Analysis of Organic Sulfides by Titration of Their Hydrogen Peroxide-Formed Sulfoxide Charles B. Puchalsky Uniroyal Chemical, Division of Uniroyal, Inc., Naugatuck, Conn.
SULFIDE SULFUR ANALYSIS is a convenient means for the assay of organic molecules containing this group. However, results tend to be high by the bromide/bromate oxidation procedure ( I ) and combustion procedures. The material that stimulated this investigation [Vitavax, Uniroyal, Inc. trademark for its fungicide, 5-carboxanilido-6 methyl-2,3dihydro-l,4-oxathiin]gave no discernible end point by the bromide/bromate procedure, using the dead-stop electrometric end point. The procedure described here is based on the room temperature oxidation of sulfide to sulfoxide by hydrogen peroxide in acetic acid. The second step of the oxidation (to sulfone) is generally much slower. Depending on the particular molecule, maximum yields of the sulfoxide are generally attained between 15 and 60 minutes. Acetic acid and unused hydrogen peroxide are removed by vacuum topping in a rotary evaporator. The sulfoxide derivative is then titrated potentiometrically in acetic anhydride ( 2 ) with perchloric acid. Excellent inflections are obtained. Interferences. Sulfoxide impurities in the sample can be titrated directly without prior peroxide oxidation, because sulfides (and sulfones) exhibit no measurable basicity in acetic anhydride. In general, any basic substance persisting through hydrogen peroxide oxidation or formed by hydrogen peroxide will interfere. These substances are limited in number and would not usually be found in organic sulfide samples.
:I I
2
3
4
MI
5
6
7
8
9
1011
12
OF T I T R A N T
EXPERIMENTAL
Figure 1. A typical titration curve
Apparatus and Reagents. The electrode system is the glass electrode/lithium perchlorate in acetic anhydride described by Siggia (3). Titrant used in this report was 0.1N perchloric acid in acetic (3), but perchloric acid in dioxane would probably improve the end point.
(1) S. Siggia, “Quantitative Organic Analysis via Functiona Groups,” 3rd Ed., John Wiley and Sons, p 614. (2) Zbid.,p 621. (3) Zbid.,p 153.
Table I. Sulfoxide Yield as Function of Time
Z of Theory Sample VITAVAX Benzyl sulfide Hexyl sulfide
Purity Recrystallized 3X from cyclohexane m.p. = 97.8-99.0 “C Eastman White Label Eastman White Label
BY bromide-bromate method
15’ Ox.
30’ Ox.
time
time
60’Ox. time
...
97.9
97 * 90
99.1
...
94.4
101.7
96.9
95.8
95.7
102.5 102.9
95.9
100.9 100.4
95.1~
100.7 100.4
Dilauryl thioTechnical 88.3 93.9 dipropionate 2,2 ’-Thio169-170 “C at ... 20 mm diethanol a Average of 10 determinations, done over a 4-month period; the standard deviation was 0.20. No end point. c Poor end point.
...
b
VOL. 41, NO. 6, MAY 1969
843
Table 11. Results of Three Controls of Vitavax Purity
Sample
A
B
A
98.0 98.9 98.4
98.1 99.2 98.2
B C
Procedure. Accurately transfer about 1 mm of the sample to a 200-ml round-bottom flask. Insert a small stirring bar covered with Teflon (Du Pont) and wash down the interior surface with 5 ml of acetic acid and mix to attain complete solution. Add another 5 ml of acetic acid and while stirring actively add 0.15 f 0.01 ml of 30% hydrogen peroxide to the stirred volume. This addition is done conveniently with a small hypodermic syringe and needle. Stopper the flask, remove it from the magnetic stirrer, and roll it at an angle to collect any droplets. Let the flask stand for the appropriate time as suggested below in Results and Discussion. Place the flask on a rotary evaporator and with a large air-bleed, begin rotation. Place a bath of room temperature water under the flask to prevent freezing of the solution. Samples judged to produce volatile sulfoxides should have a condenser interposed between the flask and the rotary evaporator. Gradually decrease the pressure to about l to 2 mm. When the solvent is gone (generally about 5 minutes), remove the room temperature bath and substitute a bath of water at 100 "C, maintaining that temperature with a hot plate. Heat the flask for 10 minutes at 1 to 2 mm to remove hydrogen peroxide.
