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).
RECEIVEDfor 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
Figure 1 .
Combustion apparatus
1. Oxygen cylinder 2. Pressure regulator 3. Flowmeter 4. KOH 5. Glass wool 6. Neoprene stoppers 7. Tygon tubing 8. Push rod 9. Furnace 10. Combustion tube 1 1 . Thermometer 12.-13. Selenium trop 14. Crushed glass, coarse 15. Crushed glass, flne 16. Glass wool 17. Neoprene ring 18. Receiving cylinder with TCM 19. Gas dispersion tube
selenium samples, as far as the sulfur analysis is concerned, was the following: in the case of copper samples, almost regardless of the size of the sample, sufficient oxygen was always available to combine with sulfur; in the case of the selenium samples, the requirement of complete oxidation of selenium involved careful balancing between the size of the sample, oxygen flow rate, and the furnace temperature. As a by-product of this investigation, important modifications were introduced into the preparation of reagents and the color development to bring about significant improvement in the sensitivity of the sulfurPRA reaction and the stability of the resulting color. EXPERIMENTAL
Apparatus. The apparatus is shown in Figure 1. It consists of: Dietert High Temperature Varitemp Furnace Model No. 3420 (H. W. Dietert Co., Detroit, Mich.). Combustion Tube and Boats. The combustion tube was in Corning Vycor Brand Glass with glazed open ends, 900 mm. long X 21-mm. i.d. X 25-mm. 0.d. It withstood the operating temperatures for several months of continued daily use. Combustion boats, Vitreosil Brand, glazed, translucent, 78 mm. long X 17 mm. wide X 10 mm. high can be reused many times if kept in a desiccator between use. Selenium Trap. Borosilicate glass tubing, standard wall, one 122 cm. long X 28-mm. 0.d. X 1.5-mm. wall is connected by means of short, 10 mm. wide glass tubing a t one end to the combustion tube and a t the other end to a second standard wall tubing, 50 cm. wide X 18-mm. 0.d. X 1.5-mm. wall. One third of the longer tubing, next to the combustion tube, is filled with coarse glass chips of about 10- to 15-mm. diameter, while the remainder of the tubing is filled with crushed glass of 1826
0
ANALYTICAL CHEMISTRY
about 2- to 3-mm. diameter. The shorter tubing is filled with 3 grams of evenly-spread glass wool. Sample Form. Selenium Powder or Shot. Spectrophotometer. Beckman B. Model, with matched 1-cm. cells. Reagents. Sodium Tetrachloromercurate, 0.2M, and Formaldehyde Solution, 0.2%. Both reagents were prepared as previously described (2). Pararosaniline Hydrochloric Acid Solution, 0.01%. One gram of rosaniline hydrochloride (Eastman P1378) is weighed in a 250-ml. beaker. One hundred milliliters of 12N hydrochloric acid is added and mixed to dissolve. This is filtered through glass wool into a 1000-ml. volumetric flask and the beaker and glass wool are washed with another 100 ml. of hydrochloric acid. Another 400 ml. of hydrochloric acid is added, then make up to the mark with distilled water. Ten milliliters of the above solution is pipetted into a 100-ml. volumetric flask and diluted to the mark with water. Standard Sodium Sulfite Solution, 100 p.p.m. of Sulfur. Anhydrous sodium sulfite (0.396 gram, approximate assay 99.4y0) is transferred into a dry, 1000-ml. volumetric flask. T C M solution (150 ml.) is added and mixed to dissolve. This is adjusted to the mark with additional T C M solution. Any desired dilutions from the above solution are always made with the T C M solution, never with water. Calibration. To a series of seven 100-ml. volumetric flasks add 1, 2, 3, 4, 6, 8, and 10 ml. of the freshly prepared standard sodium sulfite solution containing 5 pg. of sulfur per ml. Make up the volume to 12 ml. with the T C M solution. To an eighth 100-ml. flask add 12 ml. of the latter solution to serve as the reagent blank. Adjust the temperatures of all solutions to 25" f 0.5" C. by keeping the flasks in a thermostat for as long as required. Keep in the same thermostat the PRA solution, formaldehyde, and distilled
