Spectrophotometric method for the determination of small amounts of sulfur in organic compounds: A semimicro and micro method

MICROCHEMICAL

Spectrophotometric of Small Compounds:

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Method

Amounts

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(3950)

for the Determination

of Sulfur

A Semimicro

III,

in Organic

and Micro

Method

IHOR LYSYJ and JOHN E. ZAREMBO, Food Machinery and Chemical Corporation, Central Research Laboratory, Chemical Divisabns, Princeton, New Jersey

The determination of sulfur in organic compounds by the oxygen combustion technique was described by SchCjniger,’ Lysyj and In these methods, the organic matter was Zarembo,2 and Alicino.” cornbusted in an Erlenmeyer or iodine flask filled with oxygen. The products of combustion were absorbed in hydrogen peroxide solution and the sulfate ion was determined either volumetrically or gravimetrically. However, difficulties were encountered when sulfur was analyzed in the presence of other negative elements, especially large amounts of phosphorus. The visual endpoint detection of equivalent points for the sulfate ion by a number of volumetric methods4-6 was found to be unsatisfactory, and in many cases phosphorus interfered with the titration, producing higher results. The gravimetric procedure (precipitation with barium chloride) was satisfactory for most organosulfur compounds, except with compounds containing large amounts of phosphorus. In such cases higher results for sulfur were obtained, probably due to the coprecipitation of phosphorus. Volumetric and gravimetric methods when used in conjunction with an oxygen flask combustion are limited to compounds containing at least 1.0% sulfur. The limitation is due to the sample size of organic material which can be efficiently combusted in a .500-ml. iodine flask. In our experience, sample sizes above 60 mg. were incompletely combusted when t,he combustion was carried out, in a 500-ml. iodine flask. 173

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I. LYSYJ AND J. E. ZAREMBO

The extension of the oxygen flask combustion method to the determination of small amounts of sulfur in organic materials is highly desirable, since a great number of analyses for sulfur are required in this range-samples of oils, gasolines, etc. EXPERIMENTAL In a search for generally applicable methods for the determination of sulfur in organic compounds, which would cover trace, micro, and semi-micro amounts, the spectrophotometric method of Bertolacini and Barney’,* attracted our attention. These workers investigated the reaction between metallic salts of chloranilic acid and anions such as sulfate, chloride, and fluoride. The salts of chloranilic acid are only slightly soluble in water, and on reaction with anions liberate strongly colored chloranilic acid in amounts corresponding the to the amount of anion present. For sulfate determination, authors used barium chloranilate as the color reagent in a pH of 4.0 buffered solution containing 50% ethyl alcohol. The absorption for this system was investigated over a broad range of spectra and two definite absorption peaks were found-one at 530 rnp in the visual region and one at 332 rnp in the ultraviolet region. The peak in the ultraviolet region was found to be almost 30 times more sensitive than that in the visual. This made it possible to determine sulfur over extremely wide concentration ranges (0.25 to 100 p.p.m.) using the same analytical procedure by selecting the proper absorbancy peak for the given amount of sulfur. It was also found that phosphorus, nitrogen, and halides did not interfere with the determination If metals were present, they could be removed by ion of sulfur. exchange techniques. The possibility of using a spectrophotometric method for the determination of sulfur after the combustion of organic material in an oxygen-filled iodine flask on a semi-micro, micro, and trace level was The technique of combustion as investigated in this laboratory. described by Lysyj and Zarembo2 was used in our work. This consists essentially of weighing the organic sample containing sulfur on filter paper or into a gelatin capsule, placing it into a platinum spiral, igniting and inserting it into an iodine flask (500 ml.) containing 50 ml. of ammoniacal hydrogen peroxide and filled with oxygen. As soon as the sample is burned completely, the flask is shaken vigorously for a few minutes in order to absorb the products of comMICROCHEMICAL

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bustion. The solution is boiled to remove ammonium hydroxide and to destroy excess hydrogen peroxide. The residue is transferred to a loo-ml. volumetric flask, containing barium chloranilate, buffer, and ethyl alcohol, shaken, centrifuged, and subjected to spectrophotometry. Samples containing 0.3 to 10 mg. of sulfur were subjected to visual calorimetry at 530 ml.r. Any standard calorimeter can be used in this case. The absorbancy of samples containing 0.02-0.3 mg. of sulfur were measured in the ultraviolet region at 332 rnp, using a Beckman DC spectrophotometer with the ultraviolet attachment. For the determination of sulfur in trace amounts, a sample size of 20-50 mg. was used for the combustion and subjected to spectrophotometry at 332 m/l. This technique permits the determination of sulfur in concentrations to 0.05% of sulfur in the original sample.

