~~~
~
In Equation 9, the potential of the electrode in a solution containing both the ions is utilized in the calculation. Combining Equations 3 and 4, one obtains:
Table I. Selectivity Ratios for Iodide (I-) and Picrate (Pi -) on the Perchlorate-Selective Electrode with Tetra-Propylammonium Perchlorate (TPrA + Clod-) or Tetra-Butylammonium Perchlorate (TBA + C104-) as Liquid Ion Exchanger Activities (a*-) Potentials, +mV
c10-,
0.0755 0.00899 0.000964 0.000099 0.0000099
(exp E1
TBA+
TPrA+* I110.0 137.0 157.0 170.0 176.2
which can be rearranged t o give a n expression for K I Z
c10-,
Pi93.0 108.7 139.0 173.5 184.0
I107.0 145.0 171.2 197.0 215.7
- Ed
UI
- a1
RT Pi115.0 124.6 170.8 196.2 210.0
tion, as long as the two ions are a t the same activity in their solutions. If Equation 2 and 3 are combined with E1 = E z , we get: (7) where a1 and a2 are the activities of the two ions which produce the same potential when present separately. Equations 6 and 7 involve measurements in solutions containing only one of the ions in any test solution.
Plots of Equation 9 are given in Figures 1 and 2. SELECTIVITIES AND INTERFERENCES
From the studies in Table I, it is observed that the quaternary ammonium compound shows the highest selectivity when the interfering ion is highly extractable, i.e. the extraction con~ )of lower order than that of picrate. The stant ( E Q c L o -is extraction constants are found to be in the sequence Pi- > C10-4 > I- for symmetric quaternary ammonium compounds
(7,8). RECEIVED for review August 9, 1971. Accepted January 24, 1972.
(7) K. Gustavii and G. Schill., Acta. Pharm. Suecica, 3, 249 (1966). (8) G. Schill., So. Kern. Tidskr., 80, 10 (1968).
Polarographic Reduction of Tin(lV) in the Presence of a-Mercaptopropionic Acid Atsuyoshi Saito and Sadayuki Himeno Department of Chemistry, College of General Education, Kobe University, Nada-ku, Kobe, Japan THE POLAROGRAPHIC REDUCTION of tin(1V) is generally irreversible. To show two distinct waves, it is ordinarily carried out in such complexing agents as chloride ( I ) , bromide (2, 3), pyrogallol ( 4 ) , chloranilic acid (3, and P-mercaptopropionic acid (6). In the presence of a-mercaptopropionic acid (a-MPA), tin(1V) shows a reversible two-step reduction wave at p H less than 2.5 and an apparent single step reduction wave above p H 2.5. The reduction wave is well formed and appears suitable for analytical purposes. EXPERIMENTAL Apparatus. The polarograms were recorded on a Yanagimot0 Model PA-102 polarograph, with a digital voltmeter ( i l mV) (Model 156-A, Kikusui Electronics Co., Tokyo, Japan) employed for precise potential measurement. p H
1) D. E. Sellers and D. J. Roth, ANAL. CHEM., 38, 516 (1966). 2) T. Kitagawa and K. Nakano, Rec. Polurogr., 9, 121 (1961). 3) T. Kitagawa, Jup. Analyst, 6 , 603 (1961). 4) A. J. Bard, ANAL. CHEM., 34, 266 (1962). 5 ) C. C. Boyd and J. G . Wardeska, ibid., 42, 529 (1970). 6 ) S. L. Phillips and R. A. Toomey, ibid., 37, 607 (1965). 1698
values were measured with a Hitachi-Horiba type M-4 pH meter. The capillary had a value of m 2 ' 3 f 1 /of6 1.990 a t -0.35 V os. SCE and of 1.844 a t -0.80 V cs. SCE in a solution of 0.2mM tin(IV), 0.10M HCIOd, 0.10M NaC104, lOmM a-MPA, and 0.0015 Triton X-100. An external saturated calomel electrode (SCE) was used as the reference electrode. The cell was immersed in a water bath maintained a t 25 i 0.1 "C. Reagents. Standard tin(1V) solutions were prepared by dissolving granulated 99.999 tin (Mitsuwa Chemicals Co. Osaka, Japan). a-MPA was reagent grade (Tokyo Kasei Kogyo Co., Tokyo, Japan) and used without further purification. All other inorganic chemicals were reagent grade. Procedure. To a known volume of supporting electrolyte, sufficient a-MPA was added so that the final solution was 10mM, unless indicated otherwise. An aliquot of 0.01M tin(1V) stock solution was then added. This sequence of addition was followed t o avoid hydrolysis of tin(1V). After adjusting the pH to the desired value with 1M perchloric acid or 1 M sodium hydroxide, the solution was deaerated with nitrogen gas for a t least 10 minutes. The polarograms were obtained in the usual manner.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUS T 19172
