W. J., Mitchell, R. L., ANAL.CHEM.21, 1407 (1949). (3)8-12), but are limited to small samples and substantial amounts of fluoride. Sweetser (14) applied the Wickbold Oxyhydrogen flame combustion method, obtaining complete decomposition of trifluoromethane and tetrafluoromethane. ‘The technique was designed for sample: in the range of microanalysis and potentially was somewhat hazardous. Bailey and Gehring ( 1 ) applied the o\ygen bomb combustion technique to samples of u p to a few grams in the determination of traces of sulfur, fluorine] and boron in organic matwials. Eartkienicz and Robinson ( 2 ) applied the osyhldrogen flame techEVERAL
nique to the trace determination of fluoride, but were forced to rely on a less sensitive titrimetric method when metal irnpurit>iesproduced serious interference with the spectrophotometric method. By a unique combination of a modified oxyhydrogen flame-quartz combustion tube method of Hoggan and Battles ( 7 ) and a spectrophotometric measurement (5, 1 5 ) , fluoride can be determined simply and rapidly a t the low part per million level in organic materials. The procedure can accommodate samples of all sizes with comparable ease. The sample (gas, liquid, or solid in solution) is burned in an oxyhydrogen flame and passed over quartz packing at 1000° C., and the decomposition products are absorbed in boric acid solution. The resultant fluoride is determined spectrophotometrically with zirconiumSPADSS reagent. Triplicate analyses a t the I-p.11.m. level on a given sample can be made in less than 1 hour. EXPERIMENTAL
Apparatus. T h e quartz combustion apparatus is similar to t h a t of Hoggan and Battles ( 7 ) , except that the combustion gas bypass valve is removed, an improved gas control system is used, and a modified design in the blowout port is used. Essent,ially the apparatus consists of a modified oxyhydrogen burner, 1 ; packed quartz combustion tube, 2 ; with manometer fitting, 3 ; and safet,y blowout port, 4 ; condenser, 5 ; absorber section, 6 : and absorbent inlet system, 7, as shown in Figure 1. The blowout. port and manometer fitting
were placed on a side arm of the combustion tube to eliminate condensation in the manometer line. The glassware was purchased from the Greiner Glassblowing Laboratories, Los hngeles, Calif. Figure 2 is a schematic diagram of the specially designed gas control system. A pressure switch in the oxygen line actuates a solenoid in the hydrogen line so that a t pressures below a preset level-e.g., 12 p.s.i. 02-the flow of hydrogen is shut off. In addition the electrical control to the solenoid can be used a t all times to give instantaneous shutoff of the hydrogen. This is essential in the startup and shutdown procedure as described later. The manostat in the vacuum control system is used to control the vacuum a t a preset, level during the combustion step. Figure 3 shows a layout of gas, vacuum, and electrical controls which are necessary for efficient’ operation of the apparatus. Figure 4 shows a small sample reservoir suit,able for analysis of samples containing 1 to 2 p.p.m. of fluoride and higher. Reagents. T h e preparation of the spectrophotometric reagent has been described by Wharton (16). The reagent consists of zirconyl chloride octahydrate and SPL\DNS [4,5-dihydroxy3 - ( p - sulfophenylazo) - 2,7 - naphthalenedisulfonic acid, trisodium salt] in a 1 t,o 12 molar ratio in 0.4.Y HC1. Boric acid absorbent, 274, in deionized water. Procedure. Assemble the apparat,us as shown in Figures 1 to 3. Place the burner in the combustion tube and turn on the cooling air around the burner entrance. Adjust VOL. 3 6 , NO. 9, AUGUST 1964
1821
Sample Reservoir
From Control Panel
Figure 1
the furnace Powerstat to give a temperature of 1000" in the tube. Set the hydrogen pressure a t 6 p.s.i. and the oxygen pressure at 15 p.s.i. Adjust the pressure switch controlling the hydrogen solenoid to close the solenoid a t oxygen pressures below 10 to 12 p.s.i. Adjust the pressure switch in the vacuum system to activate the alarm buzzer a t manometer differences below 20 mm. and above 80 mm. of Hg. Remove the blowout port cover and adjust the auxiliary and burner oxygen flows to 3 and 2.5 liters per minute, respectively, by opening toggle valves
.
