Study of Gas-Solid Interaction,” Final Summary Rept., October 1962, Contract No. DA-44-009, U. S. -4rmy Engineer Research and Development Labs., Fort Belvoir, Va.; Proc. of ARPA Fuel Cell Conf., Whiting, Ind., February 1962. (15) Hansen, W. N., Osteryoung, R. A,, Kuwana. T.. J. Am. Chem. Xoc. 88, 1062, (1968). ‘ (16) Harrick, K. J., J . Phys. Chem. 64, 1110 (1980).
(17) Harrick, N. J Turner, A. F., J. Opt. Soc. Am. 56,’253 (1966). (18) Kuwana, T., Darlington, R. K., Leedv. D. W., ASAL. CHEY.36, 2023 (1964j. (19) Lauer, G., Schlein, H., Osteryoung, R. A., Ibid., 35, 1789 (1963). (20) Mark, H. B., Jr., Pons, B. J., Ibid., 38, 119 (1966). (21) Osteryoung, R. A,, North hmerican -4viation Science Center, Thousand
Oaks, Calif., unpublished data, 1966. ( 2 2 ) Schultz, F. A,, Xuwana, T., Case Institute of Technology, Cleveland,
Ohio, unpublished data, 1966. (23) Stratton, J. A., “Electromagnetic Theory,” p. 137, AlcGraw-Hill, New York, 1941. RECEIVEDfor review August 19, 1966. Accepted October 3, 1966.
Oxyhydrogen Flame Excitation of Refractory Elements R. K. SKOGERBOE, ANN
T.
HEYBEY, and G.
H.
Department of Chemistry, Cornell Universify, lthaca, Analytically useful atomic spectra of elements which form refractory oxides in flames have been observed in oxyhydrogen flames (operated normally or reversed) b y forced aspiration of organic solutions at above normal rates. Optimum flame conditions and emission sensitivity p a rameters have been determined for the rare earths and many of the Groups 111, IV, and V elements. Among the advantages demonstrated are: sensitivities which are highly competitive with other flames, elimination of carbon deposition and burner clogging, and the ability to analyze solutions of unusually high salt content without salt deposition.
T
to dissociate or prevent the formation of stable oxides in fuel-rich oxyacetylene (5-8) and nitrous oxide-acetylene ( 1 , 2 , 11) flames has provided striking extensions of the capabilities of flame and atomic absorption spectrophotometry. Several mechanisms for the reactions which occur in the flame have been suggested (3, 6, 7 , 8 ) but the information available to date has been inadequate to validate a particular mechanism. The favored mechanisms, however, all require the presence of carbon species in the flame, suggesting that any flame with adequate populations of these species wbuld produce the desired result. Support for this was obtained for the oxyhydrogen flame fed with organic solutions when Buell (4) and Fassel et al. ( 7 ) observed atomic spectra of several elements of high boiling point. Although high analytical sensitivity was not apparent in these reports, they did indicate that organically supported oxyhydrogen flames might produce analytically useful atomic spectra of many elements not normally excited in this flame. I n this investigation, it was established that sensitive atomic spectra of over 25 elements with a general tendency to form stable monoxides in a HE A B I L I r P
MORRISON
N. Y.
7 4850
flame can be observed in oxyhydrogen flames produced using a total consumption burner and supported by ethanol solutions of the elements. The priniary criterion for this capability requires that all defining parameters of the flame be optimized. It is further shown that the reversed oxyhydrogen flame can also be used to produce atomic spectra of these elements with little or no loss in sensitivity. The principal advantage which accrues from the use of the reversed flame is the ability to analyze solutions of high salt content without burner clogging. Under either operative condition, the flames continue to exhibit background which is significantly lower than that observed with oxyacetylene flames produced in the same manner. EXPERIMENTAL
Apparatus. The instrumentation used and the operating parameters which remained fixed throughout the study are listed in Table I. Table 1.
