Anal. Chem. 1995, 67, 2781 -2786
Sonochemical Stripping Voltammetry Nanette A. Madigan, Tammy J. Murphy,t John M. Fortune,* Carolynne R. S. Hagan, and Louis A. Couty, Jr.*
Department of Chemistty, Box 90346, Duke University, Durham, North Carolina 27708-0346
A new form of stripping analysis is described which exploits the extreme conditions produced during sonication. Sonochemical stripping voltammetry differs from other forms of stripping analysis in that the preconcentration of the analyte is sonochemical rather than electrochemical. Zero-valentmetal particles are melted onto the electrode surface after being accelerated by microjets formed due to acoustic cavitation. The electrode is then transferred to an electrolyte solution for voltammetric analysis via linear sweep or square wave voltammetry. This allows the deposition to occur in nonelectrolytic or complex matrices. Sonochemical deposition of copper onto gold electrodes is demonstrated both electrochemically and microscopically by scanning electron microscopy. Using square wave voltammetry, a linear relationship is demonstrated between extent of copper deposition from a Cu/DMSO slurry and sonication time up to 15 min. There is also a linear dependence of response on the amount of copper in the slurry. Selectivity based on the melting point of the metal in the slurry is shown, both electrochemicallyand with scanning electron microscopy, through sonication of slurries containing copper and tungsten powders. Finally, deposition of copper from lubricatingoils mixed with organic diluents illustrates the utility of the technique for samples of practical importance. It also demonstrates that information concerning the viscosity behavior of non-Newtonian fluids under conditions of extreme temperature and shear rate is obtainable, despite dilution with an organic solvent. Sonochemistry has attracted considerable attention in recent years due to the extreme conditions which occur during sonication These phenomena arise from a process known as acoustic cavitation, defined as the nucleated formation, growth, and rapid collapse of vapor-filled bubbles in a condensed medium.' During bubble collapse, transient, localized conditions characterized by temperatures4m5of 5000 K and pressures' of several hundred atmospheres have been demonstrated using commercially available, direct-immersion sonicator probes. Sonication of a liquid in contact with a solid surface causes the formation of interfacial microjets2 with velocities greater than 100 m/s which ' Current address: Methodist College, Box 12237, Fayetteville, NC 28311. Current address: Department of Biochemistry, Vanderbilt University, Nashville, TN 37232-0146. (1) Ultrasound:Its Chemical, Physical, and Biological &%cts;Suslick, K. S., Ed.; VCH Publishers: New York, 1988. (2) Suslick, K. S.Science 1990,247,1439-1445. (3) Doktycz, S.J.; Suslick, K. S. Science 1990,247,1067-1069. (4) Flint, E. B.; Suslick, K. S.Science 1991,251,1397-1399. (5) Suslick, K. S.; Flint, E. B.; Grinstaff, M. W.; Kemper, K. A. J. Phys. Chem. 1993,97, 3098-3099. 0003-2700/95/0367-2781$9.0010 0 1995 American Chemical Society
are directed toward the solid surface. Furthermore, sonication of slumes of metal powders results in particle aggregation,6 provided the melting point of the metal is less than about 2500 K. Such agglomeration is a result of heat generated from interparticle collisions during sonication. Because many aspects of sonochemistry are enhanced at surfaces, an interfacial technique such as electrochemistry is particularly well-suited for sonochemical studies. Conversely, the effects of sonication on electrochemical processes are of widespread interest. The use of high-intensity ultrasound has been explored for (i) the activation of electrode^,^-^ (ii) the ablation of insulating polymers from electrode surfaces to form an array of "microhole" electrodes,10 and (iii) the enhancement of mass transport in electrochemical experiment^.