CONCLUSION
ACKNOWLEDGMENT
The differential method permits the use of both radioisotopes for isotope dilution and current-time integration to obtain good accuracy and precision. SRDA has the advantage that exhaustive electrolysis is not required and electrolysis times of 200 seconds or less can be employed. Because preelectrolysis of each cell is necessary, the actual experimental time is approximately 1 hour. With more efficient stirring techniques, such as the ultrasonic method proposed by Bard (8),this time can be shortened considerably. The useful lower concentration limit for this cell geometry is approximately 2 x 10-6M. This could be increased somewhat by suitable modification of the electrolysis cell design.
The authors express sincere appreciation to the following members of the NBS staff: to George Marinenko for his many helpful suggestions, to E. June Maienthal for the analysis of a tracer solution, and to Fillmer C. Ruegg for his suggestion of the voltage-to-frequency converter.
(8) A. J. Bard, ANAL.CHEM., 35, 1125 (1963).
RECEIVED for review July 14, 1967. Accepted September 27, 1967. This work was in partial fulfillment for the Ph.D. degree of P. A. Pella, University of Maryland, 1967. In order to specify adequately the procedures, it has been necessary to identify commercial materials and equipment in this paper. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply the material or equipment identified is necessarily the best available for the purpose.
Polarographic Behavior of Nickel(l1) in Presence of Excess Triethylenetetrarnine D. C. Olson Shell Dewlopment Co., Emerydle, Calif.
A polarographic study of the reduction of Ni(ll) in the presence of excess triethylenetetramine (trien) was carried out in an effort to add to the available information on the relationships between the polarographic behavior of metal complexes and their structure. The dependence of the Ell2 of the Ni(ll) wave on the concentration of excess trien indicated the presence of two electroactive species, NiT(H20)2+2and Ni2T3+4, each predominating over a different range of trien concentration. NiT(H,0)2+2 is apparently directly reduced to a complex containing zero valent nickel which rapidly decomposes to the amalgam and free trien. The reversibility of the reduction shows that the replacement of water by trien in the inner coordination sphere of Ni(ll) makes its reduction easier. The reversibility may be due to the adsorption of the complex on the electrode surface where its reduction is facilitated by the high electric field present in the inner Helmholtz region.
THEPOLAROGRAPHIC BEHAVIOR of a metal ion in the presence of a complexing agent depends strongly on the nature of the ligand involved and the structure of the complex formed. Few of the relationships between structure and polarographic behavior have been worked out for Ni(I1) complexes. Some of the most informative work in this connection was done by VlEek ( I ) , who reported that when Ni(I1) is complexed by amines (ammonia, pyridine, ethylenediamine) or chloride, direct reduction of the complex occurs to a species containing zero valent nickel, which rapidly decomposes to the metal (amalgam) and ligand. In the case of ligands containing oxygen as the donor atom, the complex is decomposed before or during the electrode reaction. Mark and Reilley ( 2 ) recently reported that in the presence of a large number of amines, Ni(I1) gives a catalytic wave which occurs at a more positive potential than the Ni(H20)6+2 (1) A. A. Vlrek, Z . Electrochem., 61, 1014 (1957). 35,195 (1963). (2) H. B. Mark, Jr., and C. N. Reilley, ANAL.CHEM.,
wave. From a detailed study of the mechanism of the electrode process with o-phenylenediamine as the amine, Mark (3) concluded that the catalytic wave is due to a complex formed at the surface of the electrode. In a later paper ( 4 ) on the polarography of the Ni(I1)-ethylendiamine system, Mark discussed two mechanisms which would explain the increased ease of reduction of Ni(I1) when complexed by ophenylendiamine or ethylendiamine. One of the mechanisms, which required the formation of a more easily reduced tetrahedral complex, was ruled out on the basis of electrochemical and spectral studies. The other involves a preceding dehydration step which is facilitated by the presence of the amine in the inner coordination sphere of Ni(I1). It has been reported (5)that the replacement of one or more water molecules in the primary solvation sphere of a metal ion by a foreign ligand results in an enhanced lability of the remaining water molecules. The work reported in the present study was undertaken to gain further information on the effect of complexing amines on