are in part dependent upon the slope of the calibration curves. Based on a 0.5% relative error in making up the standard solutions by volume, and a 0.3 relative error in the readability of the recorder deflections, the error in the slope of the calibration curves could be as high as 0.8 %. This error in the slope would result in a maximum relative error of about 1 in the determination of an unknown. The results of this study generally fall within that range of error. A larger number of calibration points in the vicinity of the unknown concentration improves results. For instance, the methanol determinations are based upon calibrations at every 0.05-volume-fraction unit. The other mixtures are analyzed using calibration points at every 0.1-volumefraction unit. The relative error in the methanol analyses averages 0.1 %, compared with 0.5% for the other mixtures. The analytical accuracy of the method can be affected by the presence of one or more contaminants. Each component or impurity contributes to the measured heat according to the type of compound and its concentration. Thus, the effect of a
particular contaminant must be investigated. However, based on the overall sensitivity of the method, it is improbable that interfering substances at about 0.01-volume-fraction unit or less cause noticeable changes in accuracy. In conclusion, the described thermometric method for the analysis of miscible organic-aqueous mixtures provides good accuracy and precision, requires very little sample, and is relatively simple for routine problems. A large range of composition is suitable for analysis, although dilute solutions of either component provide limited accuracy. The procedure can be useful for the analysis of water in organic solvents, and can readily apply to on-stream analytical problems involving binary aqueous mixtures. RECEIVED for review August 16, 1967. Accepted December 14, 1967. Paper presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 6, 1967. Work supported in part by Petroleum Research Fund of the American Chemical Society.
Polarography in N,N-Dimethylacetamide Alkali Metal Ions and Ammonium Ion Viktor Gutmann, Manfred Michlmayr,’ and Gerald Peychal-Heiling Institut f ur Anorganische Chernie, Technische Hochschule Wien, Getreidemarkt 9, A-1060 Wien,Austria N,N-DIMETHYLACETAMIDE (DMA) has not been used extensively as a solvent for polarographic purposes. Although thallium, lead, cadmium, and zinc ions have been investigated in this solvent (I), and a preliminary report on DMA as possible polarographic solvent has been published (2), no systematic studies have been made on alkali metal ions. In order to establish the relationship between the polarographic half-wave potential Eliz and the donor number of the solvent (3), studies in DMA have been carried out. Though the dc polarographic behavior of alkali metal ions is well known in aqueous solutions (4-8), little information is available on the oscillopolarography of these simple ions (8-10). The polarographic behavior of alkali metal ions in acetic acid anhydride (II), ace1 Present address, Department of Chemistry, University of California, Riverside, Calif. 92502
(1) S. Musha, T. Wasa, and K. Tani, Reuiew of Polarography (Japan), 11, 169 (1963). (2) L. 0. Wheeler and D. W. Emerich, J . Miss. Acad. Sci, 9, 63 (1963). (3) V. Gutmann, G. Peychal-Heiling, and M. Michlmayr, Znorg. Nucl. Chem. Letters, 3, 501 (1967). (4) J. Heyrovskg, Chem. Listy, 16, 256 (1922). ( 5 ) E. S. Peracchio and V. W. Meloche, J. Am. Chem. SOC.,60, 1770 (1938). (6) I. Zlotowsky and I. M. Kolthoff, IND. ENG. CHEM., ANAL. ED., 14, 473 (1942). (7) 1. Zlotowsky and I. M. Kolthoff, J. Am. Chem. SOC.,64, 1297 (1942). (8) A. A. VlEek, Collection Czech. Chem. Commun. 20, 413 (1955). (9) J. E. B. Randles and K. W. Sornerton, Trans. Faraday SOC., 48, 951 (1952). (10) L. Treindl, Chem. Listy, 50, 154 (1956). (11) V. Gutmann and E. Nedbalek, Mh. Chem., 89, 203 (1958).