^'-'^ In this paper we describe a new type of electrochemical stripping analysis, based on sonochemical deposition and subsequent voltammetric stripping of zero-valent metals at a gold electrode. Stripping techniques, in general, have multielement capabilities, high sensitivities, and low limits of detection due to the chemical preconcentration step inherent in such methods.I5 The work we describe here differs fundamentally from most previous techniques in that the analyte is an insoluble, uncharged metal which is melted onto an electrode during sonication of a heterogeneous suspension (which need not be ionically conductive). The stripping step may employ any convenient form of voltammetry and is accomplished after transfer of a coated electrode to a clean electrolyte solution. While abrasive stripping ~oltammetry'~~'~ also involves the determination of solids deposited onto the electrode surface, the major drawback of that technique is the difficulty in controlling the amount of material transferred.16 For example, with the adoption of standard abrasion procedures, peak currents for square wave voltammetry of lead were reported to be 0.5 f 0.1 mA.I7 Sonochemical stripping voltammetry allows much easier control over the amount of metal deposited and thus greater precision and reproducibility (RSD = 6% for 10 min of sonication vs 20%for abrasive method^'^). This technique also (6) Suslick, K. S.; Doktycz, S.J. Adu. Sonochem. 1990,1, 197-230. (7) Zhang, H.; Coury, L. A., Jr. Anal. Chem. 1993,65,1552-1558. (8)Perusich, S.A; Alkire, R C.1. Electrochem. SOC.1991,138,708-713. (9) Compton, R G.; Eklund, J. C.; Page, S. D.; Sanders, G. H. W.; Booth, J.J. Phys. Chem. 1994,98, 12410-12414. (10) Madigan, N. A.; Hagan, C. R S.; Coury, L. A,, Jr.1. Electrochem. SOC. 1994, 141,L23-L24. (11) Bard, A. J. Anal. Chem. 1963,35, 1125-1128. (12) Dewald, H. D.; Peterson, B. A Anal. Chem. 1990,62,779-782. (13) Hagan, C. R. S.; Coury, L. A.. Jr. Anal. Chem. 1994,66, 399-405. 297(14) Klima, J.; Bernard, C.; Degrand, C.J. Electroanal. Chem. 1994,367, 300. (15) Brainina, Kh.; Neyman, E. Electroanalytical Stripping Methods, Chemical Analysis Series 126 John Wiley & Sons: New York, 1993. (16) Scholz. F.; Lange, B. Trends Anal. Chem. 1992,11, 359-367. (17) Komorsky-Lovric, S.; Lovric, M.; Bond, A. M. Anal. Chim. Acta 1992,258, 299-305.
Analytical Chemistry, Vol. 67, No. 17, September 1, 1995 2781
sonicator born
Q Au e/ectrode
Figure I. Sonochemicalcell arrangement used for the preparation of planar gold electrodes for microscopy Studies.
has advantages over traditional methods for metal determination such as ICP-AES, in that the only pretreatment of the sample necessary is dilution (rather than acid digestion), and information can be obtained about metal deposition processes at high fluid velocity. EXPERIMENTAL SECTION
Sonication experiments were performed with a Heat Systems XL2010. 475 W, 20 kHz ultrasonic processor, using a tapped Ti horn having a tip area of 1 cm2. Unless otherwise indicated, a sonicator power setting of 80%(96 pm peak-tepeak tip amplitude) was employed, with the electrodes positioned parallel to the horn tip at a separation distance of 4 f 0.5 mm. The jacketed sonochemical cell (Figure 1) held 30 f 1 mL of the metal suspension and was cooled to an initial temperature of 20.0 f 0.5 "C using a Lauda RM-6 recirculating bath. Electrochemical experiments were conducted at 20.0 f 0.5 "C with a BASlOOB electrochemical analyzer or a PAR 253 potentiostat. Gold disk (BAS) working (area, 0.020 