Figure 4 shows that the concentration of BrO,- may be determined in the concentration range 0.5-5 X 10-aM (64-640 mg/liter). On the basis of series of parallel determinations, the variability of the determination under these conditions was *7-lOz. Reactions of Other Ions. The MOO^^- ion reacts with the reagent in a neutral or slightly acidic solution (pH 1-8) to give a white precipitate that is soluble in very acidic or in basic medium, even at a concentration of 0.001M Moos*-. In a slightly acidic solution (pH about 1-3) the reagent reacts with the Fe(CN)s4- ion to give a white precipitate and with the Fe(CN)63- ion to give a yellow precipitate. The sensitivities of these two precipitation reactions are low (lowest concentration 0.03M) and therefore have no analytical significance.
DISCUSSION
In the course of the investigation, the reactions between 1,2,3-tris-(2-diethylaminoethoxy)-benzenehydrochloride and a number of inorganic anions have been examined, mainly with regard to the possibility of applying this organic base as a reagent in analytical chemistry. This investigation has shown that it is mainly the color reactions with NOn- and Br0,- ions that can be advantageously used in qualitative analysis and in quantitative determinations. The high sensitivity of the color reaction with the NO*- ion is especially significant. Very little information can be given as to the nature of the complexes forming in the reaction between the C24H4803N38+ ion and nitrites, or bromates. RECEIVED for review November 27, 1967. Accepted April 10, 1969.
Gas Chromatographic Determination of Polyhydric Alcohols in Oils and Alkyd Resins by Formation of Trimethylsilyl Derivatives George G . Esposito and M. H. Swann U.S. Army Coating and Chemical Laboratory, Aberdeen Proving Ground, M d . 21005 METHODS EXIST for the determination of a few polyhydric alcohols in alkyd resins and oils, but they are generally time consuming, tedious, nonspecific, and incapable of complete analysis. The determination of polyols in resinous products has long been an outstanding problem, and although the need for a general quantitative procedure has been firmly established, such a method could not be located in the literature. The complexity of the problem is compounded by several factors. Some polyols have similar chemical and physical properties which are difficult to distinguish. Other polyols, more diverse in nature, preclude the use of a general chemical method. Most important, polyols are combined as an integral part of an oil and alkyd resin and must be released prior to final testing. The method for dissolution of the sample must be conducted without loss of polyol and at the same time in a form suitable for subsequent analysis. Established methods ( I , 2) involve alcoholic KOH saponification, removal of fatty acids and dicarboxylic acids, and the application of tests that are capable of responding to some incremental difference in properties of the polyols. These schemes are excessively time consuming, laborious, and limited to only several of the polyols. Gas-liquid chromatography (GLC) is a rapid and convenient method for analyzing complex mixtures, but its use for the direct analysis of polyhydric alcohols is hindered by their strong polar character and low vapor pressures, the nullification of which can be accomplished by derivative formation. A GLC method for the qualitative analysis of polyols, as their acetates, in alkyd resins is available ( 3 , 4 ) ,but no provision was (1) Amer. SOC.Testing Materials, Method D-1615,Philadelphia, Pa, (2) G. M. Kline, Ed., “Analytical Chemistry of Polymers,” Vol. XII, of “High Polymers,” Interscience, New York-London, 1959 pp 25-31. (3) G. G . Esposito and M. H. Swann, ANAL. CHEM.,33, 1854
(1961). (4) Amer. SOC. Testing Materials, Method D-2456,Philadelphia, Pa. 1118
ANALYTICAL CHEMISTRY
made for quantitative determination. Aminolysis was used to liberate the polyols which were acetylated and separated by GLC. Trimethylsilyl (TMS) reagents have been used for a variety of compounds having active hydrogen; their relative reactivity to certain chemical types has been discussed in detail (5). The principal advantages derived from the formation of TMS derivatives are a reduction in polarity and an increase in vapor pressure. Although some polyols have been analyzed as their TMS derivatives (6, 7), no method has been reported demonstrating adaptability to the analysis of alkyd resins and oils. A modified aminolysis treatment combined with TMS derivatization is the basis for the proposed method. The qualitative and quantitative procedure presented in this paper is applicable to all of the polyhydric alcohols that are normally encountered in alkyd resins and oils. Polyols are first released from the resin by aminolysis and then converted to their trimethylsilyl derivatives by direct treatment of the aminolysis mixture with a special combination of TMS reagents. A portion of the reaction mixture is injected onto a silicone grease column where the polyols, as their TMS ethers, are separated and determined simultaneously. Although the procedure involves several steps, sample manipulation is minimal. All of the twelve polyols investigated responded favorably to the new method which possesses a high degree of selectivity and sensitivity. EXPERIMENTAL
Chromatographic Equipment. The equipment used to obtain the chromatograms was a Model 810 Linear Programmed Temperature Gas Chromatograph (F & M Scientific Corp.) equipped with a Brown Electronic recorder (Minneapolis-Honeywell). (5) P. S. Mason and E. D. Smith, J. Gas. Cliromatogr., 4, 398 ( 1 966). (6) B. Smith and 0. Carlsson, Acra Chem. Scand., 17,455 (1963). (7)G.G.Esposito, ANAL.CHEM., 40,1902(1968).