water. Add to each flask 5 ml. of the PRA solution, 5 ml. of the formaldehyde solution, and make up to the mark with water after 20 seconds have elapsed. Mix well and place all the flasks in the thermostat. Similarly, prepare a second series of standards, starting with 2.5, 5, 7.5, 10, 15, 20, and 25 ml. of the same sodium sulfite solution and adding 12.5 ml. of each PR-4 and formaldehyde solution into 250-ml. volumetric flasks. Allow the color to develop for 90 minutes, then read the absorbances a t 557 mp vs. water in the reference cell. Deduct the absorbance of the reagent blank from that of the standard solutions. Plot a graph showing the absorbance in abscissa us. micrograms sulfur in the ordinate. Procedure. Set the furnace temperature at 800" + 5" C. and the oxygen flow rate at 1 liter per minute. With the aid of a push rod introduce an empty preignited boat t h a t was kept in a desiccator into the hot zone of the combustion tube. Ignite the boat for 10 minutes, collecting the combustion gases into a cylinder with a bulb a t t h e top to retain the froth. The cylinder should contain either 12 or 30 ml. of the T C M solution depending on the level of sulfur concentration in the sample. At the end of the combustion period remove the fritted glass bubbler after rinsing i t with water and place it immediately into the next cylinder containing the T C M solution. Treat similarly boats with samples weighing not more than 0.7 gram and containing not more than 250 pg. of sulfur. Cleaning the -1pparatus. After collecting about 15 grams of selenium dioxide powder in the trap, the system has to be cleaned. Empty coarse and crushed glass separately into large beakers and wash first in hot nitric acid then in water. These are subsequently dried a t 110" C. for 4 hours. A11 glass tubing is washed in 1: 1 hydrochloric acid, water, and methanol. DISCUSSION
Effect of Selenium. As already mentioned in the introduction, i t was assumed at the early stages of this investigation t h a t selenium might not interfere at all in the reaction between the dichlorosulfitomercurate on one hand and the P R A and formaldehyde on the other hand. However, the preliminary tests showed clearly t h a t the amount of selenium entering the T C M solution and its effect on sulfur determination were by no means comparable to anything we experienced in the sulfur analysis of copper. In a more systematic approach aimed a t establishing the true effect of increasing amounts of selenium on the behavior of the sulfur calibration curve, 1, 10,30, 50, and 100 mg. of selenium were added to solutions containing 15, 25, 35, 50, and 100 pg. of sulfur. The resulting absorbance values are plotted in Figure 2
b
[
Figure 3. Absorbance of sulfur-PRA compound as function of selenium concentration
0.300
25 K g . of S added to a11 solutions
om0
0100
20
40 60 80 MICRWPAMS, !SULFUR
100
solution, then a portion of it in 6% HC1 Retention of Selenium Dioxide. to obtain a 0.047' PRAisolution. HowFor t h e reasons given above, it was ever, t'he concentrated PRA solution was essential t o develop in the very beginning of this investigation a t r a p more of a suspension than a solution in with the exception of those relating to t h a t would effectively remove all spitmeof being filtered t.hrough a frit,tedthe selenium additions of 50 and 100 mg. selenium dioxide carried by the oxygen glass crucible. A s a result, each portion Here, absorbances read were meaningstream and prevent it from entering the of it, taken for dilution in 6% HCl, had less since the solutions became turbid T C M solution. I n a first approach a different, PR.4 content. Thus the and the turbidity increased with sulfur some glass chips were placed near shifting of the calibrat'ion curve was and selenium