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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7 , JUNE 1978
more than a factor of 20. Phenanthrene, fluoranthene, and pyrene have very similar molecular lengths; but their respective aqueous molar solubilities at 25 "C are 5.6 x lo4, 1.0 X lo4 and 6.8 X lo-' mol/L. Tsonopoulos and Pransnitz (19) have reported that the hydrocarbon infinite dilution coefficient, y-,is the appropriate quantity for correlating the aqueous solubilities of hydrocarbons. They, along with Leinonen et al. (15) and Pierotti et al. (20) have successfullycorrelated y- with carbon number, molar volume, and degree of branching. Recently Mackay and Shiu (18) have correlated the hydrocarbon infinite dilution coefficients of 32 aromatic hydrocarbons (using the super cooled standard state) with carbon number. From this relationship, they derived a parabolic equation from which individual solubilities could be calculated. A comparison of the correlated and experimental solubility values showed that solubilities could be estimated only to within a factor of 3.
CONCLUSION The DCCLC technique is a very rapid, precise, and accurate method for determining the aqueous solubility behavior of sparingly soluble organic compounds. The agreement of PAH aqueous solubilities and calculated solubility parameters, such as AH8, with values that have been previously reported in the literature helps to confirm the validity of this approach. In all cases, replicate solubility measurements of the generated PAH solutions at constant temperature were better than &3% and in most cases better than &l%.The lack of an appropriate physical parameter for accurately extrapolating PAH solubilities, further enhances the utility of DCCLC method. LITERATURE CITED (1) D. MacKay and W. Shiu, Can. J . Chem. Eng., 53, 239 (1975). (2) C.McAuliffe, J . Phys. Chem., 70, 1274 (1966).
(3) C. Sutton and J. A. Clader, J . Chem. Eng. Data. 20, 320 (1975). (4) D. Amo(d, C. Plank, and E. Erickson. Chem. Eng. Data Ser.,3,253 (1958). (5) R. Bohon and W. Claussen. J. Am. Cbem. Soc., 73, 1571 (1951). (6) F. Franks, M. Gent, and H. Johnson, J . Chem. SOC., 2716 (1973). (7) L. Andres and R. Keffer, J . Cbem. Soc., 3819 (1952). (8) T. Morrison and F. Billet, J . Cbem. SOC., 3819 (1952). (9) H. Vermillion, Ph.D. Thesis, Duke University, Durham, N.C., 1939. (10) R. R. Stearns, H. Oppenheimer, E. Simon, and W. Harkins, J . Chem. Phys., 14, 496 (1974). (11) A. HIII, J . Am. Chem. SOC.,44, 1163(1922), (12) M. Hayashi and T. Sasaki, Bull. Cbem. SOC. Jp, 29, 857 (1956). (13) H. Booth and H. Everson, Ind. Eng. Chem.. 40, 1491 (1948). (14) R. Brown and S. Wasik, J . Res. Natl. Bur. Stand., Sect. A., (45),78, 453 (1974). (15) P. Leionen. D. MacKay, and C. Phillips, Can. J . Chern. Eng., 49, 288 (1971). (16) W. Davis, M. Krohl, and G. Clower, J . Am. Chem. Soc., 84, 198 (1942). (17) H. Kelvens, J . Phys. ColloidChem., 54, 283 (1950). (18) D. MacKay and W. Shiu, J . Chem. Eng. Data, 22, 4 (1977). (19) C. Tsonopoulos and J. Prausnitz, Ind. Eng. Chern., Fundam., IO, 593 (1971). (20) C. Pierotti, C. Deal, and E. D w , Id.Eng. Cbem.. Fundam., 51,95(1959). (21) F. Schwarz, J . Chem. Eng. Data, 22, 273 (1977). (22) R. Weimer and J. Pausnitz, J . Chem. Phys., 42, 3643 (1965). (23) R. Wauchope and F. Getzen, J . Chem. Eng. Data, 17, 38 (1972). (24) L. Andrews and R. Keefer. J . Am. Chem. Soc., 71, 3644 (1949). (25) W. May, S. Wasik, and D. Freeman, Anal. Chem., 50, 175 (1978). (26) W. May, S.Cheder, S. Cram, B. Gump, H. Hertz, D. Enagonio, and S. Dyszel, J. Chromatogr. Sci., 13, 535 (1975). (27) W. May, Ph.D. Dissertation, University of Maryland, College Park, 1977. (28) J. Setschenow, 2. Phys. Cbem., 4 117 (1889). (29) W. McDevit and F. Long, J . Am. Cbem. SOC.,74, 1773 (1952). (30) H. Davis and S. Gottlieb, Fuel, 8,37 (1962).
