.ample folio\\ ing Inetitration of the blank was usually in ( m o r \Thile -ubsequent samples were satisfactory. Therefore, the procedure followed was to pretitrate the blank to a slight excess and then add an timmonia sample of approximately the same size as the expected teyt sample and pretitrate it, noting the excess of hypobromite generated beyond the end point. Then the sample of a serics of samples n a s added and titiated a. a l m e I n the-e p-etitration procedures, a large exceqs of hypobromite -hould not be generated before adding the sample, as this may lead t o a positiL e error (1). I n the manual titrations, variations of stirring rate and total volume should have no effect on the m d point break, although the generating; reagents should be pipetted to reproduce the blank impurities. I n applying the automatic plot, however, theqe should be re-
produced fairly clowly with a volume duplication of i l ml. appearing t o be sati-factory. For a series of titrations in the same solution, satisfactory end points are obtained if the stirring rate is increased in approximately the qame proportion as the volume increase when a new sample is added. I n these titrations the recommended p H should be adhered to within -0.1 p H unit as discussed by Arcand and Swift (I). I n any event where small ammonia samples are titrated with hypobromite, which is a strong oxidant readily reduced by many different impurities, the analyst should always run a blank to determine the titration error due to impurit,ie,q. The data of this inveqtigation are shown in Tables I and 11. A series of samples ranging from 354.3 to 1.44 pg. of ammonia vcre titrated either
manually 01 :iutoiiiatically u-ing both the direct and pretitration mcthotli. Automatic titration appears t o give results slightly superior to the manual titrations. LITERATURE CITED
(1) Arcand, G. )I., Swift, E. H., . 4 s a ~ . CHEM.28,440 (1956). (2) Kolthoff, I. AI., Stenger, V. A., ISD. ENQ.CHEN.,AKAL.ED. 7, 79 (1935). i.7) Kolthoff. I. bl.. Stricks. W.. Morren. (4) Laitinin, H. -4.; Roerner, D. E., ANAL.CHEJI.27,215 (1955). (5) Willard, H.H.,Cake, W. E., tJ. A t l z . C h e m SOC.42, 2646 (1920). GARYD. CHRISTIAS EDWARD C. KNOBLOCK WILLIAMC. PTJRDY Division of Biochemistry Walter Reed Army Institute of Rcrearcli Washington 12, D. C. and DeDartnient of Chemistrv Unhersity of Maryland ' College Park, Md.
Infrared Ancdysis of Alpha-Olefins SIR: The olefin groiili type analysis preyiously reported ( 2 ) has been in use in this laboratory for a number of years. Recently a need arose for the accurate quantitative determination of olefin group types in normal a-olefin rich mixtures. I t v;as particularly desirable to achieve the highe-t possible accuracy in the a-olefin determination. Since the required accuracy was beyond the limits of the group type analysis, i t was necessary to introduce a number of modifications in the procedure to obtain a satisfactory analysii. The purpose of this cornniunication is to report these modifications. Simple averaging of the absorptii.itics of the characteristic a-olefin band (11.0 microns) to obtain a group type value results in errors too large for the present requirements This is illustrated in Table I. For esample, the absorptivity of heserie.1 deviates from the average by 14% ttnd decene-1 by 5.5%. Thus, it is nece:aary to calculate a statistically y1 eighted absorptivity to be used with each sample. -Accordingly, the following procedurcb n as developed. The absorbances for all the normal a-olefins to be incliidecl in the analysis were obtained a t all the analytical wavelength---e.g., l O . X , 11.00, 11.25, 12.03, 12.15, 12.33, 12.50, and 14.42 microns, on a Perkin-Elmer Model 21 infrared spectrophotorieter. A 0.15mm. cell was used and dilutions were made in carbon disulfide when necessary. The absorptivity of each of the pure a-olefins was calculated at their group absorptim wavelength,
11.OO microns. The talibration compounds were high-purity American Petroleum Institute sanipleq. I n the present case, we included only the even-numbered a-olefins from Ce through Cz0. Octadecene-1 and eicosene-1 nere not available and their data here determined by extrapolation of the data for the lower molecular weight compounds. When the absorptivities of pure a-olefins are u-ed and the carbon number distribution of the sample is obtained from a high temperature programmed gas chromatographic analysiq, the statistical absorptirity at 11.00 microns is calculated for the particular sample under analvsic The absorbance of the sample a t 11.00 microns divided by its Gtatiqtical absorptivity yields the concentration of a-olefins in the -nrnp:': (in mole. per liter). To determine the other olefin groups which are present as impurities in the a-olefin cample, the statistical absorbances are calculated for the sample a t all the an-
Table II.
A, l\Iicronti/
olefin group
10 35
11.00 11.25
Av. of 4
1 2 . 0 to 12.5
14.42
alytical a-avelcnyths assunling the sample contains 1 0 0 ~ onormal a-olefins. The statistically calculated absorbances are then substracted from the respective observed absorbances of the sample. These difference absorbance4
Table 1.
Absorptivities of a-Olefins at 1 1 .O Microns
Absorptivity, liter mole-' % Dev. c.rn.-' from av. 138 - 14
a-Olefin Hexene-1 Heptene-1 Octene-1 Sonene-1 Decene-1 Undecene-1 Dodecene-1 Tridecene-1 Tetradecene-1 Pentadecene-1 Hexadecene-1
I50
-6 2 -1 4 -2 G 15.5
I58 156 169
io
$6.2
1
166 165 I64 I65 I60 ~A.T.
