ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
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Vaporization and Atomization of Large Particles in an Acetylene/Air Flame J. A. Holcombe,” R. H. Eklund, and K. E. Grice Department of Chemistry, University of Texas at Austin, Austin, Texas 78712
The chloride, nitrate, and sulfate salts of Cu and Zn are separately dissolved in aqueous solutions, and a vibrating orifice droplet generator and desolvation chamber are used to introduce relatively uniformly sized, desolvated particles into a conventional acetylene/air flame. Integrated absorbance vs. height profiles are presented with a vertical resolution of 0.2 mm. The chloride systems show a 3- to 4-fold enhancement in the atomic absorbance signal in the fuel-rich flame while the free atom form of Zn was undetected when sulfate or nitrate was the counterion and when sulfate was the counterion of Cu. I t is suggested that particle inertial forces predicted by the Stokes number preferentially place the vaporizing CuCI,*2HZO particle in the center of the flame. Equilibrium data are presented to support the observed enhancements. Rate limiting vaporization processes are presented to account for the absence of any measurable atomic density for the sulfate forms of Cu and Zn and the nitrate form of Zn. Thermal decomposition data are also presented to support these arguments.
T h e chemical flame represents one of the more widely used atomization sources for atomic spectral studies in spite of the complex atomization environment present. I n most cases where a liquid sample is used, the analytical flame is involved in t h e sample desolvation, vaporization, atomization, and excitation. These processes occur in a sequential fashion and, to a first approximation, can be isolated in time with a temporal relationship dependent on the burner design and nature of the sample. The variation in droplet sizes produced by a conventional pneumatic nebulizer is primarily responsible for t h e temporal overlap of these processes. By following an isolated droplet, the desolvation process can be readily isolated from t h e other stages. This has been accomplished successfully by a number of workers using a piezoelectric coupled “droplet generator” in place of the pneumatic nebulizer (1-3). T h e vaporization process has been alluded to by a number of workers (4-9) and suitable mathematical models exist for determining rates of vaporization under somewhat restrained conditions (10-12). These models incorporate the significance of particle size in their computation as well as t h e expected dependence on such parameters as flame temperature and partial pressures of t h e vapor components a t the stated temperature. The analytical impact of low vaporization rates is relatively apparent when dealing with any refractory metal or metal oxide and this process has been suggested as a factor responsible for signal enhancement as well as depression. An attempt to study the potential analytical implications of vaporization and atomization processes was initiated using a piezoelectric, vibrating orifice droplet generator capable of producing an aerosol of uniformly sized droplets. The droplets were then desolvated t o isolate t h e desolvation process from the vaporization process within the flame cell. A conventional premix C2H2/airflame supported on a slot burner was oriented perpendicular to t h e optical path and height-dependent absorbance measurements were made using a modified optical system of a double beam spectrophotometer which permitted 0003-2700/78/0350-2097$01 .OO/O
a spacial resolution of 0.2 mm. T h e use of integrated absorbance has been discussed previously (7, 12) and was employed to measure more accurately the height dependence on the degree of atomization under various experimental conditions. Results indicate that particle size and composition have a strong influence in determining the degree of atomization because they not only influence vaporization rates b u t also play a major role in determining the spacial location of t h e atomic vapor within the flame which ultimately can result in apparent signal enhancement or depression. These effects are demonstrated on the classically well behaved Cu system using only dissolved salts of copper with no other matrix component. In most instances, it is impossible to separate in time the processes of vaporization and atomization with t h e logical exception being the case where the solute particles are in the elemental form. In this situation, vaporization and atomization are synonymous unless gas phase polymers are formed upon vaporizing from t h e molten particle. However, it is conceivable t h a t the combined processes of vaporization/ atomization can be isolated for a concentrated study by incorporating existing techniques. By using aerosols of relatively uniform diameter which are desolvated prior to their introduction into the flame, one has effectively eliminated desolvation phenomena from t h e system. Similarly, by monitoring of the time-dependent atom formation from the particles by absorption techniques in place of emission, one can effectively isolate the vaporization/atomization step from the process of excitation. Similarly, a n attempt for quantitatively following t h e change in free atom formation by absorption should require less foreknowledge of flame temperature and composition then would be the case in emission, which is strongly dependent on Boltzmann distribution and temperature, or fluorescence, which is sensitive t o the concentration of quenching species. Certain constraints must be maintained concerning the nature of the element followed in these processes, e.g., t h e existence of a sufficiently high ionization potential to assume negligible ionization under normal flame temperatures. The major complication associated with any study aimed a t elucidating vaporization/ atomization processes arises from the highly complex reaction medium Le., the flame, in which these studies must take place. However, as w ill be shown later, the observed results can often provide insight into reaction dynamics for an increased understanding of matrix and interference effects which are prevalent in t h e flame system.
EXPERIMENTAL The experimental arrangement is shown in Figure 1. A pressurized liquid feed chamber delivers liquid to the vibrating orifice droplet generator, and dispersing air is combined with a secondary air supply to carry the generated aerosol throughout the remainder of the system. A heated desolvation chamber facilitates droplet desolvation and the excess water vapor is removed in a specially designed condenser. A conventional 10-cm slot burner was used with the slot oriented perpendicular to the optical path. Alterations were made on the monochromator to permit spatially resolved viewing of the absorbance profile at different heights in the flame. 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
were retained to ensure good fuel/oxidant mixing. The burner assembly was mounted on top of a micrometer drive for precise vertical positioning, and the perpendicular orientation of the flame relative to the optical path ensured optimal spatial isolation at any given height. Certain other advantages of this flame orientation will be discussed. A Perkin-Elmer Model 303, double beam, atomic absorption spectrometer was modified slightly to permit high quality, spatially resolved absorption studies. This was accomplished by placing a narrow horizontal slit behind the spectrometer entrance slit to isolate a narrow vertical resolution element in the flame. This alteration permitted full use of the double beam character of the instrument although the higher noise level on the output signal required additional capacitative filtering. All absorption measurements were made using the null balance meter on the instrument. Chemicals. Reagent grade chemicals were used throughout and were dissolved in triply distilled water at a concentration of 1000 ppm of the metal. All solutions were filtered through nucleopore filter paper with a nominal 10-pm pore size to remove any particulate matter which might obstruct or block the droplet generator pinhole.