~
Table I. Diffusion Current for Reduction of Tin(1V)-a-MPA Complex in Various Solutions Tin(1V) concentration (mM) i d l (PA) idilC id2 ( P A ) id& (PAImM) idtotal (PA) iatotadCGAImM) 0 . 1M perchloric acida 5.02 1.074 10.74 4.92 0.502 0.10 0.492 5.14 2.166 10.83 4.93 1.028 0.20 0.986 5.09 4.188 10.47 4.96 2.036 0.40 1.984 5.15 6.510 10.85 5.11 3.090 0.60 3.066 5.05 8.36 10.45 4.94 4.04 0.80 3.952 5.01 10.53 . 10.53 5.02 5.01 1.oo 5.02 10.62 4.94 6.07 5.06 12.74 1.20 5.94 idtotal = 10.64 + 0.21 id& = 5.07 =t0.08 idl/C = 4.97 =t0.14 I, = 2.73 12 = 2.78 Itotai = 5.83 m = 1.641 mg/drop m = 1.637 mgldrop t = 4.93 secldrop t = 5.10 secldrop E = -0.35 V US. SCE E = -0.60 V US. SCE Tin(1V) concentration (mM) ia (PA) idlC ( N m M ) 0.40M acetate buffera pH 4.25 10.36 0.16 1.656 id/C = 10.33 f 0.09 0.28 2.874 10.28 I = 5.67 10.42 0.40 4.158 m = 1.665 mgldrop 10.42 0.60 6.24 t = 4.97 secjdrop 0.80 8.22 10.27 E = -0.80 V US. SCE 10.24 1.oo 10.24 10.32 1.20 12.38 0.40M citrate buffera pH 3.60 9.48 0.10 0.948 0.20 1.888 9.44 id/C = 9.48 =!z 0.07 0.40 3.782 9.46 I = 5.22 0.60 5.728 9.55 m = 1.639 mg/drop 0.80 7.597 9.49 t = 5.08 secldrop 1.oo 9.45 9.45 E = -0.70 V US. SCE 9.52 1.20 11.42 0.40M NHIOH-NH,NO~" pH 9.40 0.10 1.075 10.75 0.20 2.418 10.74 id/C = 10.76 f 0.04 10.74 I = 5.88 0.40 4.296 10.80 m = 1.649 mg/drop 0.60 6.481 10.73 t = 4.92 secldrop 0.80 8.583 1 .oo 10.77 10.77 E = -0.90 V US. SCE 1.20 12.95 10.79 a Contained lOmM a-MPA and 0,0015x Triton X-100.
RESULTS AND DISCUSSION
Typical polarograms for the reduction of the tin(1V)a-MPA complex are shown for the supporting electrolytes ; perchloric acid, citrate, acetate, and ammoniacal buffer (Figure 1). Analogous with the tin(IV)-P-MPA complex, the a-MPA complex is readily reduced from all solutions except sodium hydroxide. However, the polarographic behavior of the tin(1V)-a-MPA complex is entirely different from the tin(1V)-@MPA complex. To find the concentration of a-MPA which completely complexes the tin(IV), the variation in limiting current with a-MPA concentration while keeping the tin(1V) concentration and other conditions constant was investigated in perchloric acid (pH 1.0). If the ratio of the concentrations of tin(1V) and a-MPA was kept at a value greater than 1 :4, the limiting current was constant. A two-step polarogram is recorded with tin(1V)-a-MPA complex in perchloric acid and 0.0015% Triton X-100 as seen in Figure 1 (I). The relative wave height suggests the ratio of electron change of the two waves to be 2:2. The first wave is due to the reduction of tin(1V) to tin(I1) and the
second, the reduction of tin(I1) to tin amalgam. A series of polarograms was run to find the effect of pH on the halfwave potential of the tin(1V)-a-MPA complex. On varying the pH from 0.89 to 2.0, the half-wave potential for the first wave changes from -0.23 V to -0.41 V 6s. SCE. The E,,z for the first wave is quite pH-sensitive and shifted to more negative potentials with a pH increase, whereas the second wave is not affected at a pH less than about 2.5, and the limiting current is nearly constant. Plots of the logarithm of the limiting current for the reduction of 0.2mMtin(IV) at 25 "C in 0.1MHC10~-0.1MNaC10~ and lOmM a-MPA are linear with slopes of 32.1 mV and 29.6 mV for the first and second wave, respectively. At pH values greater than about 2.5 in acetate buffer solution as well as in citrate, phosphate, and ammoniacal buffer solutions, the tin(1V)-a-MPA complex produces a single step wave. Kitagawa et al. found that polarograms of tin(1V) in 3M NaBr 5 x 10-3M EDTA ( 2 ) and in 0.2M oxalic acid 1M NaBr (3) gave well-defined single waves due to the reduction of tin(1V) to tin amalgam.
