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ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
1.0 N HzS04
Urine sample (a)
Trapping solution (0.3 N H z S 0 4 ) (b)
Figure 1. Schematic diagram of the aeration bain. The syring attached to the sample tube contains saturated K2C03,which is to be injected into the sample
Table 11. Comparison of Titration Technique to Osmometric Technique osmometric titration sample technique, technique, no.a mmol/L mmol/L 1 57.6 58.5 2 29.5 27.9 3 36.8 35.9 4 8.0 6.0 5 23.7 23.9 6 35.5 33.0 a No. 1 is an ammonia standard at 57.5 mmol/L. Nos. 2 to 6 are samdes of urine. extracted from urine and trapped in HzS04in significant quantities. DISCUSSION NH3 formed by the alkalinization of the trapping solution is not readily released into the environment, presumably because of its extraordinarily high solubility coefficient; 10 to 15 min of vigorous aeration are required to drive NH3 from alkalinized urine. The agreement between the osmometric method and the titration method (Table 11) proves that NH3 loss prior to the measurement of osmolality is negligible. The concentration of HzS04and NaOH need not be exact, provided that NaOH is present in excess when the two solutions are mixed; NH4+is completely converted to NH3 when the solution pH is 12 or greater. The quantity of ammonia
that can be trapped is limited by the concentration of H2S04; for a solution of 0.3 N H2S04the maximal concentration of NH3 in the solution will be 300 mmol/L. For urine containing higher concentrations of NH3, smaller volumes of urine or larger volumes of trapping solution can be used. It is also possible to use trapping solutions with a greater acid concentration and proportionally greater concentrations of NaOH, provided that the final osmolality of the mixture of the trapping solution and NaOH is within the range of the osmometer. If the concentration of NH3 in the urine is very low, the use of a larger volume of urine will enhance the accuracy of the technique. Vigorous aeration is very important in ensuring complete recovery of NH3 within a given time period; incomplete recovery of NH3 results from insufficient release of NH3 from the alkalinized sample rather than from incomplete trapping in the HzS04 solution. However, overly vigorous aeration may cause the H2S04solution to coat the sides of the tube, and a spuriously high NH3 concentration may be obtained. This difficulty may be obviated by the use of tubes of larger diameter for trapping and also by inversion of the tube at the end of trapping to allow complete mixing of the HzS04solution. By using test tubes of 2.3-cm diameter, complete release and trapping of NH3 was possible in 10-15 min of aeration, whereas 20-30 min of aeration were needed with tubes of 1.5-cm diameter. Cunarro et al. concluded that the ammonia electrode provides the most accurate and rapid determination of urinary ammonia concentration (6). However, potential disadvantages of the ammonia electrode include its expense and short life span (average, 3 months). These disadvantages make the electrode method primarily suitable for laboratories doing large numbers of determinations. If an osmometer is available, ordinary laboratory supplies can be utilized to complete the aeration for the method described herein. The method is technically simple and rapid. The ease with which ammonia can be extracted and trapped and the precision of osmometry provide for accurate results. LITERATURE C I T E D (1) Oh, Man S.;Whang, Edmund M. S.;Carroll, Hugh J. Kidney Int. 1975, 8, 56-59. (2) Sobel, Albert E.; Hirschman, Albert; Besman, Lottie. Ind. Eng. Chem. 1947, 49, 927-929. (3) Conway, E. J.; O'Malley, E. Biochem. J . 1942, 36, 655-661. (4) Jorgensen, K. Scan. J . Ciin. Lab. Invest. 1957, 9 ,287-291. (5) Wilcox, Alan A,; Carroll, Wallace E.; Sterling, Rex E. Clin. Chem. 1986, 12, 151-157. (6) Cunarro, Julia A.; Weiner, Michael W. Kidney Int. 1974, 5 , 303-305.
RECEIVED for review July 9, 1979. Accepted September 4, 1979.