Figure 1. Diagram of experimental arrangement. (A) Gas cyclinders; air (1 2nd 2) and acetylene (3). (B) Precision pressure regulator. (C) Liquid feed chamber. (D) Rotameters. (E) Square-wave generator. (F) Droplet generator. (G) Thermal heating unit for desolva!ion chamber. (H) Desoivation chamber. (I) Condenser. (J) Pump for condenser cooling water. (K) Premix slot burner. (L) Height limiting aperature
Droplet Generator. A vibrating orifice droplet generator similar to that described by Berglund and Liu (13) was used throughout. Modifications of the initial design include the use of a pressurized liquid feed chamber ( 1 4 ) in place of the syringe pump employed in the original design. The feed chamber was msintained at 20.0 psig, and a calibrated capillary tube in the feed line was used to determine solution flow rates. By introducing a small air bubble into the feed line and measuring the time required to pass through the calibrated volume, an accurate measurement of solution flow rate could be made. A 25-km &meter pinhole (Optimation, Inc.) was used in conjunction with a piezoelectric ceramic disk (Gulton Industries) driven by a 80-kHz, 10-V square wave. An average solution flow rate of approximately 0.5 mL/min persisted with only small day-to-day variations. Rotameters were used for controlling gas flow, and the needle valves were oriented to place the flow dependency on the inlet gas pressure (2.5) which was held constant. Besolvation and Condenser System. A 46-cm tall, 13-cm diameter Lucite chamber was O-ring sealed t o the droplet generator base plate and equipped with a nichrome wire heating element to facilitate desolvation of the dispersed aerosol. The ice water cooled condenser was O-ring sealed to the top of the desdvation chamber, The condensate on the walls of the condeaser was collected in a calibrated trap and analyzed in the course of the study in determining sample delivery efficiencies. Burner and Spectrometer. The aerosol leaving the condenser was introduced through the "auxiliary air" inlet port of a conventiona! premix slot burner end the acetylene was fed through the inlet provided en the burner. The pneumatic nebulizer was removed from the premixing chamber, but the chamber baffles
RESULTS AND DISCUSSION Droplet Generator System. Three parameters were considered important in characterizing the processes prior to particle entrance into the flame cell: (1)system desolvation efficiency, ( 2 ) particle size distribution, (3) particle delivery efficiency to the flame. T h e degree of desolvation of the droplets from a conventional nebulizer upon leaving the burner has received some speculation by a number of workers ( 9 , 16-18). The opinions range from nearly complete desolvation upon leaving t h e burner head to an assumption t h a t only minimal solvent vaporization occurs prior to entering the primary reaction zone of the flame. This discrepancy, coupled with the fact that the 60-pm droplets generated in the study are significantly larger than the average diameters expected from a conventional pneumatic nebulizer, required empirical verification that salt particles were entering the flame. T h e desolvation efficiency was determined by diverting the air flow system after leaving the condenser to two Pyrex U-tubes connected in series. These tubes were immersed in a dry ice/isopropanol bath during a S-min operational period of the generator, and the amount of water collected in the tubes was determined by weighing the tubes after the collection period. Using this data along with a knowledge of the total amount of liquid sample introduced by the droplet generator, it was determined that an average of 69% of the water was removed by the condenser. However, simple calculations using the air and solution flow rates show that only 12% of the introduced water need be removed by the condenser for complete desolvation, i.e., desolvated aerosol in air saturated with water vapor a t the 30 "C temperature of t h e vapor leaving the condenser. The measured values are characteristic of air which was only 35% saturated in water vapor, thereby suggesting that desolvated particles were being introduced into the flame. Additional support for this assumption is provided by calculating the time required to vaporize a 60-wm water droplet. Using the equations presented by Heiftje and Clampitt ( 3 ) and assuming an average temperature of 150 "C in the desolvation chamber, a time of about 2 s can he calculated for complete solvent vaporization, and the residence time of the particles in the desolvation chamber is conservatively estimated a t 6 s. In addition, desolvation processes, if not complete in the desolvation chamber, will continue in t h e condenser and the premixing chamber of t h e burner. Additional experimental evidence supporting the assrimption of complete desolvation will be presented later in t h e paper. It was sufficient for these studies to determine the analyte delivery efficiency to the flame for each of the three dissolved salts. With this information, the absorption curves could be
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
Table I. Thermal Decomposition Scheme for Salts of Copper temp., K reaction
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0 25
n
CuS0,.5H20 + CuS0,.3H20(,) + 2H20(,)“ temperature, t)eated chamber exit CuCl, .2H,O(,) + CuCl2(,) t 2 H 2 0 CuS0,.3H20(,) CUSO,(,)~+2H2&,)‘ Cu(NO,),(,) + Cu(NO,),(, Cu(NO,),( ) - + CuO + 2 k 0 2 ( ,+ 1 / 2 0 2 ( g ) u 520 CuS0,.H28(,) + Cu!%,(,) + H2d(g)a 570-670 temperature near heating coil 890 CuC1, ,) + CUC1,(1) 4 6 B 10 12 I4 1030-1060 2CuS6, , + CuO~CuSO,(,)+ SO,(,)‘ diornetsrlpml diometerlgmi 1090-1110 CuO.Cukb, ,) 2CUO(,) + SO,(,)’ Figure 2. Size distribution of particles from (a) CuCI, and (b) CuSO, 1270 CuC1,(l) --* duCl(1) + C1( ) solutions 1400 2CuO(,) + Cu20(,) + 1510 CU,O(,) + Cu,Og) 1760 CuCl(1) + CuCl(,) 2100 CU20(1) ZCU(1) + l / 2 0 2 ( g ) 2100-2400 temperature, fuel-rich flame 2300-2600 temperature, fuel-lean flame 2850 Cup C u p 3550 CU l(,) + u(,) + Cl(,) ‘Approximate temperature at which “appreciable reaction” begins. 360 370 370 400 450-490 500