concentrations. It is apthe exit of the combustion tube and at,tributed on one hand to the slight parent from the curves plotted that for four condensers were connected in series. oxidation of the sodium sulfite solution the high Se/S ratios considered, the However, as explained earlier, this syson standing before use, and on the other effect of selenium on the absorbance of tem was ineffective since up to 50 mg. of hand to the impossibilit'y of reproducing the sulfur-PRA compound cannot be selenium could be detected in the TCM a P R h solution of identical P R h conignored. centration. I n consequence, an imsolution following the combustion of I n a second experiment, aimed at each selenium sample. I n a second trial, provement, in tmhepreparation of the two establishing the maximum selenium a glass tubing, 48 inches long X 28-mm. reagents was sought and found in discontamination that (-an be tolerated 0.d. was filled with crushed glass, solving the anhydrous sodium sulfite in without any adverse effects on sulfur inch diameter and inserted between the the TCM solution and the rosaniline analysis, to a fixed amount of sulfur combustion tube and the scrubber soluhydrochloride in -concentrated hydro(25 pg.) increasing amounts of selenium tion. Here one could follow visually the chloric acid. The latter approach was were added from 25 to 10,000 pg. The gradual progression of the white selement,ioned but not substantiated in an graph in Figure 3 shows clearly that nium dioxide powder through the tubing earlier reference ( 7 ) . Figure 4 shows selenium in concentrations up to 10 with each successive sample. Already, the erratic shifting of the calibration times that of sulfur does not affect in running the second sample, traces of curves over a period of several months seriously the sulfur analysis. This is in selenium could be detected in the in using the I\-a2SO3and P R 1 solutions accordance with the findings previously scrubber solution. The amount of selein water. Figure 5 shows, in contrajt, reported ( 2 ) . However, for selenium nium found in the scrubber solution the remarkable stability of the calihraconcentrations 40 times that of sulfur, increased with each successive sample tion curve over a comparable period of the absorbance value due to the sulfuruntil the tubing was fully saturated with time when the h'a2SOs in TCM and the PR.1 compound was reduced by about selenium dioxide. From that point on, PR;I in HC1 solutions were used. 25% in respect of a system free from each selenium sample on being ignited selenium. mould push ahead approximately the same amount of selenium dioxide powder which would end up in the scrubber solution. Following numerous modifications to the above system dictated by the experimental evidence, the final arrangement that proved fully successful was that described under Apparatus. Cnder such an arrangement, not a trace of selenium could be detected in the TCM solution, even after the apparatus was in use a full day. Instability of TCM and PRA Solutions in Water. I n running numerous tests in connection with this investigation, a serious drawback was experienced, inasmuch as the calibration curve continued shifting erratically with each new sodium sulfite ---Am a n d l o r P R L i solution. These two -Y m i w ma solutions were prepared in accordance Figure 5. Stability of calibration with what is generally reported in the curve over a 6-month period using Figure 4. Typical shi,fting of calibrasolutions of Na2S03 in TCM and PRA literature-it., Na2S03 in water and tion curve over a 1 -year period using solutions of Na2S03 and PRA in water in HCI PK.1 first in water t o obtain a 1% P R A Figure 2. Effect of selenium on varying amounts of sulfur
I
..(I
VOL. 36, NO. 9, AUGUST 1964
1827