the polarographic behavior of Ni(I1). The amine chosen for the study was triethylenetetramine (trien), a tetradentate ligand which forms an octahedral complex with Ni(I1) (6, 7) having two of the coordination sites occupied by water molecules. An irreversible polarographic wave with a large maximum has previously been reported by Jacobsen and Schroder (8) for Ni(I1) in the presence of excess trien. Millimolar solu(3) H. B. Mark, Jr., J. Electroanal. Chern., 7,276 (1964). (4) H. B. Mark, Jr., Ibid., 8, 253 (1964). ( 5 ) R. E. Connick, T. J. Swift, and E. E. Genser, paper presented at the 142nd Meeting, ACS, Inorganic Division, Atlantic City, N. J., September, 1962, abstr. p. N 22. (6) H. B. Jonassen and B. E. Douglas, J. Am. Chem. SOC.,71, 4094 (1949). (7) H. B. Jonassen and A. W. Meibohm, J . Phys. Colloid Chern., 55, 726 (1951). (8) E. Jacobsen and K. Schrgder, J. Phys. Chem.. 66, 134 (1962). VOL. 39, NO. 14, DECEMBER 1967
1785
tions of Ni(I1) were used by these authors. However, when much lower concentrations of Ni(I1) are used, a reversible * O * ' wave is obtained with either no maximum or a small one which can be completely suppressed with Triton X-100. An investigation of the mechanism of the polarographic reduction of Ni(I1) in the presence of excess trien under these conditions 0 . is reported. The reversibility of the reduction of the Ni(I1) trien complex cannot be explained by a trien labilization of the Ni(I1) hydration sphere. The reversibility may be due to the adsorption of the complex on the electrode surface where its reduction is facilitated by the high electric field present in the inner Helmholtz region.
8 1
EXPERIMENTAL Apparatus. An ORNL-controlled potential polarograph, Model Q-1988 A, and a Varian F-80 x-y recorder were employed to obtain the polarograms. The characteristics of the DME capillary in 1.OM NaC104 were m = 2.41 mg/sec and t = 3.37 sec with open circuit and h = 80 cm. A platinum wire served as the auxiliary electrode and a saturated NaCl calomel electrode (SSCE) as the reference. The potential of the SSCE has been reported (9) to be - 5 mV US. the SCE. A water-jacketed H-cell was used in which the sample and reference compartments were separated by a sintered-glass plug and an agar bridge containing NaCl. The cell temperature was maintained at 25.0 =t0.1 C. Reagents and Stock Solutions. All chemicals were reagent grade. Ni(C10& 6 H z 0 was prepared from nickelous carbonate and perchloric acid and recrystallized from water. A stock solution of the nickel perchlorate was standardized by EDTA titration using murexide as an indicator. The standard EDTA solution used in the titration was prepared from a weighed quantity of the acid form of EDTA which had been recrystallized from water and dried at 100" C . Trien disulfate was recrystallized twice using a procedure described by Reilley and Sheldon (IO). A stock solution was standardized by passing an aliquot through a Dowex-50 ion exchange column in the hydrogen form to remove the trien followed by amperometric titration of the eluted sulfate with Pbf2. The Pb+2solution used in the titration was standardized by a gravimetric method where the lead was precipitated as the sulfate and ignited to the oxide. Polarographic Solutions and Measurements. The polarographic solutions were prepared by dilution of the stock solutions. The ionic strength was maintained at 1.0 with NaC104. The solutions were buffered with 0.010M borax, although the excess trien present also served as a buffer over part of the pH range studied. The pH of the solution was adjusted with HC104or NaOH and measured with a Beckman 41260 Type E-2 glass electrode standardized with buffers good to k0.03 pH units. The solutions were purged for 20 min with nitrogen prior to each run to remove dissolved oxygen. Polarograms were obtained using a potential scan rate of 100 mV/min with no current damping for a series of solutions 5.40 X 10-jM in Ni(I1) and ranging in excess trien concentration from 9.48 X fO-4M to 4.72 X 10-2M. The pH was varied from 7 to 11. A Triton X-100 concentration of 0.0006 to 0.0012% was sufficient to suppress completely the small maximum present. The residual current was measured at each pH with solutions containing all components except Ni(I1). The dependence of the current on the mercury head was measured at constant potential and i/idratios ranging from 0.11 to 1,Ousingasolution 2.16 X IO-dMin Ni(1I)and 4.74 x 10-2M in trien at a pH of 7.00. A correction of 1.5 cm was
-----
0.2
I
N o Trien Disulfate, p H 6.5 9,48 x M Trien Disulfate, pH 6.99
M
4.73 x
I
I E
VI
(1958). (10) C. N. Reilley and M. V. Sheldon, Talanta, 1,127 (1958). 1786
a
ANALYTICAL CHEMISTRY
I
I
S.S.C.E.