-+
--f
]/b2(,)
-+
+
normalized if the efficiencies differed significantly. Knowing the amount of metal analyte retained by the system and the amount introduced by the droplet generator, the sample delivery efficiency to the flame could be determined. All apparatus was thoroughly rinsed with distilled water and all metal parts were covered with a thin sheet of polyethylene. The system was operated under normal conditions using 1000 ppm Cu solutions dissolved as the hydrated species of either CuCl,, C U ( N O ~ )or~ ,CuSO,. After 20 min of operation, the entire system was rinsed again and the collected solution diluted to a known volume. A conventional AA spectrometer then was used to determine the Cu concentration in these solutions. The results of triplicate runs of each salt showed an average delivery efficiency of 51% , 4 8% , and 43 70for the chloride, nitrate, and sulfate salts of Cu, respectively. With a standard deviation of f3%, only the sulfate solution resulted in delivery efficiencies that might be considered statistically different. Table I schematically shows the thermal decomposition scheme for the Cu salts of interest. In those reactions where a gaseous product is evolved, the temperature quoted denotes the equilibrium temperature for a partial pressure of 1 atm of the gases. In those transitions for which exact vapor pressures were not given in the literature, the temperature is indicative of the onset of “appreciable reaction”. Estimates on the temperature range within the desolvation chamber and the maximum flame temperature are also noted. Because of variations of temperatures within the desolvation chamber, the presence of water vapor, and aerosol cooling while passing through the condenser, experimental verification of particle composition was undertaken. After leaving the condenser, the gas stream was diverted to a tube-type electrostatic precipitator (Mine Safety Appliances). The tube was lined with a thin sheet of cellulose acetate onto which the particles impacted. T h e collected particles were packed in a capillary tube and X-ray powder diffraction patterns taken. As would be expected, the chloride solution resulted in CuC12.2H20particles. The sulfate solution resulted in a mixed particle composition of CuSO4-H20and Cu(OH),. While hydrated sulfate species might be expected from the temperatures present in the desolvation chamber, the absence of any measurable amounts of either the trihydrate or pentahydrate species again supports the hypothesis of complete desolvation prior to entering the condenser. Since
Figure 3. Electron micrographs of collected particles from (a) CuCI, and (b) CuSO, solutions
the temperatures of the vapor leaving the desolvation chamber and the condenser were 100 “ C and 30 “C, respectively, the presence of any solute containing droplets at either of these temperatures would result in the more highly hydrated sulfate species. The presence of C U ( O H )is~ not readily explainable from the thermal decomposition data compiled but suggests the existence of relatively high local temperatures within the desolvation chamber resulting in the conversion of the sulfate to the oxide. While it is possible that hydrolysis of the CuO species is responsible for the hydroxide formation, temperatures approaching 800 “C would be required for any significant conversion to the oxide. A negligible number of particles were collected from the nitrate and simply support the supposition that sublimation of Cu(NOJ2 takes place within the desolvation chamber. Additional compounds no doubt exist within the particles but were not detected using this technique. The same collection technique was used to collect samples for particle sizing using electron microscopy. Samples were taken along the length of the tube to ensure that size segregation was not introduced by the collector. The size distributions for the particles from the copper chloride and sulfate solutions are shown in Figure 2 and exhibit average diameters of 8.0 f 2.1 pm and 8.6 k 2.3 pm, respectively. While the size variation was larger than that reported by Berglund and Liu (13),it still represents a significant improvement over that present in a conventional nebulizer (19, 20). I t appears that some of the spread may be due to insufficient dispersion of the droplets which results in droplet recombination. For example, the abnormally high particle count a t 10 pm for the chloride would result from the combination of two droplets each of which would yield a 8-pm particle. The use of a-emitting, radioactive isotopes (13)and increased dispersing air flow rate will minimize these collisions. However, a calculated particle diameter of only 2.54 p m would be expected for the CuCl2.2H20 assuming literature values
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