The stabilities of each of the two reagent solutions were also proven separately in the following manner: SODIUhl SULFITE SOLUTION. SOlUtions of sodium sulfite in T C M containing 50 pg. of sulfur were daily prepared over a period of a week and treated with the same dilute P R h solution. The mean deviation in absorbance was less than 0.002 for an average reading of 0.355, which is insignificant. Similarly, the stability of the Xa2S03solution in TCM was proved over a period of 48 hours. Fresh portions of the same Na2S03 solution taken after 2, 24, and 48 hours and treated with the same PRA solutions gave the following absorbances: 0.351, 0.355, and 0.346, respectively. PARAROSANILINE SOLUTION.Whereas in dissolving PRA in water, anywhere from.one fourth to one third of the total rosaniline hydrochloride remains undissolved, the dissolution in hydrochloric acid is immediate and practically complete (except for an insignificant amount of residual matter). The PRA solution in water is a very dark, nontransparent green; the PRA solution in HC1 is perfectly clear and reddish. I n diluting 10-fold the PRA solution in water, the color turns from green to cherry-red; in diluting 10-fold the PRA solution in HC1, the color turns from red to light green. The most important improvement realized by changing from aqueous to acid P R h solution on one hand and from the 0.04 to O.Olyo PRA solution on the other hand was the drastic reduction in the absorbance of the reagent blank which dropped from approximately 0.20-0.25 to approximately 0.06-0.08. The boat blank using quartz boats was in the 0.01 to 0.02 range. The significance of it will be quite apparent if it is said that an absorbance of 0.2 is equivalent to approximately 25 pg. of sulfur. The mean deviation in the absorbance values of the reagent blanks over a period of a week using daily freshly prepared dilute PRrl solutions from the same concentrated PRA in HC1 solution was less than 0.004 corresponding to less than 0.5 pg. of sulfur. The comparative mean deviation using the Na2S03and PRA solutions in water was 0.025 corresponding to 3 pg. of sulfur. Figure 6 shows the effect of varying PR.4 concentration and time of color development on the absorbance of the S-PRA compound. It is apparent from the graph that although the net absorbance due to the sulfurPRA compound is reduced from 0.59 to 0.35 in changing from the 0.04% to the O.Olyo P R h solution, the fact that the reagent blank is being reduced a t the same time from 0.26 (equivalent to approximately 35 pg. of S) to 0.065 1828
ANALYTICAL CHEMISTRY
I
PRA 0.04% U
U
0.80
0.60
c
w2 0.40
I
I
1 1 NET ABS.
0.20
I 30
60
90
120
I 180
I50
TIME IN MINUTES
Figure 6. Absorbance of S-PRA compound as function of PRA concentration and time of color development .SO pg. of S added to all solutions
(equivalent to approximately 8 pg. of S) makes the change worth it. Use of still more diluted P R h solutions would not have been justified, since any further drop in'the net absorbance would not have been compensated by a comparable drop in the reagent blank. To summarize, the most important improvement realized by changing from the o.O4y0 PRA solution in water to the 0.01% P R h solution in HCl was the attainment of a greater stability of the PRA solution on one hand and an appreciable reduction in the reagent blank on the other hand. Because of the greater solubility of PRA in HCl and the different nature of the PR.4 compound in HCl against that in water, the net absorbance due to any given amount of sulfur in the O.OlyoPRA solution in HCl remained about the same as the net absorbance in the 0.04% PRX solution in water. Relationship between Sample Size, Oxygen Flow Rate, and Furnace Temperature. As previously indi-
cated, in the analysis of sulfur in selenium b y the combustion method, the demand for oxygen is far greater
Table I. Typical Sulfur Results for Some Sulfur-Doped and Other Selenium Materials
Material Lot Sulfur-doped selenium D A-2504 D A-2 505 DA-2508 DA-2509 Selenium powder
90800 39
Recovered selenium
..
S,
p.p.m.
