50 Analyst, January, 1974, Vol. 99, $9. 50-53 Gas- chromatographic Determination of Dilauryl pp’- Thiodipropionate and its Primary Oxidation Products BY J. SEDLAR, E. FONIOKOVA AND J. PAC (Research Institute of Macromolecular Chemistry, Tkalcovskd 2, Bmo, Czechoslovakia) A method is described for the gas-chromatographic determination of dilauryl pP’-thiodipropionate and its oxidation products, dilauryl sulphenyl- dipropionate and dilauryl sulphonyldipropionate. The sample is first hydrolysed in a 5 N methanolic solution of potassium hydroxide and the resulting lauryl alcohol is then determined gas chromatographically by using the internal marker technique. The method can be used to determine concentrations down to 10 p g ml-l, with a standard deviation of f3 per cent. DILAURYL PP’-thiodipropionate (DLTP, Negonox DLTP, Advastab PS 800), C12H2,0CO- CH2CH2-S-CH2CH2COOC12H25J is widely used as a component of synergistic compositions with phenolic antioxidants for the ~tabilisationl-~ of polypropylene, polyethylene, ethylene - vinyl acetate copolymers, acrylonitrile - butadiene - styrene resins and high-impact poly- styrene.The protective action of DLTP is believed2 to result from its ability to reduce hydroperoxides, via a non-radical process, to the corresponding alcohols and its sulphoxide and sulphone oxidation products. The isolation and determination of additives from polymer materials have been reviewed by several a ~ t h o r s . ~ - ~ The quantitative determination of DLTP in extracts is usually effected by determining the sulphur c ~ n t e n t , ~ , ~ although this is a time-consuming procedure.Gel-permeation chromatographys has been successfully applied to the separation and deter- mination of DLTP in multicomponent antioxidant mixtures, while infrared spectroscopy based on measurement of the extinction of carbonyl groups has been used for the direct quantitative determination of DLTP in undegraded polypropylene foils.lO A method in- volving the use of the esterification of dilauryl thiodipropionic acid by diazomethane has been reported as being suitable for the determination of DLTP in 1ard.ll Neureither and Bown2 used a high-speed polarographic technique for the quantitative determination of DLTP. Attempts to carry out a direct quantitative determination of this compound by using luminescence12 or gas-chr~matographicl~ methods have failed.No references to methods for the determination of the oxidation products of DLTP have been found. In the present work, we examined the conditions under which DLTP and its oxidation products are hydrolysed quantitatively to lauryl alcohol, thus making gas-chromatographic determination possible. The method described here has the advantages of being fairly rapid, accurate and simple. It has been applied successfully to the analysis of polypropylene samples containing 0.02 to 0.3 per cent. of DLTP and its oxidation products, the sample size being about 1.0 g of polymer. The last condition is hardly possible when the other methods are used. The additives, including DLTP, were extracted quantitatively from the sample under an atmosphere of nitrogen with a chloroform - ethanol - n-hexane (1 + 1 + 4) mixture by using a semimicro-extractor of the Soxhlet type.DLTP was separated from its oxidation products as well as from other additives by thin-layer chromatography on silica gel coated plates. The spots containing DLTP were extracted quantitatively by use of a chloroform - benzene (1 + 1) mixture and the extracts analysed by the method described under Procedure. METHOD REAGENTS- L aury 1 alcohol-Analyt ical-reagen t grade. Dilauryl PP’-thiodi$ro$ionate (DLTP)-This reagent was prepared by the esterification of thiodipropionic acid with lauryl alcohol in the presence of 9-toluenesulphonic acidl4; @ SAC and the authors.SEDLA~, FONIOKOVA AND PAC 51 sulphur assay, 6.23 per cent.(theoretical value, 6.23 per cent.); melting-point, 39.5 "C. For some analyses, commercial preparations were used. Dilaury sulphenyl-pp'-dipropionate (DLS0)-This was prepared by oxidation of DLTP with chromic acid in an acetic acid medium at 60 to 80 "C15 (melting-point, 74 "C). Dilauryl sulpJzonyl-~~'-dipropionate (DUO,)-This was also prepared from DLTP, by the method described for the oxidation of sulphides to sulphones16 with hydrogen peroxide solution at 90 "C (melting-point, 91 "C). n-Octadecane. Chloroform. Methanol. Potassium hydroxide. The last four reagents should be of analytical-reagent grade. Standard lauryl alcohol solution-Weigh accurately about 70 mg of lauryl alcohol into a Standard n-octadecane solution-Weigh accurately about 200 mg of n-octadecane into a Standard potassium hydroxide, 5 N solution in methanol-Dissolve 3 g of potassium hydrox- 100-ml calibrated flask and make the volume up to the mark with chloroform. 