V O L U M E 2 7 , NO. 3, M A R C H 1 9 5 5 matograpliy lent itself t o the use of rapidly moving solvents, because their flow rates could easily be controlled by the width of the wick. Chromatograms were completed within 2 t o 4 hours and furnished a rapid check on information obtained by column chromatography. High-temperature drying quickly removed formic acid. Complete removal of formic acid eliminated obscuring background when the papers were sprayed with indicator and brought the sensitivity of a p H indicator spray within that of silver nitrate sprays (6). Five micrograms of any of the acids showed clearly, and 0.5 to 1 y of most acids was detected. ACKNOWLEDGMENT
The author is indebted to Eugene Herrling for assistance in preparing the illustrations.
369 LlTERATURE CITED
(1) Airan, J. W., Joshi. G. V., Barnabas, J., and Master, R. W. P., ANAL.CHEM.,25, 659 (1953). (2) Bock, R. &I., and Ling, IT.S., Ibid., 26, 1543 (1954). (3) Bulen, W. A., Varner, J. E., and Burrell, R. C., Ibid., 24, 187
(1952). Isherwood, F. A., Biochem. J. (London), 40, 688 (1946). Loffler, J. E., and Reichl, E. R., Milcrochim. Acta, 1953, 79. Rao, P. S., and Dickey, E. E., Science, 117, 666 (1953). Saifer, A . , and Oreskes, I., ANAL.CHEM.,25, 1539 (1953). Zbinovsky, V., M.S. thesis, University of Wisconsin, 1951. (9) Zbinovsky, V., and Burris, R. H., ANAL.CHEM.,26, 208 (1964).
(4) (5) (6) (7) (8)
RECEIYED for review October 15, 1954.
Accepted December 10, 1964. This work was supported in part by the American Cancer Society as recommended by the Committee on Growth and by the Research Committee of the Graduate School from funds supplied by the Wisconsin Alumni Research Foundation. Published with the approval of the director of Wisconsin .4gricultursl Experiment Station.
Titration of Elemental Sulfur with Solutions of Sodium Cyanide D. A. SKOOG
and
J. K. BARTLETT'
Stanford University, Stanford, Calif.
The purpose of this investigation was to determine whether the reaction between elemental sulfur and cyanide ion could be applied to the volumetric determination of sulfur in acetone extracts. It was found that such a titration is practical and that certain acidbase indicators can be used to detect the end point. The method is simple and rapid and requires only a single, fairly stable standard reagent. The accuracy w-hich can be obtained by the proposed procedure is comparable with the other methods for this analysis.
AST of the methods for the analysis of elemental sulfur require a preliminary separation of the element by extraction with acetone; such is the casewithrubberproducts, plant spray residues, and sulfur-bearing ores. This is followed by determination of the sulfur in the acetone extract. -4mong the methods proposed for accomplishing the latter step is the oxidation of the sulfur to sulfate by bromine followed by gravimetric analysis by the usual procedures (7'). Castiglioni ( 3 ) has proposed a volumetric procedure which involves conversion of the sulfur to thiocyanate in tjhe acetone solution, followed by destruction of the excess cynnide with formaldehyde and titration of the thiocyanate with standard silver nitrate. Hardman and Barbehenn ( 4 ) recomniend conversion of the elemental sulfur to cuprous sulfide by immersing a copper gauze in the acetone extract. The sulfide formed is then liberated with acid and determined iodometrically. LIark and Hamilton (6) have determined sulfur in acetone extracts by addition of an ammoniacal solution of cuprous sulfate. The cuprous sulfide formed is measured turbidimetrically. The ;ISTLI method ( 1 ) for the determination of elemental sulfur in acetone extracts involves conversion of the sulfur t o thiosulfate by prolonged heating with a solution of sodium sulfite. The thiosulfate is then determined iodometrically. I n some recent work on the colorimetric determination of small quantities of free sulfur in hydrocarbons ( 2 ) it was observed t h a t the reaction between elemental sulfur and cyanide in aqueous acetone solutions proceeds rapidly and quantitatively toward the formation of thiocyanate. It occurred to the authors t h a t this reaction might offer a rapid and simple method for the direct volumetric determination of sulfur in acetone extracts. An investigation has indeed disclosed that such a titration of sulfur 1
Present address, Long Beach State College, Long Beach, Calif.