38.1 37.9 77.6 76.0 80.0 80.4 85.6 84.0 173 184 319 334 1120 1120
than in the comparable analysis for sulfur in metals. To release all sulfur present in the sample as sulfur dioxide, the complete oxidation of selenium to selenium dioxide is a prerequisite. I n using a 0.5-gram sample and setting the oxygen flow rate a t 1 liter per minute, it was established that for temperatures below 750' C. and above 850' C. the oxidation of selenium was incomplete as demonstrated by the condensation of some red selenium in the selenium trap. The lower the temperature below 750' C. and the higher the temperature above 850" C., the more red selenium could be seen. For the furnace temperature range 775" to 825" C. the conditions of selenium oxidation and sulfur recovery were optimum. I n the selenium trap only snow-white selenium dioxide powder could be seen. The unexpected showing of elementary selenium a t temperatures higher than 850" C. was tentatively explained by the presumable formation of some SeO which on reaching colder areas dissociated to give selenium and selenium dioxide in accordance with the reaction: 2Se0 + Se SeOz. The oxygen flow rate was maintained a t 1 liter per minute for all experiments because of the excessive foaming it generates at higher rates in the distillation receiver containing the T C M solution. In fixing the furnace temperature a t 800' C. and the oxygen flow rate a t 1 liter per minute, it was established that samples weighing up to 0.7 gram were completely ignited under the given conditions. However, in igniting larger samples incomplete oxidation of selenium resulted. This can be remedied by increasing the oxygen flow rate and providing a receiver of such size and design to avoid the overflow of the froth. In one instance, where a 1-gram sample was used, the oxidation of selenium became complete when the oxygen flow rate was increased to 1.2 liters per minute. Accuracy and Precision. Two samples of sulfur-doped selenium containing (on the basis of the chemical gravimetric analysis) 27.8 and 77.4 p.p.m. sulfur were analyzed in quadruplicate by the combustion-spectrophotometric method. The results obtained were 22.8 and 70.1 p.p.m., respectively, suggesting a sulfur recovery of approximately 807, a t the lower concentration level and 90% a t the higher level. The reproducibility of the results a t both concentration levels was remarkable, the mean deviation being 0.4 p.p.m. a t the 20p.p,m. range and 1.1 p . p m at the 70-p.p.m. range. This high precisioii of analysis makes it possible to apply a correction factor for incomplete sulfur recovery whenever this is desirable.
+
The accuracy of the PH.4 method in the sulfur analysis below 20 p.p.m. was established by mixing portions of the sulfur-doped selenium with sulfur-free selenium. Thus samples containing 11.4, 5.1, 3.7, and 2.2 p.p.m. of sulfur were assayed as 10.7, 5.4, 4.0, and 2.3 p.p.m., respectively. Analysis of Selenium Very High a n d Very Low in Sulfur. I n using Vitreosil boats and approximately 0.5- to 0.7-gram selenium samples, one can read easily from the two calibration curves from 3 to 300 pg. of sulfur corresponding to 5-600 p,p.m. sulfur in the sample. Larger sulfur concentrations can be determined using either smaller size samples or portions of the T C M solution in which SO, was collected. In this laboratory the largest
sulfur content determined in selenium was 0.112%. Attempts made to extend the applicability of the procedure to the determination of sulfur below 5 p.p.m. by collecting SOz from several 0.5-gram samples into the same T C M solution failed because of the inability to account properly for the boat blanks for each sample weight. RESULTS
Duplicate values for some typical selenium materials are listed in Table I. LITERATURE CITED
(1) Askerov, Ch. M., Aliyev, G. M., Seriya Fisiko-Mat. i Tek. Nauk 5, 45
(1952).
(2) Barabas, S., Kaminski, J., ANAL. CHEM. 35, 1702 (1963). (3) Furman, N . H., “Standard Methods of Chemical Analysis,” Sixth ed., Vol. I, Van Ir’ostrand, New York, 1962. (4) German Society of Non-Ferrous Metallurgical and Mining Engineers, “Analyse der Metalle,” Second ed., Vol. 11, Springer-Verlag, Berlin, 1961, ( 5 ) Henish, H. K., “Rectifying Semiconductor Contacts,” Clarendon Press, Oxford, 1957. (6) Kolthoff, I. M., Elving, P. J., “Treatise on Analytical Chemistry,” Part 11, Vol. 7, Interscience, New York, 1961. (7) Pate, J. B., Lodge, J. P., Wartburg, A. F., ANAL.CHEM.34, 1660 (1962). (8) Resnikov, M. Ya., Referal. Zhurnal., Metallurgiya 7, 182 (1959). RECEIVEDfor review January 9, 1964. Accepted June 4, 1964. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1964.
