~
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ABSORBANCE
r
Table I. Summary of Precision Data of Standards
H202 in standardsa
Absorbance meanb
5
0.1000
10
0.2012 0.2973
15
20 0.3838 a Standardized by KMnOc titration. Means of ten measurements of absorbance. Standard deviation(s)of absorbance mean.
Std. dev: 0.0010 0.0021 0.0025 0.0023
Figure 2. The figure shows the absorption of undissociated xylenol orange (11) at pH 1.20 i 0.05 vs. water. The curve set shows the subsequent absorption of Ti-XO-H202 measured vs. Ti-XO reagent as a blank. The values shown are in micromoles per milliliters of H202 in standard samples. The analytic range of absorbance-concentration linearity extends to 20 pM/ml Hz02. Above this, a close approximation of linearity results but the slope of the absorbance-concentration curve, measured at 550 mM, decreases. This shift occurs between 20 and 25 pM/ml. Concentrations can be determined to 1 X 10-I micromole. In typical sets of standard samples of 10, measured to 1 X 10-l pM/ml H202 the following results, Table I, illustrate the performance of the method. Sensitivity can be enhanced approximately tenfold by omission of the sample diluent and receipt of dialyzed components (11) B. gehak and J. Korbl, Collection Czech. Chem. Commun., 25, 797 (1960).
0 WAVELENGTH I N MILLIMICRONS
Figure 2. Absorbance characteristics of Ti-XO and Ti-XO HzOz The spectra were recorded on a Beckman DB-G spectrophotometer. The Ti-XO curve shows the absorption of undissociated xylenol orange in the composite reagent. The curve set shows the absorption produced by standard HZO2 samples through 50 pM/ml. The aliquots used for spectral recording were obtained from the flow cell of the AutoAnalyzer
from the sample stream into a smaller volume in the recipient stream of the dialyzer. Simple aqueous solutions not requiring separation of proteins may be processed directly, with omission of the dilutional effects which attend preparatory dialysis. We have not encountered interferents in the biological medias listed in this report. Otomo ( I ) has specified a variety of cations and anions, with comments on inhibitory concentrations, as encountered when this procedure is used to measure titanium(1V). The selectivity shown by xylenol orangetitanium(1V) toward hydrogen peroxide and the favorable absorption maxima of the complex makes possible the accurate measurement of hydrogen peroxide in complex biological solutions.
RECEIVED for review September 14, 1967. Accepted November 24,1967.
Micro Test for Thermooxidative Stability of Fluids John Mann Butler, Guthrie Wheeler, Jr., and William D. Ross Monsanto Research Corporation, Dayton Laboratory, Dayton, Ohio 45407
IN THE DEVELOPMENT of oxidatively and thermally stable fluids and lubricants and in the use of prototype and model compounds in the development of oxidatively and the thermally stable polymers, a thermooxidation test is needed that can be applied to materials that are too volatile to use in standard high temperature oxidation tests for lubricants. A test that can be used when only small quantities of materials are available is needed also. The objective of this work was to develop and demonstrate the basis for such a test. The approach employed uses melting point capillaries as micro "reactors" to contain a few milligrams of the fluid in an air atmosphere. Such sealed reactors are exposed at the desired temperatures and times. The oxygen/nitrogen ratio in the residual gas is then determined by gas chromatography.
temperatures (between 200 and 500" C). Individual capillaries containing the fluids were removed at 10-minute intervals, usually covering a time span of 90 minutes. The residual gases in the capsules were released into a gas chromatographic system using an F & M SI-4 solid sample injector to crush the capsules. The ratios of oxygen to nitrogen were determined using an F & M Scientific Corp. Gas Chromatograph, Model 700. The detector used was a thermal conductivity sensor which is very sensitive to changes in concentration of oxygen in relation to the nitrogen internal standard. Conditions of the analyses were: column, inch stainless steel tubing packed with 150-200 16 feet X mesh Porapak Q (Waters Assoc.); column temperature, -76" C ; detector temperature, 160" C ; injection port temperature, 110" C ; flow rate, 43 ml/minute. The oxygen and nitrogen components are well resolved (Figure 1) using these conditions.
EXPERIMENTAL
Quantities of 2 to 3 mg of a fluid to be tested were charged to 4 cm, size D (1.5 mm o.d., 1.0 mm i.d.) melting point capillaries using a microsyringe. The capillaries were sealed so as to entrap as nearly as possible the same amount of air in each. The sealed capillaries were placed in aluminum block holders and heated in an electric furnace a fixed 466
ANALYTICAL CHEMISTRY
RESULTS AND DISCUSSION
Using the procedure outlined above, two well characterized fluid base stocks and one exploratory compound were tested. These materials were: a five-ring polyphenyl ether c&(OCeH4)a-meHr (Monsanto Research Corp.); a diester of a
7 1
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*
I
0.20
I
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60
70
21soc
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010
6
8
IO
MINUTES
Figure 1. Chromatogram showing analysis of gases encapsulated with fluids
O'Ol
0.ooJ
10
20
30
40 50 REACTION TIME, minutes
i
Io
90
Figure 2. Residual-gas analysis us. reaction time (polyphenyl ether)
2,2-substituted acid, resorcinol dineoheptanoate (CTHieC00)2CeH4(Monsanto Research Corp.); and a fluorocarbon, perfluoro-1,6-dipheny1-hexaneyCeFs(CF&CsFb. [This new compound was furnished by M. W. Buxton (Imperial Smelting Corp., Ltd., Bristol, England) having been prepared under subcontract with Monsanto Research Corp. under U. S. Air Force Contract AF 33(615)-1344.1 The data obtained for the polyphenyl ether and the perfluoro compound at 285" C (545"F) to 410" C (770" F) are shown in Figures 2 and 3. The ester fluid was tested only at 341 " C (645O F). At this temperature all of the oxygen was consumed during the first 10 minutes. Although there is some scatter in the data points, there is no doubt as to the relative stabilities of the different materials at the various temperatures. The phenyl ether fluid and the perfluorocarbon show about equivalent stabilities and both are much superior to the ester fluid. The effects of temperature on the oxidation rates are very apparent. The slopes of these curves can be used as relative measures of the thermooxidation stabilities at the temperatures employed. The large difference in the stabilities of the ester fluid and the aromatic ether fluid is in agreement with the results from conventional tests wherein air (1 liter/hour) is bubbled through 20 ml of the fluid at test temperature and oxidation is followed by viscosity changes. As with most screening tests there are many pitfalls and care must be used in interpretation of the data. For example, the relative amounts of vapor phase and liquid phase oxidation that occur are not known. These may vary with different components and give spurious results if one is primarily interested in liquid phase oxidation. The method uses a deficiency of oxygen and only relatively little fluid is oxidized. Thus,
015.1
OOOJ
10
20
30
40 so REACllON TIME, m i n u l e i
i
60
70
IO
Figure 3. Residual-gas analysis us. reaction time (perfluoro1,ddip heny lhexane)
the method is most useful in studying the initial phase of oxidation. In spite of the limitations, some of which are also common to other oxidation tests, we believe the approach described here can be the basis of a very useful screening test for high temperature oxidation stability. This general approach possibly can be further modified and refined to give a simple rapid quantitative oxygen absorption method from which kinetic data could be developed. RECEIVED for review September 5, 1967. Accepted October 16,1967.
VOL 40,
NO. 2, FEBRUARY 1968
467
