Partition Chromatography by Electrodeposition in a Mercury Film W. J. BLAEDEL and JOHN H. STROHLI Chemistry Department, University of Wisconsin, Madison, Wis.
A study of a partition chromatographic process using a mercury film on platinum as the stationary phase was made. The retention times of TI and Pb samples were found to increase according to theory as the potential of the mercury film was made more cathodic. The retention time of TI was less sensitive to changes in potential than Pb, as would b e expected on the basis of the Nernst equation. The behavior of In and Sn deviated considerably from the theoretical under the experimental conditions used, probably because of low solubility or chemical interaction in the mercury film. The data showed that a chromatographic process can be made to occur, and that the retention time is critically and usefully dependent upon the applied potential,
A
COLUMN packing material
consisting of mercury on some support should have properties similar to the conventional partition chromatographic materials with respect t o those metals that are soluble in mercury. However, the transfer process between the mobile and stationary phases would be different since electron transfer is involved. Mercury would be the stationary liquid phase, the metal ions J i n + would be the solute in the mobile aqueous phase, and free metal Ill would be the solute, amalgamated in the stationary mercury phase. The distribution coefficient would be
where the bracketed terms indicate activities approximated by molar concentrations in this paper. I n the other types of partition chromatography, D is changed by varying the compositions of the stationary and mobile phases. With mercury the applied potential can also be used to vary D. Providing that the system is in equilibrium, D is related to the applied potential E through the Sernst equation. Present address, Chemistry Department, West Virginia 1-niversity, Morgan-
town, W. Ya.
64
ANALYTICAL CHEMISTRY
Log D = n (E" - E ) 0.059
(3)
D is highly sensitive to changes in E, each change of 0.059/n volts producing a 10-fold change in D. Equipment that would permit control of D through an applied potential might have considerable analytical use. This chromatographic method should apply to any metal that is soluble in mercury, providing that the flow rate is low enough to permit equilibrium conditions throughout the column. If equilibrium conditions are not achieved, there will be a diminution in the separation efficiency of a particular apparatus. The purpose of the work described in this paper is to find if the above electrochemical mechanism may be made the basis of a chromatographic process. Two criteria are used to indicate whether or not a chromatographic process occurs. Quantitatively the retention time of a substance is proportional to its distribution ratio, other variables held constant. I n this case, a change in potential of 0.059/n volt should cause a 10-fold change in D and the retention time. According to Equation 3, a plot of potential on a linear scale against retention time on a log scale should be linear with a slope of 0.059/n volt. There should be a difference in slope for two substances that involve different numbers of electrons. If such relationships can be found experimentally, they may be taken to indicate the occurrence of a chromatographic process through a n electrodeposition mechanism, which has not yet been reported in the chemical literature. Qualitatively, the metal band should migrate uniformly along the column during a chromatographic process. .kt any time in the process the relative position of a metal band in the column may be determined simply. If, a t any time during the migration, the applied potential is suddenly changed to a value sufficiently anodic to strip all of the metal into the aqueous phase, the resultant washout pattern will indicate the extent of migration up to the time of interruption. -1relatively short washout time would indicate a metal band
near the end of the column. The maximum washout time would correspond to a metal band a t the top of the column. diffuse washout band would indicate tailing. EXPERIMENTAL
Preparation of Mercury-Coated Platinum Packing. T h e platinum support material was prepared by cutting 0.015-inch diameter wire into short lengths (less t h a n 0.07 inch long). This material was cleaned and then plated b y precathodization and immersion in mercury ( 5 ) . Cleaning consisted of rinsing successively in alcoholic KOH, water, boiling 60% HC1O4, and water. The cleaned platinum was then placed in a platinum mesh basket in a beaker of 1M HC104, and a cathodic gassing potential applied for a few minutes, a platinum screen anode being used as a counter electrode. The basket was then lowered into a pool of mercury at the bottom of the electrolysis beaker, where the mercury coated most of the platinum. Cncoated particles were detectable by applying a cathodic potential to the basket sufficient to cause hydrogen gas evolution on platinum but not on mercury. Uncoated particles were located by a trail of bubbles, and were removed. To remove excess mercury, the coated material was placed in a centrifuge tube that had granular NaCl packed into the bottom. Centrifugation threw the excess mercury into the interstices of the qalt layer. Packings prepared this way contained 23 to 28 mg. of mercury per gram of platinum, in the form of a relatively thin and stable layer. Thicker films (up to 80 mg. of mercury per gram of platinum) were prepared by adding weighed amounts of mercury to packing prepared in the above manner. Such heavily coated packing was unstable when packed into a column, because mercury tended to drain toward the bottom. Thinner coatings were difficult to prepare uniformly. Column. T h e column proper was very similar in form t o one described previously (2). The mercury-coated packing was contained in a porous glass tube, 4-mm. i.d. and 7 em. long (Vycor 7930, Corning Glass Works, Corning, Tu'. Y.). The counter electrode (-0.03 volt against S. C. E.) consiqted of granular silver (G. F. Smith Chemical Co., Columbus, Ohio) and AgCl, packed into the annular space
