Determination of Active Hydrogen Using Exchange with Deuterium Infrared Spectrophotometric Method WILLIAM R. HARP, Jr. Shell Development Co., Emeryville, Calif.
ROBERT C. EIFFERT Martinez Research laboratory, Shell Oil Co., Martinez, Calif.
b A generalized infrared method for the determination of active hydrogen involves exchanging the active hydrogen in the sample with deuterium of D20. The sample is mixed with a relatively large amount of D 2 0 and the amount of active hydrogen calculated according to the method of Gaunt from the intensity of the 2.97micron OH band generated in the DzO. Water-soluble or insoluble samples may b e run in about '/z hour with an accuracy to about 2% of the amount of active hydrogen present.
T
HE traditional methods for determining active hydrogen in organic compounds usually involve reactions with Grignard reagents or lithium aluminum hydride ( 7 ) . Both procedures require the use of elaborate equipment and reactions are not always specific. During the past few years there have been several publications concerning the usefulness of n-ater and DzO as solvents for infrared spectroscopy (1, 5 , Q ) . Two of these (1,s) mention that the exchange of active hydrogen and deuterium which occurs in D2O solution can be followed spectroscopically. Trenner, Arison, and Walker (10) have reported a n infrared method using the 3.98-micron band to determine 0 to 5% of deuterium in water. Gaunt (3, 4), on the basis of statistical distribution of HzO, HOD, and D20, describes infrared methods for the analysis of heavy water. Among papers reporting the use of spectroscopic techniques for the determination of active hydrogen by exchange with deuterium are the following: Morowitz and Chapman (8) exchanged water or proteinaceous niaterials with DzO and analyzed the resulting mixture with an emission spectrographic technique utilizing the Balmer series of lines according t o the method of Broida, Morowitz, and Selgin ( 2 ) . Jones and Hall (6) determined water of crystallization and other active hydrogen by observing the 1.4-micron band.
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ANALYTICAL CHEMISTRY
This paper presents a generalized infrared method for the determination of active hydrogen. It is not limited to samples soluble in water. The sample is dissolved in or contacted with a large excess of DzO and the amount of O H in the DzO phase derived from the 2.97micron band in the manner described by Gaunt (3,4 ) . MECHANISM OF HYDROGEN EXCHANGE
Chemically reactive hydrogens, such as those bonded to oxygen, nitrogen, sulfur, or phosphorus, rapidly equilibrate with the deuterium of DzO to establish a statistical distribution of active hydrogen and deuterium between the exchange compound(s) and DzO. Some hydrogen bonded to carbon will exchange with deuterium but exchange is usually slow-for example, as pointed out by Gore, Barnes, and Peterson ( 5 ) , the methyl hydrogens of p-nitrotoluene will exchange completely in time. Thus, it is impossible to define unambiguously exactly what is meant b y active hydrogen, but in the method described below it is understood to be that hydrogen which exchanges rapidly with deuterium. I n systems where the exchange is slow, modifications of the method mill be necessary. I n a neutral medium, the rapid hydrogen exchange can be explained simply by a hydrogen-bonding mechanism. For example, when a n alcohol is dis-% solved in D20, the alcohol and DzO become associated in a randomly constructed polymer of the form: -[-H-O---D-O~H-O-]
I
R
I
D
n-
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R
The hydrogen and deuterium rapidly lose their identity with respect t o a specific oxygen atom, because of a shifting and re-establishment of bonds in a fashion similar to the following: -[-H-O-D-O-H-O--]
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R
n-
I
D
I
R