100-ml calibrated flask and make the volume up to the mark with chloroform.ide pellets in 2 ml of water and make the volume up to 10 ml with methanol. APPARATUS- A Perkin-Elmer, Model F 11, dual-column chromatograph equipped with flame-ionisation detectors and an isothermal column oven was used. The oven was operated isothermally at 165 "C and the injection port at 300 "C. Suitable column conditions were obtained with a 6-foot x $-inch glass column, packed with 1.5 per cent. of fluorosilicone oil FS-1265 on 80 to 100-mesh Chromosorb W AW-DMCS. Argon was used as the carrier gas at the flow-rate of 30 ml min-l.A 0-6-pl volume of the lauryl alcohol containing sample was introduced into the column by means of a 1-p1 Hamilton No. 7001 syringe. PROCEDURE- Place 5 ml of solution containing 0.1 to 3 mg of DLTP, DLSO or DLSO, in a test-tube (30 mm 0.d.) provided with a B29 ground-glass joint and evaporate off the solvent under a stream of nitrogen. Add 1 ml of freshly prepared methanolic potassium hydroxide solution, fit a reflux condenser or a cooling finger into the joint and immerse the bottom of the tube in a heating bath, the temperature of which is maintained at 80 "C. After heating for 30 minutes, transfer the contents of the tube quantitatively into a 25-ml cylindrical separating funnel (20 mm 0.d.) by using a total of 10 ml of water.Finally, rinse the tube with 2 ml of chloroform and add the rinsings to the water in the funnel. Shake the funnel well and when the layers have separated collect the bottom layer in a 10-ml calibrated flask. Repeat the extraction a further three times, using 1-5 ml of chloroform for each run. Add 1 ml of n-octadecane solution to the combined extracts in the flask and make the solution up to 10 ml with chloroform. Prepare the reference solution by placing 5 ml of standard lauryl alcohol solution in a 10-ml calibrated flask, adding 1 ml of n-octadecane solution and making the solution up to 10ml with chloroform. Run the chromatograms of both reference and sample solutions under the conditions described. Calculate the amount of lauryl alcohol formed by hydrolysis according to the equation where [LOH] is the concentration of lauryl alcohol in the sample in milligrams per 10 ml, a is the concentration of lauryl alcohol in the reference sample in milligrams per 10 ml, V, and V 3 are the peak heights due to lauryl alcohol in the reference and sample, respectively, V , and V4 are the peak heights due to n-octadecane in the reference and sample, respectively, and Q1 and Q, represent the sensitivities for the lauryl alcohol peak in the reference and sample run, respectively.RESULTS . . * * (1) == a V2V3Q2/VV,V4Q1 . . .. Under the column conditions described above the peaks due to the solvent, lauryl alcohol The retention data are summarised in and internal standard (n-octadecane) are well resolved. Table I.52 SEDLM et al.: GAS CHROMATOGRAPHY OF DILAURYL TABLE I RETENTION DATA [Analyst, Vol. 99 Relative retention Compound time Chloroform . . .. 0.10 Lauryl alcohol . . 0.68 n-Octadecane 1.00 (5.6 minutes) A series of hydrolyses at 80 "C in 5 N methanolic potassium hydroxide solution for 30 minutes was carried out within the concentration range of 0.15 to 3.00 mg of DLTP per 10 ml. The results are shown in Fig. 1, in which the amount of lauryl alcohol formed by hydrolysis is plotted against the amount of DLTP hydrolysed. 0 0.5 1 *o 1.5 2.0 2.5 DLTP hydrolysed/mg Fig. 1. Relationship between the amount of lauryl alcohol found and the amount of DLTP hydrolysed. Conditions of hydrolysis : temperature, 80 "C; reagent, 6 N potassium hydroxide solution in methanol; and time, 30 minutes The slope of the straight line, evaluated by the least-squares method, was found to be 0.7028.The mean deviation of the experimental points from the calculated values was & 2.5 per cent. over the whole concentration range. The theoretical value of the slope, assuming that the complete hydrolysis of DLTP gives two molecules of lauryl alcohol per molecule of DLTP decomposed, is 0.7238. Determination carried out under these conditions therefore gives results that are consistently low by a factor of 0.97. The conditions of hydrolysis described above were found to be the most suitable. At higher temperatures, the oxidation of lauryl alcohol occurs so that lower apparent amounts of lauryl alcohol are found. At lower temperatures, on the other hand, the hydrolysis becomes incomplete.Similar conditions hold for the hydrolysis of DLSO and DLSO,. The actual amount of the particular compound being determined in the sample can therefore be calculated by miiltiplying the observed value by a factor of 1.03, which accounts for the systematic error of the analysis. An example of the results obtained in the analysis of polypropylene sheets stabilised with pure DLTP is given in Table 11. TABLE I1 RECOVERY OF DLTP FROM POLYPROPYLENE SHEETS Concentration in polymer before Standard pressing/g kg-1 DLTP found in polymer/g kg-l deviation 3.00 2.67, 2.79, 2.79, 2.76, 2.94, 2.76 k0.09January, 19741 ,~P‘-THIODIPROPIONATE AND ITS PRIMARY OXIDATION PRODUCTS 53 The assay of lauryl alcohol in commercially produced DLTP frequently gives lower results, due mainly to the fact that these preparations contain, in addition to DLTP, other esters of thiodipropionic acid (e.g., cetyl, stearyl and possibly higher derivatives).A calibration should be carried out in each particular instance. Acknowledgement is due to Dr. M. 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Lappin, G. R., and Zannucci, J . S., Analyt. Chem., 1969, 41, 2076. “Houben-Weyl’s Methoden der Organischen Chemie,” Band VIII, Georg Thieme Verlag, Stuttgart, Knoll, R., J . prakt. Chem., 1926, 113, 40. Rheinboldt, H , and Giesbrecht, E., J . Amer. Chem. SOG., 1946, 68, 973. 65, 279. 1952, p. 522. Received June lst, 1973 Accepted August 2nd, 1973