tone (12), dimethylsulfoxide (13, 14), ethylenediamine (13, morpholine (16), liquid ammonia (17, 18), N,N-dimethylformamide (19), N-methylacetamide (20), propanediol-l,2carbonate (21), acetonitrile (22), propionitrile ( 2 3 , isobutyronitrile (24), benzonitrile (23), and phenylacetonitrile (23) has been reported. EXPERIMENTAL
A Polariter P04b (Radiometer, Copenhagen) was used for dc polarography at the dropping mercury electrode @ME) and at the stationary mercury electrode (SME); ac experiments were carried out by means of the Polaroscope P 576 (Kiiiik, Prague). A Kalousek commutator, as adapted by RBlek (25), was used to check the reversibility of the electrode processes. The capillary used was supplied (12) J. F. Coetzee and Wei-San Siao, J. Inorg. Chem., 2, 14 (1963). (13) V. Gutmann and G. Schober, Z . Anal. Chem., 171, 339 (1959). (14) G. Schober and V. Gutmann, “Advances in Polarography,” I. S. Longmuir, Ed., Pergamon Press,London 1960, p 940. (15) G. Schober and V. Gutmann, Mh. Chem., 89,401 (1958). (16) V. Gutmann and E. Nedbalek, Ibid., 88, 320 (1957). (17) H. A. Laitinen and C. J. Nyman, J. Am. Chem. SOC.,70,2241 (1948). (18) H. A. Laitinen and C. E. Shoemaker, Zbid., 72,4975 (1950). (19) G. H. Brown and R. Al-Urfali, Zbid., 80, 2113 (1958). (20) L. A. Knecht and I. M. Kolthoff, J . Znorg. Chem., 1,195 (1962). (21) V. Gutmann, M. Kogelnig, and M. Michlmayr, Mh. Chem. in press. (22) I. M. Kolthoff and J. F. Coetzee, J. Am. Chem. SOC.,79, 870 (1957). (23) R. C. Larson and R. T. Iwamoto, Ibid., 82, 3239, 3526 (1960). (24) J. F. Coetzee and J. L. Hedrick, J. Physik. Chem., 67, 221 (1963). (25) M. Kalousek and M. Rklek, Collection Czech. Chem. Commun. 19, 1099 (1954). VOL 40, NO. 3, MARCH 1968
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Table I. Polarographic Data in N,N-Dimethylacetamide DC polarography Temperature D . lo6 coefficient Slope of the [cmz sec-11 o f Ellz [mV/'C] log analysis [VI I D (35)
Compound Ellz [VI -2.38 4.0 LiC104" -2.06 1.1 NaC104 -2.08 1.55 KC104 -2.08 1.55 NH4CIO4 -2.04 1.39 RbC104 -2.03 1.63 CkClO4 5 Solution containing 3 of water (by volume).
0.066 0.060 0.059 0.060 0.059 0.059
z
by Sargent, Chicago, and its characteristics were: m = 0.65 mg sec-' and t = 4.02 sec (at h = 64 cm and a potential of -2.25 V us. SCE). A streaming mercury electrode according to Woggon and Spranger (26) was used for ac polarography; the oscillographic cell has been described elsewhere (27). The SME employed resembled the set-up described by Gerischer (28). Water-jacketed cells, thermostated at 25.0" f 0.1" C, were used in all experiments. An aqueous saturated calomel electrode (SCE) was employed as a reference electrode and was separated from the nonaqueous solution by means of a diaphragm (29). The ohmic resistance, causing a high potential drop, was measured by means of a Philips GM 4144 conductivity bridge. The temperature dependence of the SCE was calculated between 20" and 40" C from the equation given by Chateau and Pouradier (30). The water content of chemicals and solutions was determined by use of an automatic Titrator TTT 1 c (Radiometer) according to the Karl Fischer method. DMA (Fluka, reagent grade) was fractionated twice from calcium hydride in vacuo (10 mm Hg) under nitrogen over aVigreux column (80 cm); it was finally distilled in the absence of calcium hydride. Tetraethylammonium perchlorate (TEAP) was prepared as described previously (22), and 0.1M solutions of TEAP in DMA were used for all measurements. The purity of the solutions was checked by polarographic methods; the available potential range extended from +0.3 V to .-2.8 V. The solutions contained less than l.lOP3 mole/liter of water. Anhydrous perchlorates of lithium, sodium, and potassium were obtained by heating water-containing products (Lobachemie reagent grade, Merck reagent grade) at 250" in vacuo (0.1 mm Hg) for several hours. Ammonium perchlorate was prepared in the same way, but not heated above 100" C. Rubidium and cesium perchlorates were prepared by reacting the chlorides (Fluka, reagent grade) with equimolar amounts of silver perchlorate (Merck, reagent grade) in aqueous solution and evaporating the solutions after filtration. The products were dehydrated by heating at 250"C and 0.1 mm Hg. GENERAL The given half-wave potentials have been corrected both for iR losses in the cell circuit and for errors caused by damping
the polarograms. In order to investigate the nature of the electrode reactions, the conventional method of logarithmic i analysis-plotting corrected applied potentials E US. log in - i -was employed, the slope indicating the reversibility or irre(26) (27) in (28) (29) (30)
H. Woggon and J. Spranger, Chem. Zvesti, 16,250 (1962). G. Gritzner, V. Gutmann, and R. Schmid, Electrochim. Acta,
press. H. Gerischer, Z . Physik. Chem., 202, 302 (1953). V. Gutmann and G. Schober, Mh. Chem., 88, 206 (1957). H. Chateau and J. Pouradier, J. Chem. Phys., 52, 358 (1955).