$3.7
+3.1 $2.4 $3.1 1
io.
lW.1
Matrix Used in Group-Type Olefin Analysis Olbsorptivities in liter mole-' cm.-*)
Trans RHc=CHR
l-inyl RHC=CH,
Vinylidene IL'Rf'C=CHIR
Trisubstituted R'R"C= CHR
147 2 4 20 4 46 1 01
.i 59
1 65
160 4 9 45
S 29 184 7
2 43
4 19 2 23 2 53 15 7
0.24
1.35
0.41
0 31
1 88
Cis RHC=CHI: 6.09 4 71
VOL. 3 5 NO. 13 DECEMBER 1 9 6 3
3 01 2 30
24.5
2219
Table 111.
Synthetic Blend
Olefin Type Trans (Irans-heptene3) Vinyl (octene-1) Vinylidene (2-methylbutene-1 ) Trisubstituted (2methylbutene-2) Cis(cis-heptene-2) Total
Concentration (moles/liter ) DeterKnown mined 0.075 6.134
0.04 6.15
0.045
0.02
0.106 0.078
0.09 0.13
6.438
6.43
represent the departure of the sample from the assumed 100% a-olefin. When these difference absorbances are applied to the olefin group type absorptivity matrix (2) shown in Table 11, the concentrations of the various impurity olefin types are obtained.
The difference absorbance a t 11.OO microns will, of course, be a negative number if the sample is less than 100% pure a-olefin, but the negative sign should be carried along in the matrix calculations; a negative value will result representing the departure of the a-olefin concentration in the sample from the assumed 100% a-olefin. This value could be algebraically added to the total theoretical olefin concentration in the sample to obtain the a-olefin concentration. However, the concentration for the a-olefins determined by this procedure is not sufficiently accurate since the group type absorptivity is used. The mean error of this analysis when applied t o an aromatic free material that contains 80% or more a-olefins is for the a-olefin believed t o be &I% value and *1.5% for the other olefin groups. This is indicated by the analysis
of a synthetic blend shown in Table I11 where all of the mean errors are less than 1%. I n a blend of four a-olefin compounds, the error in the total vinyl concentration was 0.13%. I n some cases, diolefins and cyclomono-olefins will interfere if present in quantities greater than 3%. The diolefin and or cyclomono-olefin content may be determined by a low-voltage mass spectrometric method ( I ) . LITERATURE CITED
( 1 ) LumDkin. H. E.. ANAL. CHEM.30. ' 321 (1658)' (2) Saier, E. L., Pozefsky, A., Coggeshall, N. D., Zbid., 26, 1258 (1954).
E. L. S l I E B L. R. COUSINS ?If. R.BASILA Gulf Research and Development Co. Pittsburgh 30, Pa.
Modified Thermal Conductivity Detector for Capillary Columns SIR: Although Golay (8) initially used a thermal conductivity detector for his investigations with capillary columns, these columns have been used almost exclusively with ionization detectors. Recently, Schwartz (4) and also Camin (1) reported using primarily 0.02-inch or larger i.d. capillary columns with thermal conductivity detectors. At this laboratory, a gas chromatograph equipped with a modified thermal conductivity detector has been used for some time with 0.01-inch i.d. capillary columns.
EXPERIMENTAL
Apparatus. The gas chromatograph was fabricated along conventional lines using essentially standard components. The detector was the GowMac Micro-Cell Model JDC-015. A 2-mv. Brown recorder was used. A sample stream splitter after the design of Halasa (3) was included in the inlet system. The splitter was fabricated from a Swagelok heat exchanger tee and stainless steel tubing. The split ratio was varied with a Nupro micrometering valve. Both detector and capillary column were housed in a
Figure 2.
temperature-controlled air bath suitable for operation up to 125' C. The significant feature of the apparatus was the modification made in the sample cavity of the detector. Figure 1 shows this modification in detail. The modification consisted of positioning a 2-inch length of stainless steel, 0.010inch i d . , tubing up through the sample cavity orifice with the end of the tubing as close as possible to the thermistor bead itself. This distance was approximately 1 mm. The tubing was then silver-soldered in this position. The capillary column was then connected to the other end of the tubing.
Chromatogram of Cs through C, blend on modified cell
Column. 190-ft. X 0.01 0 In. stainless steel capillary coated with aqualane; 25' C.; helium 1.7 ml./ min. sample size, 1 &I. 1 O O : l split Componentr
1. 2. 3.
Figure 1. Modification of sample cavity in cell a. Before modification
b. After modiflcation
2220
ANALYTICAL CHEMISTRY
lsopentane n-Pentane 2,P-Dimethylbutane 4. Cyclopentane 5. 2,3-Dimethylbutane 6. 2-Methylpentane 7. 3-Methylpentane 8. n-Hexane 9. 2,2-Dimethylpentane and methylcyclopentone 10. Benzene and PA-dimethylpentane 1 1. 2,2,3-Trimethylbutane
12. 13. 14. 15. 16. 17.
1 a. 1 9. 20. 21.
3,3-Dimethylpentane Cyclohexane 2,3-dimethyl pentane 3-Methylhexone 1 ,trans-3-Dimelhylcyclopentane 3-Ethylpentane and 1 ,franr-2dimethylcyclopentane n-Heptane 1 ,cir-2-Dimethylcyclopentane Methylcyclohexane Toluene
