AH
h =
+(
y ) z
(7)
where A H represents the total error incurred in the measurement of peak height and Ah represents the error in measuring the height from the presumed base line. From the measurements of the peak heights of the peaks without base lines a direct value for the error in height measurement AH was available simply by cc,nputing the standard deviations. The relative standard deviations AH/h are shown in Figure 5 as the experimental points. The experimentally observed error in peak height may be compared with the error calculated from Equation 7 using the independently determined basic errors AB and Ah of Table I. The lines in Figure 5 are these calculated values, It is appropriate to compare the precision of height-width integration with peak-height measurement (Figures 3 and 5). Clearly, for peaks of a given height-to-width ratio, the relative indeterminate error is always smaller if only peak height is used rather than peak area. Particularly for peaks of large height-to-width ratio, the advantage in measuring the height only is enormous. The inset of Figure 5 indicates that the error to be expected in measurement of peak height is to a good approximation inversely proportional to the height and almost independent of peak shape for the idealized noise-free peaks that were measured. In practice, separate measurements that were made on less ideal peaks suggest that for two peaks of the same height where one is a sharp spike (height-to-width ratio greater than about 20), the height of the sharp spike is subject to a slightly increased error. A precision of 2 ppt should be readily attain-
able provided peaks of moderate height are measured. For example, the data of Figure 5 indicate that for a peak 10 cm high the error in height measurement was 0.13 %. Measurements of peak area have generally been preferred for quantitative work. This preference may be due to a lack of appreciation of the inherently lower precision in these measurements and also to a recognition that measurements of peak height are sensitive to instrumental and operational variations such as sample injection, column temperature, and column efficiency. For example, if two samples of the same size are injected rapidly in one case and slowly in another, the peak heights may be grossly different although the areas will be the same. Peak height is affected also by fluctuations in column temperature, which impose severe requirements on instrument design. In addition, although peak area is more sensitive to variations in gas flow rate than is peak height, the stabilization of flow rate is easier than stabilization of temperature. The detailed measurements of this present study, however, support the general conclusion often reached in practical work that with good experimental control and stability of a chromatographic system maximum quantitative precision can be obtained from measurements of peak height. The necessary degree of experimental control is often approached in repetitive and control analyses. In general, the gains in precision potentially possible through the use of height measurements should justify intensive efforts to achieve the necessary high quality of experimental performance and control. RECEIVED for review August 16,1967. Accepted November 1, 1967. Financial support to D.L.B. by the National Research Council of Canada is gratefully acknowledged.
Determination of Olefinic Unsaturation by Bromination James S. Fritz and Garth E. Wood’ Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa 50010 The direct titration of various unsaturates in 85% acetic acid-lO% water-5% carbon tetrachloride with bromine in glacial acetic acid is described. Titrations are performed both with and without utilization of a mercury(l1) chloride catalyst. I n the absence of catalyst, compounds containing an isolated olefinic linkage can be determined in the presence of compounds which possess either strongly electron-withdrawing substituents allylic to the double bond or a double bond conjugated with a carbonyl group. An indirect spectrophotometric method for the determination of small amounts of unsaturation is also described.
THE MOST WIDELY applicable methods for determination of organic unsaturation involve the reaction of bromine with the unsaturated compound. A mercury(I1) salt is usually added to catalyze the reaction, Both direct and indirect procedures have been utilized. A broad picture of these methods is presented by Polghr and Jungnickel(1). A major difficulty is that in the presence of excess bromine many com1
Present address, Celanese Chemical Co., P. 0. Box 2768,
Corpus Christi, Texas 78403. (1) A. PoIgCr and J. L. Jungnickel, “Organic Analysis,” Vol. 111, Interscience, New York, 1956, pp. 229-55.
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ANALYTICAL CHEMISTRY
pounds undergo substitution side reactions which may cause high results. Attempts have been made to correct for the substitution side reactions by determining the hydrobromic acid that is produced (2). An extrapolation procedure has been proposed to correct for side reactions such as substitution which may occur to a minor extent and cause high results (3). Direct titration methods offer several advantages over indirect procedures. No excess bromine is present until after the end point is reached; therefore, side reactions are less likely to occur. Sweetser and Bricker (4)developed a spectrophotometric method for titration of olefins and other substances with tribromide ion at wavelengths ranging from 270 to 360 mp. Miller and DeFord (5) adapted this method for electrically generated bromine. Their results were almost all low and often in poor agreement with results obtained by analyzing the same unsaturated compounds by standard methods. Leisey and Grutsch (6) obtained satisfactory results for trace unsaturation using coulometrically generated (2) Ibid.,p. 231. (3) Ibid.,p. 237. (4) P. B. Sweetser and C . E. Bricker, ANAL.CHEM., 24,1107 (1952). (5) J. W. Miller and D. D. DeFord, Ibid.,29,475(1957). (6) F. A. Leisey and J. F. Grutsch, Zbid.,28, 1553 (1956).
bromine in conjunction with an amperometric end point, although some compounds require several minutes for complete reaction. Williams et al. (7) used pyridinium bromide perbromide in acetic acid and methanol for spectrophotometric titration of phenols, aromatic ethers, and a limited number of unsaturated compounds. Like most other workers, they found it necessary to employ a mercury(I1) chloride catalyst. The aim of our research has been to develop rapid methods that will quantitatively distinguish simple olefinic unsaturation from unsaturation in which the carbon-carbon double bond is conjugated with a carbonyl, nitrile, or other electronwithdrawing group. This is accomplished by reaction with bromine in glacial acetic acid without any mercury(I1) salt or similar catalyst being added. Two procedures are presented. Macro amounts of olefins are determined by a rapid, direct titration using a spectrophotometric end point in the visible spectral region, Minor quantities of olefinic unsaturation are measured by a quick spectrophotometric procedure. This is based on the decrease in absorbance resulting from reaction of bromine with the carbon-carbon double bond in a bromine-hydrobromic acid solution in acetic acid water. Reaction of bromine with a carbon-carbon double bond in a polar solvent is generally considered to occur as follows :
Br+. . .Br-
+
\
C=C
/
( 7\ /
/ +
Br
-C-
