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Anal. Chem. 1984. 56. 1752-1753
of 0.84%. In the pH range between 3.5 and 10.5 the difference between the measured pH and the calculated desired pH was h0.002 pH units or less. Outside of this range full-on titrant generation produced a change in pH of less than 0.003 per reading. The plot of a typical titration is shown in Figure 5. The first differential of the data is shown in Figure 6. The ApH/ Amoles of titrant values were calculated directly from the sums of the time on values without using any averaging or smoothing routines. Titrations of four samples of 35.82 pmol of Na2C03produced a mean of 71.50 pmol of hydronium ion generated with a relative standard deviation of 0.27%. The titrator was less than full on in the pH regions of 3.3 to 5.3 and 6.8 to 8.9. In these ranges the measured pH was controlled to within f0.002 p H units of the desired values. The actual algorithm used was
TO = LTO + (1E+ 0.02EIEI
+ 35A.E + 0.2aElaEl)/(400ApH/LTO) (2)
Again, E , AE, and ApH were multiplied by 1000 before being used in the equation. If the AE term is omitted, or even when its multiplier is significantly decreased, T O values will oscillate around the correct control value. If the E term multiplier also is decreased to prevent this overshooting, T O will lag behind the correct control value. If only the E term multiplier is decreased or if the AE term multiplier is increased, TO will tend to be offset from the correct control value. The squared terms are necessary to bring the titrator under control rapidly without oscillations when the pH range is first reached where control
can be maintained, 3.5 for the titration of strong acids. With the indicated multipliers the squared terms have no effect within the control range. For the HC104 titrations in the 3.5 to 10.5 pH range the E value can accumulate for successive readings. In this range the standard deviation of the measured pH readings from the desired pH values was 0.0007 pH unit. With this precision the accuracy of the pH control is limited only by the accuracy of the pH measuring system itself. Without using the full algorithm we were unable to approach this degree of control. The titrator has the capability to back titrate if necessary. However, back titration only occurred when inappropriate coefficients were used in the control algorithm.
LITERATURE CITED (1) Gampp, H.; Maeder, M.; Zuberbuhler, A.; Kaden, T. Talanta 1980, 2 7 ,
5 13-5 18. (2) Martin, C.; Freiser, H. Anal. Chem. 1979, 51, 803-807. (3) Christiansen, T. F.; Busch, J. E.; Krogh, S . C. Anal. Chem. 1976, 48, 105 1-1 056. (4) Leggett, D.J. Anal. Chem. 1978, 50, 718-722. (5) Smit, J. C.; Smlt, H. C.; Stelgstra, H.; Hannema, U. Anal. Chim. Acta 1082, 143, 79-94. (6) Wu, A. H. B.; Malmstadt, H. V. Anal. Chem. 1078, 5 0 , 2090-2096. (7) Earle, W. E.; Fletcher, K. S . , 111 Chem. Insfrum. ( N . Y . ) 1976, 7 ,
101-121. (8) Harrar, J. E.; Pomernacki, C. L. Chem. Instrum. ( N . Y . ) 1978, 7 , 229-240. (9) Pomernacki, C. L.; Harrar, J. E. Anal. Chem. 1975, 4 7 , 1894-1905. (IO) Adams, R. E.; Betso, S. R.; Carr, P. W. Anal. Chem. 1978, 4 8 , 1989- 1996.
RECEIVED for review December 21,1983. Accepted March 15, 1984. This work was supported in part by funds from the Foundation of the University of North Carolina at Charlotte and from the State of North Carolina.