Ligand Exchange Chromatography of Alkyl Phenyl Sulfides Vaclav Horak," Mercedes D e Valle Gurman, and George Weeks Department of Chemistry, Georgetown University, Washington, D.C. 20057
Chromatographic behavior of eight alkyl phenyl sulfides (RS-Ph; R = Me, Et, Pr, /-PI, Bu, sec-Bu, i-Bu, and I-Bu) was examined using Hg2+,Ag+, Cd2+, and Pb2+ impregnated silica gel plates. Very efficient separation was achieved with Hg2+ and Ag' ions in solvents of medium polarRy (chloroform, ethyl acetate). The contribution of polar ((TI), molecular weight (MW), and steric effects E s to R, values was determined. For Hg2+ and Ag' impregnated plates, respectively, the foilowing equations were calculated: 0003-2700/79/035 1-2248$01 0010
+ 0.077 Es + 0.679 R M ( A g + ) = -2.397~*- 0.0077 MW + 0.156 Es + 1.152
RM(Hg*+) =
-1.933~*- 0.0061 MW
Recently, numerous selective separations have been achieved with compounds carrying either K- or n-electrons using ligand exchange chromatography. Chromatographic separations of organic sulfides on stationary phases with anchored Hgn (1,2),Znn (3), and Cun ( 4 ) have been reported in the literature. For some of the chromatographic systems 0 1979 American Chemical Society
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-
.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
preservative, meets ACS specifications,J. T. Baker Chemical Co.); ethyl acetate (certified ACS, Fisher Scientific Co.); hexane (meets ACS specifications, J. T. Baker Chemical Co.); and ferric ammonium sulfate 12-hydrate (99-100%, J. T. Baker Chemical Co.). Synthesis of Alkyl Phenyl Sulfides. Alkyl phenyl sulfides were prepared according to the general method from thiophenol and alkylhalide in 95% ethanol in the presence of potassium hydroxide (9),with the exception of tert-butyl phenyl sulfide which was prepared from thiophenol and isobutylene in 75% sulfuric acid (9). For final purification all sulfides were distilled under reduced pressure (50 mm Hg) using a short distillation column. Physical constants of phenyl sulfides are found in Table I. Thin-Layer Chromatography. Preparation of Plates. The Stahl's applicator "Model S 11" was used for the preparation of 0.2-mm thin layers on standard glass plates (20 x 20 cm). A slurry was made by thoroughly mixing 30 g of silica gel 7GF (Baker TLC reagent) and 80 mL of the aqueous solution of the particular metal salt in a 250-mL Erlenmeyer flask (shaken strongly for about 40 s ) . The following solutions of salts were used for preparing TLC plates: (i) 25% mercury acetate, (ii) 5% silver nitrate, (iii) 25% lead acetate, and (iv) 5% and 25% cadmium acetate, respectively. For the preparation of control silica gel plates, the slurry was prepared with 80 mL of water. The plates were air dried for about 1 h and activated plates were stored for a short period of time in a well-closed TLC carrying case under anhydrous CaS04. However, the plates were reactivated at 100-110 "C for 1 h if they were not used for several days. Spotting of the Samples. The samples were spotted as 0.5% solution in chloroform (about 50 pL/lO mL of chloroform) by means of a 2-pL microcap (Drummond Scientific Co.). The starting line was drawn about 2.5 cm from the bottom. A distance of 1.5 cm between each spot was maintained by using a Spotting Guide (Arthur H. Thomas Co.) leaving a 2-cm margin on each side. The spots were about 2 mm in diameter. Once spotted, the plates were left to air-dry for 5 min. Developing Procedure. Plates were developed by the ascending technique at room temperature (23 f 2 "C) to a distance of 15-18 cm. The chromatographic chamber was lined with filter paper and allowed to equilibrate for a period of at least 1 h prior to use. The developed plates were air dried for 5 min before being sprayed for detection. In one experiment a plate was developed at 7 "C. For this purpose the chamber was placed in a refrigerator. Detection. For detection the plates were sprayed with potassium permanganate reagent (10) (1:l solution of 0.1 M KMnO, and 2 M CH,COOH). The sulfides visualized as yellow spots on dark violet background were marked immediately. The limit of detection is approximately 1 pg. UV light was used for detection on cadmium-impregnated plates. R, values are reported in Tables I1 to VII. Determination of t h e Metal Content. The metal content of impregnated plate which was already used in the experiment was determined using standard analytical procedures after scraping off the silica gel (0.1-0.3 g) and extracting the metal with acidic solution (dil. "0,). Mercury and silver were titrated with ammonium thiocyanate solution using Fe(II1) as the indicator (11). Cadmium and lead were titrated with EDTA using Eriochrome Black T as the indicator (12). The determined metal contents are given in each Table of R, values.
