STABILITY OF 2-PYRIDINETHIOL-1-OXIDE 245 containing ! mL/L glacial acetic acid, and 5 mL/L of a 40% aqueous solution of tetrabutylammonium hydroxide. At a flow rate of 1.5 mL/minute the retention time was 4.0 minutes. The sulfinic acid salt was determined on a PARTISIL © SAX anion exchange column (25 cm x 3.6 mm i.d.) using 0.2M pH 7.4 borate buffer as the mobile phase. At 2.0 mL/minute the retention time was 3.0 minutes. Liquid chromatography (1) was also used to determine the presence of several hypothesized degradation products which were not found in significant amounts. These included 2-pyridinethiol 2,2'-dithiobis-pyridine 2,2'-dithiopyridine-pyridine-l-oxide 2-pyridinesulfonic acid, and 2-pyridinesulfonic acid-l-oxide. All of the above men- tioned compounds and notably 2-pyridinesulfonic acid-l-oxide (structurally very sim- ilar to the sulfinic acid) were easily separated on the reverse phase column (Figure 1). 3 4 2 ! i i o 3 6 9 min. Figure l. Separation of 2-pyridinethiol (1), 2,2'-dithiobispyridine-l,l'-dioxide (2), 2-pyridinesulfinic acid-l-oxide (3), 2-pyridinesulfonic acid-l-oxide (4), and 2-pyridinethiol-l-oxide (5). PREPARATION OF 2-PYRIDINESULFINIC ACID 2-Pyridinesulfinic acid-l-oxide was prepared by a method to make aromatic sulfinates from disulfides proposed by Danehy and Parameswaran (2). The overall equation is as follows: •S--S• + 2N.OH + 3H202 -'-- 2 •SO2- Na++ 4H20 (1) i i i o o o

246 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Approximately 4.0 g of analytical standard disulfide was dissolved in 50 ml of 1N sodium hydroxide. The solution was kept at room temperature while 2.75 mL of a solution of 30% hydrogen peroxide was slowly added over a two hour period. When nearly all the disulfide had reacted, as determined by liquid chromatographic analysis, the reaction solution was cooled in an ice water bath and adjusted to pH 5.6 with 6N hydrochloric acid. The chilled solution was immediately passed through a DOWEX © 50W-X8 cation exchange column (30 mm x 40 cm) in the H + form. The solution was eluted with deoxygenated, distilled, deionized water. Approximately 30 mL fractions were collected in 1 oz. amber bottles and immediately capped and refrigerated. All fractions were subsequently analyzed via liquid chromatography. Those fractions most free from the disulfide, pyridine-l-oxide, and sulfonic acid impurities were freeze-dried and the resulting white crystalline solid stored in a freezer. The product was characterized by its IR and V spectra, by elemental analysis, thermal analysis, and mass fragmentography. Sulfonic acid and pyridine-l-oxide impurities were quantitated by liquid chromatography. A product purity of 96.6% was determined by titration with sodium hydroxide. The sulfinic acid is thermally unstable at room temperature, decomposing to pyridine-l-oxide and sulfur dioxide. RESULTS AND DISCUSSION PRODUCT IDENTIFICATION The disulfide and the sulfinate were identified as reaction products by their liquid chromatographic retention times, as compared to authentic analytical standards, on both reverse phase and ion exchange columns. On both chromatographic systems, a clean separation was achieved between the disulfide, sulfinate and other known product impurities. Further evidence for the sulfinic acid was provided by its preparation from the alkaline hydrolysis of the disulfide. STABILITY RESULTS Although the production material (40% solution of NaPT) was quite stable at its natural pH(8.5-10) over a several month period, some degradation was observed at the low concentrations used in this study. Its stability under the various conditions of pH, temperature, and concentration is summarized in Table I. In general, stability was found to decrease with increasing temperature and with increasing pH above pH 7. Table I Hydrolysis of 100 PPM Sodium 2-Pyridinethiol-l-Oxide in 0.02M Buffers % Sodium-2-Pyridinethiol-l-Oxide Remaining at Indicated pH Values Days 5øC 40øC 4.1 7.1 10.0 4.1 7.1 10.0 0 100 100 100 100 100 100 2 98.3 101.9 99.0 109.0 105.0 97.6 7 96.6 99.0 102.9 98.2 98.1 86.9 14 97.4 102.9 99.0 101.8 102.0 83.2 21 100.0 100.0 103.8 100.9 103.0 80.0 31 98.3 100.0 100.0 98.2 98.2 74.7* *% remaining after 28 days.