Figure 1. Polarograms of 5.40 X lO+M Ni(I1) in 1.OM NaC104, 0.010M borax, 0.0006 Triton X-100 made for the back pressure to obtain h,,,,. The observed currents were corrected for residual current, The viscosities of the solutions were measured with a Zeitfuchs capillary viscometer. RESULTS AND DISCUSSION Properties of the Polarographic Wave. The polarographic wave for the reduction of 5.40 X lO-jMNi(I1j in the presence of excess trien is well defined at pH 7 as shown in Figure 1. At higher pH values a small maximum develops which can be completely suppressed with 0.0006 to 0.0012 Triton X-100. The maximum increases with increasing pH and Ni(I1) concentration but decreases with increasing trien concentration so that the amount of suppressor required depends upon these three variables. At the lower trien concentrations and pH values, the Triton X-100 was observed to shift the Eliz slightly negative if its concentration was raised above 0.0006%. The amount of the suppressor which could be tolerated with no Eliz shift increased as the trien concentration and pH increased. For example, at a pH of 7 and 4.74 x lO-3M trien, an increase in the Triton X-100 concentration from 0.0006 to 0.0012% shifted the ElIz26 mV negative; at pH 10 no shift in El,zwas observed under the same conditions. The Elt2of the wave was dependent upon both the trien concentration and the pH, an increase in either causing a negative shift in the Eiiz. This behavior is indicative of the presence of equilibria involving the electroactive species. The complex Ni(I1) species formed in the presence of excess trien have previously been determined (7) and the following equilibria may be written under the conditions of the present study :
+ T Kie N~T(HZO)~+ KS:~ 2Ni(,,,+* + 3T & NizT3+4 i
(9) H. A. Laitinen and W. J. Subcasky, J . Am. Chem. SOC.,80, 2623
Trien Disulfate, pH 9.03
Ni(,q)+
1.28,
.
1 .@O
5 . 4 8 x I O 4 M Trien, p t i 6
~~
-3.0
0.96
I
~
I
I
I
I
I
-1.0
-2.0 a)
I
Io* iz/ic-i
0 ) log
t i .n
0
i/'id-i
Figure 3. Logarithmic analysis of waves for Ni(II) in presence of excess trien
4e
+ Ni2T3+4e 2Ni(,,) + 3T
and the current-potential relationship is where T is the unprotonated form of trien and Kl:l and Kaz2 are the concentration stability constants of the indicated complexes. The concentration of T varies with pH because of the equilibria. K4
T+H+&HT+
where Kz,K3, and Kd are the concentration ionization con, HT+, respectively. stants of H B T + ~H, 2 V 2 and The regions of excess trien concentration in which the NiT(H20)2+2and the Ni2T3+4species predominate are sufficiently separated to treat each of them independently. The current-potential equations for each of the situations may be derived by the method of Lingane (11)if a reversible electrode process is assumed. When NiT(H20)2+2is the principal diffusing species, the overall electrode reaction is
2e
+ NiT(HzO)zf2 e Ni,,,) + T + 2H20
and the current-potential relationship is
with E l / *= Eo'
d1 :IKI'1 - 0.0295 log - 0.0295 log CT d,
(la)
where i and id are the current and diffusion current at the end of the drop life, E"' is the formal potential of the Ni(,Q)+2, couple, d1:1and damare the diffusion current constants of NiT(H20)2+2and Ni(am),respectively, and CT is the concentration of free trien. When Ni2T3+4is the principal Ni(1I) complex in solution, the overall electrode reaction is
E
i2
= Eli2
- 0.0148 log 7. ld - 1
(2)
with E112 =
E"' - 0.0295 log
d3;21 i2K3&'2
- 0.0443 log CT (2a)
drn
