett, and H. L. Kahn, At. Absorp. News/., 11, (2), 37 (1972). (7) R. C. Burton, K. J. Eisentraut, and R. E. Sievers, in preparation. (8) J. H. Taylor, T. T. Bartels, and N. L. Crump, Anal. Chem., 43, 1780 11971). V:A. iassel, C. A. Peterson, F. N. Abercrombie, and R. N. Kniseley, Anal. Chem., 48,516 (1976)
RECEIVEDfor review July 28, 1976. Accepted December 8, 1976. Presented in part at the 172nd National Meeting, American Chemical Society, Division of Analytical Chemistry, San Francisco, Calif., September 3, 1976. ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
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