with a standard solution of cyanide is entirely feasible. Endpoint detection can be accomplished readily because of the large change in hydrogen ion concentration which occurs when a slight excess of the standard cyanide is added t o the solution. This change may be detected by the use of suitable acid-base indicators or by potentiometric measurements. This method based upon the above appears to offer certain advantages over the other methods for the determination of sulfur in acetone extracts: The method is more rapid than most; only a single, fairly stable standard solution is required; and finally, an accuracy comparable with the other procedures can tie obtained. REAGEKTS AND SOLUTIONS
Elemental Sulfur. Flowers of sulfur were recrystallized once from carbon disulfide, dried a t 60" C., and stored over magnesium perchlorate. Solvents. The acetone and isopropyl alcohol used in this work were of technical grade. Organic Sulfur ComDounds. The organic sulfides. disulfides. and Gercaptans used ;-ere obtained froni Eastman Kodak and were not subjected to further purification. Standard Sulfur Solutions. Solutions containing known concentrations of elemental sulfur were prepared by refluxing weighed quantities of the purified sulfur in acetone until solution was complete. The resulting solutions were then transferred t o volumetric flasks and diluted t o the mark with acetone. Approximately 35 nig. of elemental sulfur w-ill remain in 100 ml. of acetone a t room temperature. Sodium Cyanide in Isopropyl Alcohol, approximately 0.05F. About 2.4 grams of sodium cyanide were dissolved in 200 ml. of water and diluted to about 1 liter with isopropyl alcohol. Silver Nitrate, 0.05F. Bromocresol Purple Indicator, 1yosolution. 7 ' solution. Bromothymol Blue Indicator, 1 PROCEDURE
Standardization of Cyanide Solutions against Elemental Sulfur. Accurately weigh about 0.16 gram of recrystallized sulfur and dissolve in 400 ml. of acetone by refluxing. Cool and dilute t o exactly 500 ml. with acetone. Transfer a 100-ml. aliquot of this solution to an Erlenmeyer flask, add about 20 ml. of water, and bring the solution just t o boiling on a hot plate. Remove from the hot plate, add 3 t o 4 drops of the bromocresol purple indicator, and titrate with t h e cyanide solution to a distinct bluish purple color. Reheat the solution. This should cause t h e indicator to return to the yellow-green color. Continue the additions of reagent and the heating until a permanent bluish
370
ANALYTICAL CHEMISTRY
purple is obtained. Near t h e end point 20 to 30 seconds are required for t h e reaction to take place. Calculate t h e formality of t h e cyanide solution as follows: nig. of S taken FN,CN = ml. of X a C S X 32.07 X 5 Standardization of Cyanide Solutions against Silver Nitrate. Transfer 50 ml. of the standard cyanide solution into a flask and add 150 ml. of water, 8 ml. of 6 N ammonium hydroxide, and 0.6 gram of potassium iodide. Titrate to the first permanent turbidity with a standard 0.05F solution of silver nitrate. This end point corresponds to the reaction of two cyanide ions with each silver ion to give the complex Ag(CiT)*-; t h e formal concentration of the cyanide solution can be calculated as folloivs: ml. of A g S 0 3 X F A ~ KXO2 ~ FN~C =N ml. of T a C N Determination of Sulfur in Acetone Extracts. Take an aliquot of the acetone solution of such a size that it contains betneen 10 and 80 mg. of elemental sulfur. Add a volume of water equal t o approximately one fifth the volume of acetone present. Bring t h e solution to boiling, add 3 to 4 drops of bromocresol purple indicator, and titrate as directed in t h e section on standardization of cyanide solutions against elemental sulfur. Calculate the milligrams of elemental sulfur in the aliquot as follows: Mg. of S = ml. of S a C S X F N ~ C XB32.07 EXPERIMEIVTAL
End-Point Detection. I n the proposed method of analysis, the end point is detected with an acid-base indicator. The sodium cyanide reagent is highly hydrolyzed in the aqueous acetone solvent, whereas the thiocyanate formed is not. As a result, the addition of a slight excess of the standard solution results in a marked decrease in the hydrogen ion concentration which can be readily detected.
application to the titration. Two of these, bromocresol purple and bromothymol blue, exhibited sharp color changes a t a point corresponding exactly n i t h the potentiometric end point. Of the two, the bromocresol purple appeared to be somewhat more satisfactory and was used in most of the work reported herein. Rate of Reaction. At room temperature, the rate of t h e reaction between cyanide ion and sulfur was found to be slow enough to make titrations inconvenient. For example, near the equivalence point a t least 5 minutes were required for equilibrium to be achieved after addition of 0.1 ml. of reagent. However, by maintaining the temperature just a t the boiling point of the solution, the reaction took place rapidly enough to make the titration practical. The higher temperature appeared to have no effect on the behavior of the indicators. 9 t the boiling temperature, the reaction rate is such that 20 to 30 seconds must be allowed between additions of reagent rvhen the titration is within 0.5 ml. of the end point. With a little practice, t h e titration can be easily completed In 5 minutes. Table I. Comparison of Sulfur and Silver Nitrate as Primary Standards for Sodium Cyanide Solutions 6 Taken, XI g . 77,50 73.50
Primary
Standard
Table 11.