Oxyhydrogen flame combustion apparatus
3 and 4 and adjusting needle valves 3a and 4a. Close the toggle valves and replace the blowout port cover. Adjust the manostat to the desired settinge.g., 80 mm. of H g difference-according to the manufacturer's directions. Pipet 25 ml. of boric acid solution into the absorbent inlet cup and attach the cup in place. Weigh accurately a sample containing 5 to 35 pg. of fluoride or more (5 to 10 grams for a 1-p.p.m. detection limit) into the sample reservoir and attach the sample reservoir to the ball joint of the burner valve. With the three-way stopcock in position
A (Figure 1) gradually open valve 2 to reduce the pressure in the system to draw the absorbent solution into the absorber,-rinsing the cup with deionized water. Place the combustion tube in the system by reversing the stopcock to position B. Open the auxiliary oxygen toggle valve, 3, and reduce the pressure to about 40 mm. of H g difference with valve 2 . When the pressure is stable, remove the blowout port cover. Turn on the hydrogen at switch 8 (Figure 2) and gradually increase the hydrogen flow by slowly opening valve 5 (with valve 5a com-
Vacuum Gauges
! -To
System
I
m
L
'
t
I
B
presourc Gauge
Pressure Gauge
Valves 3 8 4
-
/
Toggle Valves
Flowme t e rs
A l l Other Valves - N e e d l e Valves
Figure 2.
1822
rn
ANALYTICAL CHEMISTRY
Schematic diagram of gas and vacuum control system
6
Prossure &upas
Qau@
Llnr
00
System
Vacuum
Vacuum
IO cm. FLOW METERS
02
Vacuum controls
I
00s
controlr
I
Figure 3.
H2
I I
Gas control layout
pletely open) until the burner lights. Replace the blowout port cover and increase the hydrogen flow to 2.5 to 3 liters per minute. Open the burner oxygen toggle valve, 4, and open the vacuum control valve, 2, completely. Turn on the vacuum alarm switch, 7 . When the flame has stabilized, gradually open the sample valve. Once the sample has started burning, increase the combustion rat'e to give a flame 1 to 1'/2 inches long. When most, of the sample has been burned, rinse down th'e sample reservoir with spectrographic grade isopropyl alcohol. Continue burning unt.il all the alcohol has been consumed. Close the sample control valve. Turn off the vacuum alarm s;yst,em. Turn off the hydrogen by opening switch 8. Close auxiliary oxygen toggle valve 3 . Gradually close vacuum control valve 2. When the pressure in the system allproaches atmospheric, close burner oxygen toggle valve 4. Close the hydrogen needle val.;e. Remove the spray trap and rinse with deionized water, pouring the wat,er into the absorber. Drain and rinse the absorber into a 250-ml. beaker, add 5 ml. of 37y0 formaldehyde solution, and heat to boiling on a hot plate to eliminate peroxides. Depending on the fluoride content of the absorber, treat as follows: A. FORLESSTHAN 35 to 40 pg. OF FLUORIDE.Reduce the volume of the absorbent to less than 7 5 ml. by boiling. Cool the solution to room temperature and transfer quantitatively to a 100-ml. volumetric flask. Add 10 ml. of spectrophotomet~ric reagent, dilute t'o the mark with deionizrd water, and mix well. Aft,er 5 minutes and within 1 hour, measure the absorbance of the solution against the reference solution in 1-cm. cells a t 590 mjd. 1%. F O R 1 1 0 R E '1".4h- 40 p g . O F FLUORII)IC. After boiling t,he solut.ion for 5 minutes, cool to room temperat u w and t.ransfer quantitatively to a 250-m1. volumet,ric flask. Dilute to the mark with drionizcd water and mix nell. Pipet an aliquot portion (up t,o 80 ml.) rontaining less than -10 pg. of fluoridr int,o a 100-ml. volumetric flask. .\dd
f
4 cm.
