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
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7 , JUNE 1978
3rl
l
-
i /PA
I-
-.
I ~
i 01
G.5
10
15
2 .o
- E / V VSSCE
Figure 1.
Effect of NaOH. Solutions like in Figure 1 with different concentrations of NaOH. Voltammograms run from 0 V. Normality of NaOH, curve 1, 0; curve 2, 0.002; curve 3, 0.01: curve 4, 0.04; curve 5, 0.1; curve 6, 0.2 N
conditions of Figure 1,the peak potential changed from -1.47 to -1.58 V with increasing peak current. Using 0.1 M potassium perchlorate as supporting electrolyte, the peak current increased with time of standing a t potentials between -0.4 V (formation of precipitate) and -1.25 V. After 1 min a t -0.6 V, under conditions as in Figure 1, a maximum value was attained which remained constant even after keeping the electrode for 10 rnin a t -0.6 V. I t will be noted in Figure 1 that the reduction current of Co(I1) at about -1.25 V decreases with time of standing a t -0.4 V while the peak current increases. The reduction current a t ca. -1.25 V is that of the soluble Co(II), whereas the peak current increases with the thickness of the film deposited on the electrode. Using 0.1 M potassium chloride instead of perchlorate as supporting electrode, the voltammograms were almost identical with those in Figure 1 when the electrode was kept no longer than 2 rnin a t 4 . 6 V. Upon further standing a t -0.6 V, the wave had a peak potential which varied with time between -1.74 and -1.82 V. Apparently in the chloride solution the film was composed of some basic cobalt(I1) chlorides. When the solution used in Figure 1 was also made 0.01 M in tetraethylammonium perchlorate, which is strongly capillary active on the water-mercury interface, the increase of the peak currents was slower than in Figure 1, e.g., 43, 66, and 77 pA after 10, 30, and 60 s, respectively. The effect of sodium hydroxide on the voltammograms is presented in Figure 2. Note that the peak potential of the Co(II1) to Co(I1) reduction is -0.32 V and unaffected by the hydroxide ion concentration. The time of scanning between this potential at which film formation starts and the reduction potential of the film was 2.2 s. The peak potential of the film reduction wave shifted from -1.48 V in 0.003 M hydroxide to -1.60 V in 0.04 M hydroxide. With increasing hydroxide concentration, two film waves were observed, the first one a t -1.49 V with peak (curve 5 ) or limiting value (curve 6) and a peak current at -1.62 V (curves 5 and 6). The peak currents decreased continuously when the hydroxyl ion concentration became greater than 0.01 M. This decrease is attributed to formation of soluble cobaltate at -0.32 V. Apparently the peak current in 0.1 M hydroxide a t -1.49 V (Figure 2) and the limiting current a t -1.5 V in 0.2 M hydroxide are due to reduction of dissolved cobaltate, while the currents with peaks a t -1.62 or -1.64 V are due to reduction of the film. As is to be expected, the film current decreases when perchloric or another strong acid is added in small concentration to the solution in Figure 1,and no hydrogen ion reduction current is observed until the concentration of acid is about 2.5 x M or greater. A more quantitative study of the proton
consumption for the neutralization of the ammonia liberated in the reduction of the cobalt(II1) salt was made at the DME. There is no previous history at this electrode, as only a small potential range is covered during the formation of each drop, the scanning rate of our polarograph being 16 mV/s. The cobalt(II1) which diffuses to the surface of the DME is reduced in one step to cobalt(0) at potentials more negative than -1.1 V. Knowing the diffusion coefficient of the cobalt(II1) and hydrated protons, the amount of strong acid required to neutralize the liberated ammonia agreed with the calculated amount. Under specified conditions the “film current” is proportional to cobalt(II1) concentration up to a certain value. Varying the cobalt(II1) concentration in a solution of the composition as in Figure 1, and keeping the electrode for 1 rnin a t -0.6 V, the film current was proportional to concentration in the range between 5 X and 2 X M Co(II1). This range varied with the time of keeping the HMDE a t -0.6 V (or any potential between -0.4 and -1.2 V). Similar experiments in potassium chloride instead of perchlorate as supporting electrolyte yielded similar results. As expected, stirring the solution a t -0.6 V increased the rate of film formation. The film formed on the HMDE dissolved very slowly when the electrode was kept in 100 mL of cobalt-free 0.1 M potassium perchlorate solution. For example, when the HMDE after standing for 1 rnin a t -0.6 V (curve 4 in Figure 1) was placed at 21 “C in 0.1 M potassium perchlorate solution (in the absence of Co(II1)) the film currents decreased from 107 pA to 90, 50, and 38 FA, respectively, after 1, 5 , and 10 min. The effect of scan rate on the film current is very large when voltammograms are run on drops with the same film coverage. For example, when the HMDE was exposed for 5 rnin at 4 . 6 V in a solution of the composition as in Figure 1, the peak film currents were close to proportional to scan rate. At scan rates of 500,200,100,50, and 20 mV/s the peak film currents were 100, 55, 29, 12, and 8 pA, respectively. A t all these scan rates the films became completely reduced; hence the number of coulombs required for this reduction should be independent of scan rate. This was found to be the case, the number of microcoulombs required was 39 f 0.1. Of course, the above is no longer true when the extent of film coverage is different at different scan rates. For example, if the HMDE was placed in a solution like in Figure 1 and the scanning was started from 0 V, coverage with film took place during scanning between -0.3 and -1.5 V or for 2.2 s at a scan rate of 500 mV/s and for 55 s a t 20 mV/s. Under these latter conditions the film currents were 36,32, 23,15, and 9 pA and the number of pC 12, 19, 29, 36, and 39, respectively, a t scan rates of 500, 200, 100, 50, and 20 mV/s.
