Anal. Chem. 1981, 53, 651-653
85 I
Reaction Kinetics in the Determination of Silicon by Graphite Furnace Atomic Absorption Spectrometry German Muller-Vogt and Wolfgang Wendl" Kristall- und Materiallabor, Unlversitat Karlsruhe, Engesserstrasse 7, I2-7500Karlsruhe, Germany
A study has been made of the reactions occurring during the determlnatlon of silicon In furnace atomic absorptlon spectrometry in the temperature range between 1000 and 2000 O C . From the dependence of absorptlon on ashlng time and on ashlng temperature, it can be concluded that the slllcate was reduced to silicon up to an ashlng temperature of 1650 O C , and, at temperatures higher than that, the SI formed reacted to yleld silicon carbide. The reactlon klnetlcs of these processes have been analyzed. The measurements were carried out in both uncoated and nloblum-coated graphlte tubes. The enhancement of the silicon slgnal afler treatment with niobium Is caused by an Increase In reaction rate of the reductlon and a decrease In the rate of formation of carblde, In comparison to reactlons In untreated tubes.
Furnace atomic absorption spectrometry (FAAS) has recently been used for the determination of trace amounts of silicon (1,2). The relatively poor sensitivity of this method seems to be due to the formation of silicon carbide. The impregnation of the graphite tube with solutions of metals which form carbides, e.g., Ca, Ti, Hf, or W, enhanced the silicon signal noticeably (3,4). Lo and Christian (5) evaluated a possible mechanism to explain the formation of Si atoms in the graphite atomizer. According to these authors, SiOz was reduced to Si by the carbon of the furnace. Losses of silicon, responsible for lower sensitivity, were interpreted as arising from the formation of S i c and volatile SiO. Sturgeon et al. (6) and Fuller (7) have presented experimental evidence suggesting that the reduction processes are major pathways leading to gaseous atoms. In a recent work, we determined the silicon content in LiNbOs single crystals (8). In this work, we found that coating the graphite tube with Nb also enhanced the Si signal. We hypothesized that carbide formation was prevented by Nb. Up to now, no information has been found in the literature dealing with reduction and carbide forming procewes occurring during the determination of Si. The aim of our studies was to obtain data on these processes by investigations of their temperature and time dependence in the range 1000-2000 "C. The experiments were carried out in both untreated and Nb-coated graphite tubes. The results of these investigations using scanning electron microanalysis of the tube surface led to a description of the chemical reactions occurring during the determination of Si and a calculation of the reaction kinetics. EXPERIMENTAL SECTION Apparatus. A Beckman atomic absorption spectrometer (Model 1272M) was used in connection with a graphite tube atomizer (Model 1271). For all measurements, the 251.6-nm Si line was used. All other instrumental parameters were adjusted as specified by the manufacturers. Reagents. All chemicals used were of "suprapur" grade. Working standards of 100,2, and 1 mg/L Si were prepared by dilution of commercial stock solutions (E. Merck, Darmstadt) of Sic&in NaOH containing 1000 mg/L Si. The Nb solution used 0003-2700/81/0353-0651$01.25/0
for the treatment of the graphite tubes was prepared by dissolving Nb2O6in "0% All solutions were stored in prerinsed polypropylene containers to avoid contamination with Si. Procedure. A 50-pL sample of a solution containing 1mg/L Si was pipetted into the furnace and dried at 180 "C for 60 s. Ashing times varied from 30 s to 12 min. Ashing temperatures were set at 1050, 1200, 1400, 1650, 1760, 1860, and 1960 "C. Atomization occurred at 2800 OC for 10 s for all measurements in the same way. The temperature values correspond to the specifications as supplied by the manufacturer. The realistic temperatures seem to be about 50 "C lower, depending on the age of the graphite tube. The absorption signals were recorded and the peak heights measured. The treated tubes were coated by atomizing 100 pL of a solution containing 1000 mg/L Nb at 2800 "C for 10 s. After the ashing and atomizing cycles the surfaces of the tubes were examined by scanning electron microscopy, and microprobe analysis was used to identify the elements on the surface. The amount of Si used for our FAAS experiments proved to be too low for these methods. After using 50 r L of the stock solution (lo00 mg/L), we obtained well-defied results. RESULTS AND DISCUSSION Uncoated Graphite Tubes. Figure 1shows the resulting absorption of Si as a function of ashing time with ashing temperature as parameter. A relatively slow increase in absorption is observed a t 1200 and 1400 "C while at 1550 and 1650 "C the maximum absorption is reached after 2-3 min. The maximum remains constant for even longer ashing times. A decrease in absorption could not be observed a t these temperatures for up to 12 min. Figure 