(b) competing process when water is present (favored under most conditions). COz -/- 2e-
-
+ H 2 02!% HC02- + OHCOn + OH- -+ HC03-
COz-2
The reactions involving the COz-2 species must be slow to account for the two-electron stoichiometry observed with chronopotentiometry. This appears reasonable, especially if the competing process involving water is predominate.
Additional work on the electrochemical oxidation of carbon monoxide, formate ion, and oxalate ion in dimethyl sulfoxide is currently in progress. When completed this should provide additional insight concerning the intermediate species for the reduction process. RECEIVED for review September 12, 1966. Accepted January 7, 1967. Work supported by the National Science Foundation under Grant No. G P 4303. Division of Analytical Chemistry, 150th National Meeting, ACS, Atlantic City, N. J., September 1965.
Rapid Condensation Procedure for Determination of Hydroxyl in Silicone Materials R. C , Smith and G . E . Kellum Dow Corning Corp., Midland, Mich. 48640 A new silanol condensation procedure was developed which allowed rapid and reproducible analysis of total hydroxyl (silanol and water) in a wide variety of silicone materials. A new catalyst system consisting of boron trifluoride, acetic acid, and pyridine was employed. The water originally present and that formed by silanol condensation is removed by azeotropic distillation and titrated with Karl Fischer reagent.
METHODSPROPOSED for silanol determination in silicone materials include manometric LiAIH4 ( I ) and Zerewitenoff procedures (2, 3), infrared (4-6), NMR (3,phenyl isocyanate reaction ( 4 , 5), Karl Fischer reagent titration (8-IO), and silanol condensation (11-16). Condensation procedures have employed alkali, acid, or iodine as catalysts to complete the reaction. The water produced in the condensation is continuously removed and recovered by azeotropic distillation
(1) G. H. Barnes and N. E. Daughenbaugh, ANAL.CHEM.,35,
1308 (1963). (2) F. 0. Guenther, Ibid.,30, 1118 (1958). (3) J. F. Lees and R. T. Lobeck, Analyst, 88, 782 (1963). (4) K. Damm and W. Noll, Ko//oidZ., 158,97 (1958). ( 5 ) W . Noll, K. Damm, and W. Krauss, Farbe Lack, 65, 17 (1959). (6) E. R . Shull, ANAL.CHEM., 32,1627 (1960). ( 7 ) J . F. Hampton, C. W. Lacefield, and J. F. Hyde, Inorg. Chem., 4, 1659 (1965). (8) K . Damm, D. Bolitz, and W. Noll, Angew. Chem., 76, 273 ( 1964). (9) H. Gilman and L. S. Miller, J. Am. Chem. SOC., 73,2367 (1951). (10) W. T. Grubb, Ibid.,76, 3408 (1954). (11) J. Haslam and H. A . Willis, “Identification and Analysis of Plastics,” pp, 256-7, Van Nostrand, New York, 1965. (12) G. M. Kline. “Analytical Chemistry of Polymers, Part I, Analysis of Monomers and Polymeric Materials,” 9. 373, Interscience, New York, 1959. (13) G. R. Lucas and R. W. Martin, J. Am. Chem. SOC.,74, 5225 (1952). (14) L. H. Sommer and G. E. Ansul, Ibid.,77,2482 (1955). (15) L. H. Sommer, R. M. Murch, and F. A. Mitch, Zbid., 76, 1619 (1954). (16) L. H. Sommer and L. J. Tyler, Ibid.,p. 1030. 338
ANALYTICAL CHEMISTRY