cm?, 3 M NaCl Ag/AgCI reference (BAS RE-5). and Pd wire (Aldrich) auxiliary electrodes were employed for most experiments. Square wave voltammeby was used for the quantitation of copper for the gold electrodes sonicated in DMSO (frequency, 5 Hz; pulse height, 50 m V step height, 5 mv) and other organic solvents (frequency, 5 Hz: pulse height, 70 mV step height, 5 mv). Electrodes for microscopy studies were planar, polycrystalline gold electrodes prepared by magnetron sputtering of loo0 k, of Au over 100 k, of Ti on borosilicate glass (AAI-ABTECH, PME-Au-118). During sonication, these electrodes were mounted on the end of a BAS electrode using doublesided adhesive squares (3M) or EPO-TEK 377 epoxy (Epoxy Technology. Inc.). as shown schematically in Figure 1. Scanning electron micrographs (SEMs) were obtained with either a JEOL 6400 field emission SEM (acceleratingvoltage, 2.0 kv) or a Philips 501 SEM (operated at 15 kv). All solvents utilized were reagent grade or better and were used as received. The oils examined were nondetergent SAE 30 motor oil, SAE 1OW-50 motor oil both from Advance Auto Parts), SAE 1OW-30 motor oil (Pennzoil). and SOW-85W-90 gear oil (Unilube). Copper powder designated as submicrometer (99%. average particle diameter 300 nm), 5-10pm copper powder (99%). and 12 pm tungsten powder (99.9%) were obtained from Aldrich Chemicals. The submicrometer copper was used in all experiments, unless indicated otherwise. 2702 Analytical Chemisiry, Vol. 67, No. 17, Sepiember 1, 1995
Figure 2. SEMs of planar gold electrodes sonicated in Slurries of 0017 gimL submicrometer Cu in decane for 5 min wtth a tip-toelectrode separation distance of 5 f 0 5 mm and a vibrational amplitude of either (A) 96 or ( 6 )72 p m
Viscosity measurement employed a reverse flow CannonFenske opaque viscometer, s u e 400. for the pure oils, and a Cannon-Fenskeviscometer, size 100, for the oil mixtures. Measurements at 100 "C were taken in a boiling water bath, allowing 15 min for each solution to reach thermal equilibrium. Gold disk electrodes were cleaned between each experiment by sanding with first extrafine emory paper (3M) and then #800 grit pads (BAS). Next, electrodes were polished successivelywith 30, 15, 6, 3, and 1 pm diamond pastes and 0.05 pm alumina (Buehler), with sonication in a cleaning bath (Branson 1200) between each step. Square wave voltammetry was used in each case to verify complete removal of metal deposits from the Au disk. RESULTS AND DISCUSSION
Microscopy Studies. When a gold electrode is immersed in a suspension of copper particles in an organic solvent such as decane or DMSO and then irradiated with 20 kHz ultrasound for several minutes, a surface deposit of Cu forms which is visible to the unaided eye. Such deposits exist as both aggregates and single particles fused to the electrode surface, rather than as a continuous film, as may be seen in the scanning electron microscopy images in Figure 2. The planar electrodes shown were prepared by sonication of a W d e c a n e sluny for 5 min with a tiptoelectrode separation distance of 5 f 0.5 mm at a vibrational amplitude of either 96 (Figure ZA) or 72 pm (Figure 2B). These
-1200
600
0
300
E
-300
-600
("4
Figure 4. Linear sweep voltammogram (10 mV/s) in 1 M KCI with a gold disk electrode which was subjected to 5 min of sonication in a slurry of 0.017 glmL submicrometer Cu in DMSO with a tip-toelectrode separation distance of 5 0.5 mm and a vibrational amplitude of 96 pm.
+
Figure 3. High-resolution SEM of Cu aggregate sonochemically deposited on a planar gold electrode from a slurly of 0.017 gimL submicrometerCu in decane. Sonication time was 5 min with a tipto-electrode separation distance of 5 5 0.5 mm and a vibrational amplitude of 96pm.
