range of tvavelengths than have been included in the present work. In this respect, the unit is much superior to a dual system that could be constructed using lens-type condensing optics.
Leon Schwartz for photographing the illustrations; to Barbara Moyer for recording Some of the spectra; and to E. H. Keutmann for his continued interest in this work.
ACKNOWLEDGMENT
LITERATURE CITED
The authors gratefully the University of Rochester Atomic Energy Project (USAEC) for the use of its recording infrared spectrophotometer, and the Connecticut Instrument CO. for donating the ultramicro caVitY cells. The authors are also indebted to
(1) Blout, E. R., Abbate, M. J., J. Opt. ,yoe. A ~45,1028 . (1955). (2) Ford, M. A., Price, W. C., Seeds, W. E., Wilkinson, G. R., Ibzd., 48, 249 (1958). (3) Handbook of Scientific Instruments and Apparatus, p. 249, The Physical
Society, London, 1957.
(4) Mason, W. B., Van Slyke, H. N., Abstracts, Pittsburgh Conference on halytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., hfarch 1957. ( 5 ) Mohl, F., Acta Chem. Scand. 12, 1356 (1958). (6) The Perkin-Elmer Corp., Instrztrnent News 12, (3), 3 (1961): (7) White, J. U., Weiner, S., Alpert, N. L., Ward, W. M., ANAL. CHEY.30, 1694 (19%). RECEIVED for review July 12, 1961. Accepted November 9, 1961. Twelfth Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1961. Work was supported by Grant C-1003 from the National Cancer Institute.
Direct Titration of Phenols by Bromination CALVIN 0. HUBER' and JOANNE M. GILBERT2 Rockford College, Rockford, 111.
b Phenols are commonly determined by bromination titrations in which all open ortho and para positions are substituted. The titration ordinarily involves the addition of excess brominating agent and iodometric back titration of excess bromine. A direct titration in glacial acetic acid is proposed. The presence of an appropriate amount of pyridine provides a reasonable reaction rate. The end point is determined by constant current potentiometry using platinum foil electrodes. The titration curve is interpreted from voltammetric data. Quantitative and reproducible results were obtained for phenol and several substituted phenol samples. Errors and deviations were ordinarily within 1 The method is relatively simple, rapid, and convenient.
%.
T
HE DETERMINATION of phenols by bromine substitution was first proposed by Koppeschaar (6). The development and application of this classical method are reviewed by Kolthoff and Belcher (4). Sources of difficulty in the method are bromination of alkyl side groups, oxidations, replacement reactions, and precipitation of partially brominated products. The classical method involves addition of excess bromate to an aqueous bromide solution containing the sample, addition of iodide, and titration of iodine
Present address, Department of Chemistry, University of Wisconsin-Milwaukee, Milwaukee 11, !Vis. * Present address, Department of Cheniistry, University of Wisconsin, Madison 6 , Wis.
with thiosulfate using starch as indicator. Ingberman (3) has reviewed some of the difficulties of the classical method and has developed a modified titration procedure which provides stoichiometric titration for many phenols. Elemental bromine dissolved in glacial acetic acid is used as titrant, glacial acetic acid as titration solvent, and pyridine as catalyst. The end point is determined iodometrically using aqueous potassium iodide and standard thiosulfate solutions, as in the classical method. Using these modifications, Ingberman titrated a large number of phenols within a 1 to 2% error level. The development of a direct titration method with resulting improvement in speed and convenience seemed desirable. Direct titration requires a stoichiometric, rapid reaction and a suitable end point detection method. Such a direct titration was developed. Constant current potentiometry (6) was used for end point indication. Good results were obtained for several phenols tested. METHOD
In the Ingberman procedure, 25.00 ml. of 0.15M bromine in glacial acetic acid is added t o about 3 meq. of sample. About 0.25 ml. of pyridine is added, and the mixture is permitted to react for 2 to 20 minutes. The required reaction time must be ascertained for each unknown material analyzed. Investigation in this laboratory showed that titration with 0.15M bromine in glacial acetic acid in the presence of similar amounts of pyridine gave stoichiometric results, but the time required for titration was 1 to 2 hours. The replacement of pyridine with other similar compounds, to increase the rate