with benzene or toluene using Dean and Stark or sirrilar apparatus. The quantity of water collected is then measured either volumetrically or by Karl Fischer reagent titration. Most catalyst systems do not allow rapid condensation of silanol even with monomers or simple structures containing silanol. The condensation is usually incomplete and not reproducible, particularly with silicone resins. A new catalyst system consisting of a mixture of boron trifluoride, acetic acid, and pyridine was employed in our laboratory to give rapid and reproducible total hydroxyl analyses (silanol plus water) in a variety of silicone materials. This method avoids many of the interferences, empirical calibration, and miscellaneous problems as poor solubility, incomplete reaction, and interfering siloxane cleavage associated with many of the above methods. It also has broader application since monomers, fluids, and resins may be analyzed using the same procedure. The apparatus is simple and inexpensive to assemble. EXPERIMENTAL
Apparatus. Barrett type Moisture Test Receivers of 10ml capacity (Ace Glass, Inc., No. 7745) with conventional water condensers were used for azeotropic distillation of large samples. Flasks, receivers, and condensers werc all connected by 24/40 joints. For small samples a miniature distillation unit was employed. It consisted of a Bantamware Distillation Receiver of 2-ml capacity (Kontes No. 28870) with suitable condenser and 50-ml flasks with 14/20 joints. Karl Fischer reagent (KFR) titrations of azeotroped water were performed employing the recording biamperometric apparatus previously described (17). Reagents. Boron trifluoride etherate was Eastman, purified grade. Condensation catalyst was prepared by dissolving 42 ml of boron trifluoride etherate in 208 ml of dry glacial acetic acid.
(17) R. C . Smith and G. E. Kellum, ANAL.CHEM.,38,67 (1966).
Ten milliliters of this mixture contained 0.012 mole of BFI and 0.14 mole of acetic: acid. Procedure. Samples of 10 to 20 grams of silicone fluids and resins or 1 t o 5 grams of monomer silanols were placed in a 200-ml round-bottomed flask. Solid monomers or resins were dissolved in a n ,:qual amount of xylene. Addition of 10 ml of dry pyridine (except with monomers) was followed by 10 ml of boron trifluoridc-acetic acid catalyst, The flask contents were then mixed with swirling. After 5 minutes, 50 ml of tol’Jene was introduced, and a few small silicon carbide crystals were added as boiling aid. The flask was connected to the distillation apparatus which had been previously air dried. All 7 joints were well lubricated with Kel-F: Stopcock Grease. The top of the water condenser was fitted with a drying tube containing indicating Drierite. The heating mantle temperature was adjusted with a Variai: so that distillation began in 5 to 10 minutes. Azeotropic distillation was continued for 30 minutes. After cooling, the contents of the moisture trap were drained into a 100-ml volumetric flask containing dry ethanol. The condenser and trap were rinsed with three small portions of ethanol, and the flask was diluted t o the mark. Duplicate K F R water titrations were performed with 10-ml aliquots added to methanol which had been titrated to a n end point. A tilank was carried throughout the procedure. Samples of 0.3 t o 1.5 grams were employed with the small distillation apparatus. Using the same procedure as above, 1 ml of pyridine, 1 ml of catalyst, and 15 ml of toluene were introduced. The collected water was diluted to 25 ml with ethanol and duplicati: titrations made with 5- or 10-ml aliquots. Silanol plus original water was expressed as total hydroxyl using the following calculation.
