Calibration Graph and Range. The calibration graph is linear over the range 1 to 16 pg of tantalum in 10 ml of benzene. The optimum range for absorbance measurements on the spectrophotometer is 0.2 to 0.7 absorbance unit. With the recommended procedure these values correspond to 4.4 to 16 pg of tantalum in 10 ml of benzene. The effective molar absorptivity for the tantalum complex in benzene is 83,000 at 635 mp. Precision and Accuracy. In order to obtain a measure of the reproducibility of the determination of tantalum in pure tantalum solutions, the same amount of tantalum (4 pg) was determined 21 times over a period of several days. The average absorbance was 0.184 and the standard deviation 0.005 absorbance unit or 2.8%. Table I shows the results of analysis for tantalum of some simulated tantalum sample solutions which contained other ions. The table also includes the results for the determination of tantalum in several ferromolybdenum alloys. Effect of Foreign Ions on Determination of Tantalum. Solutions were prepared containing 4 pg of tantalum and a 100-fold weight excess of each ion to be tested. The tantalum was then determined by the recommended procedure. An ion was considered to interfere when an error in the net absorbance for 4 pg of tantalum of greater than 5% was produced. The presence of a 100-fold excess of the following ions caused no interference: Al, Ag, As(V), Ba, Bi, Ca, Cd, Co, Cr(III), Cu(II), Fe(III), Mo(VI), Na, Ni, Pb, Sb(III), Sn(IV), Sn(II), Sr, Ti(IV), U(VI), W(VI), Zn,Zr, acetate, chloride, bromide, citrate, nitrate, silicate, and tartrate. The presence of a 100fold excess of Ce(1V) and V(V) caused interference by oxidation of the Victoria Blue B reagent. The presence of the following ions at 100-fold excess gave rise to the error in absorb-
ance given in parentheses: NH4f (+19%), Hg(I1) (+22%), Th(1V) (loo%), and B40r2- (100%). The presence of a 100fold excess of niobium(V) caused a positive interference (f66Z). The available niobium metal, however, contained ca. 0.02% of tantalum. After allowance for the contribution of the tantalum impurity to the absorbance, this error is reduced considerably. A 10-fold excess of niobium (tantalum free) can be tolerated without interference. Nature of Complex. Victoria Blue B is blue (absorption maximum 635 mp) in aqueous solution between cu. pH 1 and pH 12, and exhibits a yellow color (absorption maximum 475 mp) in acid solution at pH lower than 1. In the recommended procedure for the determination of tantalum, therefore, the aqueous blank solution is yellow. The maximum absorbance of both the reagent blank and tantalum complex occurs at 635 mp after extraction into benzene. The slope ratio method (7, 8) indicates a reagent : tantalum combining ratio of 1 :1. The reagent-tantalum-fluoride ion association complex which is formed and extracted into benzene thus appears to result from the interaction of the protonated reagent cation with the tantalum-fluor0 complex, TaF72-, so that the empirical formula is represented by RH2+TaF72-. The mole ratio method (9) failed to give clear indication of the reagent : tantalum molar ratio, presumably owing to the low stability of the association complex.
RECEIVED for review June 7, 1968. Accepted July 26, 1968. (7) A. E. Harvey and D. L. Manning, J. Amer. Chem. SOC.,72, 4488 (1950). (8) A. E.Harvey and D. L. Manning, ibid., 74,4744 (1952). (9) J. H. Yoe and A. L. Jones, IND.ENO. CHEM.,ANAL.ED., 16, 111 (1944).
Determination of Bisphenol A and Impurities by Gas Chromatography of Their Trimethylsilyl Ether Derivatives L. E. Brydia Chemicals and Plastics, Union Carbide Corp., Bound Brook, N.J . 08805 BISPHENOL A (4,4’-isopropylidenediphenol)is an important chemical which is used in the production of polymers such as epoxy and phenoxy resins, polycarbonates, and polysulfones. Commercial bisphenol A generally exceeds 99 % in purity and a product having a purity greater than 99.8 % is normally considered a requirement for the synthesis of high polymers. The major, high boiling impurities in high purity bisphenol A, originally identified by Anderson, Carter, and Landua ( I ) , are 2,4/-bisphenol A (2,4/-isopropylidenediphenol); Dianin’s compound (4,4’-hydroxyphenyl-2,2,4-trimethylchroman),also called monophenol or codimer ; and trisphenol [2,4-bis(a,adimethyl-4-hydroxybenzyl)phenol],also referred to as BPX. Despite the widespread use of bisphenol A as a raw material for the production of polymers, no quantitative method suitable for rapid analysis of this product was found in the literature. Chromatographic procedures have most frequently been used for the analysis of bisphenol A. Paper chromatography was employed by Anderson, Carter, and Landua ( I ) , Challa (1) W. M. Anderson, G. B. Carter, and A. J. Landua, ANAL. CHEM., 31, 1214 (1959).