RECEIVEDfor review February 7,1978. Accepted April 3,1978. The authors are grateful to the Office of Air and Water Measurement, National Bureau of Standards, for partial support of this work. This work is from a dissertation submitted in September 1977 to the Graduate School, University of Maryland by Willie Eugene May, in partial fulfillment of the requirements for a Ph.D degree in Chemistry.
High-Efficiency Packed Columns Using Fine Particles of Graphitized Carbon Black Antonio Di Corcia" and Maurizio Giabbai
Istituto di Chimica Analitica, Universiti di Roma, Rome, Italy
The eflect of particle size of graphitized carbon black (Carbopack) on column efficiency has been investigated. A continuous decrease of the plate height for gas-modified solid columns has been observed by reducing the mean particle diameter from 185 down to 28 pm. As a result, a maximum of 10 000 plates per meter of column has been achieved by equal to 28 km at a carrier gas velocity of 7 cm/s using a and with a pressure drop of 19 atm. It Is noteworthy that the optimum mobile phase velocity is practlcally unaffected by the reduction of ap. This effect coupled with the use of hydrogen as carrier gas has been exploited in the fast analysis of complex mixtures of practical Interest.
a,
Numerous efforts have been made in order to obtain packed columns with high numbers of theoretical plates. This goal has been obtained by using long columns operating at medium and high pressures (1-6). This approach has some drawbacks. First, long columns under given conditions require a proportionally long analysis time; second, long columns are often inconvenient to prepare and use; third, the increase in column 0003-2700/78/0350-1000$01 .OO/O
length results in a dilution of the sample and thus may cause the undetectability of trace-contained components in complex mixtures. Another approach to achieve high theoretical plate numbers is that of improving column efficiency by varying parameters which are correlate to the plate height. The treatment and interpretation of H1r.i data obtained by varying column parameters demand a choice of rate equation. Since 1940, various developments of concepts and mathematical expressions have been formulated in order to obtain a correct relation of all parameters which can affect the plate height. A simple plate-height equation for gas-solid packed columns, which has been shown elsewhere (7-10) to give a good reproduction of experimental data over a wide range of carrier gas velocity, is:
2yD0, K =2 G p + ___
4-
U0
2k' tduoj (12' + 1)' @ 1978 American Chemical Society
wdp2uo 7 + Dm
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
1001
or in simplest terms: e
?ir= A
B
++ Cguo+ Csuoj U0
where a,, is the mean particle diameter; Dm0 is the molecular diffusion coefficient for the mobile phase at the column outlet pressure; 12’ is the capacity ratio; fd is the mean desorption time of an equilibrium population of sorbed molecules; A, w , and y are all structural factors related to particle orientation, uo is the outlet carrier gas velocity which is related to the average carrier gas velocity through the expression r.i = u o j , where j is the James-Martin compressibility factor (11). Equation 1 shows that the plate height is related to the particle diameter and it explicitly predicts that column efficiency can be improved by reducing the particle size composing the packing of the column. Plate height as low as 0.082 mm has been obtained by Myers and Giddings (12) with the use of very fine alumina particles packed in small column diameters. Recently Huber, Lauer, and Poppe (13-14) have evaluated the influence of particle size and pressure on the column efficiency. As a result, 10 000 theoretical plates per meter of column length were obtained for 30-35 pm chromosorb particles coated with 3% w/w squalane. Graphitized carbon black (GCB) modified with various liquid or solid chemical agents has proved to be effective in the gas chromatographic analysis. Various papers have dealt with optimization of selectivity of this adsorbing material not only by using suitable modifying agents but also by adjusting their surface concentration (15-18). On the contrary, only sporadic attempts have been made to improve the quality of the gas chromatographic analysis by varying parameters affecting the plate height (19-21). In the present paper, a systematic attempt is made to investigate the possibility of achieving high theoretical plate numbers with relatively short columns with a view to analyze complex mixtures in short times. For this purpose, parameters which can affect the plate height have been varied, paying particular attention to the possibility of using fine GCB particles. A minimum plate height value as low as 0.1 mm at relatively high linear carrier gas velocity has been obtained by using 25-33 pm GCB particles with practicable pressure drops. Finally, the high feasibility of columns packed with suitably modified GCB fine particles has been exploited in the elution of complex mixtures of practical interest.