L
10 ml. of spectrophotometric reagent, dilute to t,he mark, and mix well. Measure the absorbance as in A. C. FOR SOLUTIONSOF UNKNOWN FLUORIDE COXTENT. Reduce the volume to about 75 ml. and cool t o room temperature. Transfer to a 100ml. volumetric flask, add 10 ml. of reagent, dilute to the mark, and mix well. If the absorbance is outside the calibration range, pipet a n aliquot of this solution into a dry 100-ml. volumetric flask and dilute to the 100-ml. mark with reagent diluted 1 to 10 with deionized water. Measure t,he absorbance as before. Treat a blank from the combustion of isopropyl alcohol (or solvent used) in the same manner as the sample. For higher concentrations where it may be inconvenient to weigh small samples accurately, prepare a solution of t,he sample, and analyze aliquot portions. Alternatively, burn a larger sample, make t,he absorbent solution to a known volume, and take aliquots for analysis, as in Step B. Prepare a calibration curve from a standard solution of dry, reagent grade sodium fluoride. After plotting the calibration data, calculate values k and A0 to fit an equation of the type pg. F
c- 18 mm. 0.d.
=
k(.-Io - A,)
where k is the slope of the line, A. is the absorbance for 0 pg. of fluoride, and A, is t'he absorbance of the sample. The use of an equation of this type great,ly simplifies calculations when the procedure is used routinely. For routine fluoride determinations make calculations from the equation P.p.m. F = k(A40- .-1,),/grams of sample where 2 , i b is the absorbance of the blank and --I3 is the absorbance of the sample. The value of k should be about 175. In the absence of extractable interferences, a differentiation between organic and inorganic fluoride can be achieved by an extraction procedure. Extract the sample, as is or in a suitable water-immi,-cible solvent, with 1% sodium hydroxide solution. Collect, the
Figure 4.
Small sample reservoir
aqueous layer and treat with 5 ml. of formaldehyde solution. After boiling, treat the solution in the same fashion as the absorbent from the combustion procedure. Treat a blank in the same manner, if a solvent is used for the sample. DISCUSSION
Except for the identit'y of the absorbent solution, the conditions outlined here for the decomposition of organic samples have also been used in this laboratory for the determination of trace sulfur in a variety of organic matrices. For the trace fluoride determination, different solutions were studied as absorbents: water, dilute hydrochloric acid, 1% sodium hydroxide, and 2% boric acid. Erratic results were obtained with all solutions except boric acid. The reasons for this erratic behavior have not been determined. Spectrophotometric studies of the nature of the complex under the conditions of analysis indicate the formation of a l to 2 complex. The curves in Figure 5 result from the application of the slope ratio method (6). Curve I is for increasing amounts of SPADXS in excess zirconyl chloride; curve I1 is for increasing amounts of zirconyl chloride in excess SPADKS. The ratio of the slopes of these two lines is 2.2. The curve in Figure 6 is an application of the method of continuous variations. A value of 1 t o 1.7 is indicated. The studies were carried out by mixing equimolar solutions of the metal salt and dye in 0.4.Y HCI. A 1 to 2 complex of zirconium and SPADSS in HClO, has been reported (IS) and appears to exist under these conditions. A serious restriction in the use of the complex is its estreme sensitivity to perosides and other oxidizing agents. VOL. 36, NO. 9, AUGUST 1964
e
1823
C m . vwiable cmpenent, M xI0'
Figure 5.