of the reaction, as attempted. Compounds tested were: quinoline, aniline, P-picoline, ypicoline, o-toluidine, quinolinic acid, 4-(2-hydrosyethyl)pyridine, triethylamine, piperidine, and nicotinic acid. No signiiicant increase in reaction rate was observed for any of the compounds tested. Variation of the amount of pyridine used, however, resulted in a sharp change in reaction time. The use of 3 ml. of pyridine resulted in a titration time of less than 20 minutes. Further increase in pyridine concentration did not increase the rate significantly, but only added to the blank. A preliminary study of the relationship between rate of reaction and pyridine concentration was made by observing the time required for reaction when 25.00 ml. of 0.15Af bromine in glacial acetic acid was added to 40 ml. of glacial acetic acid solutions containing 0.400 grams of p-phenylphenol and various amounts of pyridine. A plot of log time vs. log concentration (Figure 1) indicates that the reaction order with respect to pyridine changes by more than a factor of two over the concentration range studied. The existence of pyridinium bromide perbromide and of pyridine perbromide complexes in acetic acid has been reported (1, 2 ) . I t seems probable that the attacking agent is a pyridine-bromine complex. Possibly a t higher pyridine concentrations, the electrophylic attack by the pyridine-bromine complex is the rate determining step. The kinetics and mechanism of this reaction deserve further study. End point indication was by constant current potentiometry, which has the advantages of being readily interpretable, applicable to nonaqueous solutions, and yielding end point signals which can be read directly. Two identical platinum foil electrodes, 1.0 X 0.5 cm., were spaced 1.0 cm. apart. The conVOL 34, NO. 2, FEBRUARY 1962
* 247
-
3.0
2.8
-
0.4
-
0.3
-
0.2
-
0.1
-
ILQ d
0
>
-
2.6
J
W
'"
Io
0
2.4-
2.2
-
2.0
-
1.8
-
0
, , , , I , , , 5
, I 10
I
,
t
I
I
1
I
I
I
I
I
20
IS
J
1.61 0.8
LOG
Figure 1. molarity
1.2
1.0
x
1.4
PYRIDINE
Table 1. Titration Results: Bromine Required to React with Open Ortho and Para Positions
2.2
CONCN.(MXlO')
Current voltage curves obtained a t a platinum foil electrode (identical to those used for titration) US. a saturated calomel electrode are shown in Figure 3. The titration curve can be interpreted by noting that voltages read during a titration correspond to the intersection of the 10-ma. constant current lines and the appropriate current-voltage curves on Figure 3. Curve 2 of Figure 3 indicates that the voltage at the anode
RelaCalcutive lated, Found, Error, Mmoles Mmoles %
6.384 6.342 6.588 6.565 6.636 6.628 o-Cresol 7.212 7.160 5.864 5.796 5.760 5.704 m-Cresol 6.636 6.624 6.096 6.182 o-Phenylphenol 5.832 5.880 6.700 6.756 5.936 5.954 6.248 6.310 p-Phenylphenol 6.320 6.389 5,844 5.910 12.12 12.01 Resorcinol 8.688 8.764 8.862 8.923 9.312 9.400 10.56 10.52 p-tert-Butyl5.560 5.586 phenol 5.312 5.324
2,4Dinitro0.00 naphthol 2,6-ditert-Butyl- 3.258 phenol 248
2.0
1.8
Logarithmic plot of reaction time vs. pyridine 1Os for bromination of p-phenylphenol
stant current of 10 ma. was provided by a 100-volt d.c. source in series with a 10 megohm resistor. Voltages across the electrodes were read with a vacuum tube voltmeter. A typical titration curve is shown in Figure 2.
Sample Phenol
1.6
is established by the oxidation of the sample phenol. The cathode voltage is established by the reduction of dissolved oxygen. This was confirmed by observhg that the cathode current of Figure 3 decreased sharply when nitrogen was bubbled through the solution. The negative shift in cathode potential observed for curve 3 is probably due to reduced solubility of oxygen caused by dissolved bromine temporarily present before complete reaction. The corresponding rise in voltage on the titration curve a t the beginning of the titration is seen on Figure 2. Curve 4 indicates that reduction of bromine takes place a t much more positive potentiah than any other species present, and its presence in a titration solution results in essentially zero voltage between the constant current electrodes. The change in titration voltage a t the end point is abrupt and large, approximately 0.5 volt per 0.1 ml.
-0.7 -0.3 -0.1 -0.7 4-1.2
ANALYTICAL CHEMISTRY
- - -
IO /LAMPS
4-1.0 -0.2 +1.4
I
+O .8 +0.8 +0.3 +1.0 1.1 +1.1 -0.9 +0.9
+
- - - - - - - - - -
- -
IO E A M P S
+0.7 f0.9 -0.4 $0.5 +0.2
VOLTS Figure 3.
0.00 4.096
- - - - -
26
V8.
S3C.E.
Current-voltage curves at Pt foil electrode vs. S.C.E. for stirred solutions 1. 30 ml. of HAC plus 3 ml. of pyridine
Curve Curve Curve Curve
2.