Table I. Physical Constants of Alkyl Phenyl Sulfides R-S-Ph, R = (1)methyla (2) ethyl (3) n-ProPYl (4) isopropyl (5) n-butyl (6) tert-butyl (7) sec-butyl (8) isobutyl
bp/50 mm, " C
n~~~
-
1.5852 1.5660 1.5563 1.5488 1.5472 1.5293 1.5440 1.5483
100
120 108-114 155 117 136-137 141
2249
Methyl phenyl sulfide (thioanisole) was supplied by Aldrich Chemical Co. (99% Duritv and bD 188 C). the retention of sulfides was so strong that sulfides were recovered ( 2 , 4 )by backwashing. Earlier, one of us reported on studies of nonchromatographic partition of sulfides between a saturated mercuric acetate solution in aqueous acetic acid and n-heptane (5). Whereas all of the reported methods offered some practical applications, the last reference included structure vs. partition coefficient relationship study as well. The study reported in this paper is a continuation of our investigations of properties and separation of organic sulfides such as the study mentioned above and similar investigations (6, 7) (reviewed in (8)). The present ligand exchange chromatographic study involves eight alkyl phenyl sulfides serving as electron donors (Ph-S-R: 1, R = Me; 2, R = Et; 3, R = Pr; 4,R = i-Pr; 5 , R = Bu; 6, R = t-Bu; 7, R = sec-Bu; 8, R = i-Bu) and four cations Hg", Ag', Cd", and Pb" serving as electron acceptors. The selected ions represent both strong and weak acceptors with respect to the sulfide ligand allowing searching for selective chromatographic system. The sulfide series allows for examination of structural effects controlling the ligand strength and, thus, the chromatographic process as well.
EXPERIMENTAL Apparatus. Refractive indexes were determined with a Bausch & Lomb Abbe type refractometer and the values were corrected to 20 "C (O.O0045/"C). Ultraviolet spectra were recorded with a Cary 15 spectrophotometer using hexane as the solvent. Chemicals. The following chemicals were used: thiophenol (97%, Aldrich Chemical Co.); bromoethane (98%, Aldrich Chemical Co.); 1-bromopropane (Matheson, Coleman and Bell); 2-bromopropane (Aldrich Chemical Co.); 1-bromobutane (9770, Aldrich Chemical Co.); 2-bromobutane (98%, Aldrich Chemical Co.); 1-bromo-2-methylpropane (Aldrich Chemical Co.); isobutylene (certified purity, Matheson Gas Products); silica gel 7GF (Baker TLC reagent, J. T. Baker Chemical Co.); mercury acetate (98.8%, Fisher Scientific Co.); cadmium acetate dihydrate (Baker analyzed reagent, J. T. Baker Chemical Co.); silver nitrate (99.970, J. T. Baker Chemical Co.); lead acetate trihydrate (100.0%, Fisher Scientific Co.); disodium ethylenedinitrilotetracetate (EDTA, 100%, J. T. Baker Chemical Co.); Eriochrome Black T (indicator grade, Aldrich Chemical Co.); chloroform (0.75% ethanol as
Table 11. R , Values on Mercury Acetate-Impregnated Layersu and Chloroformb as Mobile Phase Rf
R-S-Ph, R
=
(1)methyl
(2) ethyl ( 3 ) n-ProPYl (4) isopropyl (5) n-butyl ( 6 ) tert-butyl (7) sec-butyl (8) isobutyl (9) benzyl sulfide Mercury content 20%. ed spots.
a
b
C
e 0.480 0.585 0.557 0.560 0.529 0.400 0.511 0.494 0.491 0.454 0.443 0.565 0.546 0.520 0.517 0.348 0.460 0.554 0.474 0.420 0.486 0.582 0.552 0.586 0.546 0.300 0.395 0.420 0.454 0.385 0.414 0.511 0.506 0.537 0.506 0.491 0.602 0.575 0.600 0.575 0.300 0.306 0.286 Commercial chloroform containing 0.75% ethanol was used. d
f
Rf
0.494 0.535 i 0.041 0.426 0.463 i 0.044 0.471 0.501 ir 0.060 0.414 0.445 I0.069 0.520 0.545 i 0.038 0.388 0.390 3 0.051 0.474 0.491 * 0.043 0.540 0.562 i- 0.047 0.274 0.292 * 0.014 Pure chloroform yielded elongat-
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
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content of cadmium has no effect and large content decreases the activity of the stationary phase indicating no interaction of the sulfides with this metal. However, this ratio differs from sulfide to sulfide. The strong dependence of the equilibrium constant on the polarity of the solvent is evident from the comparison of R, = 0.0 value for butyl phenyl sulfide in the Hg2+/hexanesystem and R, = 0.545 in the Hg2+/chloroform system. The range of R, values which reflects the susceptibility of the chromatographic medium to structural differences of the sulfide molecules varies quite markedly for systems under examination. Whereas silica gel as well as Pb2+and Cd2+ions are unable to differentiate among individual molecules I-VIII, Hg2+and, particularly, Ag+ ions show remarkable