Thus, a t equilibrium, assuming no isotope effect, one finds a statistical distribution of active hydrogen and deuterium between the exchanging compound and DzO. Though it is not strictly correct, it is convenient to think of the equilibrium mixture as composed of ROH, ROD, DzO, HOD, and HzO. If a sample-Dz0 mixture contains 3 atoms of active hydrogen to 97 atoms of deuterium, then a t equilibrium the ratio of ROH t o ROD nil1 be 3 to 97. The ratio of D 2 0 to HOD to H20 will be approximately 94 to 6 to 0.1, so that again the ratio of hydrogen to deuterium in the DzO will be 3 to 97. METHOD SUMMARY
The sample containing the active hydrogen is dissolved in or contacted with DzO, and the mixture is shaken thoroughly for periods up to hour. The ratio of deuterium to active hydrogen should be greater than 30 to 1, so that the exchange can be considered essentially complete. All of the exchanged hydrogens, regardless of origin, in the equilibrium mixture become bonded to oxygen atoms and the O H bonds so formed show the typical hydrogen-bonded OH stretching absorption at 2.97 microns. Thus, after suitable calibration, the active hydrogen content of the unknown can be determined from this OH absorption of the DzO phase. APPARATUS A N D REAGENTS
Any commercial infrared spectrophotometer, manual or recording, is adequate for this analysis. Both Perkin-Elmer Model 21 and Beckman Model IR-2 spectrophotometers with sodium chloride optics have been used in these laboratories. Though rock salt is obviously unsuitable for absorption cell construction, several other window materials are available. Fused quartz about 1 mm. thick is probably the most satisfactory, as it is structurally strong, chemically inert, and transparent in the 2- to 3.5-
micron region. Cells 0.025 and 0.050 mm. thick have been used. The DzO used was purchased from Stuart Oxygen Co. and was 99.8% pure or better. Even this DzO had residual OH absorption and blank corrections mere necessary on all absorbance measurements. As DzO is extremely hygroscopic, special precautions are necessary to prevent contamination with atmospheric or adsorbed water. It is necessary to prepare and transfer samples in a good dry box. Samples have been pre ared in volumetric flasks stoppered wit{ rubber serum stoppers and sample transfers have been made with hypodermic syringes. Except when the ratio of surface area t o volume is small, it is necessary to equilibrate all samplehandling equipment with DzO. When sample size permits, it is desirable to rinse several volumes of the sample through the absorption cell before absorbance measurements are made. INSTRUMENT CALIBRATION
Water is a convenient source of active hydrogen and is used to prepare calibration samples covering the concentration range of 0 to 50 or 0 to 25 grams of mater per liter for cells of 0.025- or 0.050-mn~ thickness, respectively. A known amount of water is weighed into a 5-ml. volumetric flask, the flask is stoppered with a rubber serum stopper, and then, in a dry box, the D 2 0is added by means of a hypodermic syringe. The flask may then be removed from the dry box for weighing to determine the exact amount of DzO added. The concentrations of the calibration solutions are expressed as moles of active hydrogen per liter, taking the densities a t 25’ C. of water and DzO to be 0.9971 and 1.1023 grams per ml., respectively. The solutions thus cover the concentration range of 0 to 6 or 0 to 3 moles of active hydrogen per liter, depending on which cell is used for the calibration. Absorbances, equal to log T o / T , are determined for the calibration solutions nhere To equals the transmittance a t 2.30 microns and T equals the transmittance a t 2.97 microns, both transmittances measured relative to air. The trsnsniittance at 2.30 microns was selected as the base line reference point, because DzO has maximum transmittance a t this wave length, as do most samples to be analyzed. The sample absorbance is corrected for the blank absorbance of the cell filled with DzO. The usual calibration curve of corrected absorbance vs. concentration is constructed for use ill the analysis of unknown samples. The temperature dependence of the absorbance of the 2.97-micron OH band was not studied, and no attempt was made to thermostat the absorption cell. The absorptivity was found to be about 617 liters per mole em. a t zero concentration and it decreased linearly n i t h concentration t o about 598 liters
Table
I.
