Anal. Chem.
120
1983,5 5 , 120-122
Extension of Potentiometric Stripping Analysis to Electropositive Elements by Solvent Optimization J. F. Coetzee* and Abul Hussam Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
T.
R. Petrlck
Department of Chemistry, California State College, California, Pennsylvania 154 19
The extenslon of potentlometrlc strlpplng analysls to the Ions of such electropositive elements as the alkali and alkallne earth metals was lnvestlgated by uslng thln fllm mercury electrodes in a wide range of organic solvents and thelr mixtures with water. The alkali metal Ions can be determlned in certain organlc solvents even when several mole percent of water is present. As expected, the most effective solvents are aprotlc; however, an equally important factor is that the solvent should be a good hydrogen bond acceptor, thereby decreasing the reactivity of water toward the amalgam. The sum of sodlum and potasslum Ions can be determlned In such samples as blood serum and seawater after addltlon of dlmethyl sulfoxide. Some resolutlon of sodium from potasslum occurs In I-methyl-2-pyrrolidlnone and certain other solvents.
Potentiometric stripping analysis (PSA), which was recently introduced by Jagner ( I ) , is similar to conventional (voltammetric) stripping analysis (VSA) in that the analyte is preconcentrated by electrodeposition in a mercury electrode, but it differs from VSA in the method used to generate a signal by the preconcentrated analyte. Whereas in VSA the most sensitive and commonly used method is differential pulse voltammetry which produces a signal of differential current vs. potential, in PSA the plating potential is interrupted and the amalgam is allowed to react with an excess of a suitable oxidant, such as mercury(I1) ion M(Hg),
+ (n/2)Hg2+
-
Mn+ + ( x
+ n/2)Hgo
(1)
Consequently, the redox couple Mn+/M(Hg), determines the potential of the mercury electrode until all amalgam has been oxidized, when an abrupt change in the signal of potential vs. time occurs. Both PSA and VSA are restricted for all practical purposes to relatively noble metals when carried out in aqueous solution. However, metals forming more reactive amalgams can be determined in nonaqueous media or, more usefully, in mixtures of certain nonaqueous solvents and water. PSA offers significant advantages over VSA in media of that kind because such factors as high solution resistivity and irreversible redox couples present fewer problems. We have therefore investigated the possibility of achieving useful extensions of stripping analysis methodology by applying PSA to the determination of electropositive metals in optimized solvent mixtures. Preliminary results have been reported before (2);we now present more detailed information. Historically, "chemical" stripping analysis ( 3 )was a forerunner of PSA; such oxidants as Ce(IV), Fe(III), and Mn04were determined by allowing their solutions to react with known amounts of silver metal on a platinum electrode while monitoring potential as a function of time. Chronopotentiometric stripping analysis (CPSA) also has features ( 4 ) in
common with PSA; some of these have been compared by Buffle (5). There are nevertheless certain differences between the principles of PSA and CPSA which will be discussed elsewhere (6); we present here only our main conclusions. Under conditions of forced convection and of sufficiently long deposition time ( t d ) and a sufficiently thin mercury film (thickness I ) so that t d >> 1/30,, where D, is the diffusion coefficient of the metal in mercury, eq 2-4 represent the limiting stripping time or transition time (T),the transient potential at time t (Et),and the transient potential at a time equal to half the limiting stripping time ( E T j 2 ) .
= Ea"
Here, D , 6, and C are the diffusion coefficient, the diffusion layer thickness, and the bulk concentration of Mn+or Hg2+, as indicated, and E," is the standard (reduction) potential of the amalgam; if the convection rate is the same during deposition and stripping, then the two diffusion layer thicknesses are equal. It follows that E,/2 is more negative than the polarographic half-wave potential by the second and third terms of eq 4 and that r is directly proportional to the bulk concentration of Mn+and inversely proportional to the bulk concentration of Hg2+. Equations 2-4 therefore illustrate the potential analytical utility of PSA.