for the bulk density. This anomaly might be explained by the particle appearance as shown in Figure 3a. All chloride particles showed these “convolutions” on the surface and suggested the existence of an average density which might be significantly different from literature values. This lower particle density may be a result of the particles being hollow, which has been shown to be the case for fly ash particles (21). This surface character may be due to the rapid desolvation occurring within the chamber and the subsequent lack of time to form compact crystalline particles. The existence of an induction period in the formation of a precipitate from a saturated or supersaturated solution has been noted previously for species which are of interest in classical gravimetric methods (22-25). I t is possible that similar considerations must be given in flame spectrometric techniques where particle formation may be viewed as a simple precipitation scheme involving a rapid transition to supersaturated conditions by solvent evaporation. I t also should be mentioned that these “precipitation reactions” are occurring under unique conditions due to the initial lack of any nucleation sites within the droplet. This aspect of particle formation in flames has not been dealt with in the literature and may indeed provide some interesting information concerning fundamental processes in aerosol formation. However, Alkemade (8) has alluded to the potential impact that analyte inclusion within volatile and nonvolatile matrices might have on analytical signals, but no detailed study has been undertaken. Fassel et al. (6) have likewise suggested the importance of particle composition, the resulting effects on particle vaporization, and the preferential location of the analyte in the flame center. The particles from the sulfate solution appear to show a bimodal distribution; which is not easily explained on the basis of droplet collisions due to the large size difference. It is possible t h a t this distribution is a reflection on the two different compound forms found for these particles which will be discussed later in the paper. In contrast to the CuC12.2H20 particles, all were found to be nearly spherical with relatively smooth surfaces as is shown in Figure 3b. Again, the calculated particle densities are significantly lower than literature values. The same effect mentioned for the CuC1,.2H20 particles may also apply here. Absorption Studies. Absorption profiles were obtained on a double beam spectrometer with a horizontal, height limiting aperture placed behind the monochromator entrance slit. T h e side-by-side reflective optics in this instrument introduce astigmatism in the final images and were used to an advantage for increased spatial resolution by locating the sagittal focus of the flame cell on the monochromator entrance aperture. This arrangement resulted in an experimentally determined vertical resolution of better than 0.2 mm. The use of the sagittal focal plane also allowed for optical mixing along the horizontal direction in the flame, Le., the radiation entering the monochromator was from one selected vertical height but from various regions along the horizontal section of this observation height. Using the principle of optical reversibility, it follows that light originating along the rays defining these images would be collected a t the slit and, consequently, enter the monochromator. The flame is located a t the sagittal image and, as can be seen, isolates a narrow vertical resolution element of the flame while “mixing” the light from various horizontal resolution elements. A more detailed description of the astigmatic imaging character resulting from off-axis illumination has been recently presented by Goldstein and Walters (26,27) and its utility in obtaining increased spectral intensities and improved spatial resolution using photoelectric detection has been demonstrated by Salmon and Holcombe (28). In addition to increased spatial resolution, the “horizontal mixing” furnishes a more stable
(i-..-
c
02[
oi
0
1
5
1
10
I
15
1
20
1
,
25
Height above b u r n e r ( m m )
Figure 4. Cu atomic absorbance profiles from CuCI, solution introduced into a rich (-O-) and lean (-0-) C2H,/air flame. Similar profiles from Cu(NO,), solution for rich (4-) and lean flame (-D) are also shown
1
1
1
1
1
1
Figure 5. (a)Thermal reactions for three Cu solutions used in this study, (b) equilibrium constants at 2200 and 2600 K for selected reactions
absorbance reading since an average absorbance of a number of particles was made a t any given time. With perfect point-to-point imaging from source to slit, sporadic bursts of absorbance would be detected as single particles or their vapor clouds entered the observation zone. Absorbance Profiles: Droplet Generator. The absorbance vs. height profiles for chloride and nitrate solutions are shown for the fuel-lean and fuel-rich flames in Figure 4. The results from the sulfate solution have not been included in this figure but will be discussed later. T h e general absorbance increase followed by a plateau region characteristically reflects the expected profile based on changes in vertical flame gas propagation. The large increase in absorbance for the chloride solution in the rich flame was the most pronounced change observed. Both equilibrium and rate limited processes were considered in attempts to explain this phenomenon. The predominant equilibrium reactions for this system are shown in Figure 5 . The equilibrium values were derived from high temperature thermodynamic data and are representative of the reaction proceeding in the direction indicated in the Figure for 2000 and 2600 K. I t is assumed that only phase transitions and reactions involving the solid or liquid particles contribute to rate limited reaction processes due, in part, to thermal transport processes from the flame to the particle. It is also assumed that local thermodynamic equilibrium (LTE) exists in most regions of the flame excluding the relatively narrow primary reaction zone (29). This assumption of L T E has also been extended to include reactions involving the gas phase products involving the analyte. If thermodynamic control governs the free Cu(,, densities within the flame, then it is least likely that rate limiting vaporization processes will interfere with this control in the
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978 107-
16' IO.1 10-
1
4
6
/
8
,
I
IO
12
I
1
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14
16
18
Temp x 10.'