Active Hydrogen Contents for Known Samples
Active Hydrogens/Molecule yo theory Theory Found
Sample Water-solubles Urea 2-Amino-2-hydroxy-methyl-I ,3-propanediol Potassium acid phthalate Water-insolubles Aniline Cetyl alcohol in hydrocarbon solution Octadecyl amine in hydrocarbon Eolution Fluorene 4Methyl-2-pentene 2-tert-Butyl-6-methylphenol in hydrocarbon solution 2,6-Di-tert-butyl-4-methylphenolin hydrocarbon solution
per mole em. a t a concentration of 6 moles per liter. This is a decrease of only about 3% and indicates fairly strict adherence to Beer’s law> which one would expect for this system because: The OH band is broad, so that the absorbance is not changing rapidly across the finite wave-length interval passed by the slits. The concentration range is narrow. The medium is always the samea large excess of DzO. ANALYSIS OF SAMPLES
Techniques for Samples Soluble in D20. T o determine the active hydrogen content of a n unknown sample soluble in DzO, a portion of the sample is weighed into a volumetric flask, which is then filled to volume with DzO. All samples run thus far exchanged rapidly, so i t Tvas only necessary t o provide good mixing t o achieve equilibrium. It is sometimes impossible with water-soluble materials of higher molecular weight and having few active hydrogen atoms per molecule to prepare solutions of high enough concentration for accurate absorbance measurements. For example, in analyzing a sample of 200 molecular weight with one active hydrogen per molecule, it mould take 5 grams of sample in 5 ml. of solution to give a solution containing 5 moles of active hydrogen per liter. I n such cases, it is necessary to work a t lower concentrations and use a cell of longer path length to try to maintain the accuracy of absorbance measurements. Even with materials rrhich exchange rapidly, some active hydrogen remains in the parent molecule after exchange equilibrium has been established (4). The amount of unexchanged hydrogen depends on the relative amounts of active hydrogen and deuterium in the system. When the active hydrogen is present as OH, NH, or COOH, unex-
2.0 2.0 1.0 2.0
2.01 1.9s 0.99 (3) 1.97 ( 5 )
101
99 99 99
0 0
1.0 0.98 ( 4 ) 98 Proceeds as a first-order reaction to SOYO completion in 42 minutes
changed hydrogen TI ill contribute to the 2.97-micron absorption band and thus tend to compensate for the error caused by the incomplete exchange a t equilibrium. Compensation Kill never be exact, but will depend on the relative absorptivities of the unexchanged OH or NH and that of the HOD. Calculations. The active hydrogen content of the sample is determined as follows: The absorbance of the sample-Dz0 solution a t 2.97 microns is determined as described above and is corrected for cell blank and residual DzO background. This corrected absorbance is used to find the moles of OH groups per liter of solution from a calibration curve of corrected absorbance us. concentration. The average number of active hydrogens per molecule may then be calculated as follows: Active hydrogens/molecule = moles of OH/liter of solution moles of sample/liter of solution
If the molecular weight of the sample is unknown, the moles of active hydrogen per gram of sample are calculated as follows: Moles of active hydrogen/gram of sample - moles of OH/liter of solution grams of sample/liter of solution Techniques for Samples Insoluble in DzO. The mechanics of handling water-insoluble materials are similar t o those for water-soluble samples. For liquids, known weights of sample and DzO are placed together in a stoppered flask and shaken long enough t o assure complete exchange. If the droplets in the emulsion are small, less time will be required for complete exchange than if the droplets are large. Similarly, a material of high viscosity must be contacted longer than one of low viscosity. If the sample is very viscous, it will be advantageous to dilute it with a dry, nonreactive solvent. Almost all VOL. 32, NO. 7, JUNE 1960
795
clean, dry, inert liquids which are insoluble in DzO are suitable. Among those used are hexane, benzene, amyl acetate, and carbon tetrachloride. Thus far, all reactive samples encountered have exchanged in less than */z hour of shaking. Among the materials analyzed, the only one found which exchanges, but not rapidly, Tyas the extremely highly hindered structure present in 2,6-di-tert-butyl-4-methylphenol described in Table I. I n one month 4-methyl-2-pentene did not eschange measurably. I n the case of materials which emulsify easily, gentle shaking or centrifugation may be necessary to obtain a clear D20 phase. After the exchange is complete and the two phases have separated, the absorbance of the DzO phase is measured a t 2.97 microns and corrected for cell blank and residual DzO absorption. The active hydrogen content of the sample is calculated as described above, remembering that the volume of the sample being analyzed is only the volume of the D2O phase in the case of insoluble materials. I n these two-phase systems one can correct for incomplete exchange due to statistical distribution a t equilibrium of the exchanging hydrogen bet\yeen the D20 and the parent molecules. If the magnitude of the isotope effect is known, a n appropriate correction can be applied. Such corrections are possible because the exchanging molecule is not present in the DzO phase but remains in the organic phase and the partial compensation due to unreacted molecules present in the equilibrium mixture is not acting in these samples. Although the active hydrogen of Some
DzO-insoluble solids apparently will exchange completely, it is desirable first t o dissolve the solid in a suitable solvent. Those solvents mentioned above have been used successfully, although many others would be just as satisfactory. Once the sample is in solution, the analysis is performed as described above for insoluble liquid samples. Because most solvents contain small amounts of water, a blank determination should be run, by the same procedure, using the same amount of solvent and D20, but with no sample. Errors D u e to Presence of HzO. Because of t h e hygroscopicity of DzO i t is necessary t o be extremely careful in t h e preparation and handling of both D20 and samples t o prevent including water from t h e sample or from atmospheric water vapor. One per cent of water in a sample of molecular weight 100 containing one active hydrogen per molecule will introduce a n error of over 10%. To obtain an accurate active hydrogen figure, exclusive of water, it is often necessary to use a suitable drying procedure or to obtain a separate water determination and calculate the correction. Results of Known Samples. The results obtained from analysis of known compounds are shown in Table I. The analyses were performed on Model 21 Perkin-Elmer instruments using a n 0.025-mm. cell in t h e case of water-soluble compounds and a n 0.05-mm. cell for t h e water-insoluble ones. The modified base line technique described above was used; a solvent blank was also used where t h e water-insoluble materials were in a solvent.