SEMs of electrode deposits reveal a distribution in particle diameters. A statistical analysis of a representative image of 150 particles reveals a mean diameter of 303 nm with a standard deviation of 80 nm. Higher resolution images of deposited Cu aggregates show that particles are often fused together on their sides (Figure 3). Since the sonicator tip is positioned parallel to the electrode surface during sonication and fluid microjets should occur normal to the surface, lateral acceleration of particles across the surface resulting in agglomeration seems unlikely. A more plausible explanation is that particles undergo signifcant aggregation in solution before being accelerated toward and colliding with the electrode. This observation is consistent with solution sonication studies by Suslick and cO-workers2.'*which were aimed at surface activation of solid catalysts via removal of oxide layers. We note that interfacial aggregation is observed after a 5 min sonication time, as compared to irradiations of 15 min or longer for homogeneous studies? SEMs of particles passively adsorbed to electrodes (not shown) do not exhibit this type of agglomeration (or surface coverage); thus, aggregation must be attributed to sonication rather than to the initial properties of the powder. Voltammetry. When a gold disk voltammehy electrode which has been sonicated in a slurry of Cu particles is rinsed with deionized water and placed into a 1M KCI solution, the oxidative stripping voltammogram shown in Figure 4 is obtained, clearly demonstrating the twoelectron oxidation characteristic of copper. This electrode underwent sonication for 5 min in a suspension of 0.017 g/mL Cu in DMSO and exhibits a peak current of 1.1mk Despite the large amount of copper undergoing a change in oxidation state, however, the shipping current returns very nearly to the baseline by the termination of the potential sweep. Quantitation. The sonochemicaldeposition of metal particles onto electrodes depends on a variety of experimental parameters. In studying the effects of sonication conditions, square wave voltammetry ( S W frequency, 5 Hz; pulse height, 50 mV; step height, 5 mv) was used as a measure of the extent of copper deposition from a slurry in DMSO onto a gold disk electrode. (While all experiments discussed here involve the use of gold (18) Suslick. K S.: Doktycz. S. J./, Am. Ckm. Soc. 1989. 212. 2342-2344.
electrodes, deposition of copper onto platinum electrodes is also observed.) The first parameters to be considered are sonicator tip vibrational amplitude and distance between sonicator tip and electrode. These are related, as the cavitational intensity (and/ or microjet density) near the electrode surface must be sufficient to cause melting of the copper particles upon acceleration toward the electrode surface. A vibrational amplitude of 96 pm and a separation distance of 4 nun appears to be optimal for this system. More extreme conditions, i.e., larger vibrational amplitudes and smaller separation distances, result in lower peak currents by SWV. This is most probably a result of greater prominence of the competing process of particle removal due to increased turbulence, cavitation, and collisions by particles lacking sufficient velocity to melt upon impact. On sonicating slurries of 0.0083 g/mL Cu in DMSO, a relationship is seen between amount of copper deposited and sonication time, as shown in Figure 5A (0). The plot of peak area vs sonication time proves linear for sonication up to 13 min (correlation coefficienf 0.993). There is then a sharp increase in peak area at 15 min of sonication, followed by a decrease. The shape of the curve can be explained by considering the processes occurring during sonication of the slurry. First, the linear region does not go through the origin. This is due in part to passive adsorption of small amounts of copper onto the gold electrode in the absence of sonication (see Figure 5 4 0). It is not necessary to subtract this background term from the sonication data, since it is less than the standard deviation in the sonication measure ments (see Table 1). The decrease in peak area after 15 min of sonication may be the result of particle aggregation in the slurry producing clusters too large in s u e to be accelerated toward the electrode at rates sufficient to cause melting upon impact. Uhis is also the cause of the decreased response for the sonication of larger copper particles seen in Table 1.) Thus, the competition between deposition and removal of the copper is shifted toward removal. causing a decrease in peak area with increased sonication time. The larger than expected peak area for 15 min of sonication indicates that conditions strongly favor deposition at this point. Peak area was used for the analysis rather than the peak current because the peak width at half-height was not constant; i.e., the peaks became broader relative to the peak height as sonication time increased. This is probably due to longer sonication times producing thicker deposits of copper, with the SWVs thus involving stripping of copper from copper as well as stripping of copper from the gold substrate. Analytica, Chemistry, Vol. 67, No. 17, September 1, 1995 2783
A ,
30 a
9
time (min)
!3 15 m
t
z
Y
n
b.0
0.1
0.2
0.3
0.4
0.5
weight % Figure 5. (A) Peak area from square wave voltammograms vs time for a gold disk electrode ( 0 )sonicated and (0)dipped in slurries of 0.0083 g/mL submicrometer Cu in DMSO (error bars represent standard deviation of five measurements). (8)Peak area from square wave voltammograms vs weight percentage of copper for a gold disk electrode sonicated for 10 min in slurries of submicrometer Cu/DMSO (error bars represent standard deviation of five measurements). Table 1. Stripping Analysis of Copper Particles with and without Sonication
peak areasa (uA x V) submicrometer copper 5-10 ,um copper
exposure time (min) sonication SDb controlC sonication SDb control 1
2 5 8 10 13 15 20
3.24 3.98 6.64 10.84 13.43 16.07 24.83 17.32
0.29 0.72 0.30 0.22 0.77 0.24 0.20 0.73
0.29 0.34 0.44 0.70 0.94
2.01 2.71 5.78 8.44 10.72 9.03 8.11 8.00
0.52 0.41 0.55 0.76 0.78 0.39 0.47 0.49
0.78 0.33 0.41 0.66 0.85
a Determined by square wave voltammetry (frequency, 5 Hz; pulse height, 50 mV step height, 5 mV) with a gold disk electrode after sonication in 0.25 g of Cu per 30 mL of DMSO. Standard deviation of five measurements. Response to Cu deposited by passive adsorption from dipping electrode into slurry for given time.