It is vital that this be entirely removed or low erratic results will be obtained. Cool the flask before restoring pressure. Use a total volume of about 80 ml of acetic anhydride to transfer the contents to a 100-ml beaker, along with the stirring bar. Warm the acetic anhydride slightly, if needed, to dissolve solid derivatives. Insert the electrodes, and while stirring, adjust the pH meter to an apparent pH between 10 and 11. Add the perchloric/ acetic titrant from a 10-ml buret. Within about 2 ml of the expected end point, add titrant in 0.25-ml increments. See Figure 1 for a typical titration. RESULTS AND DISCUSSION
Table I shows the sulfoxide yield for various compounds, as a function of time of oxidation. These values are submitted as guides; recovery data on pure standards should be obtained by the user with the equipment and technique available in his laboratory. Table I1 lists the results obtained for three controls of VITAVAX, after compensation for 98.0% recovery. Column A data and column B data were obtained on different days. Chemical removal of unused hydrogen peroxide was attempted with potassium iodide and with permanganate. The products of reaction interfered with the titration. Hydrazine reacted very slowly with peroxide under the conditions of the analysis. RECEIVED for review January 2, 1969. Accepted February 6, 1969.
Polarographic Reduction of the Molybdate-Chlorate System in Biphthalate Medium Demetrios Kyriacou Research Laboratories, The Dow Chemical Co., Pittsburg, Gal$ 94565 THE POLAROGRAPHIC BEHAVIOR of the system Mo(VI)-C103in sulfuric acid media has been studied by Kolthoff and HOdara ( I ) . The present work is concerned with the above system in potassium biphthalate medium. In the presence of C103- catalytic augmentation of the Mo(V1) wave occurs. The waves are suitable for analytical purposes. EXPERIMENTAL
Solutions of NazMo042Hz0 with the desired concentration of NaC103 and potassium biphthalate were made with reagent grade materials in distilled water. Adjustment of pH to values other than 4 were made by adding HC1 or NaOH to the biphthalate solutions. The ordinary polarographic technique and also the derivative pulse technique were used (in the latter -30 mV pulses superimposed on the dc sweep). Both modes of polarography were provided with a Melabs pulse polarographic analyzer, Model CPA-3. The dropping mercury electrode had a mechanically regulated drop of 2.0 seconds and a flow of 1.67 mg per second at -0.5 V in saturated biphthalate solution. The effect of mercury column height was observed with natural drops. A platinum wire was the anode and a saturated calomel electrode was the (1) M. Kolthoff and I. Hodara, J . Electroanal. Chem., 5, 2 (1963). 844
ANALYTICAL CHEMISTRY
reference electrode. Polarograms were obtained at 23 "C after oxygen was removed by nitrogen. Reported currents are average currents corrected for residual current. RESULTS AND DISCUSSION
Electrolysis in the Absence C103-. Table I and Figure 1 summarize experimental results, Because molybdate ions are known to polymerize in acidic solutions it is not possible to deduce from this work what is the electroreducible molybdenum species. The electroreduction involves one electron and two protons, as if the electrode reaction were
Moo4*- f 2H+ 4- e --c Moo3-
+ HzO
(1)
The number of electrons and protons was deduced from microcoulometric measurements and the change of El/*with pH, as is shown below. Curve A of Figure 1 is a normal polarogram of molybdate in saturated potassium biphthalate solution, pH 4. Such polarograms were obtained with Mo(V1) concentrations up to -0.5 m M and biphthalate concentrations between 0.02 and -0.5M. Similar polarograms were obtained with molybdate in sodium chloride brines which were saturated with potassium biphthalate.