discriminating power and, thus, suggest practical applications. The effect of temperature on chromatographic data involving the Hg2+-chloroform system was tested in correlation ) (calculated from R, a t 7 OC) and RM(2p)data of R M ( , ~data (calculated from R, a t 26 OC). The plot is a straight line (correlation coefficient 0.883 for n = 8, significant at 1% probability level) with parameters: intercept = 0.238, slope = 0.707. From this resulting ARM(^) = R M ~ c ) / R M ~ =+ ~ 0.0372. In order to characterize the ligand exchange chromatographic processes, the metal-sulfide interactions were examined quantitatively in a nonchromatographic experiment. For this purpose four different concentrations of butyl phenyl sulfide were distributed between Hg2+-impregnatedsilica gel (same quantities, scraped from a TLC plate) and hexane, and the concentration of the free sulfide was determined by UV. The linear l~g[sulfide]~,vs. 10g[SUlfide]a&,,bplot suggests a Freundlich isotherm controlled adsorption process. The effect of the molecular structure on the chromatographic mobility of sulfide I-VI11 was examined in some detail for data obtained in both the Hgn-chloroform system and the Ag+-ethyl acetate system. For this purpose, averaged Rs, and l?M(mm) data were calculated from the experimental R, values using the following relationships:
Table 111. R f Values on Mercury Acetate-Impregnated Layers and Chloroforma as Mobile Phase at Two Different Temperatures 7 "C
a
R-S-Ph,R=
Rf
(1)methyl ( 2 ) ethyl ( 3 ) n-propyl ( 4 ) isopropyl ( 5 ) n-butyl ( 6 ) tert-butyl ( 7 ) sec-butyl ( 8 ) isobutyl
0.583 0.576 0.523 0.400 0.606 0.363 0.486 0.636
26 " C
RM -0.146 -0.133 -0.040 0.176 -0.187 0.244 0.024 -0.242
Rf 0.451 0.389 0.451 0.323 0.411 0.251 0.343 0.450
RM 0.085 0.196 0.085 0.321 0.156 0.475 0.282 0.087
See Table 11,
Determination of Adsorption Isotherm ( 1 3 ) for n-Butyl Phenyl Sulfide in Mercury Acetate Impregnated Silica Gel. Five milliliters of a solution of n-butyl phenyl sulfide in hexane (concentration S) and 0.2 g of exactly weighed adsorbent containing 20% Hg were added to a stoppered vial. The vial was shaken for 10 min to establish equilibrium and centrifuged for 5 min to separate the phases. In the top layer the concentration of organic sulfides was determined from reading of the A, in the range 250-260 mm and using a calibration graph. Table VI11 shows concentration of the free sulfide SFand calculated amount of the adsorbed sulfide SA (per 1 mL of the solution) for four original concentrations of n-butyl phenyl sulfide (S). Plot of log SFvs. log SAis a straight line with a correlation coefficient r = 0.994.
DISCUSSION In order to obtain maximum reproducibility, the plates were prepared from silica gel and solution of the respective metal salt using the same quantities, concentrations, and procedures. Fluctuation in R f values resulted primarily from fluctuations in temperature since the plates were developed without temperature control. Only in an experiment in which temperature dependence of R, values was examined (Table 111) the chromatographic chamber was kept at a constant temperature. However, by using benzyl sulfide as a standard using Rs or Rf(corr) (for definition see Table IX) such fluctuation is substantially reduced; e.g., in the Hg2+-chloroform system, the standard deviation of R, values is about f1070, for Rf(mrr) about f2.5%. However, this method does not increase the accuracy for the Ag+-ethyl acetate sufficiently for meaningful correlation. Here, the use of Dean and Dixon's method (14) increases the accuracy to about 2.5%. The strength of the respective metal to complex the sulfide ligand is measured by the increased sulfide retention compared to the retention on untreated (plain) silica gel. This effect gives for butyl phenyl sulfide the expressed as Rf(med)/Rf(pk) following values: AgN03(EtAOc) 0.51, Hg(OA~)~(chloroform) ~%, 0.85, Pb(OAc)2(chloroform)0.9, C ~ ( O A C ) ~ (chloroform) 0.99, Cd(OA&(25%, chloroform) 1.08. Whereas the fist three metals increase the retention of the butyl phenyl sulfide, low
,
The "correction" of R, values (Rf(corr)) makes it possible to substantially reduce effects of those factors (e.g., temperature, plate activity, etc.) which are responsible for R, differences observed in different experiments. The benzylsulfide standard is chosen arbitrarily (in Table I1 represented data was R, value from column c).