EXPERIMENTAL Carbopack C, which is an example of GCB, was supplied in the 8C-100 mesh range by Supelco, Bellefonte, Pa. It was ground and sieved to obtain the appropriate particle size ranges. Column packings were prepared in the usual way (16,21,22). Acetone was used as solvent for TCEP and column conditioning temperature was 140 “C. Glass columns of variable diameters were filled with the packing material. When glass was substituted with stainless steel, a systematic loss of efficiency as large as -25% was noted. The packing operation, which is very critical was described in detail elsewhere (21). If this operation is correctly carried out, the weight of carbon fiiing a 108 cm X 0.9 mm i.d. column is within 0.60-0.65 g. Columns filled with greater amounts of carbon yield a considerable decrease of permeability and a loss of efficiency. Another critical operation is the introduction of the carrier gas in columns packed with “medium fine” (100-60 pm) and very “fine” (60-25 Mm) GCB particles. Even from the start, the pressure of the carrier gas must be increased very slowly by using a fine pressure regulator. The lower the particle size range is, the more important this precaution is. If this operation is not correctly carried out, the column shows an anomalously low permeability and some void spaces in the packing material can be observed.
L v ; - r
3
d’
-.-.
&---4-
^ I d _
“i
-
w
c
.G
1 . .
Figure 1. Avs. u curves for packed columns with various particle diameter ranges. Column: length, 108 cm; i.d. variable from 1.5 to 0.9 mm; packing Carbopack C 0.2% PEG 1500;carrier gas, nitogen; temperature, 56 O C ; sample, pentane
+
The drop in pressure just needed to obtain a dead time equal to 14.2 s using methane at 56 “C, eluted with nitrogen as carrier gas on a 108-cm column packed with GCB particles of different size ranges coated with 0.2% PEG 1500,are reported here. They are as follows: 15Cb125pm, 1.2 atm; 106--90, 2.3; W75, 3.0; 75-61, 4.0; 61-33, 7.3; 33-25, 19.0. A gas chromatograph (Carlo Erba, Model GI, Milano, Italy) equipped with a flame ionization detector was used. To obtain a fine pressure regulation, the line of the carrier gas was directly connected to a DE 38/50 pressure regulator of the Air Liquid (Paris, France).
DISCUSSION OF RESULTS Effect of Column Diameter on Column Efficiency. The average theoretical plate height was measured as a function of the average mobile phase velocity for columns of four different diameters, that is 2.0, 1.5, 0.9, and 0.5 mm, packed with Carbopack C (73-60 pm) + 0.2% PEG 1500. I t was observed that the effect of the column diameter is not important in the range where the tube-to-particle diameter ratio is 8 to 25. Where the ratio was higher than 25, a serious loss of efficiency especially a t high linear carrier gas velocity was noted. The steep rise of the right hand branch of the curve suggests that the coefficient, o,of Equation 1 is increased because of the increase of the transcolumn effect (23). In the course of our experiments, we maintained the tubeto-particle diameter ratio around the value of 15 to obtain a good arrangement between efficiency and the capacity of the column. Effect of Particle Size on Column Efficiency. A number of columns were prepared using different particle sizes of Carbopack C partially coated wit.h 0.2% PEG 1500. As reported in Figure 1,for each column the average theoretical plate height was determined as function of the average mobile phase velocity. From the observation of the H/r.i curves, it can be deduced that the reduction of d, has the following consequences: (1)The minimum value of the mean plate height, Rmh,decreases gradually; (2) the Rmin value for each curve occurs a t about the same linear carrier gas velocity. Moreover, it can be seen that by using a particle size range between 25 and 33 pm, 10000 plates per meter of column can be achieved. In Figure 2, Rminvalues for each H/Li curve reported above are plotted vs. d,. I t can be noted that in the range of high particle diameter values, there is a linear decrease of Rminby reducing d p . This decrease is, however, smoothed down a t low particle sizes. This means that by using GCB particle sizes lower than about 30 pm there will be little improvements on the column efficiency compared to severe practical disad-
1002
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
i
01
0
. / I
l
l
1
I
I
I
E
I
I
I
I
J
U i
o
1s 30
6 5 60 75 l,sec
Figure 3. Chromatogram showing the separation of C, to C, alcohols.