Another parameter of importance is the dependence of the stripping signal on the amount of copper in the slurry. Using a sonication time of 10 min, the behavior shown in Figure 5B was seen. A linear region (correlation coefficient, 0.998) is observed from 0.0013% to 0.1878% (w/w) copper in DMSO. For larger percentages of copper, there is no clear trend in the peak area. Again, this is likely due to competitive processes of deposition, removal, and bulk agglomeration of particles. Selectivity. Another important aspect of the sonochemical deposition of metal particles onto electrodes is selectivity. Since Suslick and co-workers demonstrated that particle aggregation is dependent on the melting point of the metals,6 it is reasonable to expect the same correlation in sonochemical metal deposition. To show this, a gold disk electrode was sonicated in a slurry containing 0.25 g each of 5-10 pm Cu powder (mp = 1083 "C) 2784
Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
and 12 pm W powder (mp = 3410 "C) in 30 mL of DMSO. ('The larger copper particles were used rather than the submicrometer powder to show that the size was not the factor preventing deposition, as smaller W particle$ were not available.) Figure 6B shows the resultant stripping peak after 10 min of sonication. The response is indistinguishable from the peak which results from the sonication under the same conditions of the slurry containing only copper (Figure 6A). Thus, the presence of the tungsten particles does not interfere with the deposition and quantitation of the copper. (Note that the peak is only half the size of that produced using the submicrometer powder, demonstrating that particle size is an important factor. This observation is consistent with particle aggregation reducing the amount of copper deposition for long sonication times and large weight percentages of copper.) It is not straightforward to determine electrochemically if any tungsten deposited on the electrode, since W is not easily oxidized in the working region for a gold electrode. Thus, a planar gold electrode was sonicated in a slurry of 0.0083 g/mL W in DMSO for 10 min, and a SEM image was obtained (Figure 7). No particles are visible on the surface. Instead, there is a "cratering" of the surface which is not seen in the images of the electrodes sonicated in copper slumes (Figure 2). This is most likely due to the W particles impacting the surface at velocities which cannot cause localized melting of the particle, but which produce melting of the gold. Thus, the particle is not immobilized on the surface but leaves an impression where the collision occurred. Therefore, sonochemical deposition appears to be sensitive to the melting point of the metal particle. Oil Studies. Ascertaining the presence of metal particles in used lubricating oils is of importance in monitoring engine wear, with a knowledge of the nature and concentration of the particles present allowing the determination of the location of the damage.'g-21 Atomic absorption spectroscopy has been widely employed for this oil analysis. However, there are still several problems in sample preparation and nebulization which can cause errors in the determination.*' Sonochemical stripping voltammetry studies using lubricating oils as sonication solvents demonstrate that metal deposition can occur from nonconductive, complex matrices, since the voltammetry is performed subsequently in a separate electrolyte solution. Figure 8 and Table 2 show the stripping peak responses for copper deposited from various mixtures of oils. The square wave parameters were changed in order to maximize the peak currents, as the voltammograms show a decrease in reversibility of the stripping reaction relative to that of samples sonicated in DMSO (frequency, 5 Hz; pulse height, 70 mV step height, 5 mV) . This change in reversibility may be due to the formation of elemental carbon on the copper deposits before or after melting onto the gold electrode, resulting from the decomposition of alkane components during sonication?* There is no detectable deposition of copper from the pure lubricating oils. This is evidently due to the high kinematic viscosities of the oils. However, dilution with mesitylene (chosen because of its miscibility with the oils and its low vapor pressure) provides interesting results upon sonication. (19) Najjar, Y. Fuel 1987,66,431-434. (20) Leugner. L. 0. Lubr. Eng. 1989,45, 618-624. (21) Barbooti, M. M.; Zaki. N. S.;Baha-Uddin, S. S.; Hassan, E. E. Analyst 1990. 115, 1059-1061. (22) Suslick, K. S.: Choe, S.; Cichowlas, A. A.; Grinstaff, M. W. Nature 1991, 353. 414-416.