Figure 1. Effect of plating procedure
initial sample
Eluent, 1 M HC101-0.05M NaCI, a t 0 . 2 6 ml./min. Sample, 0.1 ml. of 0.01M TIN08 Curve A. Sample injected a t zero time into column a t - 0 . 2 0 volt, sufficiently anodic to prevent retention of TI Curve 6 . Sample injected into column at - 0 . 6 0 volt, ond 10 min. allowed for plating. Column potential changed to - 0 . 2 0 volt a t zero time
between the porous glass tube and a concentric outer jacket, the electrolyte being 6d1 NaC1. The annular space also contained a silver wire reference electrode. Voltages were applied between the mercury-coated plat~inumand the working electrode, and were measured with a microvoltammeter (model 425X, Hewlett-Packard Co., I'alo Alto, Calif.) between the mercury-coated packing and the silver wire reference electrode. Solutions were pumped through the column via small diameter Tygon tubing with a peristaltic pump (model P.\ 6, S e w 13runswick S'cientific Co., New 13runnwick, S . J.). The column was vertically mounted, and solutions were passed from hottom to top, to facilitate removal of occasiona'l bubbles. Column Operation. T h e column was operated as a n integral part of a flowing system. Deaerated solutions were pumped through the column and the effluent was passed through a continuous polarographic cell ( I ) ; the dropping mercury electrode was held a t a potential such that the polarogra1)hic current was a measure of the concentration of the electroactive species in the column effluent. For all work in this paper, the dropping' mercury electrode was 0.70 volt, cathodic with respect to the Ag-AgC1 electrode in 6M S a C l . The sample was introduced by syringe injection through a short section of gum rubber tubing located between the pump and the column. This inlet system had considerable holdup, and produced spreading of the band before it reached the column. See Figure 1, curve A . To counteract this spreading, the sample was plated at3 a relatively sharp band ont,o the front' part of the column, by applying a relat'ively high cathodic potential for 10 minutes during and after sample injection. Figure 1: curve B , shows a typical band of a metal that was plated on by this procedure, and then eluted a t a potential sufficiently anodic to put the metal completely into the aqueous phase. The potentials a t which various sample metals were initially plated onto the column
were -0.60 volt for T1, Pb, and Sn, and -0.80 volt for In. ,2ft,er sample deposition was complete, the band was eluted by applying a new potential to the column, and the metal ion concentrat'ion in the effluent was observed through its polarographic current. When the new potential was sufficiently anodic, the material was stripped from the packing in a short time, and a sharp elution curve resulted, similar t'o that in Figure 1B. The curves in Figure 1 are smoothed reproductions of the polarographic currents measured in the flow-through polarographic cell. Sample Preparation. Samples used in this study consisted of 0.10 ml. of a 0.010,11 solution of the metal salt, unless otherwise noted. Lead samples were prepared from reagent grade nitrate, and thallium samples from C.P. grade nitrate. Indium solutions were prepared b y dissolving the metal in a small amount of HC1 and then diluting. Tin samples were prepared by dissolving reagent grade SnC12 in deaerated I.\[ HC104-0.05;2d XaC1. The SnC12solutions were prepared fresh daily, because Sn(I1) became oxidized to Sn(1V) on standing in air. Measurement of Retention Time. T h e experimentally observed retention time (measured as the time between application of the eluting potential and observance of the polarographically measured elution peak) had to be corrected b y subtracting the unretained peak time (measured as the time required for elution of the peak a t a highly anodic eluting potential). Because of this subtraction, short retention times were considered to be less accurate than long ones. .\verage deviations of retention times were of the order of 0.3 minutes. Measurement of Tailing Factors. Limited space prevents publication of all of t h e different kinds of elution curves t h a t were obtained. Instead, a rough b u t semiquantitative description of the peak asymmetry t h a t results from tailing is given b y the tailing factor, defined as the ratio of the tailing half-width to the leading half-width. The tailing factor is unity for a symmetrical curve, and becomes larger as the tailing increases. I t is an empirical quantity, used for descriptive purposes only, and has not bcen theoretically related t'o fundamental column or operating parameters. RESULTS A N D DISCUSSION