Figure 2.
-E/V
VS.
SCE
Voltammogram in 0.1 M solution of potassium perchlorate, 5X M in Co(II1). Aging time at -0.6 V: curve 1, 0.5 s; curve 2 , 5 s; curve 3, 10 s; curve 4, 60 s
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
Several other examples can be added involving soluble oxidants which are reduced to an insoluble oxidation form which becomes attached to the HMDE and yields a film current in a certain potential range. The HMDE was kept in solutions of 0.1 M potassium perchlorate and chromic acid of concentrations between M for 1 or 5 min and 2 X a t -0.6 V. Upon cathodic scanning, large film currents with peak potentials between -1.78 and -1.86 V were observed. These currents were proportional to concentration in the range between and M chromic acid. A solution 2 x M in potassium chromate and 0.1 M in perchlorate was kept for 1min a t potentials between 0 and -0.8 V; upon cathodic scanning they yielded the same film current with peak potential a t -1.86 V, as was observed in the above solutions of chromic acid. The HMDE was placed for 1 to 5 min a t 0 V in solutions 0.1 M in potassium perchlorate and to 2 x M in permanganate. Large film currents with peak
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potentials varying between -1.75 and -1.9 V were observed. From the observations presented in this paper, it would appear that the HMDE can provide interesting information in studies of complex redox reactions in which an insoluble reducible or oxidizable product is formed which yields a film on the surface of the hanging but not of the dropping mercury electrode.
I. M. Kolthoff* Sorin Kihara Department of Chemistry University of Minnesota Minneapolis, Minnesota 55455 RECEIVED for review October 24, 1977. Accepted March 3, 1978. This work was supported by Public Health Service Grant CA16466-03 from the National Cancer Institute.
Comments on Unrecognized Systematic Errors in Quantitative Analysis in Gas Chromatography Sir: A recent paper ( I ) in this journal attempts to show the dangers of using the “internal standard” method for quantitative gas chromatography unless volumes of solution injected and component concentrations are the same as those used for calibration. The authors correctly state that the method of internal standards assumes that the area ratio of analyte/internal standard ( R A ) is independent of the volume injected. They have presented an abundance of data to show that a change in the volume injected or a change in both analyte and internal standard concentrations (at a constant weight ratio), produces a dramatic change in the observed R A . Such unrecognized systematic errors would indeed lead to very poor quantitative gas chromatography. We have conducted several experiments designed to test the validity of these authors’ conclusions as related to hydrocarbons (Figure 9) ( I ) . I t is our belief that the reported large changes in R A could easily be demonstrated by relatively few tests. A Hewlett-Packard 5711 A (FID) was equipped with a 6 f t X l/s-in. column packed with 10% OV-101 on 100/120 mesh Chromosorb W AW-DMCS and operated a t 130 “C. Injector and detector temperatures were 250 OC. Gas flows were: H2, 30 cm3/min; N2, 25 cm3/min; and air, 240 cm3/min. Area measurements were made with a Perkin-Elmer Model 1 computing integrator. The electrometer was kept on the 100 range for all tests. A solution containing 2.16 x g/cm3 n-nonane and 3.41 x g/cm3 n-decane in chloroform was used to determine the effect of the volume injected upon the C9/Cloarea ratio ( R A ) . All injections were made with a Hamilton 701 N syringe. Data below show no significant variation in R A as a function of a tenfold variation in injection volume. Injection volume, p L R A found 0.5 1.o
2.0 3.0
5.0
0.642 0.636 0.639 0.643 0.642 Mean R A = 0.640 s = 0.003 0003-2700/78/0350-1005$01 .OO/O
For the following tests, solutions were prepared by diluting n-C9 and n-Clo stock solutions (10 g hydrocarbon/100 mL CHC13)to the concentrations shown below. Injection volume was 1.0 pL in all cases. (C, = G o ) Concentrations, gIcm3 4 x 10-3 8x 2 x 10-2 4 x 10-2
R A found 1.003 0.997
1.015 1.013
Mean RA = 1.01
s = 0.008
These results show that a tenfold variation in concentration had no significant effect on R A , Sir: We We would like to make a few additional comments in response to Shatkay’s reply to our previous correspondence. In his letter to us, he replied that with the all-hydrocarbon system the “volume effect” should be so negligible as to easily escape detection. However, in his paper ( I ) he stated that while the phenomenon illustrated by Figure 9 (Le., the dependence of R A on volume injected) “is much less noticeable than for the pair trimethyl phosphate-undecane (cf., Figure 2), it is still quite significant.” The data we presented in our first correspondence were directed at the above conclusion. We did not find a significant variation in R A as a result of changing the injection volume. LITERATURE C I T E D (1) A. Shatkay and S. Flavian. Anal. Chem, 49, 2222 (1977).
G . C. Royston Research Department Copolymer Rubber and Chemical Corporation Baton Rouge, Louisiana 70821 RECEIVED for review January 16, 1978. Accepted March 10, 1978. 0 1978 American Chemical Society