Column, 1.5-mm i.d., length 108 cm; packing, Carbopack C (73-60 km) 0.2% TCEP; carrier gas, hydrogen; pressure drop, 5 atm; dead time, 4 s; temperature, 134 OC. Eluates, (1) methanol, (2) ethanol, (3) isopropanol, (4) propanol, (5) 2-methyI-2-propano1, (6) 2-butano1, (7) 2-methyCl-propanol, (8) 1-butanol, (9) 2-methyl-2-butanol, (10) neopentanol, (1 1) 3-methyl-2-butanol, (12) 3-pentano1, (13) 2-pentano1, (14) 2-methyl-1-butanol, (15) 3-methyCl-butanol, (16) 1-pentanol
+
a
0,5
1
1.5
2 t, rnrn
Figure 4. Chromatogram showing the separation of Ceto C, aromatics. Column, 0.9-mm i.d., length, 108 cm; packing, Carbopack C (60-33 0.3% Te NF; carrier gas, hydrogen; pressure drop, 14 atm; rm) dead time, 4.6 s; temperature, 184 OC. Eluates, (1) benzene, (2) toluene, (3) ethylbenzene, (4) isopropylbenzene; (5) rn-xylene, (6) p-xylene, (7) o-xylene, (8) styrene, (9) propylbenzene
+
ANALYTICAL APPLICATIONS T o give experimental evidence of the effectiveness of columns packed with fine particles of Carbopack, some interesting separations were performed. It has to be pointed out that attention was paid to carry out rapid analysis of mixtures of interest by both short columns with a sufficient number of plates and hydrogen as carrier gas. So, the development of long columns packed with fine particles having efficiencies comparable with capillary columns was neglected.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
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is noteworthy that by using 60-33 pm GCB particles the elution time is seven times lower than that obtained by using a 3-m long column packed with the same material but having a conventional particle diameter (21). Finally, in Figure 5 is shown a chromatogram concerning the separation of C4 hydrocarbons including isopentane and pentane. Also in this case, a very large decrease of the analysis time with respect to our previous results (22) can be noted. This is due to the double effect of a very efficient column and hydrogen as carrier gas. Another interesting result is that by decreasing the size of GCB particles, there is no significant effect in the GCB specific surface area. As a matter of fact, the same relative amount of picric acid yields the same separation factors passing from a mean particle diameter of 137 to 46 pm.
LITERATURE CITED (1) R. P. W . Scott, in "Gas Chromatography 1958",D. H. Desty, Ed., Butterworth, London, 1958,p 189. (2) M. N. Myers and J. C. Giddings, Sep. Sci., 1, 761 (1965). (3) C. A. Cramers, J. A. Rijks, and P. Bocek, J. Chromatcgr., 65, 29 (1972) (4) C. A. Cramers, J. A. Rijks, and P. Bocek, Ciln. Chim. Acta, 34, 159 (1971). (5) F. Bruner, C. Canuili, A. Di Corcia, and A. Liberti, Nature(London), 231,
Flgure 5. Chromatogram showing the separation of C, hydrocarbons including isopentane and pentane. Column, 0.9-mm i.d., length, 108 cm; packing, Carbopack C (60-33 pm) 0.19% picric acid; carrier gas, hydrogen; pressure drop, 12 atm; dead time, 4.4 s;temperature, 50 'C. Eluates, (1) methane, (2) ethane, (3) propane, (4) propene, (5) isobutane, (6) 1-butene, (7)butane, (8) isobutene, (9) cis-2-butene, (10) trans-2-butene, (11) butadiene, (12) isopentane, (13) pentane
+