Table IV. R f Values on Silver Nitrate-Impregnated Layersa and Ethyl Acetate as Mobile Phase Rf
R-S-Ph, R (1)methyl ( 2 ) ethyl
=
( 3 ) n-ProPYl ( 4 ) isopropyl ( 5 ) n-butyl ( 6 ) tert-butyl ( 7 ) sec-butyl (8) isobutyl ( 9 ) benzyl sulfide a
Silver content 2.4%.
a
b
0.395 0.260b 0.395 0.310 0.407b 0.319 0.344b 0.500 0.243
0.392 0.313 0.392 0.326 O.36gb 0.307 0.375 0.466 0.250
C
0.438b 0.301 0.411 0.384b 0.445 0.390b 0.45gb 0.541b 0.308
d
e
0.407 0.311 0.412 0.316 0.430 0.316 0.373 0.480 0.288
0.41 2 0.333b 0.440b 0.322 0.443 0.327 0.390 0.503 0.282
Zf 0.409 t 0.304 i 0 . 4 1 0 _c 0.332 r 0.419 k 0.332 t 0.388 i 0.498 5 0.274 5
0.018
0.027 0.019 0.030 0.032 0.033 0.043 0.028 0.027
Values which were rejected (using method 1 7 ) in calculation. Rf' values i n Table XI.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
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Table V. R f Values on Cadmium Acetate-Impregnated Layersa and Chloroformb as Mobile Phase layers prepared with solution containing 5% Cd( AcO), 25% Cd( AcO),a R-S-Ph, R = (1)methyl (2) ethyl (3) n-propyl (4) isopropyl (5) n-butyl (6) tert-butyl (7) sec-butyl ( 8 ) isobutyl (9) benzyl
a
b
0.669 0.693 0.705 0.687 0.687 0.687 0.693 0.687 0.687
0.682 0.680 0.697 0.691 0.691 0.691 0.691 0.691 0.691
C
0.716 0.727 0.739 0.722 0.753 0.730 0.737 0.7 37 0.767
d 0.710 0.710 0.719 0.727 0.756 0.744 0.744 0.744 0.764
-0.100.
\
sulfide
mxJ Cadmium content 14%.
-0300
\I
Rf(hexane)
(1)methyl ( 2 ) ethyl (3) n-ProPYl (4) isopropyl (5) n-butyl (6) tert-butyl (7) sec-butyl (8) isobutyl (9) benzyl
--
Rf(chloroform)
a
b
C
d
0.494 0.534 0.557 0.545 0.537 0.545 0.545 0.537 0.437
0.463 0.477 0.503 0.497 0.503 0.520 0.508 0.497 0.404
0.600 0.600 0.600 0.603 0.600 0.603 0.603 0.600 0.594
0.600 0.600 0.600 0.600 0.600 0.603 0.603 0.600
=
0.0
Figure 1. Plot of Rwcar, from Hg2+/chioroformsystem vs. Taft u* constants for alkyl R in Ph-S-R (I-VIII)
Table VI. R f Values on Lead Acetate-Impregnated Layersa R-S-Ph, R
-0;o
-0200 U*
0.7% ethanol.
-
sulfide a
Lead content 19%.
In order to determine the effect of electron donation by the sulfur atom on chromatographic behavior of individual alkyl phenyl sulfides the RM(corr)for Hg2+-chloroform data were plotted against Taft u* substituent constants (Figure 1). The ~ ~ ~= , -1.400 u* negative slope of the straight line R M ( C,H&Ph) -0.320, (r = 0.994) provides an evidence of an increase in ligand strength of the sulfur atom with an increase in electron donation from alkyl groups in the series of four isomeric butyl phenyl sulfides. Another line is separated by an increment ARM(corr)which corresponds to the contribution of one carbon atom to the migration of individual compounds within each homologous series of alkyl phenyl sulfides (e.g., n-alkyl-, sec-alkyl-, etc.). Thus, RM(corr)values of sulfides I-VI11 are functions particularly of the polar and molecular weight effects. The minor role of steric effect compared to these two effects appears to be evident from the “normal” behavior of tert-butyl phenyl sulfide. Figure 2 visualizes the molecular
Figure 2. Plot of Rwm) from Hg2+/chloroform system vs. number of carbon atoms in alkyl R in Ph-S-R (I-V, VII)
weight effect on RM(corr) values. The straight line plotted through ethyl, propyl, and butyl phenyl sulfide points, R M ( c H ~ ( c ~= ~ )-0.0777 , ~ ~ ~ ) n 0.111, [ r = 0.999, n = 1,2, 31, suggests linearity for compounds from one and the same structural class, n-alkyl phenyl sulfides. Points for isopropyl phenyl sulfide and sec-butyl phenyl sulfide lie on a line comprising another structurally related class of compounds, sec-alkyl phenyl sulfides. In this diagram, the separating two isomeric sulfides reflects the difference in electron donation effect of an alkyl group attached through the primary and secondary carbon atom, respectively. These results are comparable with those obtained by one of us in a study mentioned earlier involving Hg2+-sulfidecomplexation
+
Table VII. Rf Values on Nonimpregnated Silica Gel Layers hexane R-S-Ph, R (1)methyl ( 2 ) ethyl (3) n-propyl
=
( 4 ) isopropyl (5) n-butyl (6) tert-butyl (7) sec-butyl (8) isobutyl
(9) benzyl sulfide
See Table 11.