Time of Standing, Days
fi
13 26
MILLILITERS
I 4
I 6
J!
lk
;1
!I
OF SODIUM CYANIDE REAGENT
Figure 1. Titration curve for sulfur with sodium cyanide reagent Concentration of cyanide solution 0.0462F; 13.6 mg. of sulfur
Figure 1 illustrates the magnitude of the changes in hydrogen ion concentration which occur in a typical titration. I n this example, 13.5 mg. of elemental sulfur were dissolved in 100 ml. of acetone, about 20 ml. of water were added, and the solution was titrated with a 0.0462B solution of sodium cyanide. Empirical p H readings were obtained with an ordinary glass-calomel electrode system which had been calibrated against a n aqueous buffer solution. A well defined end point was obtained. Several acid-base indicators were investigated for possible
Formality of NaCN
Stability of .4queous Isopropyl .4lcohol Solutions of Sodium Cyanide
T
6b !
NaCN Used, MI.
S ... 10.37a 0.0466 s 9.86a 0.0466 AgN03 ... is' is 25.00 0.0465 AgNOs , . . 18 20 25.00 0.0465 a Volumes required for titration of aliquot8 equivalent to one fifth of sulfur taken.
0
7
0 06393 AgNOa Used. 111.
Formality us.
s
0 464 0.464 0 462 0.402 0 462 0 461
Standardization of Sodium Cyanide Solutions. Both elemental sulfur and silver nitrate were found to he suitable for the standardization of the aqueous isopropyl alcohol solution of sodium cyanide used as the standard reagent in the method. When sulfur was used it was found necessary to recrystallize t h e ordinary flowers of sulfur once from carbon disulfide before application as a primary standard. I n standardizing against silver nitrate a modification of the Liebig-Denigks titration (6) for cyanide was used. The modification involved increasing t h e amount of potassium iodide and decreasing the quantity of ammonia recommended for the titration in aqueous solution. This was necessary in the presence of isopropyl alcohol as was shown by titration of several aqueous solutions of exactly known cyanide concentration, to Tvhich had been added amounts of isopropyl alcohol corresponding to the amount which would be present in the standardization of an alcoholic cyanide solution. Table I shows a comparison of results of the standardization of a cyanide solution by the two methods. Searly identical results are ohtained and either substance is suitable as a primary standard. The data are of additional interest inasmuch as they indicate that the reaction betn-een sulfur and cyanide proceeds quantitatively and involves a 1 to 1 ratio of the reactants. Stability of Cyanide Solutions in Isopropyl Alcohol. During the preliminary work on this method, standard solutions of sodium cyanide in acetone were used. However, these were found to be unstable, decreasing in formality by several per cent each day. This lack of stability probably arises from reactions between the cyanide and acetone to give the cyanohydrin which may further decompose by hydrolysis. Solutions of sodium
V O L U M E 27, NO. 3, M A R C H 1 9 5 5 Table 111.
371
Effect of Certain Compounds on Determination of Elemental Sulfur
Compound Added n-Butyl sulfide Ethyl disulfide n-Butyl mercaptan
wt. of
Compound Ilg. 200 40 20 20 8000
Petroleum ether
Table I\’.
Wt. Present 17.80 15.60 23.04 33.05
Found 17 87 15.60 21.05 2G. 27 15.60
15.60
Analysis of Acetone Solutions of Elemental Sulfur Relative Error,
Sulfur, Ala. Taken 7.80 10.10 15.60 15.60 15.60 15.60 23,40 31.20
s, M g .
Found 7.79
70 0.1 0.1 0.5 0.0 0.3 0.3 0.4 0.0
10.11
15.53 1.5 . IiO 15.64 l5.5,j 23.49 31.20
Average
cyanide to precipitate and become unavailable for reaction with the sulfur. Concentrations of water higher than 20% appeared to cause the rate of the reaction to decrease appreciably. Interferences. The effect of a number of substances on the proposed procedure was investigated. Table I11 shows t h a t a n excess of either a typical aliphatic sulfide or disulfide does not cause a n y alteration of the results. On the other hand, me!captans d o interfere. This interference undoubtedly results from the reaction between the mercaptans and elemental sulfur which is catalyzed by the basic reagent. Metallic ions such as silver, mercury, or cadmium interfere with the titration b y forming stable complexes with the cyanide reagent. Apparently the complesed cyanide ions do not readily react with the sulfur. Results. Table I V shows result< obtained by the proposed method of aliquots of acetone solutions containing known quantities of elemental sulfur. The average relative error of these determinations mas found t o be 0 . 2 7 . The standard deviation was 0 3%.
