RESULTS AND DISCUSSION Direct Gas Chromatographic Method, Internal Standard CHZC12. The internal standard CHJOH used by Hogan and coworkers (2) is converted by the reaction mixture too soon. T o eliminate this effect, we use CH2C12 as an internal standard. A half-time for this standard of more than 24 hours is not a serious drawback in our determinations. The results of a calibration are shown in Figure 2. Figure 3 gives a representative chromatogram. Table I1 lists the data for Figure 2. The relative standard deviation a t each experiment proved to be 3%. Water concentrations varied from 70 to 260 ppm wt. In conformity with Hogan ( 2 ) and Hollis and Hayes ( 3 ) ,we measured a linear relation between relative area HZO/CHZC12 and H20 content. The additional HzO content caused by handling is 30 ppm wt, considerably less than the ca. 45 ppm wt found by Hollis and Hayes (3). The calculated correction factor is 0.535. The results of a second calibration procedure, executed by coupling the conical flask directly to the Karl Fischer apparatus, are given in Figure 4 and Table 111. Within experimental accuracy, the value found for residual water is zero, in contrast with Joseph ( 4 ) , who measures a peak height corresponding with about 55 ppm wt H20 content. A correction factor 0.417 is calculated. The difference between the correction factors measured is caused by the use of two different T C detectors. In accordance with Hollis and Hayes ( 3 ) , we find that the use of a back-flush valve reduces the accuracy and reliability of the determination. An example of water determination in chlorination reaction media is given in Table IV. The water content in this chlorination of cyclohexanone varies from 56 to 400 ppm wt. The confidence interval is *17 ppm wt for the duplicate and &14 ppm wt for the triplicate determinations. A second series of 24 samples taken from a continuous benzene chlorination process results in a mean value of 24 ppm wt HzO and a standard deviation of 27 ppm wt. A method for measuring the upper limit of the error caused by handling the sample is given below. The water added by the supply of internal standard solution and pyridine should be subtracted from the values measured gas chromatographically. To determine this correction, varying quantities of internal standard and pyridine are added to samples with a like amount of water (3) 0. L. Hollis and W. V. Hayes, J. Gas Chrornatogr., 4 , 235 (1966). (4) H. Joseph. IsraeiJ. Chem. 8, 575 (1970).
quantities of CH2CI2 and p y r . water analysis
Figure 5. S y s t e m a t i c e r r o r s in
a . H 2 0 added by CH2C12 and pyridine. b . H 2 0 added by handllng and residue in dry benzene
content, e . g . , benzene dried on molecular sieve. The value which is to be subtracted from the water content measured is indicated in Figure 5 by a . The value b on the ordinate is the sum of residual water in dry benzene and water introduced by handling the sample. Values for a and b of the order of magnitude of 20 ppm wt are normal. We showed that an accurate water determination is possible in the range of 50-440 ppm wt in chlorination reaction media, uiz., in the presence of chlorine, hydrogen chloride, and catalysts. In determining concentrations of 20 ppm wt, the accuracy decreases sharply ( D = 120%). Because a decrease in the accuracy in the 50-400 ppm wt range may occur accidentally, research is now concentrated on finding the factors affecting the reliability of the determination.
ACKNOWLEDGMENT Thanks are due to L. Balt for his helpful discussions. Received for review August 30, 1972. Accepted February 23, 1973.
Characterization of Pellicular Porosity by Gel Permeation Chromatography E. P. Otocka Bell Laboratories, Murray Hill, N.J. 07974
Pellicular ( i .e . , layered) chromatographic substrates have become a very important part of the resurgence of liquid chromatography (LC). These particles retain the advantage of large particle packings (low to medium pres-
sure drop across the column) while adding the advantage of small particle packings (greatly reduced plate heights). Since the porous surface layer on these beads represents such a small fraction of their total volume, characteriza-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1969
Table I. Pellicular Porosity Material Corasil I I
Dmax
Determined at
Dav,
300
V O i- ( V t
-
V0)/2.
Aa
Drn,,,. A
. .
74 900
> 1900
Zipax a
A
200
IO6
k-.~~5
-
a
-
3
a
-
5 lo4
0
:
A 0
-
w
I
-
-
(narrow)
RESULTS AND DISCUSSION The data of Figure 1 show that the two materials have greatly different pore sizes, distributions, and total porosities. To calculate the pore diameter, the criterion of Cantow and Johnson (2) was used as a first approximation, namely, Dpore= 2h ( h = the mean square end to end distance of the random coil in the solution). Further refinement would have to take into account a number of different parameters (3, 4 ) . The results of the simplified approach are shown in Table I, where the 6 values for the individual polystyrenes were determined by standard statistical methods (5). The agreement between available porosimeter data and the GPC method is good. The indicated breadths of the pore size distributions also agree with those furnished by the suppliers. The total pore volume was determined by the exclusion limits and the weight of packing in the column. The high value of Corasil I1 is a result of the relatively thick pellicular layer.
1 0
I.o
1.10
V,(ml) Figure 1. Molecular in dioxane
-
800-1000
1.
-
:
w
0.070 0.029
Dav! A (manufacturer)b 140 (broad)
fill technique ( I ) . For Zipax, 3.46 g of material was required, while 3.23 g of Corasil I1 filled the column. The columns were equilibrated with dioxane a t 1.0 ml/min flow rate on a PerkinElmer Model 1210 LC unit. The Corasil 11 column showed a AP of -400 psi, a t this flow rate, while the Zipax column had a AP of -450 psi. This difference is explained by the slightly smaller particle size of the Zipax material. The flow was reduced to 0.25 ml/min and 2 pl of 2 pg/pl Pressure Chemical narrow molecular weight distribution standard polystyrenes ( g 900 to 2 X 106) were injected using the on-column septum injector. An ultraviolet (254 nm) detector was employed, and the output recorded a t a chart speed which corresponded to 2.0 in./ml. The average peak retention volume from three injections of each polystyrene was used in generating Figure
L
I
mlig
Porosimeter data.
1 0 ~ ~
a -
Pore vol,
I.
0
weight vs. retention volume for polystyrenes
( m ) Corasil I I packing, ( 0 )Zipax
ACKNOWLEDGMENT tion by porosimetry presents some difficulties. In this work, the use of narrow molecular weight distribution polystyrenes and gel permeation chromatography (GPC) is examined as an alternate means of characterizing the pore size distribution and total pore volume of such particles.
The author wishes to thank P. M. Webster for capable assistance in carrying out these experiments. Received for review March 9, 1973. Accepted April 12, 1973.
EXPERIMENTAL Two pellicular chromatographic substrates were examined. Corasil 11, available from Waters Associates, is a solid glass core particle with silica gel deposited on the surface. Zipax chromatographic support was obtained from E. I. Du Pont. This material again has a solid glass core on which submicrometer particles of a silica gel have been attached. The support materials were dry packed into 0.26- X 50-cm stainless steel chromatography columns, using the modified tap-
1970
ANALYTICAL
CHEMISTRY,
VOL.
(1) J . J. Kirkland, J. Chromatogr. Sci., 10, 129 (1972). ( 2 ) M . J. R. Cantow and J. F. Johnson, J. Polym. Sci., Part 4-7, 5, 2835 (1967). (3) K. A. Boni, F. A . Sliemers. and P. B. Stickney, J. Polym. Sci., Part A-2. 6, 1579 (1968). (4) E. F. Casassa and T. Yagami, Macromolecules.2, 14 (1969). (5) P. J . Flory, "Statistical Mechanics of Chain Molecules," Interscience, New York. N . Y . , 1969, Chapter 2.
45, NO. 11, SEPTEMBER 1973