2 shows the corresponding absorption curves at 1760, 1860, and 1960 OC ashing temperatures. At 1760 "C an increase in absorption to almost the maximum value appears within 3 min, followed by a decrease to almost one-tenth of this value within the next 2 min. At higher ashing temperatures the maximum cannot be observed. Obviously, it is reached a t ashing times which are too short. After firing with 50 pL of the concentrated Si solution, we took scanning photomicrographs of the surfaces of the graphite tubes. The elements found on the surfaces had been indentified by electron microprobe analysis after the various steps of the heating treatment. After the drying cycle, the elements of the stock solution (Na, C1, Si) were found in the center of the tube, Le., the hot zone. Figure 3 shows the surface of a tube after ashing a t 1650 "C. Beads containing only Si, Na, and C1 could solely be identifed on the colder ends of the tube. At ashing temperatures higher than 1650 "C, the beads could still be found in quantities comparable to those a t 1650 "C, but some of them were no longer completely spherical. After the atomization cycle, no more silicon could be found in the hot zone, whereas on the ends of the tube, beads containing Si had been formed. Nb-Coated Tubes. Figure 4 shows the absorption at ashing temperatures of 1200, 1400, and 1760 "C as a function of ashing time in Nb-coated graphite tubes. The increase in absorption is much faster in comparison to the rate in uncoated tubes. The maximum reached is about 10% higher. For temperatures above 1650 "C, the absorption in Nb-coated tubes decreases more slowly compared with uncoated tubes. 0 1981 American Chemical Society
652
ANALYTICAL CHEMISTRY. VOL. 53. NO. 4,APRIL 1981
250
r-----1
501
I/'
P
Figure 3. Scanning electron miaophotcgraph of the surface of a tube treated wiih 50 @I of a 1000 mg/L Si solution: ashing temperature. 1650 'C; ashing time, 10 min.
yl
1505
I\
A
176OOC 1860OC 1960'C
0'
Absuptbn of 1.78 X IO" pml of Si vs. ashlng time for ashing temperatures above 1650 OC in uncoated tubes. Flgure 2.
Thermodynamic and Kinetic Considerations. The Na&i03 remaining in the tube after the drying cycle can react with carbon in the following ways: Na2Si03 + 3C 2Na Si 3CO (1)
-- ++ -+ - +
+ 4C
2Na
Na2Si03 + 2C
2Na
Na2Si03
Si
C
+ Sic + 3CO S i 0 + 2CO
Sic
(2) (3) (4)
The formation of nitrides was not taken intn account because of the low N2 content of the AI used as the inert gas. The temperatures a t which these reactions become thermcdynamically favorable have been calculated as described by Campbell and Ottaway (9). This results in starting temper-
I 2
L
6
8
10
12
14
oshing time lmtnl
Flgun 4. Absaptlon of 1.78 X 10" pmol of S i vs. ashing time In Nb-coated tubes. The dashed line cwresponds to the absorption for 1200 OC ashing temperature in uncoated tubes.
atures of 15OCb1700 'C, assuming chemical equilibrium. The reaction kinetics of these processes can be determined from the absorption curves (Figures 1,2, and 4). The measured absorption of Si is proportional to the amount of Si which has been formed during the ashing cycle. The ab. sorption curves for 1550 and 1650 "C (Figure 1) remain constant even for longer asbingtimes. This leads to the conclusion that all available Na,SiO, was reduced to Si after reaching the maximum value of absorption. The solidified Si droplets are shown in Figure 3. Simultaneous formation of SIC and/or S i 0 acmrding to eq 2 and 3 seems to be rather unlikely. These reactions would lead to various Si:SiC or Si:SiO molar ratios
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
Table 11. Reaction Rates of Sic Formation in Uncoated and Nb-Coated Graphite Tubes kc, mol mol'' min-' 1760 "C 1660 "C 1960 "C uncoated 0.96 0.92 0.94 Nb-coated 0.37 0.39 0.4
Table I. Reaction Rates of Reduction in Uncoated and Nb-Coated Graphite Tubes k R , mol mol-' min-' 1050 1200 1400 1550 1650 uncoated Nb-coated
"C
"C
"C
"C
"C
0.12
0.46
0.73
0.5 1
0.97 1
0.97 1
a t different ashing temperatures resulting in different absorpion maxima. Loss of Si as a vapor or by diffusion through the walls of the graphite tube seems to be negligible at these ashing temperatures. This loss of Si during the ashing cycle would lead to a decrease in absorption with ashing time after the maximum of absorption had been reached. Additionally, the results of the electron microprobe analysis with more concentrated solutions gave no hints to this process. The reduction of Na2Si03may be described as a pseudofirst-order process by the equation -dmNazSi03/dt = k R mNa#iO3
(5)
which yields after integration mNazSi03 = mo e - k R t
(6)
where kR is the reaction rate of the reduction, nothe initial amount of Na2Si03,and mNazSiOsthe amount of unreduced Na2Si03at time t. ~ N ~may ~ be s expressed ~ o ~ by mNazSiOa =
mO
- mSi
(7)