;:m
.-
-60
I
-80
-IO870
0.50
0.30
0.10
-0.10
E (VI
E (VI
Figure 6. Square wave voltammograms of gold disk electrode sonicaled in a slurry of 0.0083 g h L 5-10 pm (A) absence and (6) presence of 0.0083 gl" 12 pm tungsten powder.
Cu in DMSO for 10 min in the
Table 1. Sonochemical Stripping Analysis of Copper in Various Oil Mixtures
sonication solvent
ratid
mesitylene SAE30 PUR pure SAE 1OW-30 pure SAE 2OW-50 SAE 8OW-85W-90 pure
15 15 SAE 2OW-50 15 SAE 8OW-85W-90 15 SAE 30 19 19 SAE 1OW-30 SAE 2OW-50 19 SAE 8OW-85W-90 19 SAE 30 SAE 1OW-30
Figure 7. SEM of a planar gold electrode sonicated lor 10 min in a slurry of 0.0083 gimL 12 pm tungsten in DMSO with a tip-toelectrode Separation distance of 5 i 0.5 mm and a vibrational amplitude of 96 am.
\v/-.
-40}
-58.b5
0.45
0.25
I 0.05
E IV) Figure 8. Square wave voltammograms obtained by sonicating a gold disk electrode in slurries of 0.25 g of submicrometer Cu per 30 mL of solution. Sonication solutions were (A) mesitylene, (6) a 1:5 mixture of SAE 8OW-85W-90gear oil and mesitylene. (C) a 1:5 mixture of SAE 2OW-50 motor oil and mesitylene, (D) a 1:5 mixture of SAE 1OW-30motor oil and mesitylene, and (E) a 1:5 mixture 01 SAE 30 motor oil and mesitylene.
It can be seen that the SAE 80W45W-90 gear oil provides the greatest extent of deposition for both mixtures. The motor oils give much lower peak currents, with SAE 2OW-50 somewhat higher than the SAE 1OW-30 and SAE 30. These results are explained by considering the viscosities of the oils. Since particles only deposit onto the electrode if they have sufficient kinetic energy to melt upon impact, solutions which significantly slow the particles after acceleration by a microjet,
peak currentb kinematic viscosity (cS) (u.4 20 "C 100 "C 48 0
0 0 0 0 0 4 27 3
8 11
37
0.82 281.45 149.78 472.48 425.08 1.42 1.42 1.64 1.44 1.17 1.17 1.28 1.25
0.44 10.84 9.40 18.81 13.96 0.63 0.66 0.73 0.63 0.53 0.55 0.60 0.56
Solvent: mesitylene. Determined by square wave voltammeby (frequency: 5 Hz: pulse height, 70 mV; step height, 5 mv)with a gold disk electrode after sonlcahon for 15 min in 0.25 g of submicrometer Cu per 30 mL of solution.