Slope ratio method
Constant component 1 X HCI 0 Excess SPADNS Excess ZrOClz
10-3M in 0.4M
I n the absence of fluoride, phosphate, and possibly other interferences, the reagent could be valuable in the trace detection of peroxides. Formaldehyde was found to remove this interference
Table
I.
Recovery of Fluoride Combustion Technique
Micrograms fluoride Taken Found 2 5 11 14 29
9 8 6 5 0
by
Difference, pg.
2 6 6 0 115 140 267
-03fO25 O2fO52 - 0 l f 0 1 -05+10 - 2 3 i 0 3
~
Table II.
Application to Higher Fluoride Levels
Fluoride, p g . Compound Taken Found p-Fluorobenzoic acid 144 8 145 0 Trifluoroacetamide 125 4 125 6 C Y - ( Perfluoropropy1)145 6 145 5 p-cresol
Std. dev. 4 5 3 2 2 0
Table 111. Comparison of Results of Combustion-Spectrophotometric Method and Activation Analysis
Sample number 2924 2925 2926 2927 2928 2929 2930 2932
1824
Fluoride, p.p.m. Combustion Activation 36 10 20 46 8 20 23 20
5 0 4 1
2 3 5 3
45.4 11.3 26.1 48.2 7.9 24.3 25.8 19.2
ANALYTICAL CHEMISTRY
quantitatively after boiling for about 5 minutes and had no adverse effects on the reagent. To illustrate the sensitivity of the reagent to peroxides, 0.1 gram of a dioxane solution of hydrofluoric acid was diluted to 100 ml. with decane. Erratic and high results were obtained for water-extractant solutions which were not treated with formaldehyde, presumably due to peroxides in the d'ioxane. A significant, but constant, blank was found for burning the flame alone for 2 to 12 minutes. Within the limits of experimental error no differences were noted in the absorbance of the solutions resulting from the burnings. An identical blank can be expected for the combustion of varied amounts of isopropyl alcohol. For example, an absorbance of about 0.50 was found for the reagent alone under the conditions specified. I n the presence of 25 ml. of boric acid, the absorbance was about 0.47. A value of 0.45 to 0.46 was found for the flame alone or combustion of alcohol up to 20 ml. Slight leaks around the burner are presumably responsible for the lowered absorbance. With a tight seal a t the burner entrance, the blank will be that of the boric acid only. The blank has been found to remain constant during the course of a day's analyses. The recovery data in Table I were obtained from the combustion of p-fluorobenzoic acid in isopropyl alcohol solution. I n all cases the absorbent solutions were evaporated as in step A of the absorbent treatment. The data indicate that some fluoride is lost during the evaporation step. For most purposes an acceptable recovery level for analysis in the low part per million fluoride range was obtained. In the range where evaporation is unnecessary, losses do not occur and accuracy is high, as shown in Table 11. Table I11 compares results obtained by the combustion-spectrophotometric method and by activation analysis. Although the results are somewhat low, agreement is good in most cases. Once the combustion has been initiated, the only adjustment normally required is in the sample feed rate. With a properly functioning manostat adjusted to the desired vacuum setting, a minimum of operator attention is required. Using the st'artup procedure given, about 3 minutes are required between samples. The absorbent inlet cup can be disconnected and filled during combustion. Vsing a series of small sample reservoirs the nest $ample to be burned can also be weighed out during the combustion step. In this manner three or four samples of about 10 grams each can be burned in 1 hour. The rate of combustion is about 1 gram per minute. The actual rate of combustion can be observed if a sample reservoir
