Determinaition of Microgram Amounts of Total Sulfur in Rocks Rapid and Accurate Analysis by a Combustion Method J. G. SEN GUPTA Geological Survey of Canada, Ottawa, Ontario, Canada
b For r a p i d and accurate determination of micro amoimtsof total sulfur in a variety of rocks, the sulfur trioxide formed b y igniting the sample with vanadium pentoxide at 900" to 950" C. in a current of nitrogen i s reduced b y heated copper to sulfur dioxide. The latter i s determined titrimetrically after absorption in a known excess of potassium iodate-iodide solution b y back-titrclting the excess with thiosulfate, or spectrophotometrically with pararosariiline and formaldehyde after absorption in sodium tetrachloromercurate solution. Results for several National Bureau of Standards standard samplcs agreed closely with certificate values. Application to several acidic, basic, and ultrabasic rock samples (sulfur contents 0.01 to 0.8%) yielded resuhs considered superior to those obtained b y the conventional gravimetric: barium sulfate method.
T
need for an accurate and rapid method for t'he determination of small amounts of sulfur in rocks has Iwen cmphasized by the ivork of the Geological Survey in connection with thc International Upper Uantle Project. The conventiond method of fusion of the sample with sodium carbonate and sodium peroside Followed by leaching n i t h water, filtration, acidification, and precipitation of sulfate as barium sulfate is time-consuming and is susceptible t o serious erIors due to the incomplete precipitation of small amounts of sulfate and coprecipitation of silica which i> usually prcs~intin overwhelming amounts. In basic and ultrabasic rock samples chromium may interfere hy the coprecipitatio.1 of barium chromate with barium sul'ate. Sulfur can be effectively rcmoi-ed from ores, concentrates, refractory. and other inorganic materials by comhuation \vith air or oxygen ( 1 , 3-5, S I O ) , but combustion in a current of oxygen rc,,:iiiies a prohibitivdy high temperature of 1450" C. if < h e sulfur trioside formed in the initial stage is to be completely converted to dioxide ( 9 ) . HI:
COMBUSTION TUBE
NITROGEN
--
COPPER
-
--
~
I =2
RUBBER STOPPER
COMBUSTION BOAT
,
1-
--'>-GLASS.-(
WOOL I TETRACHLOROMERCURATE SOLUTION
-2
Figure 1.
HEAT
PUARTZ-
DEFLECTOR
WOOL
\
\
BOROSI LlCATE GLASS JOINT
BOROSILICATE GLASS RECEIVERS
Combustion tube and assembly
This considerably shortens the life of the combustion tube and requires an expensive high-temperature furnace. Hagerman and Faust (6) determined wlfur in inorganic sulfates, silicaalumina catalysts, and suspensions of barium sulfate in oil by igniting the material with vanadium pentoxide in a current of air at 900' t o 950" C., and making the final determination by alkalimetric titration of sulfuric acid. Larsen, Ross, and Ingber (?') modified this method by using nitrogen as the carrier gas and reducing sulfur trioxide to dioxide b y heated copper and determined sulfur in uranium trioxide, sodium zirconium fluoride, and hydrofluoric acid. This latter technique mas employed by Bloomfield (a) for the determination of sulfur in soils, but it has not been applied to the analysis of rock.. Bloomfield'5 technique has been improved by the present author by employing an electric furnace in place of burners for more uniform temperature control and uqing a feries of t n o receivers for efficient trapping of sulfur dioxide. The proposed titrimetric method for final determination of wlfur dioxide is more rapid than the colorimetric method employed by Lar-cn. ROSYand Ingber ( 7 ) and by Bloomfield ( 2 ) , and is now used in the Geological Survey for the determination of d f u r in a I ariety of rock types. EXPERIMENTAL
The combustion furnace was a Burrell high temperature electric furnace, Model €€-2-9, Apparatus.