Excitation system Burner Burner mount Gas regulation Pressure Flow meters Control valves Sample delivery Dispersing system Monochromator Grating Slits External optics Readout system Power supply and amplifier Photomultiplier Recorder
Solutions. The preparation of ethanol solutions of the rare earths has previously been described (7). High purity metals or oxides of the other elements studied were dissolved in an appropriate acid and diluted with absolute ethanol to produce a final solution mhich was less than 1 volunie $& aqueous. Stock solutions of 0.1% metal concentration prepared in this manner mere subsequently diluted with absolute ethanol to prepare the required concentration series. Selection of Analysis Conditions. Flame conditions were optimized by varying the fuel and oxygen flow rates, the flame region viewed by the spectrometer. and the sample feed rate, Regulation of these variables for a certain type of flame fed with a particular solvent effectively provides control over flame variables such as: temperature, flame geometry, sample feed rate, droplet-size distribution, and the populations of species which may be participants in dissociation and/or excitation producing reactions. The use of an infusion pump as the external sample delivery system extends
Experimental Apparatus and Conditions
Beckman fic4020, oxyhydrogen Custom built rack and pinion Custom built Beckman type regulators; 10-p.s.i. hydrogen pressure, 20-p.s.1. oxygen pressure =tly0rotameters; Brooks Instrument Co., Model $6-111024-MFB l/a-inch needle valves at rotameter outlet; Hoke Inc., 280 Series Continuously variable infusion pump; Harvard Apparatus Co., Model #600-900VDC Jarrell-Ash $32000 scanning Ebert, 0.5-meter focal length 1200 line/mm., blazed for 5000 A. Unilateral adiustable straight slits 10-cm. sphescal quartz condensing lens focused on the entrance slit to sample a 5-mm.-high segment of the flame ORNL design ( 9 ) EM1 6255B, 13 stage, 5-13 response, operated at 1900 volts Leeds and h-orthrup, Model G, modified for continuously adjustable 1-20-mv. range and h50-mv. zero suppression; 10-mv. span used
VOL. 38, NO. 13, DECEMBER 1966
a
1821
the delivery range, provides more reproducible delivery rates, and produces an undefined but reproducible control over the many flame variables involved in excitation. For the limited amount of work in which the infusion pump was not used, the sample aspiration rate was controlled by including the total flow rate of fuel plus oxygen as an experimental variable. This approach, however, did result in a significant reduction in the experimental flexibility and the maximum sensitivity because only a limited aspiration rate range was possible. The spectrometer slit width, amplifier sensithity, and recorder response parameters were subsequently adjusted under the optimum flame conditions t o maximize the signal-to-noise ratios and/or minimize spectral interference from adjacent lines or bands when observed. In some instances, higher signal-to-background ratios could be obtained by reducing the slit height, thus reducing the extraneous signal contribution (background, line, or band) from the regions immediately above or below the localiaed region of maximum excitation. The optimum conditions determined for. the systenis studied are presented m Table 11. Changes in the required conditions
Table
Element -4s
Wavelength, mr 235.0
Be Bi
234.9 223.1
DY Er
404 6
ELI
459 I 4
Gd
2
405.8 253.7 410.4
La RiI 0 Nd
461.8 379.8 492.4
Pr
Se Si Sm
495.1 289.8 391.2 204.0 251.6 478.3
Sn Tb Te Ti
243.0 432.7 214.3 399.8
Flame type" N R N N
R
N R N
I
400.8
Sb SC
IS.
R N R N
Tu' N
R
w N
N
R
N
x
N N N N R N N
x
mere observed when different burners were used but these changes mere normally less than 10% for any single variable.
calibration curve, d l / d C , were used t o define the sensitivity after the method proposed by Mandel and Stiehler (10)i.e., the sensitivity, y, is given by
RESULTS AND DISCUSSION
.y=-
Sensitivity Evaluation. Table II also presents the pertinent sensitivity data determined for the elements studied. The wavelengths selected were generally those recommended by Fassel et aZ. (6, 7 ), Gilbert (8),Buell (4) and Dean et al. (5). The sensitivity data were determined by establishing analytical calibration curves using four concentrations ranging from within 'a factor of two of the detection limit to an order of magnitude higher. These determinations using the parameters specified in Tables I and I1 were made by successively scanning the analytical mwelength four times for each concentration standard at a scan speed of 5 A. per minute and a chart speed of 1 inch per minute. The standard deviation was calculated for the difference between the signal and the background (12) using the four measurements made on the lowest concentration. This datum, s, and the slope of the
dI
dC.8 The equation as given conforms with the mthors' preferred definition of analytical sensitivity : the ability to discern a small change in concentration (or amount). Because the same units (mv.) are used for both the standard deviation and the line intensity, the sensitivity i s derived in reciprocal concentration p.p.m.-I Thus, an inunits-eg., crease in the slope of the analytical curve and/or an improvement in the measurement reproducibility produces higher sensitivity. The reciprocal of the sensitivity is directly indicative of the minimum concentration required to produce a discernable change in the signal intensity. To obtain an objective evaluation of the detection limit, some statement concerning the degree of confidence that can be associated with the limit is required. It can be shown that division of the appropriate value of
Optimum Conditions for Analysis and Detection Limits
Slit width, 30 75 75 30 40 40c 20 25 200 30 20 20 30 30 30 40 30 305 30 30 50 30 75 40 35c 40 50 3 5" 70 30 20 260 28 30 30 40
Flame
H Pflow rate, region, mm.