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0.95 1.28 1.43 1.42 1.53 1.56
2.44 4.54 5.56 5.52 6.42 6.68
Temperature coefficient of in [ z/"C]
AC oscillopolarograph Incision potential [Vl
0.8 1.35 0.9 0.9 0.8 0.5
-2.42 -2.06 -2.08 -2.01 -2.05 -2.03
versibility of the process at the DME. Further reversibility tests were performed by using the temperature coefficient of the Eli2 between 20" and 40" C, the Kalousek commutator, anodic stripping, and ac oscillopolarography (31). The evaluation of the oscillopolarograms was made as described recently (27) by using T1+ and K+ for the calibration of the potential scale. The cathodic and anodic incision potentials in ac polarography showed a certain difference even for reversible processes, caused by the iR drop. The mean values are given, inasmuch as incision potentials on dE/dt - E curves cannot be determined with an accuracy higher than 5 %. At the SME the so-called anodic stripping method was applied (32-34). Pre-electrolysis was carried out at a potential more negative than the reduction potential of a given ion in order to form reduction product. After a short period of rest, the hanging mercury drop was polarized to more positive values and the polarogram was recorded. An anodic peak is found in the polarographic curve, if any product accumulating at the electrode has been formed. For reversible reduction, this peak appears at the same potential as the cathodic wave, while for an irreversible process the peak is either shifted to more positive values or it does not occur at all. In order to establish the nature of the limiting current, the dependence of the wave height on the concentration, on the temperature between 20" and 40" C, and on the height of the mercury head was checked. Diffusion coefficients D and diffusion current constants ID (35) have been calculated from the Ilkovi; equation.
*
RESULTS Results are given in Table I, and the dependences of the wave heights on the concentration and on the square root of the mercury height are shown in Figures 1 and 2, respectively. No reduction wave was found for Li+ in the potential range available under strictly anhydrous conditions. If water is present, a wave appears at very negative potentials which is shifted to more positive values by increasing the water content. The wave height reaches a maximum at 3 % water (by volume) and then remains constant up to 7 % water. The half-wave potential Ellz is -2.38 V for Li+ in the presence of 3 % water. According to the slope of 0.066 V for the logarithmic analysis, and the temperature coefficient of the half-wave potential (Table I), the process is partly irreversible (31) J. Heyrovskf and J. Forejt, Z . Phys. Chem., 193, 77 (1943). (32) W. Kemula, Z. Kublik, and S . Glodowski, J. Electrounal. Chem., 1,91 (1959). (33) R. de Mars and I. Shain, J . Am. Chem. Soc., 81,2654 (1959). (34) H. W. Nuernberg, Z . Anal. Chem., 186,1(1962). (35) J. J. Lingane, IND.ENG.CHEM., ANAL.ED., 15, 583 (1943).
All other ions investigated (Na+, K+, NH4+,Rb+, Cs+) are polarographically active under strictly nonaqueous conditions. The half-wave potential values are rather similar (Table I). The limiting currents were shown to be due to diffusion in the range from 5.10-5 to 3.10-3 moles/liter for Na+, from 8.10-6to 3.10-3 moles/liter for K+, from 6.10-6 to 3.10-3 moles/liter for NHn+,and from l.10-4to 3.10-3 moles/liter for both Rb+and Cs+. Addition of water up to 3 does not alter the shape or height of the waves. At higher water contents the wave heights decrease, and the half-wave potentials are shifted to more positive potentials in the presence of more than 5 % water.
as has been confirmed by means of the Kalousek commutator. The Kalousek polarogram exhibits an anodic wave at - 1.8 V (Figure 3), if a potential of- 2.5 V is applied during the producing half-cycle. The limiting current is controlled by diffusion in the range from l.10-4 to 3.10-3moles/liter; at higher concentrations the wave becomes ill-defined. At the hanging mercury drop electrode, Li+ gives an anodic peak indicating the formation of an amalgym. In oscillopolarography Li+ does not give any incisions without the addition of 3 % water to the solution. The incisions at - 2.42 V (Figure 4d,together with Na+) indicate a reversible electrode process under these conditions.