I NALGON TUBING
having silicon carbide heating elenieiits with aluminized ends and provided v i t h a thermocouple and a pyrometer indicator. A 13urrell high temperature Zircum 30-inch combustion tube (lljz-inch 0.d.) with t'apered end lvas used in this h d y , but a similar mullite combust'ion tube could be used. The t'ube was packed a t the hot zone 11-ith either copper turnings or a roll of 100-mesh copper gauze, followed by a roll of quartz i ~ o oto l filt,er out any combustion dusts (Figure 1). The tapered end of the combustion tube was joined to two borosilicate glass receivers by a short piece of Salgon tubing. A heat deflector guarded the rubber stopper u>ed a t the inlet (cf. Figure 1). Translucent silica combust,ion boats, i 8 X 17 X 10 mm., were preheated in a muffle furnace a t 1000" C. for several hours to free them from sulfur and then stored in a desiccator. After use the boats were cleaned for further use by heat'ing them with concentrated hydrochloric acid. Matheson prepurified nitrogen gas was used after successive passage through solutions of sodium hydroxide and tetrachloromercurate to absorb any t'race of SOs present. 1 Ueckman Ir\Iodel I3 spectrophotometer nit,h matched 1-cm. borosilicate glass rectailgular cells was used for absorption measurements. Reagents and Solutions. Deionized water was used for t h e preparation of all reagent solutions. Sbandard sulfate solution was prepared from anhydrous potassium sulfate and standardized by the gravimetric barium sulfate method (11). A solution containing 60 pg. of sulfur per ml. VOL. 35, NO. 12, NOVEMBER 1963
1971
was obtained by diluting an aliquot of this stock solution. Vanadium pentoxide was prepared from analytical reagent grade ammonium metavanadate by first heating in a quartz dish for 4 to 5 hours at 550" C. with occasional stirring and finally heating at 1000" C. for several hours to free from any sulfur impurity. The molten mass was cooled and stored in a desiccator. The hardened mass was removed from the dish by gentle tapping at the back. As finely powdered vanadium pentoxide may react with atmospheric sulfur dioxide if stored for a long time, it was ground to a very fine powder in an agate mortar immediately before use. Analytical grade Celite (diatomaceous silica) was used. Standard potassium iodate solution was made b y dissolving 1.0703 grams of potassium iodate in 1 liter of water, 1 ml. = 0.000481 gram of sulfur. An exact empirical factor was determined following the general procedure outlined below for a blank combustion. Sodium thiosulfate solution was made u p so that 4.5 ml. were equivalent to 1 ml. of the above potassium iodate solution. A solution of lower concentration was obtained by diluting an aliquot. Starch-iodide solution was made by adding 4.5 grams of Dotassium iodide to 100 $1. of- starch solution containing 0.45 gram of soluble starch. Hydrochloric acid, 0.2N, was used. Sodium tetrachloromercurate and pararosaniline hydrochloride solutions were prepared as described (2). A 2% (v./v.) aqueous solution of formaldehyde was used.
Table I. Recovery of Sulfur f r o m Potassium Sulfate b y Combustion with Vanadium Pentoxid e 0.158126 gram NazS& 3 0.03567 gram KIOs KIOa Mean Sulfur Sulfur soln. error, found, added, taken, ml. mg. mg. mg. 0.025 11.05 11.0 24 11.0 0.007 12 5.48 5.50 5.50 5.50
500
504
Mean error, Pg. 0.67
250
250
0.00
Sulfur taken, I%*
Sulfur found, rg.
500 498 252
248
120 60
118 120 119 60 59
60
1972
ANALYTICAL CHEMISTRY
1.00 0.33
Procedure 1 (Titrimetric). T h e extent of recovery of sulfur by t h e combustion of potassium sulfate with vanadium pentoxide and Celite was studied.
A known aliquot of standard potassium sulfate was placed in a silica combustion boat and dried in an oven at 100' C., and 1 gram of vanadium pentoxide and 0.2 gram of Celite were added. The combustion tube, containing copper turnings and quartz wool (Figure l ) , was heated to 900" to 950" C. in a current of nitrogen. For milligram amounts of sulfur (5 to 11 mg.) the two receivers contained, respectively, 35 and 30 ml. of 0.2.V hydrochloric acid and 4 and 1 ml. of starch-iodide solution. A measured escess of potassium iodate solution was added to the first receiver. For microgram amounts of sulfur (60 to 500 pg.) the receivers contained 25 ml. of 0.2.V hydrochloric acid, and 2 and 1 ml. of starch-iodide solution, respectively. The boat was quickly introduced into the hot zone of the tube and, after being stoppercd, was heated in a slow current of nitrogen for 30 minutes. During the combustion some iodine wa5 lost immediately from the first receiver but was effectively trapped in the second receiver, giving a blue color. When the exit gas was passed through a third receiver containing acid and starchiodide solution, no blue color was observed. After 30 minutes the receivers were disconnected and the contents transferred to a 250-ml. Erlenmeyer flask. The tubes and joints of the receivers were washed with water and the rrashings added to the main flaik The blue sdution was then immediately titrated with sodium thiosulfate until the addition of one drop of the solution rendered i t colorless. A blank combustion showed excellent recovery (Table I). The applicability of the combustion method was also tested colorimetrically by determining the sulfur contents of fi;e Yational- Bureau of Standards Of these, two standard samples. (Flint clay and Plastic clay) had compositions approximating rocks. The final colorimetric determination of sulfur dioxide m-as carried out by adapting the method of West and Gaeke (18).