U
0.217 0.211 0.221 0.206 0.223 0.194 0.211 0.253 0.080
ethyl acetate
chloroforma
b
C
d
e
0.249 0.243 0.254 0.237 0.271 0.220 0.260 0.288 0.090
0.706 0.7 34 0.751 0.718 0.728 0.712 0.700 0.720 0.734
0.696 0.702 0.713 0.696 0.713 0.685 0.690 0.708 0.713
0.665 0.676 0.686 0.665 0.665 0.665 0.665 0.665 0.670
f
-
0.726 0.727 0.727 0.716 0.727 0.716 0.716 0.716 0.727
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
Table VIII. Determination of Adsorption Isotherm for Butyl Phenyl Sulfide Using Mercury Acetate-Impregnated Silica Gel and Hexane S , clg/mLa
SF, wdmLb
5.92 5.03 3.55 2.37
4.58 3.78 2.54 1.60
SA
1.34 1.27 1.01 0.77
Table IX. Calculated xs,Rf(corr), and R M ( Values ~ ~ ~ ~ ) for Mercury Acetate/Chloroform System =
(1)methyl (2) ethyl (3) n-propyl (4) isopropyl (5) n-butyl (6) tert-butyl ( 7 ) sec-butyl ( 8 ) isobutyl
%(torr?
77s” 1.835 i 1.598 i 1.762 t 1.510 ? 1.891 i 1.412 i 1.738 t 1.964 2
0.024 0.038 0.060 0.034 0.035 0.056 0.029 0.038
0.550 i 0.007 0.479 t 0.012 0.529 f 0.018 0.459 i 0.016 0.568 t 0.010 0.424 i 0.017 0.521 i 0.011 0.590 0.011
*
RM(corr)
-0.0872 0.0365 -0.0 504 0.0714 -0.1189 0.1330 -0.0365 -0.1563
Average Rs values calculated using data in columns values calculated using c-f in Table 11. Average R f Rj(corrvalues from data in coiumns d-f and R f value for benzytsulfide from column c as a standard value from Table 11. Table X. Calculated R s , Rf(corr), and R M Values Silver Nitrate/Ethyl Acetate System R-S-Ph, R
=
RS
(2) ethyl ( 3) n-propyl (4) isopropyl ( 5 ) n-butyl (6) tert-butyl (7) sec-butyl (8) isobutyl
1.500 i 1.112 t 1.504 i 1.241 i 1.532 i 1.213 i 1.417 i 1.839
(1)methyl
*
0.094 0.106 0.118 0.088 0.092 0.085 0.084 0.173
R-S-Ph, R = (1) methyl (2) ethyl (3) n-ProPYl (4) isopropyl ( 5 ) n-butyl (6) tert-butyl (7) sec-butyl (8) isobutyl
wz/mLC
SF = equilibrium a S = original sulfide concentration. SA = amount of sulfide adsorbsulfide concentration. ed.
R-S-Ph, R
Table XI. Calculated E,’ and RM’Valuesa for Silver Nitrate/Ethyl Acetate System
* for
Rf(corr)
RM(corr)
0.364 i 0.023 0.270 i 0.026 0.366 t 0.029 0.296 t 0.020 0.373 i 0.022 0.295 t 0.021 0.340 i 0.017 0.444 i 0.036
0.242 0.432 0.239 0.376 0.226 0.378 0.288 0.122
-
R; 0.402 0.308 0.402 0.318 0.439 0,317 0.379 0.487
RM’
0.010 0.006 t 0.010 i 0.007 i 0.008 r 0.008 r 0.009 t 0.017 t i
0.1725 0.3 5 1.6 0.1725 0.3314 0.1059 0.3334 0.207 1 0.0226
a values were derived by rejecting (using method 14) questionable values (marked1ith in Table IV). RM’ values were calculated from R ; values.