Curve 1 solution plus 0.20 grams of p-phenylphenol
3. Curve 2 solution plus one half the amount of Br2 required for end point 4.
Curve 3 solution plus excess Brz
PROCEDURE
Bromine, National Formulary, and reagent grade acetic acid and pyridine were used. Sample phenols (“purified” and reagent) were used as supplied. An amount of sample to require about 20 ml. of 0.15M bromine solution was dissolved in 37 ml. of glacial acetic acid plus 3 ml. of pyridine. This solution was titrated to a permanent voltage drop. +4 blank titration was made on the glacial acetic acid and pyridine minus sample. Blanks were 0.1 to 0.2 ml. Presumably, further purification of the pyridine would reduce this small blank; however, this was not attempted. Standard bromine solutions were prepared both by standardizing approsimate solutions with thiosulfate and by :tccurately weighing chilled bromine and diluting to volume. The solutions, once prepared, gave consistent results over
a period of a month indicating that, with reasonable handling, there is not significant loss of bromine during this time. The titration volume was not critical, and smaller volumes could be accommodated with the apparatus described. The usual time for titration was about 20 minutes. RESULTS AND DISCUSSION
Results obtained for a series of phenols are shown in Table I. Three of the phenols titrated are of special interest. Theoretically, 2,4dinitronaphthol does not brominate in any position. Only an amount of titrant corresponding to the blank was necessary to bring about the end point voltage change. p tert-Butylphenol was titrated satisfactorily, but 2,6-ditert-butylphenol overbrominated. These results cor-
respond to those found by lngberman (3). Troublesome precipitation of bromination products, noted in some cases by Ingberman, was prevented, since no aqueous solutions were used. LITERATURE CITED
(1) Djerassi, C., Scholz, C. R., Chem. SOC.70,417 (1948).
J. Am.
( 2 ) Englert, S. , M. E., McElvain, S. M., IIbid., b 51,863 (1929). (3) Ingberman, A. K., ANAL. CHEM.30, \ - ,
1003 (1958). (4) Kolthoff, I. M., Belcher, R., “Volumetric Analysis,” Vol. 111, pp. 534 ff., Interscience, New Ynrh York, 1957. 1~157 (5) Komeschaar. M. F.. 2. anal. Chem. . ,15,259 (1876).’ (6) Reilley, C. N., Cooke, W. D., Furman, N. H., ANAL.CHEM.23,1223 ( 1951). RECEIVEDfor review August 7, 1961. Accepted December 4, 1961. Division of Analytical Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961.
Structural Group Analysis of High Sulfur Content Mineral Oils CLARENCE KARR, JR.,l R. T. WENDLANDI2 and W. E. HANSON Mellon Institute, Pittsburgh, Pa.
b A method for the calculation of the average ring composition for a high sulfur content mineral oil is based on the observation that replacement of one or two methylene groups of a hydrocarbon molecule by a sulfur atom will raise the refractive index and density, on the average, b y amounts which are proportional to the weight per cent of sulfur so introduced. The proportionality factor appears not to vary greatly with hydrocarbon type and, for mixtures of the sort encountered in petroleum, it can b e taken as essentially constant. Adjusting the observed refractive index and density values of an oil in accordance with its sulfur content thus gives the values which the mixture would have if all sulfur compounds present were converted to their hydrocarbon analogs. The average ring analysis can then b e calculated by any of the well known structural group methods. The technique is illustrated by application to mixtures of hydrocarbons and sulfur compounds and to selected sulfur concentrates from high sulfur content crude oils. Nomographic charts are provided for routine work.
0
the past 20 years, numerous methods have appeared in the literature for determining the structural group composition of mineral oils. Following the original “direct method,” which was based on the molecular weight and ultimate analysis of the oil fraction before and after exhaustive hydrogenation, the procedure was gradually simplified to the point where physical property correlations, derived from measurements on petroleum oils of diverse types, substituted for the difficult hydrogenation. The method most commonly employed is that proposed by van Xes and van Westen (3). The aim of every structural group analysis scheme is to estimate the proportion of the carbon present as aromatic ring structures, as naphthenic ring structures, and as paraffinic side chains. With certain assumptions as to the type of rings present and the extent t o which they are condensed, it is possible to express the carbon distribution data as the mean number of aromatic and cycloparaffinic rings per molecule. Hydrocarbon type analysis has found its greatest use in the heavier oils-Le., for oils of average molecular VER
weight of 200 and above, and in particular for wax-free (alkane-free) oils. This latter simplification permits the allocation of the “paraffinic carbon” strictly to alkyl side chains on the ring structures. In developing analytical methods, one must be cognizant of the fact that sulfur and, to a considerably lesser extent, nitrogen and oxygen may be present. The latter two elements have been dismissed because of their relatively slight contribution on a mole percentage basis in most mineral oil fractions, Sulfur, on the other hand, may be an important component of oils but, since the nature of the sulfur compounds present is usually unknown, the correction has often been ignored. van Nes and van Westen have advocated an empirical correction, but the sulfur contrnt of thc oils r.mployed in deriving the factor to be applied did not exceed 2.3y0. Caniisa and Fratta (1) have shovn that oils of higher sul-
Present address, U. S. Bureau Mines, Morgantomn, W. Va. 2 Present address, Winona State College, Winona, Minn. VOL. 34, NO. 2, FEBRUARY 1962
249