0,2
LITERATURE CITED
cyanide in a solvent consisting of 80% by volume of isopropyl alcohol and 20y0by volume of water were found to be remarkably stable This is illustrated in Tahle 11. These data indicate that the alcoholic cyanide solutions are considerably more stable than simple aqueous solutions of sodium cyanide which are reported to decrease in formality by 0.3% per day (5). Effect of Water Concentration on Titrations. It was found desirable t o have between 15 and 20T by volume of water in the acetone solvent at the beginning of the titration. With smaller amounts of water, there was a tendency for sodium
(1) Am. SOC.Testing Materials, Philadelphia, Pa., ”Book of ASTLI Standards,” Part 6, p. 52, 1952. (2) Bartlett, J. K., and Skoog. D. .I AKAI.. .,CHEM..26, 1008 (1954). (3) Castiglioni, h..Z.a n d . Chem., 91, 3 2 (1932). (4) Hardrnan, A. F., and Barhehenn. H. E., I K D . ENG. CHEW., i l x a ~ED.,7, 103 (1935). (5) Kolthoff, I. lI., and Stenger V. A.. ”Volumetric Analysia,” Vol. 11. pp. 282-3, Interscience. Sew York, 1947. (6) Mark. G. L., and Hamilton, J. 31.. IVD. E m . CHEW.,A K ~ L E D . , 14, 604 (1942). (7) Tuttle, J. B., “Analysii of Rubber,” p. 58, Chemical Catalog
Co., Xew York, 1922.
RECEIVED for review September 7, 1954. Accepted November 15, 1954.
I r o n W Perchlorate as a Reductant in Glacial Acetic Acid 0.N. HINSVARK
and K. G. STONE
Kedzie Chemical Laboratory, Michigan State College, East Lansing, M i c h .
A reducing agent was required which could be used for nonaqueous titrations of oxidants. Iron(I1) perchlorate in glacial acetic acid was satisfactory for the determination of chromium trioxide and sodium permanganate in glacial acetic acid without the addition of water. .4n amperometric end point with two active electrodes was most suitable.
S
TUDIES still in progress in this laboratory led t o the investigation of iron(I1) perchlorate as a n analytical reagent for the determination of oxidants in glacial acetic acid. Aqueous iron(I1) perchlorate has received limited attention as a reductant in the establishment of oxidation potentials of various cerium( IV) complexes ( 1 ) . Iron(I1) chloride has been used in glacial acetic acid (3))b u t its low solubility in this solvent limits its applicability. Iron(I1) pel chlorate is much more soluble; solutions in ewess of 0.1S n i t h respect t o iron(I1) are easily prepared. Using acetic acid solutions of iron( 11) perchlorate standardized against aqueous potassium dichromate, solutions of sodium . permanganate and chromium trioxide in glacial acetic acid can be analyzed M ithout the introduction of aqueous reagents. REAGENTS AXD APPARhTUS
Baker’s analyzed acetic acid was further purified by distillation away from chromium trioxide followed by a second distillation from potassium permanganate. T h e iron(I1) perchlorate hesa-
hydrate and perchloric arid ( T O % ) were obtained from the G. Frederick Smith Chemical Co. Baker’s analyzed chromium trioxide and Fisher Scientific Co. C.P. grade sodium permanganate were used as oxidants. A Fisher Elecdropode, sensitivity 0.025 pa. per scale division, equipped with 2-em. 18-gage platinum wire electrodes, was used for detection of t h e equivalence point ( 2 ) . T h e solution being titrated was stirred under a nitrogen atmosphere with a magnetic stirrer. T h e iron(I1) perchlorate solutions were prepared in the following manner: Acetic anhydride in slight excess over t h a t necessary to react with t h e water present in the iron(I1) perchlorate was added to t h e acetic acid. After the acetic acid had been flushed with nitrogen, t h e approximate weight of iron(I1) perchlorate hexahydrate was added. This solution was left under a nitrogen atmosphere for a minimum of 2 hours but frequentlj. much longer. A4measured volume of this solution was added to a solution of 5 ml. of 85% phosphoric acid in 20 ml. of water T h e resultant solution was analyzed by titrating to the diphenylamine color change Ll-ith a solution of primary standard potassium dichromate ( 4 ) . .4 potentiometric titration showed the equivalence point to be coincident with t h e color change when the iron(I1) solution was titrated under these conditions. I n t h e experimental work the iron(I1) perchlorate solution9 were used for t h e analysis of acetic acid solutions of sodium permanganate and chromium trioxide. I n the permanganate studies an approsimate weight of sodium permanganate was added to t h e desired amount of acetic acid. Sodium permanganate was used in preference to the corresponding potassium salt because of its much greater solubility in this medium. T h e actual concentration of the solution prepared in this manner was found by titrating a weighed quantity of