Teflon Tubing as a Removable Replacement for Steel Ferrules in High-pressure Liquid Chromatography James L. Meek Laboratory of Preclinical Pharmacology, St. Elizabeths Hospital, National Institute of Mental Health, Washington, D.C. 20032 Most modern liquid chromatographs use 1/16 in. 0.d. steel tubing to connect the pump, sample injector, column, and detector. A leaktight s e d is made by compressing steel ferrules onto the tubing. Unfortunately, the compression fittings made by different manufacturers are incompatible. When columns or other components from different sources are interchanged, adapters may be required for each combination. An alternative to nonremovable steel ferrules is to use a plastic material as part of the fitting to grip the tubing and form a seal. Teflon ferrules can be used for this purpose but are limited to about 1500 psi. “Universal” adapters are now available from Alltech Associates, Deerfield, IL, and Rainin Inst, Woburn, MA, that can be used to 5000 psi. These adapters are rather expensive for use throughout an HPLC system and are too bulky to use in some cases (e.g., on Rheodyne sample valves). A less expensive solution that will work both with external threaded fittings (e.g., Swagelok) and with internal fittings (e.g., Waters, Rheodyne, Valco, Parker, SSI) can be made without machining from the ordinary Teflon tubing used in the inlets to most HPLC pumps. A Teflon tubing seal will withstand a t least 5000 psi, is reusable and removable, and allows ready interchange of different types of fittings, even This article not subject to
those which have been damaged by scratches or overtightening.
EXPERIMENTAL SECTION A Teflon sleeve is made by inserting Teflon tubing (l/s in. o.d., 1/16 in. id.) into the female part of the fitting and cutting it off flush with a razor blade. The tubing is removed. An additional 1-2 mm is cut off and discarded. The remaining Teflon sleeve is slipped over the steel tubing to be connected and inserted into the fitting, and the fitting is tightened slowly. The Teflon sleeve is thereby compressed and flows into and fills both a portion of the threaded part of the fitting and the space normally occupied by the ferrules. Maximum pressure tolerance develops if the fitting is retightened after a few minutes to allow completion of the flow. When the fitting is disassembled, the sleeve remains with the female portion of the fitting. The tubing can be removed from the sleeve and reinserted 10 or more times if, after loosening the nut, the tubing is wiggled in the sleeve to enlarge it. When the steel tubing is then reinserted, only moderate care is needed to center the tubing in the sleeve and avoid shaving off a piece of Teflon. A fitting can be used alternatelywith both conventional steel ferrules and Teflon sleeves since the latter can be removed by unscrewing them with a small Phillips head screwdriver (without damage to the sleeve) or with a flat screwdriver or a small
U.S.Copyright. Published 1984 by the American Chemical Society
Anal. Chem. 1984, 56, 1753-1755
sheet metal screw (destroying the sleeve).
RESULTS AND DISCUSSION Tubing connections made in this way are leakproof at the maximum pump pressure available in this laboratory (5000 psi). The high-pressure tolerance of the sleeve probably results from the largo surface area, which grips the tubing, and the ability of Teflon to conform exactly to both the tubing and the tapered portion of the fitting. Use of Teflon sleeves does not affect the dead volume of the system. The sleeves can be used a t elevated temperatures: in this laboratory they are used on ion exchange columns operated at 65 "C. Sleeves can also be used on plastic tubing if they are first formed against
1753
steel tubing. Care must be exercised subsequently to not tighten the sleeve too tightly against the plastic tubing. Teflon sleeves may also be useful when valuable components have been damaged such that they cannot be made leaktight using steel ferrules. Sleeves about 1 mm thick before compression will stop leaks past steel ferrules in existing conections, or the steel tubing can be cut and full size Teflon sleeves used to totally replace the ferrules. Registry No. Teflon, 9002-84-0.
RECEIVED for review January 12, 1984. Resubmitted March 22, 1984. Accepted March 30, 1984.