Le.. those with higher viscosities under sonication conditions, will result in less deposition. When considering the viscosities of the solutions, it is important to remember that sonication produces extreme localized temperatures as well as micmjets. Thus, the temperature and shear rate (the velocity gradient in fluid flown) dependences of the viscosities are important. Table 2 lists the kinematic viscosity measured at 20 and 100 "Cfor each oil mixture used, and Table 3 shows the SAE standards for the pure oils. Note that the viscosity of each diluted oil varies with that of the pure oil, with the Same trends observed in the temperature dependence. The dependence of viscosity on shear rate is more complicated. While single grade oils, e.g., SAE 30. act as nearly Newtonian fluids (a fluid that at a given temperature exhibits a constant viscosity at all shear rates or shear stresses"), multigrade oils, e.g., 1OW-30,are non-Newtonian as a result of polymeric additives used to improve the viscosity indexz6 (which refers to the temperature dependence of viscosity). Thus, for oils under (23) ASTM Method D468590. 199ZAn~wlL?mkofASTMStondordr.Acan Soeiety for Testing and Mated&: Philaddphia. PA 1992. Vol. 05.03. (24) Stondord Handbook of Lvbrieofioi E n p i n w i w O'Connor. J. J., Ed.: McCraw-Hill: New York. 1968: Chapter 12. (25) Ginhick. F. SAE Tmm. 1992. IO2 (4). 1574-1585 (Paper 922288). (26) Si&icnificorce of Tprb fir Petroleum Prodmu: Boldf K. Hall. B. R, Eds.: Amelican Society IorTesting and Materiais Philadelphia. PA IW?:Chapter 8.
Analyiical Chemishy, Vol. 67, No. 17, September 1, 1995
2705
Table 3. SAE Viscosity Ranges for Oils and Literature Values of Temporary Viscosity Loss
viscosity range (cS)24 -17.8 "C 98.9 "C
oil
SAE 30 SAE 1OW-30 SAE 2OW-50 SAE 8OW-85W-90
1303-2606 2606-10423 3257-21716n
9.62-12.93 9.62-12.93 16.77-22.68 14.24-25.0
m b . 2 5
1.2 11.4 15.1 not available
"Viscosity quoted at -17.8 "C is for grade 80W. bTemporary viscosity loss; see text for details.
sonication, a more appropriate measure of viscosity is the temporary viscosity loss (TVL, see Table 3). This is defined as
TvL= 1 - (HTHS/DYN) where HTHS is the high temperature and high shear rate viscosity23 (which can be related to engine performance27) and DYN is the kinematic viscosity at 150 "C. The trend in TVL for the motor oils, Le., SAE 30 < SAE 1OW-30 < SAE 2OW-50, is the same as the trend in peak currents for the diluted oils. It is interesting to note that any trend occurs, since the diluted samples are predominantly mesitylene. Thus, sonochemical deposition of metals appears to give qualitative information about the viscosity (27) Spearot, J. A. In High-Temperature, High-Shear (HTHS) Oil Viscosity: Measurement and Relationship to Engine Operation, ASTM STP 1068; Spearot,J. A, Ed.; American Society for Testing and Materials: Philadelphia, PA, 1989; pp 43-59.
2786
Analytical Chemistry, Vol. 67,No. 77,September 7, 7995
of some complex solvents under extreme conditions. Because the method is sensitive to sample viscosity, calibration of a system involving a complex solvent would require the solution viscosity to be matched. Standard addition is a viable option to remove this difficulty. CONCLUSIONS Sonochemical stripping voltammetry has been shown to be a useful technique for the determination of zero-valent metal particles in organic suspensions. The method is both sensitive and selective and allows the preconcentration of metals from complex matrices, such as lubricating oils mixed with organic diluents, with little or no pretreatment of the sample required. This is accomplished by eliminating the need to have the metal present as reducible ions in an electrolytic solution. This may be of importance in such areas as the determination of diesel engine wear, which often involves detection of metal particles as a guide for preventative m a i n t e n a n ~ e . ~Sonochemical ~-~~ stripping analysis also gives qualitative information about the viscosity of a solution under extreme conditions, which could find application as a cost-effective way to test the efficacy of motor oil additives.
Received for review December 21, 1994. Accepted June 5. 1995.@ AC941229Y Abstract published in Advance ACS Abstracts, July 15, 1995.