REAGENTS Barium chloranilate, obtainable from Eastman Kodak Chemical Company. 6y0 ammonium hydroxide sol&on. 6% hydrogen peroxide solution. Buffer solution, 0.05 M potassium acid phthalate, pH 4.0.

APPARATUS Combustion apparatus: Platinum wire, forming a spiral at one end and sealed into 24/40 ground glass tube. 250 and 500 ml. iodine flasks (ground glass). Beckman DU spectrophotometer with ultraviolet attachments.

PROCEDURE Preparation of Standard Curve An 824.4-mg. portion of ammonium sulfate, previously dried at 120°C. for 4 hours, is weighed into a 200-ml. volumetric flask. The crystals are dissolved in 150 ml. of distilled water. After dissolution the flask is filled up to the mark. One milliliter of this solution is equivalent to 1 mg. of sulfur. To 9 volumetric flasks (100 ml.) the following increments (in milliliters) of standard solution are added:

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I. LYSYJ AND J. E. ZAREMBO

mg. S/100 ml

Fig 1. Absorbanceof sulfur at 530 mp.

mg. S/l00

ml.

Fig. 2. Absorbanceof sulfur at 332 mp. 0, 1, 2, 4, 6, 8, 10, 12, 14. Ten milliliters of buffer solution, 50 ml. of ethyl alcohol, and 300 mg. of barium chloranilate are added to each of the flasks. The solution is diluted to volume with distilled water, The absorbance thoroughly shaken for 10 minutes, and centrifuged. of the centrifuged solution is measured at 530 mp versus a blank. MICROCHEMICAL

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For the preparation of a curve for trace analysis, the stock solution of ammonium sulfate is diluted ten times and the following increments transferred to 100 ml. volumetric flasks: (1 ml. = 0.1 mg. of sulfur) 0.2 ml., 0.6 ml., 1.O ml., 1.4 ml., 2.0 ml. The same procedure as above is used for the development of the color but the absorbancy measurements are taken at 332 mp. The curves for sulfur in the visual and ultraviolet region are presented in Figures 1 and 2.

Combustion of Sample Twenty-five milliliters of 6% hydrogen peroxide and 25 ml. of 6% ammonium hydroxide are added to a 250- or 500-ml. ‘Erlenmeyer or iodine flask, and the sample is cornbusted according to the procedure of Lysyj and Zarembo.2

Determination of Sulfur One hundred milliliters of distilled water is added to the combustion flask and the solution is boiled until the total volume is reduced to lo-15 ml. This destroys the excess ammonium hydroxide and hydrogen peroxide. The solution is cooled and transferred to a lOOml. volumetric flask. The combustion flask is rinsed twice with 10 ml. portions of distilled water and twice with 25 ml. portions of ethyl alcohol. The washings are collected in a volumetric flask to which 10 ml. of buffer solution and 300 mg. of barium chloranilate have been added. The volume of the flask is brought up to the mark with distilled water. Then it is shaken for 10 minutes, centrifuged, and the absorbance of the clear solution is measured with a spectrophotometer versus a reagent blank. The blank is prepared by adding IO ml. of buffer, 50 ml. of ethyl alcohol, and 300 mg. of barium chloranilate to a loo-ml. volumetric flask and diluting with distilled water up to the mark. For samples containing 0.3 to 10 mg. of sulfur, measurements are taken at 530 rnp; for samples containing sulfur in amounts below 0.3 mg., at 332 rnp.