liters/minuteb 7 9 4 4 9 13 8 11 9 5 7 12 3 13 8 15 9 15 10 10 5 8 4.5 4 10 7 8.5 7 4.5 6 8.5 1Ob 7 9 8 8
above tip 26 23 21 23 23 29 26 26 19 30 32 25
21 26 20 27 25 29 20 27 23 24 25 19 28 30 23 25 26 25 17 25 25 25 25 31
Sample feed rate, ml./minute 2.7 2.7 2.7 2.4 2.0
2.0 2.4 2.4 2.4 2.0 2.7 1.6 2.7 2.0 2.7 2.0 2.7 2.4 2.0 2.4 3.1 2.7 2.7 2.7 2.0 2.0 2.7 2.6 3.9 2.7 2.7 1.2 2.0 2.7 2.7 2.4
Sensitivity, p.p.m.-l 1.3 0.9 1.0 1.0 0.6 7.1 1.6 1.6 1.1 520 650 0.4 4.2 7.8 1.7 0.5 1.8 0.2 0.2 0.4 17 1.7 1.0 0.7 1.2 0.9 6.1 0.4 5.2 1.3 0.6 7.3 3.5 1.6 1.7 3300
Detection limit, p.p.m. 2 3 3 3 5 0.4 2 2 3 0,006 0.005 20d 0.7 0.4 2 6 2 15 20d 8 0.2 2 3 8d
3 3 0.5 40* 0.6 2
N 5 R 0.4 409.4 N Tm 0.9 R 2 N 411.2 v 2 N 410.2 Y 0.001 R 398.8 Yb a i\T = normal fiame; R = reversed flame. Optimum oxygen flow rate was found to be 3.0 =t0.2 liter/minute for all elements except Tm in a regular flame for which case it was 4.0 literelminute. c Slit height reduced from 20 mm. t o 10 mm. Preliminary studies on the highly refractory elements; Nb, W, Zr, Re, Ta, La, d Limited by adjacent line or band interference. and Ce indicate their detection limits to be in excess of 500 p.p.m.
1822
e
ANALYTICAL
CHEMISTRY
V 41 1.2 rnF
3.7r 0
5 3.0 0 +
5 1.7
u 15
19
23
27
31
Flame region , rnm. above burner tip
Figure 2. Interaction between the gas flow ratio and flame region for vanadium 12-
Numerical values on the contours are relative signal intensities. The sample feed rate and oxygen flow rate were held at their optimal values
d
Te 2i4.3mp
b IO
-
E-
Hg 253.7 mp
x ._ r
E6 ._
-
._ 234 vr
2
~
~
*
,
:
:
~
;
mized for each point
b. Effect of feed rate on line intensity.
Gas flow rates and flame region were
optimized for each point
the t statistic by the sensitivity effectively accomplishes this purpose-Le., the detection limit is given by
D.L.