Because a large excess of trien (18 to 88-fold) was used, CT can be calculated directly from K z , K 3 ,and K4 and the pH of the solutions. Values of the ionization constants in 1.OM NaC104 at 25.0" C were calculated with the Van't Hoff equation from reported values (12) of the constants in 1.OM N a N 0 3at 30" and 40" C. Equations l a and 2a predict rectilinear plots of Elizus log C, with a slope of 29.5 mV in the region where NiT(H20)2+2 is the predominant complex in solution and a slope of 44.3 mV where Ni2T3+4predominates. The experimental plot, shown in Figure 2, is in good agreement with theory, having two linear regions with slopes of 28 and 43 mV. The Eli2 values in the region of lower trien concentration were obtained from plots of Equation 1. The plots gave good straight lines, as illustrated by a typical example in Figure 3, with slopes ranging from 28 to 35 mV. Plots of Equation 2 were used to obtain the Eli2values in the region of higher trien concentration with i and id expressed in microamperes. The plots deviated from linearity at potentials near the top of the wave as seen from the example given in Figure 3. The slopes, which range from 13 to 18 mV for the linear portions of the plots, agreed well with theory. Plots of Equation 1 actually gave better straight lines than Equation 2 in the region of higher trien concentration, but the values obtained with Equation 1 showed the same dependence on -log CT as those obtained with Equation 2. Because the slope of the plot of El:2c's -log CT is equal to 2 9 . 5 ~( I ] ) , where p is the number. of ligand molecules per metal ion involved in the electrode process, the same conclusion is reached in either case-Le., the depolarizing complex in the region of higher trien concentration is composed of 1.5 trien molecules per nickel ion. The diffusion current of the wave decreased somewhat with increasing pH and trien concentration. A decrease in id (12) A. E. Martell, "Stability Constants of Metal-Ion Complexes,
(11) J. J. Lingane, Ckem. R e m , 29, 1 (1941).
Section 11, Organic Ligands," The Chemical Society, Burlington House, London, 1964. VOL. 39, NO. 14, DECEMBER 1967
e
1787
10.1
4.0,
Ciope t0.6
-
+0.4
-
i/id Ratios
0
0.50
1 .o
0
0.47
0.59
0
0.43
0.45
A
0.47
0.20
v
0.34
0.1 1
3.8
3.6
a
e
t0.2
:
3.4
c
3.2
C
._ - -0.2
0 0.568 M
3.0
in N a i S 0 4
0
0.283 M in T r i e n D i s u l f a t e , pH 7
0
0.238 M i n NiT(H,0)2t2
A
0.253 M i n T r i e n Disulfate, pH
\
IO I
2 .a
-0.4
c
-0.8
- 1 .o 1.2 1
I
I
1.6
1.8 '09 h c o r r .
Figure 4. Dependence of current on mercury head at various ilid ratios would be expected in going from the region of lower to higher trien Concentration because the diffusing species changes from . an increase NiT(H20)2+2to the larger N ~ z T ~ +In~ addition, in the viscosity of the solutions was observed to occur under the same conditions, which would also cause a decrease in id. The variation of id and the viscosity with trien concentration and pH is given in Table 1. Reversibility of the Electrode Process. The good fit of the polarographic waves obtained in the region of lower trien concentration to Equation 1 indicates that the reduction of NiT(H20)2+2corresponds to a two-electron reversible electrode process. The reduction becomes slightly irreversible below pH 7 as evidenced by slopes >29.5 mV obtained from plots o f Equation 1. Because a plot of the current-potential equation for a reversible system is not always a reliable criterion of reversibility, an independent method was employed to test the reversibility. The dependence of the current at various [/id ratios on the mercury head was studied. The dependency may be expressed by the equation i = kh,,,,5 where i is the
Table I. Dependence of id on Trien Concentration pH, and Viscosity id, pa [Trien] PH 7, CP. 1.19 x 10-3 7.0 0.871 0.453 1.19 X 10.0 0.887 0.400 4.72 X 10.0 0.931 0.326 4.72 X 9.0 0.926 0.360 4.72 X 7.0 0.915 0.420 5.92 x 10-3 8.0 0.878 0.442
1788
-0.2
-0.4
-1.0
-0.6 -Os% Volts vs S.S.C.E.