2 -
0
1
2
3
4
5
6
7
8
* 9
10
Figure 2. Dependence of wave height on square root of mercury height VOL. 40, NO. 3, MARCH 1968
a
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Figure 3. Kalousek polarogram of Lis The reversibility of the electrode processes Na(1)-Na(O), K(1)-K(O), NH4(I)-NH4(0), Rb(1)-Rb(O), and Cs(I)-Cs(O) has been shown by logarithmic analysis and the temperature coefficient of the half-wave potentials (Table I), as well as by means of the Kalousek switch and the oscillopolarographic results. A conventional polarographic curve of Cs+ can be seen in Figure 5 ; a Kalousek polarogram is shown in Figure 6 (for NH4+); oscillographic curves are given in Figure 4b (for K+ and TI+)and Figure 4c (for Rb+). The reversibility for sodium, potassium, rubidium, and cesium ions was confirmed by the anodic stripping method (Figure 7 for K+). Ammonium ions did not show an anodic amalgam dissolution peak, indicating a low stability of the amalgam. DISCUSSION The polarographic characteristics of NH4+ and the alkali * metal ions (with the exception of Li+) are very similar in DMA. Simple reversible one-electron reductions occur and the products are amalgams; these can be reoxidized by apply-
Table 11. Diffusion Current Constants of Alkali Metal Ions in Various Solvents PDC DMF Acetone CHsCN (21) DMA (19) (12) (22) Li+ 0.97 0.95 1.09 ... 2.56 Na+ 1.09 1.28 1.61 ... 3.22 K+
1.32 1.43 1.63 1.30 1.53 1.68 1.26 1.56 1.98 NH4+ 1.15 1.42 ... PDC = propanediol-1,2-carbonate. DMF = N,N-dirnethylforrnamide.
Rb+ cs+
2.70 2.76
3.38 3.25
...
3.05
...
Figure 4. Oscillographic dE/dt-E-curves a. Supporting electrolyte b. Calibration ions TI+ and K +
added Rb+ in solution d. Separation of Na+ from Li+ c.
...
Table 111. Viscosity and Donor Number of Various Solvents Viscosity Solvent Donor number (25' C) tcpl CHSCN 14.1 0.35 PDC 15.1 3.37 Acetone 17.0 0.32 DMF 26.6 0.80 DMA 27.8 0.92
ing the Kalosek commutator or oscillopolarography. If the anodic stripping method is used, the amalgam has to be stable over a considerable period of time in order to allow reoxidation and to exhibit an anodic current. Only ammonium amalgam does not show sufficiently high stability for this procedure. Lithium ion can be reduced only if water is present in the solution. It has been reported (36) that the number of solvent molecules at the ion decreases in the order Li+ > K+ > Na+ > Rb+. Lithium ion can be expected to be surrounded by more DMA molecules than the other alkali metal icjns; therefore Li+ can form a very stable, probably symmetrical, complex (36) B. H.van Ruyven, Rec. Trao. Chim., 72,739 (1953).
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ANALYTICAL CHEMISTRY
I
Figure 7. Cyclic voltammetry polarogram of K+ at the SME
E
Figure 5. DC polarogram of Cs+ at the DME with the strongly solvating solvent. This solvate complex with DMA causes a high polarographic overvoltage, by which the reduction potential is shifted to such a negative value that no reduction can take place under anhydrous conditions. On the addition of water, the symmetry of the complexes in solution becomes distorted; consequently, the overvoltage decreases, and the reduction occurs within the potential range available. Li+ is irreversibly reduced in dc polarography at the DME, but a reversible electrode process occurs in oscillopolarography. A slow inactivation of the primary reduction product, following the electrode reaction, can explain this behavior; the secondary product is oxidized at a more positive potential than the primary product of the reduction. Under oscillopolarographic conditions, there is not enough time for the slow inactivation process, no conversion to secondary product takes place, and the oxidation of the primary product occurs at the same potential as the reduction to it. The half-wave potentials obtained in DMA have been discussed with respect to results reported in other solvents (3), and it has been concluded that the donor number of the solvent plays a more important role than physical properties such as dielectric constant or viscosity. The reductions occur at more negative potentials with increasing donor number of the solvent. Some influence of the solvation properties on the diffusion current constants in different solvents can be found. Table I1 shows IO values in five solvents. The diffusion current constants depend mainly on the viscosity of the solvent,
but also on the solvation (determining the size of the diffusing solvate complexes); In decreases with increasing viscosity and increasing donor number (Table 111). Acetonitrile shows the highest ID values in agreement with the low viscosity and the low donor number; acetone should give higher diffusion current constants if only viscosity is important, but they are found lower than in acetonitrile, corresponding to the larger donor number. A similar argument applies in other solvents; it should be pointed out, however, that diffusion current constants in PDC are similar to those in DMA because of the high viscosity and the low donor number of PDC. All polarographic waves can be used for analytical purposes because of the diffusion-controlled limiting current over a wide range of concentrations. Because of the E112values, only Li+ can be separated from the other alkali metal ions. Figure 4d shows the curve of an oscillopolarographic analysis of Li' and Na+; the separation is also possible by dc polarography and anodic stripping. A subsequent paper (37) describes more extensively the analytical applicability of DMA for alkali and alkaline earth metal ions. RECEIVED for review September 8, 1967. Accepted December
15, 1967. (37) V. Gutmann, M. Michlmayr, and G. Peychal-Heiling, J . Electroanal. Chem., in press.
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