EXPERIMENTAL SECTION Chemicals. Acetonitrile, propylene carbonate, dimethyl sulfoxide, ethanol, 2-propanol, and water were purified as described before (7). Other solventstested were reagent quality and were used without further purification. Tetraethylammonium perchlorate (TEAP) and tetrabutylammonium perchlorate (TBAP) were prepared as described elsewhere (8);these salts contained 1.5 and 2.7 ppm sodium ion, respectively. Mercury(I1) chloride (Baker analyzed reagent) contained 0.1 ppm sodium ion. Lyophillized human blood serum (General Diagnostics, Versatol acid-base normal) had the following composition: Na+, 143.0; K+, 4.7; Ca2+,2.5; and Mg2+,3.0 mM. Synthetic serum was prepared by dissolving NaCl and KC1 in water at concentrations
0003-2700/83/0355-0120$01.50/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
of 143.0 mM Na+ and 4.7 mM K+. Synthetic seawater was prepared to contain (in units of mM) 478.6 NaCl, 10.7 KCl, 10.7 CaClZ.2H2O,54.7 MgCl2GH20,1.9 X loa CuS04-5Hz0,3.2 X lo4 ZnC12. CdCl,, 6.3 x lo4 PbCl,, and 6.3 X Instrumentation. PSA was performed with an ISS-820 ion scanning unit (Radiometer, Copenhagen) interfaced with a TTA-80 titration assembly. An electronic circuit similar to that described by Jagner (9) was built for derivative PSA. The three-electrode cell contained a planar glassy carbon electrode (Radiometer Model F 35100) with a geometric area of 0.05 cm2 as working electrode, a 1-cni2platinum foil as counterelectrode,and an Ag/(O.Ol M AgC104G 0.1 M TEAP in acetonitrile) reference electrode (hereafter designated as Ag+/Ag) coupled to the analyte solution through a 0.1 M[TEAP salt bridge solution in the same solvent as the analyte; ithe two junctions were asbestos fibers. Procedure. The glasiay carbon working electrode was cleaned before each set of experiments with a polishing grade of alumina (0.3 pm particle size) and was then washed successively with 1 M nitric acid, water, and acetone. Analyte and plating solutions were deaerated with ultrapure nitrogen presaturated with the solvent in question. Deaeration was carried out in the absence of the working electrode to avoid the deposition of gas bubbles on the electrode surface. The working electrode was then inserted and (typically) mercury was plated from M HgClz lo-’ M TEAP for 4 min, after which the analyte solution was introduced with a microsyringe and three to five deposition-stripping cycles were carried out. Background corrections for the presence of sodium ion in the supporting and stripping electrolytes were applied.
+
RESULTS A N D DISCUSSION Potentiometric St ripping Analysis of Alkali Metal Ions. We investigated the PSA of sodium ion in a wide range of solvents, including methanol, ethanol (EtOH), 2-propanol (2-PrOH), ethylene glycol, 1,2-dimethoxyethane (DME), ethylenediamine, pyridine, acetone, acetonitrile (AN), benzonitrile, dimethyl sulfoxide (Me2SO), dimethylformamide (DMF), sulfolane, dimethyl carbonate, propylene carbonate (PC), propylene oxide, y-butyrolactone (GBL), dioxane, 1-3dioxolane (DL), and 1-methyl-Qpyrrolidinone(NMP). This list contains mostly aprotic solvents, several of which have found commercial application in lithium batteries, but selected protic solvents were also included for comparison. Best results were obtained in DME, Me2S0, DMF, PC, GBL, DL, NMP, and, surprisingly, EtOlH and 2-PrOH. The remaining alkali metal ions as well as the alkaline earth metal ions were also tested in those solvents in which sodium ion gave good response. Several stripping agents were tested; none of these (oxygen, hydrogen peroxide, iodine, permanganate ion) had an advantage over mercury(I1) ion. The question of how much water can be tolerated in different solvents is of crucial importance. As would be expected, the reactivity of residual water toward amalgams is lowest in those solvents that are good hydrogen bond acceptors. We have reported elsewhere (IO) equilibrium constants for the formation of the hydrogen bonded complexes H O H . 4 and S--HOH-.S between solvents S and low concentrations of (monomeric) water. Typical formation constants [(mole fraction)-l at 30 “C]for the 1:l complexes vary from 4.1 for AN to 14.2 for DME, 43.4 for DMF, and 59.4 for Me2S0. Stepwise formation constants for the 1:2 complexes vary less with the solvent and typically fall between 0.2 and 0.9 (mol fraction)-l. Distribution fractions calculated from these formation constants show that, in such solvents as DMF or Me2S0 containing up to a few centimolar water (typical levels in these solvents when they are purified by other than the most rigorous procedures), liittle free water is present and S-.HOH-.S is the predominant water species. On the other hand, in such solvents as AN, HOH.43 is the predominant species while considerable free water is also present. At higher water concentrations the systems become more complex owing to progressive polymerization of water, but the trends remain
2.0
t
121
i
DMSO
I 1
0
0.04
I
0.08
0 ‘
0.12
Mole Fraction of Water Flgure 1. Tolerance of dimethyl sulfoxide and acetonitrile to water in the otentiometrlc stripping of sodium amalgam. Conditions: analyte, 10-9M NaCl + M HgCI, 5X M TEAP; E,, -2.99 V vs. Ag+/Ag; t,, 120 s. Dashed lines show where hydrogen evolution becomes visible to the unaided eye.