-1.2
-
A
Figure 5. Dependence of drop time on potential for solutions 1.OM in NaC104, 0.010M in borax, 0.0006% Triton X-100
-0.6
-
0
ANALYTICAL CHEMISTRY
current at the end of the drop life, k is a constant, h,,,, is the mercury head corrected for back pressure, and the value of x depends on the reversibility of the electrode reaction (13). For a reversible system x is constant at 0.50 for all i/id ratios, whereas for an irreversible process it varies from 0 at the bottom of the wave to 9.50 at the top of the wave. Figure 4 shows log-log plots of the results obtained using a NiT(H20)2+Z solution containing excess trien at pH 7 at i/id ratios ranging from 0.11 to 1.0. The slopes, which are equal to x, are in fair agreement with the reversible value, although the small decrease observed at lower i/id ratios indicates some irreversibility at pH 7. Mechanism o f the Electrode Process. The reduction of NiT(HzO)2+2 may proceed by either of three mechanisms. A. The complex is dissociated, followed by reduction of the hydrated metal ion : -T
2e
NiT(Hz0)2+2-+ Ni(nqlT
Ni,,,,
B. The complex is decomposed during the electrode process : 2e
NiT(Hz0)2-2
-+
N&,)
+ T + 2H20
C. The complex is directly reduced, followed by its decomposition: 2e
NiT(Hz0)2-2
--f
+ T + 2H20
NiT(H20)20 Ni,am) -+
The first mechanism may be eliminated for three reasons. First, it Lollows from Equation l a that the Elizof the wave of NiT(H20)2+2would have to be more negative than that of Ni(aq)-2at all trien concentrations. However, as shown in Figure 1, the El,?ofthe NiT(H20)2i wave at the lowest trien concentration used is more positive than the of the Nicaq)12 wave. Second, because the reduction of Ni(,Q)+Pis irreversible ( 1 4 , a reversible wave would not be expected if the hydrated nickel ion were the electroactive species. Third, the first mechanism requires that the rate of dissociation of NiT(13) A. A. VlEek, Progr, Inorg. Chem., 5, 229 (1963). (14) 1. N. Kolthoff. and S. J. Lingane, "Polarography," Vol. 11, second ed., p. 486, Interscience, New York, 1952.
(Hz0)?+2be more rapid than its rate of diffusion because a diffusion-controlled wave was observed. To meet this requirement, the rate constant for dissociation must exceed 10; second-I (15). The rate of dissociation of NiT(Hz0)2+2 has been reported in the literature (16) and is many orders of magnitude slower than the above requirement. For example, kdlss is equal to 10-7 sec-’ at pH 7 (16). The rate of dissociation is, in fact, so slow that no wave at all would be obtained if prior dissociation were necessary. The reduction must, therefore, proceed by either mechanism B or C. The present study does not allow a distinction to be made between the two, but C appears to be the more plausible because VlEek ( I ) previously showed by oscillopolarography that the Ni(I1)-ethylenediamine complex is reduced by this mechanism. Because the reduction of NiT(Hz0)2+2is reversible while the reduction of Ni(H20)612is irreversible, the question arises of why complexation of Ni(I1) by trien accelerates the electrontransfer step. It has previously been observed that certain other ligands also facilitate the reduction of Ni(I1) ( 2 , 3 , 17) and other metal ions (18, 19). This effect may be due to the labilization of the hydration sphere of the metal ion as a result of the replacement of one or more of the inner coordinated water molecules by a foreign ligand. This explanation arises from the theory that metal ions must undergo partial dehydration prior to reduction. Hush and Scarrot (20) have measured the heat of activation associated with the kinetic step in the reduction of Ni(H20)6-2in solutions of high ionic