70 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Alternatively, two samples were analyzed in which either amino group was blocked with methyl iodide (8) or the N-oxide function was reacted with acetic anhydride (9). Good results have been obtained by using redox titration, but this method yields the content of amine oxide only. In most cases, TiCI 3 was used as titrant and the titration was carried out in an acidic medium (10-14). The use of SnCI 2 (15) and KI or HI (16) has also been reported. The two-phase titration has been used infrequently for the analysis of amine oxides. It has been used to determine the sum of amine oxide and free amine (17) or for deter- mining N-oxide in the presence of anionic surfactant (18). Among the chromatographic methods used for the analysis of amine oxides were gas chromatography (GLC), thin-layer chromatography (TLC), or high-pressure liquid chromatography (HPLC) (9). The aim of this work was to examine the accuracy of two selected methods of analysis of cocamidopropylamine oxide which serves as an example of a commercial amine oxide- free amine mixture. The methods selected were potentiometric titration in isopropyl alcohol (method I) and the combination of two-phase and titanometric titration (method II). The accuracy of these methods for simultaneous determination of amine oxide and unreacted amine, particularly with respect to the amine impurity, has not been studied. In addition, the effect of temperature on the stability of the active component in the commercial amine oxide during determination of the "solids content" was examined. EXPERIMENTAL MATERIALS Laurylamidopropylamine oxide (C•2NNO) was obtained in two steps according to the following scheme: C•.•H23COOH + H2N(CH2) 3 CH 3 I N I CH 3 C 11H23CONH(CH2)3 H202 C 11H23CONH(CH2)3 CH 3 N + I CH3 n-propanol 60øC CH 3 I N I CH 3 H20 O ß I-I20 (1) (2)

ANALYSIS OF COCAMIDOPROPYLAMINE OXIDE 71 Laurylamidopropylamine (C•2NN): b.p.: 161øC, 0.24 mm Hg, m.p. 33-36øC, ter- tiary nitrogen amount 99.1%, was obtained according to Muzyczko et al. (19). The oxidation step was carried out in the same way as described by Hoh et al. (12). The separated amine oxide was laurylamidopropylamine oxide monohydrate: % N for C•2NNO ß H20, calculated 8.80% found 8.71%. The amine C•2NN and its oxide C•2NNO were used for preparing standard mixtures simulating commercial composi- tions. These mixtures were then used for assessing accuracy of analytical procedures. Commercial cocamidopropylamine oxide (Gamidox K-40, aqueous solution) was kindly supplied by GZChG "Pollena" (Gdafisk, Poland). It had the following characteristics: ß active substance: min 32%, ß free amine: ca. 2%, -- ß average mol. weight of N-oxide: M = 309, -- ß average mol. weight of amine: M - 293. POTENTIOMETRIC TITRATION The titration procedure was the same as described by Metcalfe (8). A pehameter N-512 (Mera-Elmat, Poland) equipped with combined glass electrode SAg P-201W (WPL- Gliwice, Poland) was used. A sample containing 5-7 millimoles of amine oxide dissolved in isopropyl alcohol was titrated with 0.20 N HC1 solution in the same alcohol (VA). Another sample was quaternized with 3 ml of methyl iodide at 50 --- 2øC for 15 min and after cooling to room temperature titrated as before (VB). The difference V A - V B yielded the free amine content. TITANOMETRIC TITRATION The procedure was similar to that described by Brooks and Sternglanz (10). A sample containing ca. 0.35 mmole of amine oxide was placed in a conical flask and dissolved in 20 ml of ethanol-water mixture (1:1, V:V). 1.5 ml of 3M NH4SCN complex-forming reagent solution was added. Then the content of the flask was flushed with nitrogen, and 10.0 ml of ca. 0.20 N TiC13 solution was introduced. After flushing the contents with nitrogen again, the flask was firmly closed and left in the dark for 0.5 h. Then 7.5 ml of 18% HC1 (1:1, V:V) solution was added and the sample titrated with a 0.10 N solution of NH4Fe(SO4) 2 until the pale yellow or colorless solution turned light red. The same operations were carried out for a blank sample. TWO-PHASE TITRATION These titrations were made using two alternative indicators, i.e., methyl orange or a mixture of dimidium bromide and disulfine blue VN150 (mixed indicator). To a stoppered cylinder, 10.0 ml of a solution of a known amount of sample in 0.05 N HC1, 10 ml of citric acid/disodium phosphate buffer (pH = 3.0), 15 ml of chloroform, and 1 ml of 0.01% indicator solution were introduced. The resulting two-phase system was titrated with a standard 0.004 M sodium tetrapropylenebenzenesulphonate solu- tion. The titration was carried out with methyl orange until the chloroform layer lost