=
t(n - 1, 1 Y
cy)
(2)
where n is the number of measurements made for the computation of the standard deviation and 100 (1 - ). is the percentage confidence level required. The use of a value of 3 for t to obtain the data given in Table I1 closely approximates the 9575 confidence level. The use of the standard deviation and the slope a t the detection limit level produced conservative estimates of the detection limits presented because the values specified have all been verified by experimental observation. Even though the definitions used by different investigators vary, useful sensitivity comparisons are obtained when the detection limits presented herein are considered relative to those presented for the oxyacetylene flame (6). The oxyhydrogen flame detection limits given are equivalent to or better than those presented for the fuel-rich oxyacetylene flame operated normally for fourteen of the sixteen elements for which comparisons are possible. When
the premixed oxyacetylene flame (6) is used, the detection limits are better than those for the oxyhydrogen flame for 12 of the 14 elements for which data are available. Extension of these comparisons to atomic absorption data ( I S ) indicates improved sensitivity for the oxyhydrogen flanie emission in 12 of 21 instances. In only a few cases do any of the above detection limits differ by more than an order of magnitude. The data presented in Table I1 also show that there is little or no loss in ultimate sensitivity when the oxyhydrogen flame is reversed. Because the reversed flame floats 4 to 5 mm. above the burner tip, extremely high solids content solutions can be aspirated tvithout appreciable deposition of salts on the burner orifice. This ability extends the analytical sensitivity with respect to solid samples since larger sample weights per unit volume of solvent can be analyzed. Flame Parameter Interaction. Several factors pertinent to the success of this investigation require amplification. Of the flame variables studied, the oxygen flow rate exhibited the least effect on the emission intensities except, of course, when the
burner was allowed to perform its own aspiration function. The use of an infusion pump t o regulate the sample feed rate effectively provides an additional flexible experimental parameter for the specification of flame conditions. Changing the aspiration rate produces ; ~ on the# flame temsignificant effects perature, geometry, fuel-to-oxidant ratio, the droplet-size distribution, the residence time of the sample in the flame, and the populations of potential reaction participants. In essence, there is a high degree of interaction between the flame variables-i.e., a change in the level of one variable produces concomitant changes in the levels of the others. Interactions between the feed rate and the region of maximum excitation and the effect of feed rate on signal intensity a t the optimum flame conditions for each feed rate are demonstrated for three systems in Figure 1. AS expected, the effect depends on the system. In the case of Hg 253.7 mp the optimum fuel and oxygen flow rates remained constant on rariation of the sample delivery rate. I n contrast, the optimum gas flow rates for both T e 214.3 ni,u and V 411.2 mp changed with the delivery rate. The latter situation was most typical for the systems studied. For all the elements studied, a sample feed rate considerably higher than that attainable when the burner performed its own aspiration (1.0-1.5 ml./minute) was required to maxiniioe sensitivity. As indicated by Figure l b , the use of higher sample feed rates enhanced the sensitivity by factors of 3 to 10 or more. Figure 2 presents an example of the effects of changing the fuel-to-oxygen ratio and the flame region viewed a t the optimuni feed rate. The oxygen flow was held constant a t the optinium level, 3 liters/minute. Both primary and secondary maxima are apparent in this example and selection of any of the VOL. 38, NO. 13, DECEMBER 1966
1823
variable combinations specified by these maxima would not have a pronounced effect on the nieasurement sensitivity. Only a few of the systems investigated, howel-er, allow this latitude. The localized nature of the response niaxima in Figure 2 are generally typical and indicate the degree of regulation required to maintain maximum sensitivity and precision of measurement. For the majority of the systems studied, an increase in the signal intensity due to B change in flame parameters was accompanied by a parallel decrease in the flame background. Consequently, the signal intensity changes presented in Figures 1 and 2 are indicative of the
effects of the variables on the signal-tonoise ratio. LITERATURE CITED
(1) Amos, M. D., Thomas, P. E., Anal, Chznz. Acta 32, 139 (1965). (2) Amos, M. D., Willis, J. B., Spectrochim. Acta 22, 1325 (1966). (3) Broida, K. P., Shuler, K. E., J . Chern. Phys. 27, 933 (1959). (4) Buell, B. E., ANAL.CHEM.35, 372
(1963).
( 5 ) Dean, J. A., Carnes, W. J., Analyst 87, 743 (1962). (6) D'Silva, A. P., Kniseley, R. N., Fassel, V. A., AXAL. CHEM.36, 1287
(1964). (7) Fassel, 5:. A., Curry, R. H., Kniseley, R. N., Spectrochim. Acta 18, 1127 (1962).
otornetris cti
( 8 ) Gilbert, P. T., Jr., Xth Colloq. Spec. Intern., 171, Spartan Books, 1963.
(9) Kellev, PI. T.. Fisher. D. J.. Jones.' H. e., h . 4 L . CHEM. 31,'178 (1959). (10) Mandel, J., Stiehler, R. D., J . Res.
Natl. Bur. Std.. A 53. 155 119541. (11) Manning, D. d, A i - Abiorplion Newsletter 4, 267 (1965). (12) bIorrison, G. EI., Skogerboe, R. K
"Trace Anal.: Physical Methods,;' G. H. Morrison, ed., p. 6, Interscience, New York, 1965. (13) Slavin, W., Sprague, S., Manning, D. C., At. Absorption Xewsletter, No. 18, Perkin-Elmer Gorp., Norwalk, Conn., February 1964. in
RECEIVEDfor review July 21, 1966. Accepted September 12, 1966. Research supported by the Advanced Research Projects Agency.