Discussion The method as described above was developed as a part of a general study undertaken in this laboratory to provide a simple, rapid, safe, and accurate method for organic elemental analysis. This method for sulfur combines the simplicity of oxygen flask combustion with

s N

6

:: F

s r

8 3 2 r

k

z

is 5

99.46

12.62

12.79

H0&CsH:~(OH).COzH.2H,O

Sulfosalicylic acid dihydrate

Average recovery and standard deviation

28.28

4.90

28.42

4.79

26.79

(CsH3N.CSz.Nn

Go&O&r&S

27.14

Sodium biphenyldithiocarbamate

Tetrabromophenolsulfonphthalein

CaH6.SOz

41.88

42.12

H2NCSNH,

Thiourea

38ulfolene

12.29

12.51

% S

% S

f

0.94?&

98.67

99.30

102.29

98.85

99.43

102.44 18.54 12.59 12.27 98.08 20.32 97.20 12.16 7.65 101.13 42.60 18.61 100.52 42.64 17.94 100.85 5.40 42.58 100.70 18.97 27.83 100.99 17.52 27.41 99.41 4.90 26.98 102.50 4.91 25.59 100.62 4.82 28.60 104.80 14.02 5.02 101.09 18.32 28.93 100.35 20.54 28.52 100.98 6.08 28.70 12.32 99.92 12.78 12.82 100.35 12.85 12.42 97.11 4.09 100.72 f 0.73%

% recovery

98.24

% S

procedure

Mg. sample

Proposed

% recovev

Parr Bomb

CBHSN: NCSNH.NHCeHb

Formula

I

Diphenylthiocarbazone

Compound

TABLE

F2

iz z

z

m

5 4

3

2

.-

SIJLFIJR

IN ORGANIC

COMPOUNDS

179

the efficiency of a calorimetric method and considerably shortens the time previously required for the determination of sulfur by classical methods. A number of organic compounds containing sulfur were analyzed using this procedure. The same compounds were analyzed for sulfur by the Parr bomb method, followed by gravimetric determination of sulfate. The results are presented in Table I. Analyses of phosphorus-containing compounds are given in Table II. TABLE II of Sulfur in Organic Compounds Phosphorus

Determination

Containing

Mg. Formula

Compound

Sulfur and

%S

%S

sample

found

Recoverv

o,o’-Diethyldithiophosphoric acid

G&(W’S~H

34.44

17.00 12.42 26.52

35.00 34.31 34.69

101.63 99.62 100.73

o,o’,o”-Tributylthiophosphate

(CdH,0)3PP

11.36

20.19 21.78 20.90

11.02 11.39 11.48

97.01 100.26 101.06

16.18

27.06 21.99 25.27

16.25 16.49 16.31

100.43 101.92 100.80

o,o’,o”-Triethylthiophosphate Average

(C~H,O)ZPS

and standard

deviation

Determination Synthetic mixture containing Sulfur,

0. lO”,0

Sulfur,

0.10%

Sulfur,

0.05%

100.38 f

0.95yo

TABLE III of Sulfur in Small Quantities Sample size, mg.

y. S found

44.12 25.03 19.23 44.9 49.6 47.9 47.96 58.31 55.15

0.498 0.484 0.535 0.109 0.090 0.086 0.058 0.072 0.068

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I. LYSYJ

AND

J. E. ZAREMBO

For the determination of sulfur in small amounts, a synthetic mixture was prepared by dissolving 3.333, 0.666, and 0.333 g. of benzyl sulfide in a small volume of benzene and diluting it with ethyl alcohol to a total weight of 100 g. This corresponds to 0.5, 0.1, and O.O5oj, sulfur. The samples were analyzed in triplicate, taking readings at 332 mp. The results of this experiment are presented in Table III. It was found that filtering in this case is preferable to centrifuging. It was also necessary to use deionized and distilled water for trace analysis.

Summary A rapid calorimetric method for the determination of sulfur in organic compounds is described. The method is very general in scope and permits the determination of organic sulfur in the presence of other negative elements such as chlorine, phosphorus, and nitrogen, without cumbersome and time-consuming separations. Jt is possible to determine sulfur in semi-micro, micro, and trace amounts using essentially the same technique.

References 1. W. ScMniger, Microchem. Acta, 1955, 123. 2. I. Lysyj and J. E. Zarembo, Anal. Chem., 30, 428 (1958). 3. J. F. Alicino, Microchem. J., 2, 83 (1958). 4. J. S. Fritz and S. S. Yamamura, Anal. Chem., 27,146l (1955). 5. A. Steyermark, Quantitative Organic Microanalysis, Blakiston, Philadelphia, 1951. 6. H. Wagner, Microchem. Acta, 1957, 19. 7. R. F. Bertolacini and J. E. Barney, II, Anal. Chem., 29, 281 (1951). 8. Ibid., 30, 202 (1958).

Received November 26, 1958

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