1.6 -
1.4-
2 1.2-
-2 0
1.0-
-E
i 0.8
-
0.0
0.2
Figure 6. tions
0.3
0.4
0.5
0,6
0.7
0,8
0.S
I
Method of continuous varia-
Concentration of each solution. in 0.4M HCl
5 X 10-4M
with a calibrated side tube is used (7). The decomposition of organic fluorides in the oxyhydrogen flame presumably produces hydrogen fluoride. Studies are underway to determine if the quartz tube is attacked, with the production of silicon tetrafluoride. After about' 8 months' usage the combustion tube collapsed because of thermal devitrification. The inner surface of the tube devitrified to a-cristobalite, presumably because of the extreme thermal conditions of the combustion. The fluoride in the absorbent solution could be determined by some other procedure (3, 4). The zirconium-SPhDXS reagent was chosen for its sensitivity, simplicity, and tolerance of interference. The reagent consists of a stable single phase and displays linear spectral absorbance in a range suitable for trace work. Spectrographic grade isopropyl alcohol was found to be ideal as a rinse solvent. The alcohol possesses no interfering materials, so that the amount' used in rinsing need not be precisely measured. .kiditionally, the alcohol burns smoothly. Decomposition as carried out in this apparatus possesses several advantages over other reported methods. The technique, performed au described! is perfectly safe, in contrast to reported experience with the Kickbold apparatus ( 7 ) . The blowout port coinj)letrljeliminates the possibility of shattering of the quartz tube by esl)losions. The gas control system eliminates the possibility of a hydrogen flow in the absence of oxygen. This is important in laboratories in which the apparatu.5 is connected to a common or house uppl ply
of oxygen feeding several locations. Another advantage is that the operator a t all times has independent control of gas flows, pressure, and sample introduction rate. The flexibility appears to be greater than in the technique of Uartkiewicz and Robinson (2’). LITERATURE CITED
( 1 ) Bailey, J. J., Gehring, D. G., ANAL. CHEW 33, 1760 (1961). ( 2 ) Bartkiewicz, S. A,, Robinson, J. W., Anal. Chim. Acta 22,427 (1960).
( 3 ) Belcher, R., Leonard, M. A,, West, T. S.,J . Chem. SOC. 1959,3577. (~, 4 ) Belcher. R.. Leonard. M. A.. West. T. S..TaZanti2. 92 (1959). ( 5 ) Beilack, E., Schouboe,’P. J., ANAL. CHEM.30,2032 (1958). ( 6 ) Harvey, A. E., Jr., Manning, D. L., J.Am. Chem. Soc. 72,4488 (1950). ( 7 ) Hoggan, D., Battles, W: R., ANAL. CHEM.34, 1019 (1962). ( 8 ) Horton, C; A,, “Treatise on Analytical Chemistry, I. M. Kolthoff and P. J. Elving, eds., Part: 11, Vol. 7, pp. 207334, Interscience, New York, 1961. ( 9 ) Hubbard, D. M., Henne, A. I,., J . Am. Chem. SOC. 56, 1078 (1934). (10) Levy, R., “Proceedings of the Inter-
national Symposium on Microchemistry, 1958,” pp. 112-31, Pergamon Press, New York, 1960. ( 1 1 ) Ma, T. S.,ANAL. CHEM.30, 1557 (1958). (12) Milner, 0. I., Ibid., 22,315 (1950). (13) Peshkova, V. M., Mel’chakova, N. N., Sinitsyna, E. D., I z v . Ucheb. Zavedenii Khim. i Khim. Tekhnol. 3. 72 (1960); Anal. Abstr. 8, 3659 (1961): (14) Sweetser, P. B., ANAL. CHEM.28, 1766 (1956). (15) Wharton, H. W., Ibid., 34, 1296 (1962).
RECEIVED for review February 14, 1964. Accepted May 7, 1964.