.
rnmonia-
DENNIS G. PETERS and ABDOLREZA SALAJEGHEH Department o f Chemistry, Indiana University, Bloomington, Ind.
b Spectrophotometric measurements of the rate of disappearanceof thioacetamide have substantiated and extended previous information concerning the ammonia-thioacetamide reaction. The rate of this reaction exhibits a first-order dependence on the thioacetamide concentration and a second-order dependence on the ammonia concentration. Third-order rate constants for the ammoniathioacetamide reaction are 0.0055, 0.014, 0.024, 0.037,and 0.059 liter2 mole-2 minute-1 at 40°,60°, 70°,80") and 90" C., respectively, At a given temperature, an apparent increase in the third-order rate constant is observed for decreasing initial ammonia concentrations. This is due to a contribution by the hydroxide-catalyzed hydrolysis of thioacetamide to the overall rate of disappearance of In addition, sulfide thioacetamide. produced by the ammonia-thioacetamide reaction is oxidized by dissolved oxygen. If allowance is made for the latter two side reactions, the rate of disappearance of thioacetamide and the rate of formation of sulfide are equal.
quantitative studies of the kinetics of precipitation of zinc and nickel sulfides from ammoniaammonium ion buffer solutions by thioacetamide have been reported recently (8-4, 9). The rate of precipitation of each of these sulfides is controlled in part by a direct reaction between thioacetamide and the various metal ammine species and also by an amEVERAL
1824
e
ANALYTICAL CHEMISTRY
monia-thioacetamide reaction. This latter reaction is second order in animonia and first order in thioacetamide and produces sulfide ions which react rapidly with a metal cation to form the sulfide precipitate. Interpretation of precipitation reactions involving thioacetamide in aqueous ammonia solutions, and especially elucidation of the nature of the direct reaction between thioacetamide and metal ions, requires knowledge of the rate constant for the ammonia-thioacetamide reaction. The only published investigation of the ammonia-thioacetamide reaction is that of Peters and Swift ( 7 ) , whose results were based solely on measurements of the rate of formation of sulfide. Therefore, it was of interest to measure directly the rate of disappearance of thioacetamide by spectrophotometric means and to test the assumption made by Peters and Swift that the rate of formation of sulfide and the rate of disappearance of thioacetaniide are equal. In ammonia-ammonium ion buffer solutions, the role of the hydroxidecatalyzed hydrolysis of thioacetamide (1) as a source of sulfide is usually unimportant in comparison to the ammoniathioacetaniide reaction. However, as the ammonia concentration decreases and the reaction temperature increases, the contribution of the hydroxide-catalyzed hydrolysis to the rate of disappearance of thioacetamide and the rate of formation of sulfide can become significant. The effect of this hydrolysis on the apparent rate of the ammoniathioacetamide reaction is taken into account in the present study. For some experiments with reaction
times of 2 to 3 hours, Peters and Swift found a marked decrease in the rate of formation of sulfide. Moreover, the sulfide ion concentration itself was frequently less after 2 or 3 hours than after much shorter reaction times. These observations were attributed to oxidation of sulfide by dissolved oxygen. The work sunimarized below includes the results of specific experiments showing the effect of oxygen on the rates of formation of sulfide and disappearance of thioacetamide. Finally, as an aid to the verification of the reaction mechanism proposed earlier, measurements have been performed to determine the influence of the amnionium ion concentration and ionic strength on the rate of the ammoniathioacetamide reaction. EXPERIMENTAL
Reagents. Reagent grade thioacetamide was recrystallized from a two-to-one mixture of benzene and ethanol and was dried and stored over Drierite in a vacuum desiccator. The white solid did not change color over a period of a t least 6 months and gave clear solutions when dissolved in water. Stock solutions, usually 1.00F in thioacetamide, were prepared by weight, but were never kept longer than 1 day. Aqueous stock solutions of ammonia, animonium chloride, and sodium perchlorate were oreoared as described previously ( 7 ) . Amaratus and Procedure. The reac'tion vessel was similar to those used in previous studies ( 1 , 7 ) except that it was fitted with a sintered-glass gas-dispersion tube so that prepurified nitrogen could be bubbled through the a
*