Thallium. Figure 2 shows the dependence of the retention time of T1 upon the column potential and eluent flow rate. Each point on Figure 2 represents a n elution experiment. I n accord with theory, the semilog plots of Figure 2 are reasonably good straight lines over a range ot retention times of 10-fold or more. Further, the slope.. of the lines are 54 a n d 56 mv., which agree with the theoretical value of 59 mv. for a 1-electron transition (shown dotted on Figure 2). The
-
. 1.' "
TI
Pb
Z
I
t
i
6 Figure 2. Dependence of retention time on column potential Eluent, 1M HCIOa-0.05M NaCI, a t 0.056 (low) or 0.26 (high) rnl./minute Sample, 0.1 ml. of 0.01M TlNOa or Pb(NO& Dotted lines indicate theoretical slopes for 1 a n d 2-electron transitions Unretained p e a k times (in minutes) a t high a n d low Aow rates, respectively, were: 3.3 and 1 4 . 7 (for TI) and 3.6 and 14.6 (far Pb)
-
tailing factors for the elution curves of Figure 2 are given in Table I. The tailing was greater a t longer retention times (more cathodic potentials) and at higher flow rates. The curves were fairly sharp and looked much like curve B in Figure 1. Thallium samples showed a definite and fairly uniform migration along the columns during elution. When a sample was plated and then eluted by changing to a very anodic potential, the peak appeared in 3.2 minutes at a flow rate of 0.26 ml. per minute, and in 14 minutes at 0.056 ml. per minute. Samples that were permitted to migrate along the column at 0.52 volt for 4 minutes a t 0.26 ml. per minute and 20 minutes a t 0.056 i d . per minute before stripping were washed out with retention times of 1.1 and 5 minutes, respectively. The tailing factors for these curves were 1.31 and 1.18, slightly higher than for peaks that were stripped from the front of the column. (These data were taken in a column somewhat larger than that used for the data of Figure 2.) Lead. T h e relationship between retention time and applied potential
Table 1.
Tailing Factors for Elution of Thallium (Conditions of Figure 2)
Tailing factor Potential,
0.056
volts
ml./min.
ml./min.
-0.48 -0.50 -0.52
1.07 1.14 1.43
1.25 1.64 2.04
0.26
~
VOL. 3 7 , N O . 1 , JANUARY 1 9 6 5
65
is shown in Figure 2 . When the less accurate points a t low retention times are ignored, the semilog plots are reasonably good straight lines, in accord with theory. The slopes of the lines are 36 and 38 mv. a t 0.26 and 0.056 nil. per minute, considerably larger than the theoretical slope of 30 mv. for a 2-electron transition (shown dotted on Figure 2 ) . Tailing factors for the elution of Pb, shown in Table 11, are similar to those for T1 a t the low flow rate, but significantly higher a t the high flow rate. Lead, like T1, also migrated along the packing during elution, but not so uniformly as TI. Under conditions permitting a small amount of migration before stripping, pretailing of the washout peak was observed. At 0.056 ml. per minute, an unretained peak showed a washout time of 14 minutes and a tailing factor of 1.14, while a sample that had migrated for 1 hour a t -0.42 volt showed a washout time of 9.5 minutes and a tailing factor of 0.74. It was postulated that the column was overloaded with respect to Pb, which could result in more rapid elution for a portion of the lead. Support for this postulate was obtained through experiments with smaller amounts of Pb. Under similar conditions, but with 0.1 ml. samples of 0.001M solution (0.1 pmole), there was no pretailing. Tin and Indium. The behavior of Sn and I n was far from ideal. With TI and P b , increased retention times (more cathodic potentials) caused
Table II. Tailing Factors for Elution of Lead (Conditions of Figure 2)
Potential, volt -0.30 -0.36 -0.38 -0.41
Tailing factor 0.056 0.26 ml./min. ml./min. 1.05 1.14 2.33 1.17 2.28 1.16 1.37 2.78
a drop in elution peak heights, as would be expected, since the sample was spread over a larger volume. With In, and to a smaller extent with Sn, as the potential was made more cathodic, the drop in peak height occurred without any appreciable increase in retention time. Figure 3 shows elution curves of In a t various potentials. Table I11 compares elution curve heights with retention times for various metals. The migration of Sn and In along the column was not of a chromatographic nature. .ifter most of the Sn or In had been eluted from the column, the remaining material appeared to be largely still a t the influent end of the column. This conclusion was supported by washout experiments. In a typical experiment with In, when elution was interrupted and a highly anodic potential applied to strip the remaining In, the washout time was 2.8 minutes, compared to 1.0 minute for TI under similar conditions. Qualitatively, the above behavior resembled stripping from a solid surface instead of an amalgam. The nonideal behavior of In was probably due to its limited solubility in mercury, reported as 0.007% (6). The I n samples (1 pmole) used in these experiments were large enough to saturate about 1.5 grams of mercury, or about five times the total amount of mercury in the column. The behavior of Sn could not be explained on the same basis, since its solubility (0.62oj,) was much larger than that of In, and close to that of Pb (1.3'%), which behaved ideally. Also, experiments with three fold smaller Sn samples showed the same behavior. On the other hand, there is evidence that mercury-coated platinum does not behave like mercury in regard to the stripping of some metals. Even though they strip readily from a hanging drop, Sn and Zn do not strip readily from mercury-coated platinum. The retention has been presumed to be due to intermetallic compound formation between the metal and platinum dissolved in the mercury (3).