+
Table I. Features of Typical Calibration Hots for Potentiometric Stripping Analysis of Alkali Metal Ions (M+)in Various Solvents solvent Ma slope” rc Me,SO
AN EtOH
Na K Rb cs Na K Na
21.8 23.3 23.0 21.4 11.0 12.3 17.5
0.9999 0.9997 0.9962 0.9998 0.9972 0.9939 0.9983
a Conditions: ca. to MMCl t M HgC1, t- lo-’ M TEAP; E,, -2.99 V VS. Ag*/Ag; t d , 60 s. Units: s mM-’ ; represents the sensitivity of PSA. Correlation coefficient.
the same. The marked differences in the reactivity of water in Me2S0 and AN toward sodium amalgam are illustrated in Figure 1. In MezSO, the limiting stripping time (7) decreases little until ca. 8 mol % of water has been added, while in AN r is smaller and actually increases until ca. 5 mol % of water has been added (after which r decreases rapidly). This curious behavior in AN may be caused by the fact that AN itself is polymerized by sodium amalgam; such polymerization may block deposition of sodium more effectively than the formation of sodium hydroxide does. Under certain conditions, significant amounts of sodium hydroxide undoubtedly form in MezSO as well, even at low water concentrations, because T does not remain directly proportional to t d for large values of t d and/or &+. We therefore recommend that t d be optimized empirically for a given metal ion concentration and water content; reasonable values of t d for “pure” Me2S0 are 120 s for M Na+ and 30 s for M Na+. Nevertheless, reproducibility of the limiting stripping time in a given MezSO-water mixture remains satisfactory (u 0.08 s) up to ca. 20 mol % water for Na+, and 14,12, and 4 mol % water for K+, Rb+, and Cs+. Typical results obtained for the alkali metal ions in Me2S0, AN, and EtOH are listed in Table I. We have devoted less attention to the PSA of lithium ion. Stripping signals were obtained in a few solvents, including EtOH, 2-PrOH, and DMF, but not in MezSO. Stripping signals were more drawn out and had poorer sensitivity than those obtained with the other alkali metal ions. Resolution of stripping signals of mixtures of alkali metal ions is generally modest; this is in agreement with the poor resolution previously observed in polarographic half-wave potentials (11). Nevertheless, sodium and potassium ions a t comparable concentrations can be resolved in GBL and NMP;
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983 I
c
m
I
I
I
Stripping Time ( s e c )
Figure 2. Stripping signals of human blood serum in dimethyl sulfoxMe. Conditions: Successive injection of 10+L aliquots of serum into 20 mL Of M HgCI, 4- IO-' M TEAP; E,, -2.99 V VS. Ag+/Ag; t d , 60 S.