Determination of the Composition of Acrylamide/Acrylate Copolymers Using Thermogravimetric Analysis Nissanke L. Dassanayake* and Richard W. Phillips The Western Company of North America, P.O. Box 186, Fort Worth, Texas 76101 The copolymers of acrylamide and acrylic acid salts or substituted amides and acrylic acid salts are being widely used as friction reducers in the oil well service industry. Most preparations of these additives involve emulsion polymerizations and the copolymers are available as emulsions. The composition of the copolymer is important with regard to the application and performance. The carboxylate salt moiety of the copolymer increases solubility in aqueous solvents and the amide group may enhance the viscosifying property of the copolymer. Therefore, it is necessary to know the composition of the copolymer. The accepted method used to determine the composition of acrylamide/acrylate copolymers makes use of infrared spectrophotometry ( I , 2). The amide carbonyl group absorbs near 1650 cm-l and the carboxylate carbonyl group absorbs near 1563 cm-l. The ratio of the area of the amide absorption to that of the carboxylate absorption should give the relative amounts of the two components present in the copolymer. If the polymer spectrum can be easily obtained, and the carbonyl absorptions of the two moieties are well resolved, the infrared spectrophotometric method gives accurate results. Most available samples are water based. Water and carbonate ion int,erfere with the infrared spectrophotometric method of analysis. The method most generally adaptable to polymer studies is the film technique. This procedure requires the spreading of a thin film of solvent-polymer paste or emulsion on a sodium chloride disk and evaporating the solvent. This procedure is not convenient for water-based emulsions. Proximity of the two carbonyl absorptions sometimes results in poorly resolved spectra with the carboxylate carbonyl absorption appearing as a shoulder on the amide carbonyl absorption. This makes an accurate determination of the composition of the copolymer difficult. Hence, another method involving the use of thermogravimetric analysis is described. The controlled pyrolysis of polymer samples to give characteristic degradation products has been rather extensively studied (3). The method uses the observation that C-N bonds are weaker bonds than C-0 bonds (C-N, 69.5 kcal/mol; C-0,84.0 kcal/mol) (3). Other factors, such as the formation of products with higher bond energies, e.g., ammonia, hydrogen chloride, and water, may, in addition to bond dissociation energies, increase reaction 0003-2700/84/0356-1753$01.50/0
Table I. Infrared Data of Polymer Residues Obtained by Pyrolyzing an Acrylamide/Acrylate Copolymer to 272 "C, 365 "C, 412 "C, and 492 "C
temp, "C
amide NH, 3400 cm-'
23 212 365 412 492 a
a a
a a
a
shoulder shoulder b
b b
Presence of peak.
functional groups amide carboxylate c= 0 c= 0 1650 cm-' 1565 cm-' a a a a a
Absence of peak.
rates and therefore reduce pyrolysis temperatures.
EXPERIMENTAL SECTION The infrared spectra were obtained on a Perkin-Elmer Model 283B spectrophotometer. Thermogravimetric analyses were carried out on a Mettler TA3000 system. The copolymers were prepared by using standard methods described in the literature (4).Emulsions of copolymers from different vendors were used. The copolymer was precipitated by the addition of isopropyl alcohol and oven dried to yield a powder. The pyrolysis of the copolymer was observed by thermogravimetry. The dry powder sample (8-9 mg) was heated at 10, 25, and 50 OC/min from 35 to 985 O C and a thermogravimetric curve obtained. Since there were no significant differences in the thermograms, the fastest heating rate was chosen to provide the shortest analysis time. The heating was carried out with compressed air as the purge gas at the manufacturers recommended flow rate of 100 cm3/min. The thermal decomposition pattern of the copolymer was examined and four end temperatures were selected for futher study. Thermograms were obtained as the copolymer was heated to these four end temperatures (Figure 1). The residue remaining following pyrolysis to each of the end temperatures was analyzed by infrared spectrophotometry. Three absorptions, the amide -NH2 at 3400 cm-', the amide carbonyl at 1650 cm-l, and the carboxylate carbonyl at 1565 cm-', were monitored vs. temperature (Table I).
RESULTS AND DISCUSSION Table I outlines the method used to obtain the temperature a t which the amide moiety of the copolymer was completely 0 1984 American Chemical Society