221 2
e
ANALYTICAL CHEMISTRY
and Hermans (2), and Reinking and Barnabeo (3). Aurenge, Degeorges, and Normand (4), and Zowall and Lewandowska ( 5 ) used thin-layer chromatography for this analysis. However, quantitative results are difficult to obtain with these techniques. Gas chromatography has also been used for the analysis of bisphenol A. Direct methods have been reported by Tominaga (6> and Davis and Golden (7), but Gill was unable to determine bisphenol A directly because of serious peak tailing (8) and Anderson, Carter, and Landua ( I ) and Freudewald (9) observed decomposition when a direct procedure was employed. (2) G. Challa and P. H. Hermans, ibid., 32, 778 (1960). (3) N. H. Reinking and A. E. Barnabeo, ibid., 37, 395 (1965). (4) J. Aurenge, M. Degeorges, and J. Normand, Bull. SOC.Chim. Fr., 1963, 1732. (5) H. Zowall and T. Lewandowska, Chem. Anal. (Warsaw), 10, 947 (1965). (6) S. Tominaga, Bunseki Kuguku, 12, 137 (1963). (7) A. Davis and J. H. Golden, J. Chromutogr., 26,255(1967). 36, 1201 (1964). (8) H. H. Gill, ANAL.CHEM., (9) J. E. Freudewald, unpublished work, Union Carbide Corp., Bound Brook, N. J., 1964.
Table I. Analysis of Bisphenol A Synthetics by Gas Chromatography of TMS Ether Derivatives Wt. %added Wt found. 2,4’-BPAb Dianin’s 4,4’-BPA Trisphenol 2,4’-BPA Dianin’s 4,4’-BPA 0.20 0.10 99.50 0.20 0.20 0.11 99.49 0.10 0.11 99.79 ... 0.10 0.10 99.80 0.48 0.19 98.85 0.48 0.48 0.18 98.91 0.09 0.05 99.77 0.09 0.09 0.05 99.76 0.05 0.01 99,89 0.05 0.06 0.01 99.88 a All results are averages of two or more determinations. BPA = bisphenol A.
The direct method of Tominaga is not desirable for the rapid determination of trace impurities in bisphenol A because the procedure requires an extraction with boiling xylene prior to chromatographic analysis. Furthermore, a different chromatographic procedure is required for the determination of di- and trinuclear phenolic impurities. Davis and Golden state that bisphenol A can be quantitatively determined by direct analysis providing an all glass inlet and column system is used (7), but these authors provide no quantitative data to support this claim nor do they indicate whether their chromatographic procedure is capable of determining separately the individual impurities in bisphenol A. Gill determined bisphenol A and associated impurities by gas chromatography after converting them to acetates by reaction with acetic anhydride. However, the Gill procedure has the disadvantage of being extremely time consuming. An analysis which determines each impurity individually takes over 2.5 hours. This paper describes a method for the analysis of bisphenol A and the determination of the major, high boiling impurities in bisphenol A by gas chromatography of their trimethylsilyl ether derivatives. EXPERIMENTAL
Apparatus. An F and M Model 810 dual column gas chromatograph equipped with a hydrogen flame ionization detector was used. The chromatographic columns were stainless steel, 6 feet X 0.25 inch, Siliclad treated, and packed with 1% OV-1 on 80 to 100 mesh Chromosorb G (HP), (Supelco, Inc.). The injection port and detector were maintained at 300 “C. Gas flows were: helium, 65 ml per minute; hydrogen, 65 ml per minute; and air, 530 ml per minute. The column temperature was held isothermally at 200 O C until the bisphenol A derivative eluted, then programmed to 275 OC at a rate of 20 “C per minute. All peak areas were measured with a planimeter and the weight per cents were calculated by using the appropriate response factors and normalizing. Chemicals. Bis(trimethylsily1)acetamidewas obtained either from Supelco, Inc. or Applied Science Laboratories, Inc. and was used as received. Only reagent packed in glass sealed ampoules was used. The 4,4‘-bisphenol A was the high purity product of Union Carbide Corp. and was recrystallized twice from benzene before use. The Dianin’s compound, 2,4 ’-bisphenol A, and trisphenol were isolated from commercial bisphenol A by fractional crystallization and distillation. Procedure. Approximately 100 mg of sample were weighed into a two-ml serum bottle, and the bottle was stoppered with a rubber septum cap. To aid the reaction a partial vacuum was pulled by evacuating air from the bottle with a 10-ml hypodermic syringe. One-half milliliter of bis(trimethylsily1)acetamide reagent was then added to the serum bottle with a hypodermic syringe. The derivative preparation was shaken continually until the sample was completely dissolved (15 minutes). A 0.5-pl aliquot was then withdrawn and injected into the gas chromatograph.