Table I. Relative Retention Time of Trimethylsilyl Ethers Retention time relative to Poly01 1&butanediol Ethylene glycol 0.58 Propylene glycol 0.63 2,3-Butanediol 0.73 1,ZButanediol 0.83 0.86 Neopentyl glycol 1,4-Butanediol 1 .oo 1.19 Diethylene glycol 1.29 Glycerine 1.46 Trimethylol ethane 1.63 Trimethylol propane 1.83 Triethylene glycol 1.88 Pentaerythritol
A
0
5
10 MINUTES
Quantitation. Peak height measurements were used to obtain data. The height of each poly01 peak was related to the height of a known amount of internal standard. Correction factors were established by dissolving known amounts of poly01 and internal standard in butylamine which were then treated with the TMS reagent and chromatographed in the same manner as the sample.
I5
Figure 1. Separation of trimethylsilyl ethers of polyhydric alcohols A . Ethylene glycol
G . Diethylene glycol
B. Propylene glycol
H. Glycerine
C. 2,3-Butanediol D. 1,3-Butanediol E . Neopentyl glycol F. 1,4-Butanediol
1. Trimethylol ethane
J . Trimethylol propane
K. Triethylene glycol L. Pentaerythritol
Operating Conditions Detector cell temp, "C Detector cell current, mA Injection port temp, "C Helium flow at exit, cc/min Column heating rate, "C/min Initial column temp, "C Terminal column temp, "C
300 160 300 80 6 100 300
A column was prepared from 16 ft of '/a-inch copper tubing packed with 20'3 by weight of silicone grease (DC-11) on 60- to 80-mesh Chromosorb W. Reagent. Mix 20 parts by volume of Bis(trimethylsily1)trifluoroacetamide with 80 parts of hexamethyldisilizane. Protect from moisture. Reagent remains active and stable when stored at 60" C in an oven. Procedure. Weigh a sample containing approximately 2 grams of nonvolatile into a 125-ml flask. Add 3 ml of methylene chloride and evaporate solvents in a 60 "C water bath using a current of air. Repeat the drying procedure two more times, dissolving the resin each time with 3 ml of methylene chloride. All of the paint solvents should be eliminated by the final drying. Weigh accurately about 200 mg of 1,Cbutanediol into the flask followed by 10 ml of butylamine. Reflux under a water condenser for two hours, add 0.5 ml of water to the flask and continue refluxing for two additional hours. Place 5 drops of the aminolysis mixture into a micro test tube; add 0.5 ml of TMS reagent and mix. Heat for 30 min at 60 "C. in an oven. Inject 40 p1 onto the silicone grease column and follow the recommended operating conditions.
RESULTS
A synthetic mixture of polyols was converted to their TMS ethers; their separation on the silicone grease column is shown in Figure 1. Only 2,3-butanediol and neopentyl glycol are not completely resolved. The same calibration mixture was used to obtain the retention time data presented in Table I. Figure 2 illustrates the results obtained from the analysis of soybean oil. An unknown alkyd was subjected to the method and produced the chromatogram shown in Figure 3. Comparison to the calibration chart in Table I proved the polyols to be ethylene glycol, glycerine, and pentaerythritol. Peaks appearing between 20 min and 35 min are reaction products from phthalic anhydride and fatty acids. Table I1 shows the results obtained from coating materials of known composition. The amounts determined are consistent with the known quantities of polyol.
REI 512
0
5
IO
Ib
20
2'5
30
35
MINUTES
Figure 2. Determination of glycerine in soybean oil A . 1,4-Butanediol (internal standard) B.
Glycerine VOL. 41, NO.