20 22 24 26 28 30 IKeIvinl
Figure 6. Gas phase partial pressure ratio of CuO/Cu under equilibrium conditions as a function of temperature for (a) 0.01, (b) 3.2 X (c) 4.1 X lo-', and (d) 7.7 X lo-'' atm 02.The dashed line designates conditions necessary for a 10-fold excess of Cu over CuO
nitrate system since t h e form of Cu entering the flame is gaseous C u ( N 0 J 2 or CuO. Calculations predicting free Cu densities are extremely tenuous due to t h e nonuniform temperature and chemical environment present. However, from the values for the equilibrium constants and the likelihood of CuO equilibrium representing the primary source of bound Cu, one need only consider the general dependency of the Cu/CuO ratio on the oxygen partial pressure, Po,, and the flame temperature. These plots are shown in Figure 6 where t h e Cu/CuO ratio is plotted as a function of temperature for various values of Po,. The range of values chosen for Po, brackets those given by the computer simulation of Chester e t al. (30). Curve b shows the results using the Po, for a 2600 K lean flame, while curve d represents Po, a t 2000 K for a rich flame. These two values cover the two extremes in PO, presented in t h e published data (30). Since air entrainment in the flame is likely but was not considered in their model, the extension of Po, to 0.01 atm was included as curve a. As can be seen, only a t reduced temperatures or increased values for Po, d o appreciable amounts of CuO form. Thus, it is suggested that enhanced signals from CuC1, introduction originate from preferential injection of the particles into the center of t h e flame where reduced Po, and elevated temperatures persist and, consequently, promote the increased free atom population. T h e reducing nature of this center region likely scavenges most of the chlorine resulting from the thermal decomposition of both the CuC1, and CuC1, thereby minimizing the possibility of the Cu-C1 recombination reaction which is thermodynamically more favorable than t h e C u - 0 reaction. T h e Cu from t h e nitrate sample enters the flame as either gaseous Cu(N03), or CuO and is likely to follow the flame gases and will appear throughout the flame, including the cooler outer fringes where the formation of compounds is increased. A similar enhancement of t h e chloride over the nitrate is not observed using the conventional nebulizer and reflects the difference in particle sizes between the two systems. I t is suggested that the larger particles from the droplet generator have sufficient vertical momentum upon encountering the turbulent gas flow region near the primary reaction zone that they are not diverted laterally but enter t h e center of t h e flame. To justify the assumption of minimal deflection of the chloride particles, one must consider the forces acting on the particles which would cause deviations from t h e vertical trajectory. The environment above the burner orifice a t the boundary between the preheating zone and the primary reaction zone can be characterized as having an extremely large temperature gradient along with turbulent gas flow. While the gas flow probably retains its strong vertical component, the combustion reaction introduces additional flow patterns orthogonal to this vector. The temperature gradient and shear forces present at this boundary can alter the initial particle
2101
trajectory, e.g., the particle will follow the gas flow exactly in the limiting case of negligible particle mass and size. T h e temperature gradient also sets u p thermal forces which are derived from radiometer theory and tend to accelerate t h e particles away from the high temperature zone (31). I t can be shown by using the equations for these thermal forces that they will exert a negligible influence on most particles encountered in flame spectrometric techniques. Using the equations for calculating the force encountered when a 9-pm CuC12.2H20 particle enters a thermal zone with a thermal gradient of 20000 OC/cm, a stopping distance in excess of approximately 2 cm is required. While the thermal gradient might exemplify t h a t encountered a t the boundary between the preheating zone and the primary reaction zone, this gradient persists for a considerably smaller distance than is necessary to alter the particle trajectory or velocity. It is more probable t h a t t h e interplay between particle inertia and turbulent gas flow a t the primary reaction zone is responsible in determining the extent to which the particles are diverted from their initial flow direction. The Stokes number S can be used to assess the magnitude of this inertial force with respect to fluid shear forces (32).
where u is the particle velocity, p and r a r e the particle density and radius, respectively, p is the gas viscosity, and u, can be taken, in this instance, as t h e width of aerosol jet (33). Qualitatively, as S approaches zero, the particles tend to follow the gas flow. At large values of S, the particle inertia becomes significant, Le.. the stopping distance is increased and the influence of gas pressure on changes in particle direction become negligible. Consider, for example, the CuC12.2H20 particles generated in this study with a 8-pm diameter, a density of 1.0 g/cm3, and velocity leaving the burner orifice of 400 cm/s. Using an approximate figure for the gas viscosity of 350 fipoise for N2 a t 500 K and assuming a jet width of 0.5 mm, a Stokes number of 0.81 is obtained. If a horizontal solid plate replaced the turbulent gas flow region, viz., the primary reaction zone, t h e calculated Stokes number would predict t h a t all of the particles would impact on the plate (33). I t should be recognized that the replacement of the flame front with a rigid plate represents a barrier with no vertically directed force vector. Thus, passage of these particles through the primary reaction zone should be more easily accomplished than attempting impaction on this hypothetical plate under similar flow conditions. Using the flow model, it can be shown that particles which exhibit a Stokes number of 0.16 would still impact with 20% efficiency. This would be typical of a particle of similar density to those studied but having only a 3.5-pm diameter. A more exacting model for particle penetration would require information concerning pressure gradients and flow patterns a t this boundary and will not be attempted in this paper. I t is useful to contrast the particles described above to those encountered in a conventional nebulizer which introduces droplets with an average diameter approximately 10 times smaller. Using t h e same analyte solution, a Stokes number of 0.0081 is arrived at, and the impaction model suggests that negligible inertial forces will exist and impaction efficiency will be correspondingly low. Increasing the analyte solution concentration C by 10-fold, Le., 1%Cu as CuCl,, would result in a Stokes number of only 0.038 since S increases only as (C)2'3. While this value does not satisfy the minimal Stokes value of approximately 0.12 for finite impaction efficiency on a solid boundary, these particles may not be significantly diverted upon encountering the turbulent vapor region of the primary reaction zone. While it is not the intent of this paper