A Rapid Method for the Determination of Micro Amounts of Sulfur in Selenium LASZLO ACS and SILVlO BARABAS’ Canadian Copper Refhers limited, Montreal East, Quebec, Canada
b Recently published combustion spectrophotometric techniques for the determination of sulfur in metals and alloys, resulting in the formation of the purple-colored sulfur-pararosaniline compound, were investigated as a basis for a method of analysis of sulfur in selenium. The low-melting selenium, however, unlike metals covered by the published methods, has a great demand of its own for oxygen, which dictates the establishment of carefully balanced conditions between the oxygen flow rate, sample size, and the furnace temperature. Moreover, selenium has a marked negative influence on the absorbance of the sullfur-pararosaniline compound if allowed to enter the chromogenic system. The procedure devised takes care of all these factors, and moreover, incorporates numerous modifications in the preparation of colorimetric reagents and the technique of color development, providing greater stability of the pararosaniline solution, substantial increase in riensitivity, reduction in the value of the reagent blank, and complete elimination of the annoying shifting of the calibration curve reported in literature. The procedure has been successfully applied to the analysis of selenium samples containing 5 to 1200 p.p.m. wlfur. It takes 2 hours to complete as compared to two days by the conventional procedure previously used.
S
is an important impurity in high purity selenium .llthough the mechanism under which it affects the physical properties of selenium is not fully understood, it is an experimental fact that sulfur has definite effects on the surface crystallization of selenium ULFUR
( 5 ) as well as on its photo, thermal, and electrical conductivities ( I , 8). For certain uses, the presence of sulfur, even in micro and submicro arnounts, is most undesirable; for other uses, selenium is carefully and purposefully doped with micro and macro quantities of sulfur. I n either case, the most rigorous control of the sulfur content of selenium is demanded. To the best of our knowledge, there is no satisfactory procedure for sulfur in selenium recorded anywhere in the literature. Even the most exhaustive and authoritative treatises on analytical chemistry recently published (3, 4,6), which dedicate considerable space to the selenium analysis, either overlook the analysis of sulfur as an impurity or refer to it only briefly and vaguely. Among the procedures suggested, those based on the separation of sulfur as barium sulfate prevail. Usually, the recommended finish is either turbidimetric or gravimetric. I n our experience, however, this approach is absolutely unreliable for the determination of either micro or macro amounts of sulfur. For example, we found that even on adding as much as 100 rg. of sulfur as sulfate to a concentrated solution of selenious acid and subsequently treating with barium chloride, no visible or measurable turbidity could be observed. On the other hand, where considerably larger amounts of sulfate were present, the results tended to be high because of some coprecipitation of barium selenite with the barium sulfate. T h a t is why, in this laboratory, the analysis of sulfur in selenium always involved the preliminary complete volatilization of selenium by repeated treatment with hydrochloric and hydrobromic acids followed by gravimetric measurement of sulfur
as barium sulfate. I n the case of micro amounts of sulfur where 20-gram samples were used, the procedure took two days to complete. I n a recent article ( 2 ) , a rapid procedure for the determination of sulfur in refined and blister copper was described. The procedure involves igniting copper in a stream of oxygen at 1150” C. and absorbing the evolved sulfur dioxide in a solution of sodium tetrachloromercurate (TCM). A purple color is developed by adding pararosaniline (PR.l) and formaldehyde. I n the course of the same work it was established that, selenium present in concentrations up to I O times t.hat of sulfur had no effect on sulfur analysis. Hence, it was assumed that an analogous procedure for sulfur in selenium should offer no serious difficulties. However, it was soon realized in the course of the experiment,al work that. the sulfur analysis of selenium presents entirely new problems which were not met in the sulfur analysis of copper. For example, in the course of selenium ignition in oxygen, in spite of using several condensers linked in series to retain the fine selenium dioxide powder, appreciable amounts of selenium dioxide were carried over along with sulfur dioxide by the oxygen stream into the receiving cylinder containing the T C M solution. Here the ratio of selenium t.0 sulfur would be over 1 O O O : l . At such high ratios, the effect of selenium was to depress the absorbance due to the sulfur-PRA compound. With increasing amounts of selenium, lower absorbances were obtained for the same amount>sof sulfur. A second, apparent dissimilarity between the copper and Present address, Noranda Research Centre, Pointe Claire, Quebec. VOL. 36, NO. 9, AUGUST 1964
1825