CONCLUSIONS
I n general, a determination is accurate t o h2Yc of the amount of active hydrogen present, while repeatability is somewhat better. As little as 0.005 weight % ’ active hydrogen can be determined with no method modification. Determinations may be made in about ‘/z hour per sample a t a fairly low cost. DzO costs approximately 50 cents per gram, and 1to 5 grams are ordinarily used for each sample.
LITERATURE CITED
(1) Blout, E. R., Lenormant, W., J . Opt. SOC.Am. 43,1093 (1953).
(2) Broida, H. P., Morowitz, H. J., Selgin, M., J . Research h’utl. Bur. Standurds 52,293 (1954). (3) Gaunt, J., Analyst 79, 580 (1954). (4) Gaunt, J., Spectrochim. Acta 8, 57 (1956). (5) Gore, R. C., Barnes, R. B., Peterson, E., ANAL.CHEM.21,382 (1949). (6) Jones, J. H., Hall, M. A., “NearInfrared Determination of H20 in DgO. Application to Determination of n’ater of Crystallization and Readily Exchangeable Hydrogen in Organic and Inorganic Compounds,” Pittsburgh Conference on ilnalytical Chemistry and Applied Spectroscop March 1956. ( 7 ) Mitchell, John, Jr., dilthoff, I. M., Proskauer, E. S., Weissberger, A., “Organic Analysis,’’ Vol. I, Chap. 1, 3, 4; Vol. 11, Chap. 4, Interscience, New York, 1953 and 1954. (81 Morowitz. H. J.. ChaDman. XI. W.. Arch. Biochem. Bidphys. 5 6 , 110 (1955): (9) Potts, W. J., Wright, X., ANAL. CHEM.28,1255 (1956). (10) Trenner, N. R., Arison, B. H., Walker, R. JT., Ibid., 28, 530 (1956). \
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RECEIVEDfor review June 3, 1959. Accepted March 23, 1960.
Twenty-one New X-Ray Diffraction Powder Patterns H. G. NORMENT,l P. 1. HENDERSON,2 and R. L. SOUTH3 Callery Chemical Co., Callery, Pa.
A collection of 21 standard x-ray diffraction powder patterns, mostly of boron compounds, is presented. The data have been collected by both diffractometer and Debye-Scherrer methods.
I
THE COURSE of several years of analysis by x-ray diffraction in this laboratory, many powder patterns have been obtained which are not included in existing reference compilations. The best of these data are presented here as well as a brief description of the source of the compounds and their preparation (when this information is not restricted),
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ANALYTICAL CHEMISTRY
purification, and method of intensity measurement. Unit cell parameters are given where these are available. Except where specifically mentioned, the powder patterns have not been checked against the single crystal data. The Bragg d-spacings and intensities are found in Table I. APPARATUS AND TECHNIQUES
When sufficient sample was available, the patterns were measured with the Xorelco Geiger counter diffractometer using standard procedures ( I S ) . Nickel filtered C U R , radiation was used. The intensities reported are peak heights
above background, so scaled that the most intense line is given a value of 100. Samples were finely powdered by a Wig-L-Bug vibration grinder. All diffractometer patterns were checked against 114.6-mm. Debye-Scherrer films for completeness and accuracy of dvalues. 1 Present address, Diffraction Branch, Optics Division, U. S. Saval Research Laboratory, Washington 25, D. C. 2 Present address, Graham Research Laboratories, Jones and Laughlin Steel Co., Pittsburgh, Pa 3 Present address, Department of Chemistry, Western Reserve University, Cleveland, Ohio