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
to pursue this aspect rigorously, it is possible that further studies may show t h a t observed signal enhancements of an analyte in a matrix with a high salt content actually result from preferentially placing the vaporizing particle in the flame center. As an example, this would account for the observations reported by Fassel (6). Additionally, the importance of a nebulizer which produces reproducible distributional droplet diameters is suggested by these results. Long or short term variations in the droplet size distribution could lead to significant changes in sensitivity and matrix immunity or susceptibility in samples containing appreciable amounts of dissolved solids. More recently, Skogerboe and Olson ( 3 4 ) have shown the importance of the aerodynamic particle size on sample loss by settling along a verticle introduction tube prior to entering a plasma source and used the particle inertia to account for matrix effects on P b analysis caused by an excess of dissolved Na salts in the sample. In comparing the absorbance profiles for the fuel lean flame, the chloride solution shows a slightly depressed absorbance signal. T h e increased Po, in all regions of the lean flame as well as the lack of as strong a reducing environment for scavenging the liberated chlorine might be responsible for this decrease. As in the rich flame, it is expected that the CuC1, particles will be preferentially located in the flame center where the temperatures and Po2 should be more conducive to free atom formation than in the fringes. Thus, to account for the larger signal of the nitrate over the chloride system which favorably competes for the atomic Cu is suggested and the Cu/CuCl equilibrium plays such a role. The CuS04 solution gave no measurable absorbance signal under either flame condition. This suggests negligible vaporization of the final product of the CuS04 salt within the observation zone studies. By referring to the thermal data presented previously in Table I, it can be seen that Cu(1) is the final condensed thermal product of this species through the reduction of Cu20(l) a t 2100 K. The equation relating rate of vaporization of a droplet is given as ( I O ) :
_-am - 2n-MDPd at
RT
where m is the mass, t the time, M the molecular weight of the particle salt, D the diffusion coefficient, P the partial pressure above the molten droplet a t the flame temperature T , R is the gas constant, and d is the diameter of the particle or droplet. Integration of this equation and rearrangement to solve for the total mass lost ml a t any given time t gives the following:
where m, and p are the initial mass of the particle with a density p. Substitution of the initial particle diameter do for m, in Equation 3 gives the following:
where p is the particle density. The fraction of mass lost is more easily expressed as:
Using the thermal decomposition scheme and thermodynamic data presented previously, the sulfate particles will be converted to Cuilj,and vaporization should proceed from this form of the analyte. Simple calculations show that a 4-pm diameter Cu(l, droplet will result from the decomposition of the sulfate
/
p 08-
limn (mancl
Figure 7. Fraction of mass vaporized for a 4-prn diameter Cu,,, droplet assuming an isothermal environment at (a) 1800, (b) 2000, and (c)2200 K. (See text for explanation of axis presentation)
Table 11. Thermal Decomposition Scheme for Salts of Zinc temp., K reaction 310 Zn(NO3);6H,O(,) Zn(N0,),.6H,0(1) ZnS04.7H,0(1) 370 ZnS0,.7H,O(,) 380-400 Zn(NO,), .6H20(1)+ Zn(NO,),(,) + 6H,O(,) 510 Zn(NO,), ,) ZnO(,) t 2 N 0 , + l/202(g)a 550 Z n S O , . d , O +ZnSO,(, t #I,O 1110-1170 5ZnSO,(, %nSO,.2Znd(,) T 2S6g,)(,)‘ 1230-1290 3ZnS04.2)Zn0(,)-+ 5Zn0(,) + 3SO,(,)a 2250 ZnO(,) -+ ZnO(l) Approximate temperature at which “appreciable reaction” begins. --f
+
+
+
particle. Using Equation 5 , a family of curves relating the fraction of particle mass vaporized as a function of time is shown in Figure 7. Isothermal conditions are shown in curves a-c for temperatures of 1800, 2000, and 2200 K. No attempt was made to build in the changes in particle velocity as a function of time that would be expected (351, but an average velocity of 800 cm/s was assumed and the alternate labeling of the abscissa in distance units reflects this assumption. While all reactions and phase transitions preceding the formation of Cql, are thermodynamically favorable within the flame temperatures available, all reactions are endothermic. For this reason ultimate conversion to Cui], may be further delayed due to the time dependent thermal transport properties alluded to previously. In brief, the underlying assumption used in explaining the absence of a Cu signal for the sulfate sample rests on the refractory nature of the particle relative to its size and is not dependent on gas phase reactions which scavenge the Cu. To further verify this hypothesis, analogous solutions of Zn(NO& and ZnSOl were made and introduced into the flame. The thermal reaction scheme for these species is presented in Table 11. As can be seen, the transition temperatures and products formed are similar to those for Cu with the exception that the Zn(N03), does not sublimate but proceeds along a decomposition path similar to that seen for the sulfate species of both Cu and Zn. As would be expected from the model presented, neither of these samples produced a measurable absorbance signal for Zn. It is also interesting to note that the dissolved ZnClz produced absorbanceheight profiles similar to those observed for the chloride form of Cu. The profiles for the fuel-rich and fuel-lean flame are shown in Figure 8. Absorbance Profiles: Pneumatic Nebulizer. As would be expected using a conventional pneumatic nebulizer, no significant difference in the absorbance profile was observed when the three salts of Cu or Zn were used. The absorbance profile for 40 ppm Cu solutions introduced into a fuel-rich and fuel-lean flame are shown in Figure 9. The concentration was chosen to give similar absorbance values to those obtained
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
2103
These deviations from a smooth increase in absorbance may he due to increased flame gas propagation rates resulting from the combustion process occurring in these regions which will tend to reduce the analyte residence time within these zones. Conversely, these plateaus might reflect the depletion of the free Cu resulting from recombinant reactions with flame gas products unique to these zones, e.g., CuH (36).