strength. The fact that it lies close to the heat of activation for the exchange of water between the inner coordination sphere of Ni(a9)+2and solvent, measured by NMR (21), provides evidence that the kinetic step in the reduction of Ni(HzO)6+2involves water loss (or gain). An increase in the rate of water exchange would, therefore, be expected to cause an increase in the rate of the reduction. Several authors ( 5 , 22) have reported that complexation by a foreign ligand does labilize the remaining water molecules in the hydration sphere of a metal ion. For example, while the rate constant for the is 2.74 X lo4 sec-I (21), it exchange of water for Ni(HpO)6+2 appears to be 1.2 X l o 6sec-1 for Ni(glycine)(H20), (22). In the case of NiT(Hz0)2+2,the above explanation cannot account for the reversibility of its polarographic wave because the rate of water exchange is actually slightly slower for (15) A. A. VlEek, Progr. Znorg. Chem., 5,217 (1963). (16) D. W. Margerum. D. B. Rorabacker, and J. F. G. Clarke, Jr., ’ inorg. Chem.,2, 667 (1963). (17) i. V. Nelson and R. T. Iwamoto. J . Electroanal. Chem.. 6, 234 ‘ (1963). (18) A. J. Engel, J. Lawson, and D. A. Aikens, ANAL.CHEM., 37, 203 (1965). (19) J. A. Schufle, M. E. Stubbs, and K. E. Whitman, J . Am. Chem. SOC.,73, 1013 (1951). (20) N. S . Hush and J. W. Scarrot, J . Electroanal. Cliem., 7, 26 (1964). (21) T. S. Swift and R. E. Connick, J. Chem. Phys., 37, 307 (1962). (22) G . C. Hammes and J. I. Steinfeld, J. Am. Chem. SOC., 84, 4639 (1962).
NiT(H20)2+2than for Ni(Hz0)8+3 (23). The reversibility may be due to the adsorption of the NiT(H20)2+2complex at the electrode surface where its reduction is facilitated by the high electric field in the inner Helmholtz region. Figure 5, which shows the potential dependence of the drop time of the DME in solutions with and without trien and NiT(H20)2+Z9 indicates that both the ligand and the complex are adsorbed at the surface of the drop. The adsorbed NiT(Hz0)2+2 complex would be under the influence of a stronger electric field than the nonadsorbed Ni(Hz0)6+2 species. The strong field existing in the vicinity of the electrode surface can alter the structure and properties of a complex ion (24). It is possible in the case of NiT(Hz0),-2 that a more readily reduced species is produced, or the proper orientation for reduction is facilitated, as a result of the influence of the electric field. This mechanism does not preclude a preceding hydration step, The influence of the electric field on the adsorbed complex may labilize the coordinated water molecules. RECEIVED for review August 11, 1967. Accepted September 28, 1967. (23) D. W. Margerum, and H. M. Roseii, Ibid.,89, 1058 (1967). (24) A. A. VlEek, Progr. Znorg. Chem., 5, 249, 257, 295 (19633.
An Improved icrswave Emission Gas Chromatography Detector for Pesticide Residue Analysis In this article by H. A. Moye [ANAL.CHEM., 39,1441 (1967)j there is an error in the recording of data. All references to monochromator slit width are incorrect by a factor of 10. The 6-micron slit width mentioned twice in the text should in actuality be 60 microns. Figures 1 and 2 on page 1442 should have ‘‘X 10-1’’ included in the legends of the horizontal axes.
Determination of
uantities of Phenol in on C a p t ~ r ~
In this article by R. E. Long and 6. C. Guvernator 111, [ANAL.CHEM.,39, 1493 (1967)] on page 1495 the credit for financial support is incorrect. Instead, the credit pertains to the paper beginning on page 1495, A Statistical Method for Evaluation of Limiting Detectable Sample Concentrations, by P. A. St. John, et al.
VOL. 39, NO. 14, DECEMBER 1967
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