Human Blood S e r u m
0
Synthetic Serum S o d i u m Ion Only
A
v
I-
-
6.0
-
2.0 V
0
I
I
0.2
I
I
I
0.4
I
0.6
Concentration ( m M ) Figure 3. Concentration dependence of limting stripping time of human blood serum in dimethyl sulfoxide: (0) human blood serum, C,,+ 4CK+= 147.7 mM; (0) synthetic serum, CNat CKt = 147.7 mM; (A) sodium ion only, Cy+ = 143.0 mM. Conditions: samples added to M HgCI, IO- M TEAP; Ed, -2.99 V vs. Ag+/Ag; t,, 60 s. Correlation coefficients for all three lines are equal to 0.9998.
+
+
we are investigating the possibility of optimizing solvent mixtures for this purpose. Potentiometric Stripping Analysis of Alkaline Earth Metal Ions. The PSA of the alkaline earth metal ions is less satisfactory than that of the alkali metal ions and was studied in less detail. Nevertheless, stripping signals were obtained for Ca2+,Sr2+,and Ba2+ions, as well as Mg2+ion, in EtOH, and for Ca2+ ion also in Me2S0, DMF, GBL, and NMP.
However, signals were less sensitive, more drawn out, and less reproducible than those observed with the alkali metal ions; it is likely that these problems are caused by precipitation of the hydroxides on the electrode surface. In Me2S0, the stripping signal of Ca2+is so much less sensitive than that of Na+ or K+ that low concentrations of Ca2+do not significantly interfere in the PSA of alkali metal ions in such samples as blood serum (see below); this is, of course, a blessing in disguise. Potentiometric Stripping Analysis of Sodium Plus Potassium Ions in Blood Serum and Synthetic Seawater. In Figure 2, stripping signals are shown for different concentrations of human blood serum in MezSO while, in Figure 3, limiting stripping times are compared for different concentrations of human blood serum, of synthetic serum containing only sodium and potassium ions, and of sodium ion only. Figure 3 shows that calcium and magnesium and other metal ions, as well as proteins, in human blood serum do not significantly interfere in the PSA of sodium and potassium ions. (It is also evident that the sensitivity for the sodiumpotassium mixture is higher than that for sodium ion only at the same concentration.) Analogous results were obtained with synthetic seawater; stripping signals were essentially those produced by sodium and potassium ions, with no measurable interference from calcium and magnesium ions or from the more noble metals which are present in much lower concentrations. In conclusion, the fact that stripping analysis, whether PSA or VSA, is a valuable complementary technique to spectroscopic methods for the determination of the more noble metals has been well established (12j. We have shown in this paper that the scope of stripping analysis can be extended to some of the more electropositive metals in a simple, practical manner. We are investigating further the determination of lithium and calcium ions and the resolution of sodium ion from potassium ion.
LITERATURE CITED (1) Jagner, D.; Granell, A. Anal. Chlm. Acta 1976, 8 3 , 19-26. (2) Coetzee, J. F.; Hussam, A.; Petrick, T. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1981; ABC Press: Monroevllle, PA, 1981; paper 77. (3) Bruckensteln, S.; Blxler, J. W. Anal. Chem. 1965, 3 7 , 786-790. (4) Perone, S. P.; Davenport, K. K. J . Electroanal. Chem. 1966, 12, 269-276. (5) Buffle, J. J . Nectroanal. Chem. 1981, 125, 273-294. (6) Coetzee, J. F.; Hussam, A., unpublished results. (7) Coetzee, J. F.; Istone, W. K. Anal. Chem. 1980, 52, 53-59. (8) Coetzee, J. F.; Martin, M. W. Anal. Chem. 1980, 52, 2412-2416. (9) Jagner, D.; Aren, K. Anal. Chim. Acta 1978, 100, 375-386. (10) Coetzee, J. F.; Hussam, A. J . Solution Chern. 1982, 1 7 , 395-407. (1 1) Mann, C. K.; Barnes, K. K. "Electrochemlcal Reactions In Nonaqueous Solvents"; Marcel Dekker: New York, 1970. (12) Ryan, M. D.; Wilson, G. S. Anal. Chem. 1982, 5 4 , 20R-27R.
RECEIVED for review July 12,1982. Accepted October 18,1982. This work was supported by the National Science Foundation under Grant Numbers CHE-7727699 and CHE-8106778.