0
0
5
10
15
Height above b u r n e r
,
20
I
25
Imml
Figure 8. Zn atomic absorbance profiles for ZnCI, solutions in a fuel-rich (-0-)and fuel-lean (-0-)C,H,/air flame
0
Figure 9. Cu
5
IO
Height
o b o v r burner ( m m i
I5
20
2S
atomic absorbance profiles using a pneumatic nebulizer .-. -) and fuel-lean (-) conditions
under fuel-rich (-
using the droplet generator. The final absorbance values after about 1.7 cm are approximately equal under both flame conditions with t h e slight enhancement under fuel-rich conditions likely due to the somewhat reduced flame propagation rates and subsequent increase in the local free atom densities. Assuming an average droplet diameter of 10 pm a t the concentration cited, a 0.4-pm diameter desolvated particle is expected prior to vaporization. Treatment of this particle with Equation 1 produces a Stokes number of 2 x lo-' and implies minimal inertial forces exerted by the particle. Thus, one would expect no spatial segregation within the flame from the sulfate or chloride particles as was suggested in the case of the previous studies, and the particles are likely to follow the flame gas flow pattern t o a first approximation. T h e CuSO, particles in their final condensed form, C q , , were analyzed using Equation 4 t o determine vaporization times. Complete vaporization is suggested within 0.15, 0.05, and 0.008 ms for 1800, 2000, and 2200 K, respectively. These times are roughly a factor of 2000 smaller than those times calculated previously for the particles resulting from the operation of the droplet generator system. Thus, one is left with a situation in which the analyte distribution throughout the flame is independent of the chemical form of the dissolved form of the metal in the original solution. Similarly, if one assumes t h a t t h e counteranion of the introduced analyte is in low concentrations relative to the flame gas species capable of recombining with the free atomic form of the analyte, then t h e observed independence of the absorbance signal on the dissolved form of the metal would be expected. One unique feature of the curve shapes in Figure 9 that has not been observed previously, is the inflection points at approximately 4 and 12 m m in the rich flame and a t 4 mm in t h e lean flame. T h e locations correspond roughly to the top of the primary reaction zone and the interconal zone.
CONCLUSION The results and observations suggest the importance of the sample introduction technique and point out the potential impact that particle size can play in determining: (1) the delivery efficiency of analyte to the excitation region whenever it is necessary to pass through a pressure barrier generated by a thermal process, (2) the spatial region within the thermal source where the processes of vaporization and atomization occur, and (3) the rate of vaporization. I t is anticipated t h a t an awareness of the impact of t h e Stokes number as well as studies aimed a t increasing thti fundamental knowledge t h a t characterizes the pressiirc' gradients existing at the boundaries of thermal sources should provide a useful pathway for optimizing sample delivery efficiencies for a number of sources. The analytical implications of local chemical environments within the flame that can favorably or adversely affect the degree of atomization has also been shown in this study. Preferential sample introduction into these zones may account for some of t h e observed analytical phenomena historically classified under "matrix" or "interference effects". Similarly, it may be possible to experimentally exploit these regions for optimal sensitivity and freedom from "matrix effects". Additionally, these studies emphasize that the desolvated particle composition can result in "matrix effects" which, in fact, can be explained from well documented fundamental information such as thermal decomposition processes, vaporization rate dependencies on particle size, or gas phase equilibrium considerations within t h e source. While the results from the droplet generator experiment do not quantitatively reflect the expected analytical results using a conventional nebulizer with samples containing low concentrations of dissolved solids for the analysis of Cu and Zn, it is felt that they should underline the importance of particle size and composition for elements which can form refractory particles and on high salt matrices for a numher of sample types.
LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
G. M. Hieftje and H. V. Malmstadt, Anal. Chem.. 40, 1860 (1968). G. M. Hieftje and H. V. Malmstadt, Anal. Chem., 41, 1735 (1989). N. C. Clampitt and G. M. Hieftje, Anal. Chem.. 44, 1211 (1972). V. A. Fassel and D. A. Becker. Anal. Chem., 41, 1522 (1969). D. T. Coker, J. M. Ottaway, and N. K. Pradhan, Nature Phys. Scl., 233, 69 (1971). A. C. West, V. A . Fassel. and R. N. Kniseley, Anal. Chem., 45, 1586 (1973). 6.V. L'vov, 0. A. Katskov, L. P. Kruglikova, and L. K . Polzik, Spectrochim. Acta, Part B , 31, 49 (1976). C. Th. J. Alkemade, Anal. Chem., 38, 1252 (1966). P. J. T. Zeegers, R. Smith, and J. D. Winefordner, Anal. Chem., 40 (13), 26A (1968). B. V. L'vov, "Atomic Absorption Spectrochemical Analysis", English ed , American Elsevier Publishing Co.. New York, 1970. K . P. Li, Anal. Chem.. 48, 2050 (1976). K. P. Li, Anal. Chem., 50, 628 (1978). R . N. Berglund and 6. Y. H. Liu. Environ. S c i . Techno/.,7, 147 (1973). B. M. Joshi, Ph.D. Thesis, University of Michigan, Ann Arbor, Mich., 1977. C. Veillon and J. Y. Park, Anal. Chem., 42, 684 (1970). R. Herrmann, C. Th. J. Alkemade, and P. T. Gilbert, "Chemical Analysis by Flame Photometry", Interscience, New York. 1963. C. Th. J. Alkemade in "Flame Emission and Atomic Absorption Spectroscopy", J. A. Dean and T. C. Rains, Ed.. Marcel Dekker. New York, 1969, Chapter 4. J. 8. Willis, Spectrochim. Acta, Part 6 , 25, 487 (1970). J. A. Dean and W. J. Carnes, Anal. Chem., 34, 192 (1962). J. Stupar and J. 6.Dawson, Appl. Opt., 7, 1351 (1968). J. A. Campbell, J. C. Laul, K. K. Nielson, and R. D. Smith, Anal. Chem. 50, 1032 (1978). V. K. Lamer, Ind. Eng. Chem., 4 4 , 1270 (1952).
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
2104 (23) (24) (25) (26) (27) (28) (29) (30)
R. A. Johnsonand J. D. O'Rourke, J. Am. Chem. Soc., 76, 2124(1954). A. E. Nielson, J . Colloid. Sci., IO, 576 (1955). J. D. O'Rourke and R. A. Johnson, Anal. Chem., 27, 1699 (1955). S. A. Goidstein and J. P. Walters, Spectrochim. Acta, Part 6 ,31, 201 (1976). S. A. Goldstein and J. P. Walters, Spectrochim. Acta, Part B , 31, 295 (1976). S. G. Salmon and J. A. Holcombe, Anal. Chem., 50, 1714 (1978). L. de Gabn and G. F. Samaey, Spectrochim. Acta. Part 6,25, 245 (1970). J. E. Chester, R. M. Dagnall, and M. R . G. Taylor, Anal. Chim. Acta, 51, 95 (1970).