Trisphenol 0.20
...
0.43 0.10 0.05
Table 11. Relative Retentions and Relative Responses for TMSa Ether Derivatives of Components of High Purity Bisphenol A Samples Structure of Relative Relative Component derivative retention response 2,4’-Bisphenol A
Dianin’s compound
0.655
1.12
0.807
1.42
CH3 a C
\’
CH3
4,4’-Bisphenol A
1.00
1.00
2.07
1.22
H3C-C-CH3
Q
OTMS
Trisphenol
OTMS
OTMS
CHa a
TMS = Si-CHa
I
CHs
RESULTS AND DISCUSSION
In the determination of relatively nonvolatile (e.g., trisphenol) or heat sensitive compounds (e.g., bisphenol A) gas chromatographic analysis is frequently preceded by the preparation of stable, volatile derivatives. Trimethylsilyl (TMS) ether derivatives have been widely used for this purpose. Bis(trimethylsily1)acetamide (BSA) is a relatively new silylating reagent which reacts with a variety of active hydrogen-containing compounds to form TMS derivatives (IO). The phenolic hydroxyl groups of bisphenol A and of the major, high boiling impurities in bisphenol A are quantitatively converted to the corresponding TMS ethers within 15 minutes (10) J. F. Klebe, H. Finkbeiner, and D. M. White, J. Amer. Chern. Soc., 88,3390 (1966).
VOL. 40, NO. 14, DECEMBER 1968
2213
4,4’-BISPHENOL A 64 X 102
Figure 1. Chromatogram of the TMS ether derivatives of bisphenol A and of the major, high boiling impurities in bisphenol A
I
0
2
4
6
8
10
12
14
16
I8
20
22
I
I
24
26
28
30
32
RETENTION TIME, MINUTES
after the BSA reagent is added to the bisphenol A sample, providing the sample is completely dissolved in this time. The products of these reactions were verified by mass spectrometric analysis of traps of the individual gas chromatographic peaks. Each of the compounds gave a parent ion of the proper size for all OH’Sbeing replaced by OSi(CH&’s. The emphasis in this work was on the analysis of high purity (>99x) bisphenol A . The method was developed with a hydrogen flame ionization detector because the concentration of impurities in high purity material is frequently below the detection limit of the thermal conductivity detector. The present procedure can readily determine impurities at the 50 ppm level without a concentration step. The separation of the Dianin’s compound derivative from the 4,4’-bisphenol A derivative is critical to this analysis. The best results were obtained by operating the oven isothermally until the 4,4’-isomer derivative eluted, then rapidly programming the temperature to elute the trisphenol derivative. Chromatographing time for a complete analysis-Le., separate determination of each component-by this method is approximately 30 minutes and this time can be cut in half if it is not necessary to determine the low concentration of Dianin’s compound which is normally present in high purity bisphenol A. To minimize analysis time, the excess BSA reagent is not removed from the derivative preparation prior to chromatographic analysis. The excess reagent and the derivatives burn at the detector to form silica, some of which is deposited on the collector electrode. These deposits adversely affect the response of the hydrogen flame ionization detector. Daily cleaning of the collector electrode mitigates this problem. This preventive maintenance takes less than 5 minutes a day. Table I shows data which were obtained by analyzing synthetic mixtures of the major, high-boiling impurities in bisphenol A and illustrates the reliability of the method for the analysis of high purity material. The synthetics were pre-
pared by accurately adding acetone solutions of the impurities to weighed amounts of twice recrystallized bisphenol A and evaporating the acetone with a stream of nitrogen. The derivatives were then prepared in the usual way. The relative retentions and the relative responses of the TMS ether derivatives of the compounds of interest are listed in Table 11. The results of a study on the precision of the method are summarized in Table 111. These data were obtained from 5 successive analyses of a synthetic mixture. Figure 1 shows a chromatogram of the TMS derivatives of a sample of high purity bisphenol A. In addition to the high boiling impurities, low concentraof water and phenol may also be present in tions (