the RMM(corr, for the Hg2+-chloroform system and RM‘for the Ag+-ethyl acetate system, respectively, is correlated producing the following information: Hg *+-chloroform Ag+-ethyl acetate a
b
c d
r(n =
-1.933 t 0.132 -0.00611 t 0.00059 +0.0774 r 0.0231 1-0.6792 r 0.0816 8)= 0.993
-2.397 i 0.254 -0.0077 f 0.00114 +0.1559 t 0.0445 +1.1518 ~t 0.1575 r(n = 8)= 0.983
Interestingly, the RM data from one set of data on nonimpregnated plates and hexane as solvent analyzed using the same linear relationship reveal similar character with regard to the participation of polar, steric, and molecular weight effects as those from Hg2+and Ag+ impregnated plates: a = 4.833 f 0.137, b = 4.00372 f 0.00061, c = +0.0203 f 0.0240, d = 0.935 f 0.085, r(n=8)= 0.969. The large values of standard deviations in the various slopes are accounted for by both the experimental errors and the fact that the various factors (u*, MW, and Es) are not mathematically independent; just coincidentally correlating vs. each other with a correlation coefficient r = 0.87. Based on the above data average percent contribution of individual structural factors is calculated as follows: (for average u* contribution)
+
100 Clau;*l
R s , Rf(corr and R M ( have ~ ~ meanings ~ ) similar to those from Takle IX. a
(5). In the quoted paper, sulfides from a similar series were partitioned between a polar ligand phase (AcOH/H20) saturated with Hg(0Ac)z and heptane and logarithms of the partition coefficient was plotted vs. n number of carbon atoms of the R substituent in the formulae R-S-Bu. The plot showed separation into classes: one in which R was n-alkyl and another in which R was sec-alkyl. Separation into structural classes has been observed in gas chromatography in log rV vs. n plots (rV is relative elution volume and n is number of carbon atoms) involving different homologous series of hydrocarbons (e.g., n-paraffins, 2-methyl paraffins, 2,2dimethyl paraffins, etc.) (15). The structural effects on elution parameters are here associated with the differences in boiling points which exist for isomeric hydrocarbons. “Anomalous” behavior of the methyl derivative (I),RM(corr) of which does not fit on the line of regression comprising n-alkyl phenyl sulfides (Figure l),is similar to that observed for a methyl derivative in studies of the partition processes involving Hg2+-sulfide complexation quoted above (5) as well as to other known examples (16). This apparent inconsistency is, however, clarified by applying linear equation in which RM is expressed in terms of polar (u*), molecular weight (MW), and steric (17)(Es) effects. Using a computer program by M. Charton (18) and the equation RM = au* bMW + cEs + d,
+
(similar formulations were used for MW and Es) producing the following information: Hg2+-chlo- &+-ethyl unimroform acetate pregnated a*
MW
ES
22% 7 4% 4%
21% 73% 6%
17% 81% 2%
As a result of the above presented analysis of chromatographic behavior of alkyl phenylsulfides on Hg2+-and Ag+impregnated silica gel plates based on the contribution of individual structural factors shows, the molecular weight is far more influential than polar and steric effects. However, the steric effect, the contribution of which to the RM is generally the smallest, is a factor by which the behavior of the methyl derivative is explained. Finally, whereas the retention of sulfides with Ag+ is much stronger than with Hg2+ the contribution of polar, molecular weight and steric factors to the RM is identical. Since the vast increase of retention on impregnated plates is evidently due to the interaction of the particular metal ion with the electron donating site of the sulfide molecule, it seems that plain silica gel interacts with the same site, probably the sulfur atom. However, in the latter case the energy of the
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
interaction is rather small resulting in much smaller retention and poor resolution. The results presented in this paper demonstrate the efficiency of the ligand exchange chromatography for separation of alkyl phenyl sulfides using Hg2+and Ag+ ion-loaded stationary phase and suggest analytical applications. Furthermore, the method may be appplicable to other types of sulfides as well. Resulting from our preliminary experiments, leaching of the metal salts from the stationary phase is a problem which should be solved to allow successful application of the method discussed in this paper in HPLC.
2253
(7) V. Horak, J. Pecka, and J. Jurjev, Collect. Czech. Chem. Commun., 32,3386 (1967). (8) V. Horak and J. Pedta, "Mechanisms of Reactions of Sulfv COmpouW', Vol. 4,Intra-Science Research Foundation, Santa Monica, Calif.. 1969, p 43. (9) V. N. Ipatieff, H. Pines, and S. M. Friedman, J . Chem. SOC.,80,2731
(1938).
(IO) S.Stahl, "Thin Layer Chromatography", Academic Press, New York, 1965,
p 494. (11) I.M. Kolthoff and E. E. Sandell, "A Textbook of Quantitative Inorganic Analysis", 3rd ed., Macmillan, New York, p 461. (12) K. A. Connors, "A Textbook of Pharmaceutical Analysis", Wiley, New York, 1967,p 49. (13) R. Aiger, H. Spitzy, and R. W . Frei, Anal. Chem., 48,3 (1976). (14) R. D. Dean and W. J. Dixon, Anal. Chem., 23,636 (1951). (15) A. T. Jams, A. J. P. Martin, and G. H. Smith, Biochem. J , 52,238 (1952). (16) I. E. Bush, "The Chromatography of Steroids", Pergamon Press, London, 1961,p 31. (17)R. W. Taft, Jr., "Steric Effects in Organic Chemistry", M. S. Newman, Ed., John Wiley & Sons, New York, 1956. (18) M. Charton, Pratt Institute, Brooklyn, New York.