where Fl C
= =
grams H?C)/ml K F R in methanol diluent Conversion factor for the condensation reaction: 2(=SiOH,,
+
H20
+ =SiOSi=
2p3 = 1.888 (--o express results as hydroxyl) H20
(2) (3)
A = Aliquot fa2tor The original water W.IS determined using the modified K F R titration with high molxular weight alcohol diluent as previously reported (17). ‘The silanol content expressed as per cent hydroxyl may then be calculated by difference as follows:
% O H (silanol)
=
% O H (total) - % OH (H20)
(4)
where
Fz
=
grams H%O/mlK F R in high molecular weight alcohol C =
2 (OH) 1.888 as above --, €120
RESULTS AND DISCUSSION
Typical total hydroxyl determinations in several monomer silanol compounds are presented in Table I. The results are compared with those obtained with the LiAlH4 method ( I ) . Pyridine was not added in the condensation procedure with monomers since someu hat high results were encountered in its presence. Hydroxyl analysis of various types of poly-
Table I. Total Hydroxyl Determination in Monomer Silanols Condensation % OH new % OH Sample& theory method LiA1H4 HO(CH,),SiC6H,Si(CH,)20H
15.0
13.0
13.0
CeH>Si(OH), 32.1 27.0 6.64 (CGH,)aSiOH 5.92 5.92 (C6HJ2CH3SiOH 7.95 1.87 1.68 (C6H,),Si(OH)? 15.8 14.8 14.8 (CGHi)2Si(OH)Jh 15.8 16.2 16.2 a These samples were not pure silanols. Detection of siloxane by IR generally accounted for the differences from theory. b Sample contained “free” water. . . I
Table 11. Total Hydroxyl Determination in Fluids Sample
X OH condensation new method
Z OH LiA1H4
HO[(CHa)2SiO],H HO[(CHa)2SiO],H HO[(C6HJ(CH1)SiOl,H HO[CFa(CH?)~CH3SiO],H HO[(CH2=CH,)CH2SiO].H HO[(CH2=CH,)CHaSiO],H
4.13 2.20 5.16 2.37 3.80 4.27
4.19 2.60 5.85 2.35 3.73 4.35
siloxanediol fluids are similarly tabulated in Table 11. Agreement with the LiAIHJ method was generally within a relative 2.5 % with the monomer and fluid samples. Total hydroxyl determinations with different types of silicone resins and representative precision of the new condensation method are given in Table 111. Alkali catalyzed condensation and LiAlH4 method analyses are also noted for comparison. Results obtained with the boron trifluoride, acetic acid, and pyridine catalyst system were consistently more precise, and the relative standard deviation was generally less than 3 % with all types of resins tested. Precision with the miniature condensation apparatus and smaller samples was about half as good as that found using the larger system. Resins with an absolute known hydroxyl content were not available for accuracy evaluation ; however similar results are obtained with the other methods (relative 5 to 19% deviation with the K O H catalyzed condensation procedure and 4 t o 20% with the LiAIH, method). The alkali condensation method required lengthy distillations for complete reaction, usually 2.5 to 3 hours, which is about six times longer than needed with the new condensation procedure. The relative standard deviation typically varied from 4 to 18% with resins tested in Table 111. LiA1H4 method results were obtained using rate plots of milliliters of hydrogen evolved cs. time (I) since resins generally release volatile silanes with this reagent. This method is unsatisfactory for many resins since gels rapidly form and prevent quantitative reaction and evolution of hydrogen. I n cases where suitable rate plots could be obtained, relative standard deviation of 1.6 to 6 . 6 x was encountered. A study was made relating duration of azeotropic distillation and recovery of water from condensed silanol. The distillation was initiated 5 minutes after catalyst addition, Results with two resin samples containinp 0.7 and 3.5 % total hydroxyl indicated that 15 to 20 minutcs of distillation were sufficient to allow for complete condensation of the silanol and removal of water in these materials. It appeared that the VOL. 39, NO. 3 , MARCH 1967
339
Table 111. Total Hydroxyl Determination in Silicone Resins New condensation method Rel. OH N u std.dev.
Resinn 1 2 3 4 5
0.786 3.32 0,732 6.96 5.23
6 14 8 10 10
0.0230 0.0760 0,0442 0,186 0.0910
2.93 2.28 6.04 2.67 1.74
Cond. with KOH OH 0.651 3.51 0,785 6.27 5.62
LiA1H4
x OH
0.685 3.58 0,648 6.66 4.26
Mixtures of RSi0,/2 and R2Si0in which R was alkyl or phenyl substituents. a
Table I$’.
Hydroxyl Condensation in Polymers and Resins with Catalyst Variation Total silanol condensed, % Without Sample With BF3 BF, Solvent Resin 45-50 0 Toluene Polymer 30 0 Toluene Resin 90