(31) R. D. Cadle, "Particle Size Theory and Industrial Applications", Reinhold Publishing Corp., New York, 1965. (32) G. M. Hidy and J. R . Brock, "The Dynamics of Aerocolloidal Systems", Pergamon Press, New York, 1970. (33) W. E. Ranz and V. B. Wong, Ind. Eng. Chem., 4 4 , 1371 (1952). (34) R. K. Skogerboe and K. W. Olson, Appl. Spectrosc., 32, 181 (1978). (35) C. B. Boss and G. M. Hieftje, Anal. Chem., 49, 2112 (1977). (36) D. J. Halls, Anal. Chim. Acta, 88, 69 (1977).
RECEIVED for review June 1, 1978. Accepted October 2, 1978.
Reversibility of Copper in Dilute Aqueous Carbonate and Its Significance to Anodic Stripping Voltammetry of Copper in Natural Waters Mark S. Shuman" and Larry C. Michael Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina, Chapel Hi//, North Carolina
275 14
Cyclic voltammetry and anodic stripping voltammetry of copper in dilute carbonate solutions approximating natural fresh water indicated carbonate alkalinity and pH affected copper reversibility, possibly through variations in buffer capacity or rate of C 0 2 hydration. Recommendation was made that shifts in anodic stripping peak potential or changes in stripping current magnitude with gross changes in solution composition of pH and alkalinity not be used to indicate changes in trace metal speciation.
Knowledge of the chemical state of trace metals in solution is important to understanding trace metal transport, reactivity, and toxicity in aquatic and marine environments. Ion selective electrodes and anodic stripping voltammetry (ASV) are the most popular techniques for studying speciation because of their selectivity and sensitivity. There are several approaches to the application of ASV. In one approach, shifts in stripping peak potentials are interpreted in t h e same way half-wave potentials are interpreted in classical polarography, as a n indication of the formation of reducible complexes in solution (1-4). There is no theoretical basis for this analogy with polarography, but assumptions about electrochemical reversibility are usually made to make the analogy plausible. In a second approach, peak current variations with pH, organic chelate content or carbonate alkalinity are observed and related to t h e formation or dissolution of nonreducible complexes, solid phases, or colloidal species (1.5-9). In yet another approach, ASV is used to measure reducible metal during a complexometric titration. Serial additions of metal are made t o a solution containing ligands which form nonreducible complexes (1,10-12). T h e end point k r e l a t e d to t h e solution concentration of these ligands and the current magnitude is related t o t h e conditional formation constant of t h e complex (13). The first two approaches rely on strict electrochemical reversibility, or adherence to a given degree of irreversibility, throughout gross solution changes of pH, alkalinity, dissolved organic and dissolved solid content, a requirement that is probably unrealistic. The p H and alkalinity of natural water samples vary widely depending on sample location, season, dissolved organic carbon content, temperature, and other factors, and these variations can be expected to affect one or more steps in the electrochemical 0003-2700/78/0350-2104$01.OO/O
reduction of metals during ASV analysis. Therefore, it is important if these approaches are to be used that solution variation effects on reversibility be investigated. Although equilibrium models of trace metals in natural waters are available t h a t consider wide ranges of solution conditions (14-16), there is considerable disagreement among models for Cu in fresh water (3,17,18) and none can be used with certainity to predict principal Cu species a t near neutral p H or to predict electrochemical reversibility. No systematic electrochemical study of Cu reduction in dilute neutral carbonate solutions has been reported although Meites (19) studied the polarographic reduction of copper in 0.25 M carbonate, p H 9.5 to 11.0, and found a doublet wave which he attributed to a stepwise reduction through the cuprous state. Above p H 11.0, a blue precipitate of cupric hydroxide appeared in his solutions. Ernst et al. ( 3 ) performed differential pulse polarography and differential pulse anodic stripping on solutions of varying carbonate alkalinity and pH and reported Cu reduction approximated reversible behavior throughout these changes; however, they reported formation constants for CuC03based on measurements of peak potential shifts an order of magnitude lower than previously reported values. Low ligand numbers and low formation constants were also obtained for a suggested C U ( C O ~ ) ~species. 'In addition to carbonate, their solutions contained 0.1 M KN03. The principal objective of the work reported here was to investigate reversibility of Cu reduction in dilute aqueous carbonate solutions that approximated conditions in natural fresh water (except elevated Cu concentrations were used) and to relate ASV of Cu in these solutions to reversibility. Cyclic voltammetry was used and carbonate alkalinity, pH, and scan rate were varied over a limited range. A secondary objective was to evaluate the method of using voltammetric peak potentials to identify Cu species undergoing reduction.
EXPERIMENTAL All reagents were reagent grade and used without further purification. Standard copper(I1) solution was prepared by dissolving 1.00000 g of 99.999% pure copper wire (J. T. Baker and lo-* N alkalinity were Chemical Co.). Solutions of W4, prepared from 0.1 N sodium carbonate (J.T. Baker Chemical Co.). Potassium perchlorate (Fisher Chemical Co.) and potassium nitrate (Matheson, Coleman and Bell) were used to reduce cell resistance in some experiments. The pH was maintained by purging the cell solutions with a mixture of seaford grade N2 C 1978 American Chemical Society