In Figure 3 is shown a chromatogram concerning the separation performed within 1 min of all aliphatic alcohols u p to C5 contained in a water mixture. This separation was accomplished by using medium fine particles (73-60 pm). It can be noted that the first peaks are not completely separated. This is due to the presence in our chromatographic apparatus of some dead volumes which cause peak broadening for too quickly eluted components. This effect external to the column prevented us to use for practical purposes very fine GCB particles, e.g., 25-33 pm packed in very short columns. In Figure 4 is shown the separation of aromatic hydrocarbons from benzene to propylbenzene including styrene. It
175 (1971). (6) F. Bruner, P. Ciccioii, and A. Di Corcia, Anal. Chem., 44, 894 (1972). (7) R. H. Perrett and J. H. Purneii, Anal. Chem., 34, 1336 (1962). (8) R. H. Perrett and J. H. Purneil, Anal. Chem., 35, 430 (1963). (9) G. L. Hargrove and D. T. Sawyer, Anal. Chern., 39, 945 (1967). (10) G. L. Hargrove and D. T. Sawyer, Anal. Chem., 40, 409 (1968). (11) A. T. James and A. J. P. Martin, Biochem. J . , 50, 679 (1952). (12) M. N. Myers and J. C. Giddings, Anal. Chem., 38, 294 (1966). (13) J. F. K. Huber, H. H. Lauer, and H. Poppe, J . Chromatogr., 112, 377 (1975). (14) J. F. K. Huber, H. H. Lauer, and H. Poppe, J , Chromatogr., 132, l(1977). (15) A. Di Corcia, D. Fritz, and F. Bruner, Anal. Chern., 42, 1500 (1970). (16) A . Di Corcia, A. Liberti, and R. Samperi, Anal. Chem., 45, 1228 (1973). (17) F. Bruner, P. Ciccioli, G. Crescentini, and M. T. Pistolesi, Anal. Chern., 45, 1851 (1973). (18) A. Di Cwcia and A. Liberti, in "Advances in Chromatography", E. Grushka, Ed., Marcel Dekker, New York, N.Y., Voi. 14, 1976,p 305. (19) A. Di Corcia and R. Samperi, J . Chromafogr., 107, 99 (1975). (20) A. Di Corcia and R. Samperi, J . Chromatogr., 117, 199 (1976). (21) A. Di Corcia, A. Liberti, and R. Samperi, J. Chromatcgr., 122, 459 (1976). (22) A . Di Corcia and R. Samperi, Anal. Chem., 47, 1853 (1975). (23) J. C. Giddings, "Dynamics of chromatography", Part I , J. C. Giddings and R. A . Keiler, Ed., Marcel Dekker, New York. N.Y., 1965,p 45. (24)A. B. Littlewood, Proceedings of 5th International Symposium on Gas Chromatography, Brighton, September 1964,A. Goidup, Ed., Institute of Petroleum, London, 1965.
RECEIVED October 17, 19'77. Accepted January 23, 1978.
CORRESPONDENCE Reduction Currents of Films Formed during Reductions at the Hanging Mercury Drop Electrode Sir: There are several examples of stepwise reductions in which one or more reduction products are insoluble and reducible to a lower oxidation state. The shapes of voltammograms obtained under such conditions a t the hanging mercury drop electrode (HMDE) are quite different from those of polarograms a t the dropping mercury electrode (DME). A fairly detailed study has been made of the voltammetry of cobalt(II1) hexammine chloride, Co(NH,),Cl,, a t the HMDE. The surface of each mercury drop was 0.023 cm2. Unless stated otherwise, the scanning rate was 500 mV/s. Potentials refer to the saturated calomel electrode. The voltammograms 0003-2700/78/0350-1003$01 .OO/O
were run a t 2 1 "C. In ammoniacal buffers or in dilute acid solutions, the cobalt(II1) salt yields 2 waves, the first one with a peak potential a t about -0.35 V (Co(II1) to Co(II)), the second one of dissolved (Co(I1)to Co(0))at -1.23 V in ammoniacal buffers of pH 9.5 or less and in dilute acid, and of -1.36 V in a buffer of pH 10.4. Upon reduction of Co(II1) to Co(II), C O ( I I ) ( N H ~ ) ~is* + formed which is unstable and in neutral medium or in excess alkali hydroxide yields a precipitate of variable composition. This precipitate forms a film on the surface of the HMDE and is reduced, yielding a wave with a peak (see Figure 1). Under 0 1978 American Chemical Society