+
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
+
1699
c (
f
:
L
3
u
0
0
0
0 -0.3 -0.G
-0.9
-1.2
-1.5
V vs SCE Figure 1. Polarogram of tin(1V) in the presence of a-mercaptopropionic acid Solution contained 4 X 10-4M tin(IV), 10mM LYMPA, and 0.0015 Triton X-100 1. 0.1M perchloric acid 0.1M sodium per-
+
chlorate 2. 0.2M citrate buffer, pH 3.93 3. 0.2M acetate buffer, pH 4.34 4. 0.4M ammoniacal buffer, pH 9.22 Even when no separation of the two waves is apparent on the recorded polarogram, it is possible to show, using the logarithmic analysis of the waves in the above buffer solutions, that two forms are present (7). The logarithmic analysis indicates the formation of two waves. For example, tin(1V) complex in acetate buffer solution of pH 3.70 with 0.0015% Triton X-100 shows two straight lines with slopes of 21.7 mV and 29.0 mV. Results obtained by the logarithmic analysis show that the reduction step in these buffer solutions does not proceed directly to tin amalgam but proceeds partly through tin(I1). The value, 21.7 mV, which is close to the theoretical value for a reversible three-electron reduction, differs from the theoretical slope for a reversible two-electron reduction, 29.5 mV. The conclusion is reached that the reduction proceeds partly through a direct four-electron reduction of tin(1V) to tin amalgam and partly through a two-electron reduction of tin(1V) to tin(I1) and then tin(I1) to tin amalgam. However, it is not possible to obtain the fraction of the current due to each reduction step from the sum of the current at a given potential. T o determine the composition of the tin(1V)-a-MPA complexes, the variation of half-wave potential with a-MPA con-
centration was measured in perchloric acid and other buffer solutions. The number of a-MPA molecules complexed with tin was determined from plots of El,, us. log a-MPA concentration. In perchloric acid solution, a value of 2.09 molecules of a-MPA per tin(1V) ion was calculated. The deviation from 2.00 is within experimental error. The Ellz of the second wave is independent of a-MPA concentration and its average is -0.420 =k 0.03 V us. SCE, This agrees with the E112 value of tin(I1) found in perchloric acid, the value -0.382 V calculated for the reduction of uncomplexed tin(I1) from thermal data, and the value for tin(I1) in pyrogallol media (4). It indicates that tin(I1) in perchloric acid solution containing a-MPA is uncomplexed. The height of the two-step wave obtained in a solution of pH value less than about 2.5, and the single-step wave obtained in a solution of pH value greater than 2.5 are both proportional to the square root of the corrected height of the mercury column, indicating that the currents are diffusion controlled. The diffusion currents were obtained by subtracting the previously measured residual currents and averaging the results. The diffusion currents for both the first, id, and second, id, waves and corresponding to the fourelectron reduction of tin(1V) to tin amalgam in perchloric acid increases linearly with increasing tin(1V) concentration as shown in Table I. The variation in diffusion current with tin(1V) concentration was also studied in other buffered electrolytes, and the relationship was found to be linear over the concentration range studied in acetate, citrate, and ammoniacal electrolytes. In ammoniacal buffer solution, when the pH was raised to a higher value than 10, the diffusion current decreased by about considerable amounts one-half. As with tin(IV)-P-MPA of stannate ion are formed at the higher pH value, and stannate ion does not form a complex with a-MPA. It was also confirmed that an electroreducible complex is not present in sodium hydroxide solution where the predominant tin(1V) species is stannate ion. The variation of the current with temperature, about +1.48 per degree, is further evidence that the process is diffusion controlled, since the temperature coefficient for a rate controlled reaction is generally larger and for an adsorption reaction smaller than this value. A maximum is not observed in perchloric acid media but it appears and becomes larger with increasing pH and tin(1V) concentration. It was completely suppressed by the addition of 0.0015 %Triton X-100. It should be noted in Figure 1 that the polarographic reduction waves at a pH value higher than about 4, exhibit a sharply decreasing minimum after attaining a flat diffusion limited plateau. It occurs at about - 1.2V us. SCE so that it does not interfere with reductions at more positive potentials. An effect of interference by other metal ions on the determination of tin(1V) ion in the solution containing a-MPA is currently being carried out in our laboratory. In addition, tin(1V) complexes of thioglycollic acid and thiomalic acid has been found to have a behavior similar to the a-MPA complexes.
(a,
RECEIVED for review September 13, 1971. Accepted January (7) I. Ruzic and M. Branica, J . Electromal. Chem., 22, 243 (1969).
1700
31,1972.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