where msi is the amount of Si formed during time t. The absorption of rnsi may be calibrated by totally reducing and then atomizing various amounts of Na2Si03. This calibration curve is valid only for measurements where the ashing is done at 1650 "C. If it is applied to reduction processes at lower temperatures, it may simulate a greater amount of msi, since an additional amount of Si is reduced in the atomizing cycle between the ashing temperature and 1650 "C. It has been proved that these effects can be neglected. With the aid of the calibration curve, we can determine the reaction rate kR of the reduction process between 1050 and 1650 "C from the slope of the plot -log (no - msi) vs. ashing time t. The maximum and the following decrease in absorption, shown by the curve at 1760 "C (Figure 2), may be interpreted as a reduction of Na2Si03to Si, which reacts afterward with carbon to form S i c according to eq 4. The kinetic approach to these reactions may be written as dmSi/dt = kR mNazSiO3 - k C mSi
853
(8)
where kc is the reaction rate of the formation of Sic. In the part of the absorption curve showing the decrease, the remaining amount of NapSi03is very low and rnNa#iOa may be neglected. Therefore, the reaction rate kc has been determined from this part of the absorption curve from the slope of the plot -log ma vs. t. msi is again taken from the calibration curve. At this temperature losses of Si by diffusion through the tube walls and evaporation are also not considered. It is unlikely that a change of temperature of about 100 OC (from 1650 to 1760 "C) should result in such a drastic increase in evaporation or diffusion rate to decrease the absorption within 3 min to one-tenth of its maximum value. Discussion. The reaction rates kR and Itc in uncoated and Nb-coated tubes for different temperatures are listed in Tables I and 11,respectively. The activation energy of the reduction was determined from the slope of the Arrhenius plot log kR vs. 1/T. A value of 32 kcal/mol for uncoated tubes and a value of 9.2 kcal/mol for Nb-coated tubes was obtained. As can be
seen from Table I1 no temperature dependence of kc was observed. These experimental results provide evidence for the following reactions of NazSi03in a graphite furnace during the ashing cycle. The reduction of Na2Si03is observed to begin at about 1200 "C in uncoated and at 1000 "C in Nb-coated tubes. Up to a temperature of 1650 "C neither carbide formation nor losses of Si are detected in either kind of tube. Surprisingly, the reduction rate increases and the activation energy decreases by coating the tubes with Nb. It is possible that the Nb-carbide, which exists in large excess over Na&3i03, takes part in the reduction process, e.g., by incorporation of oxygen into the Nb-carbide lattice. The same maximum reaction rate occurs in both uncoated and Nb-coated tubes. It is possible that this maximum reaction rate is governed by the rate of diffusion of the molecules taking part in the reaction process. At temperatures higher than approximately 1700 "C Sic is formed. A simultaneous reduction of Na2Si03to volatile S i 0 seems rather unlikely. This would yield an absorption curve of different shape. Following microprobe analysis no traces of Si at the colder parts of the tubes could be found until the atomization step. With the formation of S i c in uncoated tubes, diffusion may also play a major role, as the reaction rate is the same as the reduction rate at 1650 "C.In Nb-coated tubes the carbide formation is slowed down considerably. It is possible that the rates of diffusion of Si and C are lowered by the Nb-carbide layer on the surface of the tubes. The increase in the reduction rate and the decrease of the formation of Sic both explain the enhancement of the silicon signal after the carbon tubes have been treated with Nb. A mechanism similar to that found in our studies may be responsible for the enhancement by other metals mentioned in the literature. Further experimental work must be done to obtain a more detailed understanding of the processes which occur on the surface of the graphite furnaces in atomic absorption spectrometry. ACKNOWLEDGMENT The authors thank A. Bienhaus for technical assistance, P. Pfundstein for carrying out the electron microprobe analysis, and R. Hannawald for assistance in preparing the manuscript. LITERATURE CITED (1) Rawa, J. A.; Henn, E. L. Anal. Chem. 1979, 57, 452. (2) Tanaka, T.; Kumamara, T.; Yamamoto, Y . Bunsekl Kagaku 1977, 26, 519. (3) Thompson, K. C.; Godden, R. G.; Thomerson, D. R. Anal. Chlm. Acfa 1975, 289, 74. (4) Ortner, H. M.; Kantuscher, E. Talenta 1975, 22, 581. (5) Lo. D. G.;Chrlstlan, Q. D. Can. J . Specfrosc. 1977, 22, 45. (6) Sturgeon, R. E.; Chakrabartl, C. L.; Langford, C. H. Anal. Chem. 1976, 48, 1792; (7) Fuller, C. W. Electrothermal Atomization for Atomic Absorption Spectrometry"; The Chemical Society: London, 1977. (8) Muller-Vogt, G.; Wendl, W. Mater. Res. Bull., In press. (9) Campbell, W. C.; Ottaway, J. M. Talenta 1974, 27, 837.
RECEIVED for review September 9,1980. Accepted December 19, 1980. The authors are grateful to the Deutsche Forschungsgemeinschaft for financial support of this research project.