LITERATURE CITED (1) L. R. Snyder, Anal. Chem.. 41, 314 (1969). (2) W. L. Orr, Anal. Chem., 38. 1558 (1966). (3) W. L. Orr, Anal. Chem., 39, 1163 (1967). (4) J. W. Vogh and J. E. Dooley, Anal. Chem., 47, 816 (1975). (5) V. Horak and J. Pedta, Collect. Czech.Chem.Commn., 32,3394(1967). (6) V. Horak and J. Pecka, Cokct. Czech. chem.Cornnun., 32,3055(1967).
RECEIVED for review May 2, 1979. Accepted August 7, 1979.
Observation of Electrochemical Concentration Profiles by Absorption Spectroelectrochemistry Richard Pruiksma and Richard L. McCreery' Department of Chemistry, Ohio State University, Columbus, Ohio 43210
A laser beam passing parallel to a working electrode surface was used to monitor an electrogenerated chromophore employing absorptlon spectrophotometry. By placing a 10-pm slit parallel to the electrode and intercepting the beam after passage by the electrode, a portion of the diffusion layer was sampled. Movement of the SUI relathre to the electrode allowed monitoring of concentration as a function of distance from the electrode surface. The resulting concentration profiles agree well with theory for both single- and double-step experiments for distances of 50-200 pm from the surface. Because of long optical path length (0.5 cm or greater), the present method Is much more sensitive than previous spectroelectrochemlcal methods. In addition, spatial resolution of the diffusion layer provides both fundamental information about mass transfer and addttlonal insight Into reactions accompanylng charge transfer.
The use of spectroscopic probes to monitor electrochemical events has become widespread and spectroelectrochemistry has been used for a variety of purposes (1-3). The majority of applications of spectroelectrochemistry involves light absorption, with the beam transmitted through the electrode or reflected off its surface. For the techniques used to examine solution species in the vicinity of the electrode, two major objectives have been realized, spectral characterization of electrogenerated materials and kinetic monitoring of reactive species. The most common techniques use optically transparent electrodes, with the spectrophotometric beam being perpendicular to the electrode plane. This approach yields an integrated absorbance throughout the entire diffusion layer, and the time course of this absorbance has been used to diagnose reaction mechanisms for electrogenerated species and to spectrally characterize reactive intermediates. Internal reflection spectroscopy (IRS) has been used for the same purposes, with the region of the diffusion layer within one wavelength of the electrode surface being sampled by the evanescent wave ( 4 ) . 0003-2700/79/0351-2253$01.00/0
These previous spectroelectrochemical methods based on absorption suffer from two major drawbacks when applied to the monitoring of transient electrogenerated species. First, the optical path length is very short, being limited to the thickness of the diffusion layer or the length of the evanescent wave. Thus these techniques have been applied primarily to strong chromophores or species with relatively long lifetimes, allowing measurement of absorbance value vs. time transients with acceptable signal-to-noise ratios. Second, these techniques cannot supply information about the concentrations of electroactive species (or their reaction products) as a function of distance from the electrode surface. IRS can provide surface concentrations, and a transmission or reflection experiment can provide total concentration in the diffusion layer, but neither allows spatial resolution of the diffusion layer. Reflection spectroscopy at a glancing incidence angle (5) can greatly extend the bptical pathlength but still does not resolve the concentration gradients of electroactive species. Despite the fundamental importance of concentration vs. distance profiles to electrochemistry, they have not been observed using absorption techniques. Several experiments have been reported which make use of the refractive index gradient of a diffusion layer to construct a concentration vs. distance profile (6-10). While these interferometric techniques have succeeded in some cases, they lack both sensitivity and selectivity because they are based on refractive index changes accompanying electrochemical events. It is very difficult to monitor more than one solution component using changes in refractive index, and fairly large refractive index gradients are required to be measured interferometrically. Hence refractive index techniques have allowed observation of diffusion profiles only for fairly concentrated solutions (ca. 0.1 M) of single components (e.g., CuS04). Application of such techniques to reactive systems a t millimolar levels would be extremely difficult. The objective of the present work is to construct concentration vs. distance profiles for an electrogenerated species using absorption spectrophotometry. The light beam is or@ 1979 American Chemical Society