Table 111. Comparison of Elution Peak Heights with Retention Times Conditions: Eluent, 1M HC104-0.05M NaCl (for T1, Pb, and Sn), or 1M NaCl (for In)! at 0.26 ml./minute Sample, 0.1 ml. of 0.01M solution, plated for 10 minutes at -0.60 volt (for T1, Pb, and Sn) or -0.80 volt (for In) Symbols: E, elution potential (volt), against Ag-AgC1 in 6M NaCl H , relative peak height, % R,a corrected retention time, minutes TI Pb Sn In E H R E H R E H R E H R -0.40 100 0 -0.30 100 0 -0.30 100 0 -0.50 100 0 -0.46 67 0 . 5 -0.36 93 0 . 4 -0.34 73 0 . 2 -0.54 55 0 . 4 -0.48 40 1 . 1 -0.38 49 1 . 8 -0.36 27 0 . 4 -0.56 36 0 . 4 16 5 . 3 -0.38 9 1 . 4 -0.60 13 0 . 7 -0.50 30 2 . 7 -0.40 -0.52 17 7 . 1 -0.40 0 . 5 4 . 0 -0.62 3 3.8 a I-nretained peak times were 3.3 minutes for T1, and 3.6 minutes for Pb, Sn, and In'
66
ANALYTICAL CHEMISTRY
R-"
33
2
4
6
TIME (MIN.1 Figure 3.
Elution curves for indium
Eluent, 1 M NaCI, a t 0.26 ml./minute Sample, 0.1 ml. of 0.01M InCla, plated onto column a t - 0.80 volt for 10 minutes Elution potentials listed on curves, applied at zero time
Number of Plates in the Column. An estimate of the number of theoretical plates ( p ) in the column was obtained through measurement of the retention time ( R ) and base width (W)for T1 and P b elution curves. Twice the half width of the peak was used as the base width. Use of the common relationship (4)
gave values of p ranging from 4 a t long retention times to 10 a t short times. Because of the finite lag and spread of an unretained peak, fairly large corrections had to be applied to the observed values of R and W . Because of these corrections, and because of tailing, the calculated values of p are not regarded as accurate. Another method of estimating the number of plates gave values around 4. This estimate was obtained by comparing the separation possible in an ideal countercurrent distribution apparatus with the hypothetical column separation based on two experimental elution curves of T1. CONCLUSIONS
This study, although of only relatively short retention times, indicates that a chromatographic process can be based upon transfer of a metal between a mercury or anialgani film and an aqueous solution. Bands of T I and Pb migrate uniformly along the column packing, and the experimentally observed peak retention time is dependent upon column potential in harmony with the Nernst equation. However, tailing of the elution curves indicates
that the behavior is less than ideal. At least part of the deviation from ideality is due to the inefficient column that was used, because of its short length, low capacit?:, and large-sized and nonuniform pa'cking. For some systems, nonideality may be due to limited solubility of the metal (In) or chemical interaction (Sn) in the mercury film. Hecause of its outstanding amenability to control by aelection of the elution potential., chromatography by electrodeposition is a promising method for analytical separations. However, until column preformance can be improved. and until the nonideal be-
havior of some metals can be circumvented, the full advantages of this new chromatographic method will not be realized. ACKNOWLEDGMENT
h stimulating discussion of the feasibility of the process with Professors L. B. Rogers and J. IT. Ross a t the Massachusetts Institute of Technology in .ipril 1959 is acknowledged. LITERATURE CITED
(1) Blaedel, W. J., Strohl, J. H., AKAL. CHEM.36, 445 (1964). ( 2 ) Ibid., p. 1245.