Procedure 2 (Spectrophotometric). One gram of vanadium pentoxide was weighed out, a portion was spread along the bottom of a silica combustion boat, and the boat and contents were accurately weighed. A finely ground sample (0.1 to 1.0gram containing about 60 pg. of sulfur) was added to the boat, spread evenly along the bottom, and weighed again. More vanadium pentoxide was added and the contents of the boat were thoroughly mixed with a long, narrow spatula. The contents were covered with the rest of the vanadium pentoxide and approximately 0.2 gram of Celite was spread on the sur-
face. Alternatively a knon n TI eight of the sample could be taken in the boat and treated with vanadium pentoside and Celite as above. The combustion tube was preheated together with the copper turnings and quartz woo1 as previously described, for 1 hour a t 950" C. in a current of nitrogen. The receivers, each containing 25 ml. of sodium tetrachloromercurate solution, were connected to the combustion tube by a short Salgon tube, the boat and contents were introduced into the hot zone, and the heat deflector and the stopper were quickly replaced. The tube was heated a t 900' to 950' C for 30 minutes in a slow current of nitrogen (2 to 3 bubbles per second). The Salgon connection was detached and the contents of the receivers nere transferred into a 100-mi. volumetric flask; the joints of the receivers and the Kalgon tubing were washed mith water. The solution was diluted to volume with water, a n aliquot containing not more than 20 pg. of sulfur was transferred to a 50-ml. volumetric flask, 5 ml. each of pararosaniline hydrochloride and formaldehyde solutions were added, and the solution was diluted to volume with water. After 30 minutes the absorbance was measured a t 550 mp against a reagent blank. It i> important that this measurement be completed within 60 minutes of mising. The sulfur content was obtained from a standard curve prepared as follows: One milliliter of standard potassium sulfate solution (containing 60 pg. of sulfur) was placed in a silica combustion boat and evaporated to dryness in an oven a t 100' C., 1 gram of vanadium pentoxide and 0 2 gram of Celite were added, and the sample was ignited as previouqly described; the sulfur dioxide mas collected in sodium tetrachloromercurate solution. Then lo-, 20-, and 30-ml. portions of the 100-ml. final volume were taken in three 50-ml. volumetric flasks and treated in the same way as the sample, using the same amount of pararosaniline hydrochloride and formaldehyde solutions and measuring the absorbances after 30 minutes a t 550 mp. This standard curve should be prepared every day. The results for National Bureau of Standards analyzed samples, using this spectrophotometric procedure, are given in Table 11. Sulfur was determined on these samples also by the titrimetric method, following combustion of samples ranging from 0.1 to 1.0 gram each. Both sets of results are very close or identical to the NBS certified values. Reproducibility. T h e reproducibility of the method was tested b y taking 0.4 gram of Flint clay and 0.2 gram of Plastic clay and determining the sulfur dioxide formed in the combustion by both titrimetric and spectrophotometric methods. The results (Table 111) demonstrate the reliability of the methods. Each result represents a separately weighed sample taken through the entire procedure.
~~
Table
II.
~~
~
Determination of Sulfur in National Bureau of Standards Samples
Sample No.
Certificate value 0.27 (recalcd.)
Name Argillaceous li nestone Fer “osilicon Flint clajPlatitic clay Phcsphate rock
1A 58
97 98
120
Table 111.
0.008
0.017 0.03 0.13
Total sulfur, yo Found by combustion method SpectrophotoTitrimetric metric 0.27 0.275 0.005 0.018 0.03 0.13
0.006 0.016 0.03 0.137
Reproducibility of Combustion Method
(Sational Bureau of Standards samples)
SamPle No. Same 97 Flint clay DS
Plastic clay
Certificate value 0.017 0 03
Tlgble IV.