Ana lysis of Bound Styrene-Bu tad iene Content of Copolymers by Infrared Absorbance Ratio Method ARTHUR S. WEXLER Dewey and Almy Chemical Division, W. R. Grace and Co., Cambridge 40, Mass.
b Butadiene-styrene composition ratios of insoluble emulsion copolymers are readily determined by infrared analysis of pressed or cast films. The composition ratio is a linear function of the ratio of Ihe 1639 cm.-1 vinyl and the 1601 cm.-l phenyl absorption bands measured from suitable base lines. Standard deviations of 0.2% are obtainable. The absorbance ratio method is readily adaptable to data transfer between different spectrophotometers.
A
INFRARED procedure for determination of copolymer composition of butadiene-styrene polymers does not seem to have been reported either in the literature or in the methods manuals of several of the rubber companies. Therefore, we are reporting a rapid, precke procedure for determination of styrene-butadiene emulsion copolymer (composition by an infrared absorbance ratio method. Most of the infrared procedures reported in the literature ( I , 9, 4, 6) are not applicable to commercial emulsion copolymers because of their low solubility in suitable solvents. 1:’rocedures based on film analysis such as those reported by Dinsmore and Smith (2) and Miller and Willis (5) are either too sketchy or are not applicable for ,& wide range of copolymer composition We have been successful in developing a procedure applicable to emulsion copolymers of SATISFACTORY
butadiene and styrene over a wide range of composition ratios. The technique is applicable also to mixtures and more complex polymers containing butadiene and styrene. EXPERIMENTAL
Rubber samples were either coagulated (latex) or extracted (slab rubber) with either neutral or acidified (0.05N sulfuric acid) isopropanol. Usually 5 ml. of latex was diluted with 5 ml. of water and poured slowly into 100 ml. of isopropanol under agitation in a Waring Blendor. After centrifuging, the coagulated sample was partly dried between filter paper layers and then pressed in a cold press to a thin film. The thin film was subjected to complete drying at 100” C. (30 minutes) in an oven and then repressed to clear films of about 0.05- to 0.1-mm. thickness in a steam press between Tefloncoated aluminum plates. Films were mounted in cardboard frames for transmission measurements. Films of poor mechanical stability were pressed on a supporting layer of aluminum for specular reflectance measurements. Good results were also obtained with samples of high butadiene content by specular reflectance measurement of air-dried films cast directly from latex on sheets of highly reflecting aluminum. Peak heights were measured from suitable baselines with a ruler to the nearest 0.25 mm. which corresponded to 0.001 absorbance on 10-inch charts. Peaks less than 20 mm. were measured
with a Bausch & Lomb magnifier with a suitable reticule. A Beckman IR-9 spectrophotometer was used for primary standardization work. The instrument was purged with dry air to keep water vapor to a minimum. Where necessary, instrumental balance was obtained with either reference or sample beam attenuation. Conditions were set as follows in most cases: slit, 0:60 a t 1600 crn-1; gain, 0.075-0.125; SB/DB, about 1-1.5; period, 2.0; scan speed, 40-80 ern.-’ per minute; slit control in select; absorbance, 0-1. The absorbance per 0.1-mm. film thickness was determined to be 1.8 for the polystyrene 1601-cm.-’ band and 0.75 for the polybutadiene 1639cm.-l band under these conditions. The ratio of the absorbances of the 1639-cm.-’ peak to the 1601-cm.-l peak (absorbance ratio) is the experimental datum required for analysis of polymer films for S B copolymer composition. RESULTS
The absorbance ratio-composition data for emulsion copolymers of styrene and butadiene are closely approximated by the equation: Butadiene weight fraction = 2.37R
+ 0.07
(1)
where R is the absorbance ratio of the 1639-cm.-l vinyl band of butadiene to the 1601-cm.-l phenyl band of styrene measured from suitable baselines as shown in Figure 1, for the 46.6y0 exVOL. 36, NO. 9, AUGUST 1964
* 1829