(3) Kemula, W., Kublik, Z., Galus, Z., A\7ature 184, 179.5 (1959). ( 4 ) Keulemans, A . I. lI., "Gas Chro-
mat,ography," 2nd ed., Reinhold, Kew
L., Brubaker, R . I,., Enke, C. G., AXAL.CHEM.35, 1088 (tO63j. ( 6 ) Sidgewick, X.I-,,',Chyniical E1ement)s and Their Compounds, Oxford I-niv. Press, S e w York, 1950.
RECEIVEDfor review hugu-it 5, 1964. Arcepted October 14, 1964. This work was support,ed by (:rant So. AT( 11-1 )10232,froni the I'. 8. Atomic Energy Cornmission. Taken in part from a thesis submitted by John H. Strohl in partial fulfillment of the requirements for the Ph.1). degree at the Universit,>- uf Wisconsin, January 1964.
Simultaneous Determination of Oxygen and Nitrogen in Refractory Metals by the Direct Current Ca r bo n-Arc, Gas Chro ma to g ra phic Te c hniq ue ROYCE K. WINGE and VELMER A. FASSEL lnstitute for Afomic Research and Department o f Chemistry, lowa State University, Ames, lowa
b The d.c. carbon-arc, gas chromatographic technique has been applied to the simultcrneous determination of the oxygen and nitrogen content of the refractory metals. With the aid of a platinum flux, the oxygen and nitrogen are liberated from the metal as carbon monoxide and molecular nitrogen into a static helium atmosphere. An aliquot of the resulting gas mixture i s then passed through a gas chromatograph and the respective peak heights are related to the oxygen and nitrogen contents of the metal. Analytical data olbtained from 1 2 different base metals suggest that single oxygen and nitrogen analytical curves can b e used for determining these impurities in most refractory metals and alloys.
T
of oxygen and nitrogen commonly occur in the refractory metals ai; solid solutions in the interstices of the metal lattice or as inclusions of the oxides or nitrides of one or more of the m.etals present in the specimen. Molecular gases may be entrapped in voids in some metals, but this type of occurrence is not likely in the refractory metals (8). When oxygen and nitrogen are present as interstitial impurities, they usually exert a profound effect on the physical and mechanical properties of the host metal. ;\s a ronsequence, there is an el-er-increasing interest in analytical methods for the determination of these RACE IMPURITII:S
impurities in metals and alloys. Sitrogen is commonly determined by variants of the classical Kjeldahl procedure (15) with acceptable accuracy and precision. However, as more corrosion-resistant alloys are being developed. complete dissolution of the sample by the acid solvents is becoming increasingly difficult. Uncertainties also exist as to whether all of the combined nitrogen is quantitatively converted to ammonium salts for all of the alloy systems encountered. I n view of these considerations it is desirable to explore other techniques for performing the nitrogen determinations. Various modifications of the vacuumfusion technique (16, 17) have been applied successfully to the determination of the total combined oxygen content of many metals. I n principle, nitrogen can be determined simultaneously, but the analytical data so obtained have been repeatedly questioned (1, 13, 1 4 ) . The various factors which appear to contribute to the frequently observed low vacuumfusion nitrogen results have been discussed by several investigators (1, b j 9). I n 1957, Booth, Bryant, and Parker ( I ) concluded "that the vacuum-fusion technique, as presently operated, does not furnish very reliable nitrogen figures for those elements forming very stable nitrides." On the other hand, there have been scattered reports that vacuum or inert gas fusion nitrogen results on several refractory metals were in accord with
the Kjeldahl values. I n all instances, these determinations involved a platinum-bath environment a t teniperatures equal to or exceeding 1900" C. Thus, successful comparisons were found for Zircalloy 2 a t 1900" C. (8); for zirconium a t 1900" C. ( d ) ,and 1950" C. (10); and for niobium a t 1900°C. (11). One of the distinct advantages of the d.c. carbon-arc extraction technique is that the molten globule attains local temperatures in the 3000" C. range. This feature plus the precipitous temperature gradient in the molten globule appears to contribute to the rapid extraction of the oxygen and nitrogen content of many metals. The success achieved in simultaneously determining the oxygen and nitrogen content in low and high alloy steels by an arc extraction, gas chromatographic technique suggested its further application to the determination of these impurities in the refractory metals. EXPERIMENTAL
Apparatus and Procedure. The experimental facilities and their mode of operation have been described (3). The procedure for preparing the samples followed the practices outlined previously ('7). The degassing procedure used for this study was as follows. .liter loading the de3ired number of supporting electrodes into the chamber, they and the inner surfaces of the chamber are degassed by arcing each electrode for 30 seconds at 25 ampere:; in a helium supporting atmosphere at 250 torr. All of the electrodes are deVOL. 37, NO. l , JANUARY 1965
* 67