8,JULY 1969
1119
Table 11. Determination of Polyols in Coating Materials Sample Alkyd Soybean oil Linseed oil Alkyd
Polyester
Poly01 Glycerine Pentaerythritol Glycerine Glycerine Ethylene glycol Glycerine Pentaerythritol Propylene glycol
z
z
Found 10.3 11.8
Present 9.9 12.1
10.7 10.4
10.6 10.6
4.6 11.2 8.2 43.4
4.4 11.1 8.4 43.2
DISCUSSION
It was felt that a direct decomposition of alkyd resin, which permits the use oflarge samples but does not require removal of the reactant, would improve the yield of polyol. Accordingly, a method of derivatization not involving many intermediate steps would also be advantageous. These objectives were stressed during the research leading to the development of the procedure described in this paper. Butylamine serves as both a solvent and reactant. As a consequence, dried films as well as resin solutions, oils, and fats can be analyzed. Apparently, the amine reaction with TMS reagents does not constitute a source of trouble. Peak height measurements are more convenient then area integration, a principal advantage being the unattended operation of the chromatograph. At the inception of this investigation, both peak height and area were used for quantitative determinations. Contrary to expectation, no significant difference was observed. At the start of the investigation relative peak height correction factors were secured for the various polyols. Periodic examinations were conducted and the factors were found to be permanent when the same column and operating conditions were employed. Various TMS reagents were evaluated and their suitability was judged on the basis of purity and performance. Hexamethyldisilizane (HMDS) is not adequately reactive in the absence of a catalyst, and when the catalyst, trimethylchlorosilane, was used the large amount of ammonium chloride which formed during the reaction was found to be objectionable. Other TMS reagents tested were N,O-Bis(trimethy1silyl)acetamide, N-trimethylsilyldiethylamine, and Bis(trimethylsily1)trifluoroacetamide (BSTA). Good conversion to the TMS ether was observed for each, but impurities from all of these reagents interfered with the determination of ethylene and propylene glycol. It was reasoned that a mixture of HMDS with one of the more reactive TMS reagents would provide the necessary reactivity without interference from impurities. The 20:80 blend of BSTA:HMDS satisfied these conditions. The possibility of extending the method to the analysis of mono- and dibasic acids has not been investigated. Peaks attributed to the presence of these materials appear on a chromatogram during the determination of polyols and the concurrent identification of some of the carboxylic acids has been realized. The first impression is that the peaks are too small to have quantitative significance, but apparently, this is because of their having a low ratio of reactive groups per carbon atom as compared to polyols. When the mixture of TMS reagents is used, the peak from phthalic anhydride appears just following pentaerythritol ; however, if BSTA is
1120
0
ANALYTICAL CHEMISTRY
J 0
5
IO
I5
LO
25
30
35
MINUTES
Figure 3. Analysis of polyhydric alcohols in unknown alkyd resin A . Ethylene glycol B. 1,4-Butanediol (internal standard)
C. Glycerine D. Pentaerythritol
used alone the phthalic peak shifts to a new position further along the chromatogram without affecting the position or magnitude of the poly01 peaks. The shift is probably due to the reaction of BSTA with dibutyl phthalamide. The qualitative aspects of the method have not been emphasized because of the availability of a suitable method (3)for this purpose. However, the new method is an improvement, inasmuch as it requires less effort and it is more sensitive. Polyester resins respond qualitatively to the method, but quantitative application is not certain. The results from the analysis of a known polyester are shown in Table 11. However, a polyester prepared from neopentyl glycol, not shown, gave low yields. A four-hour aminolysis is not always necessary; two hours without water suffices for drying oils. Some alkyds gave slightly low results without the addition of water. The effect of water is not completely understood; however, it has been postulated that the majority reaction is that of aminolysis with the more resistant ester groups finally yielding to a hydrolytic cleavage. If water is added at the beginning, substantially lower results are obtained which is probably because of the change in solubility properties of the reaction mixture. Efforts to optimize the aminolysis were made by varying the time duration of the aminolysis and by changing the amount of water. Because variations in the physical and chemical properties of alkyd resins can often be related to polyol composition, the method presented in this paper should prove valuable for conducting performance studies and quality control testing. The method should have application to other fields such as plastics, oils, and biomedical. ACKNOWLEDGMENT
The advisory assistance of C. F. Pickett, Director of the Laboratory is acknowledged and appreciated.
RECEIVED for review March 3,1969. Accepted April 25,1969.