so. 1 2
3 4
5 6 7 8
9
10
11 12
13 14
Total sulfur, yo Found Mean SpectroTitrimetric error photometric 0.017, 0.017, 0.015, 0.015, 0.016, 0.016, -0.0007 0.016, 0.016, 0.016, 0.016 0.016, 0.017 0.032, 0.032 0 030, 0.030, 0.032, 0.030, +0.001 0.031, 0.031, 0.030, 0.030 0.031, 0 030
Mean error -0.0012 +0.0005
Determination of Sulfur in Rocks
Name of sample Quartzite Quartzite Quartzite Quartzite Gray dolomite 0ldi:r diahase Gat’bro Gak’bro Gabbro Gabbro Gahbro dyke C1ir.o pyroxenite Welisterite Per: dotite
Gravimetric 0.09 0.12 0.29 0.01 0.04
0.15
0.05
0.034 0,015 0.05
0.05 0.019 0.025 0.87
Total sulfur, % Combustion method SpectrophotoTitrimetric metric 0.07 0.09 0.10 0.11 0.30 0.34 0,005 0.007 0.02%
0.13
0.05
0.04
0.02
0.04 0.03 0.016 0.027 0.83
0.02 0.10 0.05
0.03 0.02 0.032 0.03 0.014 0.020 0.81
DETERMINATION OF SULFUR IN ROCKS
DISCUSSION
K h e n Procedureai 1 and 2 were applied to several rock samples, the results were very satisfactory and compared well with the gravimetric values obtained by the usual barium sulfate precipitation, followed by treatment of the barium sulfate with hydrofluoric acid to remove coprecipitated silica (cf. Table IV). However, the gravimetric values were slightly higher in some samples containing chromium, presumably because of coprecipitation of barium chromate.
In the final colorimetric determination with pararosaniline hydrochloride and formaldehyde, the use of 5 ml. of 2y0 (v./v.) formaldehyde, instead of the O.ZoI, solution recommended by earlier authors (2, 7 , l a ) , produced a more intense color (almost double absorbance) and was reproducible and stable u p to 1 hour. The system also obeyed Beer’s law u p to 18 pg. of sulfur per 50 ml. of total volume, but the color faded after 1 hour. The complex formed by reaction of the sulfur dioxide with sodium
tetrachloromercurate was very stable and was not oxidized by atmospheric oxygen even after many hours’ standing; it is therefore advantageous to burn as many samples as possible a t one time and to determine the sulfur contents spectrophotometrically a t the end of the day. The spectrophotometric method is suitable only for the microdetermination of sulfur, nhereas the titrimetric method is suitable for determination of both micro and macro amounts and is much more rapid. The furnace used in this study could accommodate two tubes and, because two samples could be run at the same time, a t least 10 samples could be completed in 1 day. The classical barium sulfate method requires 2 days for completion. After five analyses the copper turnings in the tube become dark because of coating of oxide films and hence should be replaced by fresh material. The quartz wool should be replaced every 3 days. The combustion tube should be changed after about 4 weeks, as the yield of sulfur dioxide is lower subsequently because of saturation of the active walls of the tube with the dust particles which have a tendency to adsorb sulfur dioxide. The end of the tube should be brought to the hot zone after each week to burn out any accumulated dust. ACKNOWLEDGMENT
The author is grateful to J. A. Maxwell and S. Abbey for their interest and helpful suggestions and to Ez. G. Hoops for gravimetric determination of sulfur in some rock samples. LITERATURE CITED
(1) Aidinyan, R. Kh., Pochvovedenie 1957, (9), 49; Ref. Zhur. Khim. 1958, Abstr. 21, 191; Anal. Abslr. 1958, 3930. (2) Bloomfield, C., Analyst 87,586 (1962). (3) Coller, M. E., Leininger, R. K., ANAL. CHEM.27, 949 (1955). (4) Fedoseev, P. N. Lagoehnaya, R.hf., Zh. Anal. Khim., SSSR 9, 224 (1954); Anal. Abstr. 1955, 656. (5) Fernlund, U.,Zechner, S., Jernkontorets Ann. 138, 665 (1954). (6) Hagerman, D. B., Faust, R.A., ASAL. CHEM.27, 1970 (1955). (7) Larsen, R. P., Ross, L. E., Ingber, N. B., Ibid., 31, 1596 (1959). (8) Poutet, M., Boulin, R.,Chim. Anal. 36. 98 (1954). (9) Rice-Jones; W. G., ANAL. CHEM.25, 1383 (1953). (10) Takahashi, M., Japan Analyst 2, 26 (1953). (11) Vogel, A. I., “Textbook of Quantitative Inorganic Analysis,” 2nd ed., Longmans, Green, New York, 1951. (12) West, P. W., Gaeke, G. C., ANAL. CHEM.28, 1816 (1956). RECEIVED for review February 14, 1963. Accepted August 12, 1963.
VOL. 35, NO. 12, NOVEMBER 1963
1973
