be accounted for by assuming that these four coordination positions are filled by formation of two chelate rings, each linking to the metallic ion a nitrogen atom and a hydroxyl oxygen atom from which the proton has been removed by reaction with the hydroxide ion. Each chelate ring accounts for an increase of approximately two log units in the value of log pz2. For the Cu(A)2(0H)2 complexes, the log pZ2values decrease as the ligand changes from the primary to the secondary to the tertiary alkanolamine. The decrease is not as great as would be expected because of the increased steric hindrance and the decrease of the value of pK*= of the amine. It is evident that an important factor is the increased probability of forming chelate rings as the number of alkanol substituents on the nitrogen atom of the ligand is increased. It is possible, also, that with the diethanolamine and triethanolamine complexes, additional chelate rings may be formed by coordination of a hydroxyl group at a fifth or sixth coordination position of the copper(I1) ion. For the diethanolamine and triethanolamine complexes it is not possible to determine, from the present data, whether the two chelate rings of the CU(A)Z(OH)~ complexes involve the same or different amine molecules. The steric effect may be noted by comparison of the log pZz values for the monoethanolamine and ethylethanolamine complexes, each of which may form one chelate ring per ligand molecule. Although the value of ~ K Afor H ethylethanolamine is about 0.5 unit greater than for monoethanolamine, the log fizzvalue of the copper(I1) complex is about one unit less. Thus the additional ethyl substituent of ethylethanolamine significantly reduces the formation constant of the complex, as compared to the monoethanolamine complex. A less pro-
nounced change in the same direction is shown by the log fizz values of diethanolamine and ethyldiethanolamine. Similar comparisons may be made for the log ,BIZvalues and the values of PKAHfor the corresponding amines for the Cu(A)(OH)* complexes. It appears that the complexes with one alkanol group have coordinated one chelate ring with one proton lost, and one hydroxide ion, while the complexes of amines with two or more alkanol groups per nitrogen atom have formed, from the same amine molecule, two chelate rings with two alcoholic protons being removed. An inspection of the distribution curves of the various figures indicates “tat as the number of substituents on the nitrogen atom increases, the Cu(A)(OH)z species are favored over the Cu(A)%(OH)2species. ACKNOWLEDGMENT
Grateful acknowledgment is given to the Data Processing Center, West Virginia University, for use of the IBM 1620 and 7040 Data Processing Systems, and to personnel of the Center for help in working out programs. RECEIVED for review January 9, 1967. Accepted August 22, 1967. In part abstracted from the Ph.D. dissertation of J. F. Fisher, 1963. Southeastern Regional Meeting, American Chemical Society, Charleston, W. Va., October 15-17, 1964. Financial assistance was provided J.F.F. by the U. S . Department of Health, Education, and Welfare, through West Virginia University, in the form of a Title IV National Defense Education Act Fellowship.
Analytical Applications of Kalousek Polarography W . F. Kinard, R. H. Philp,’ and R.C. Propst Sacannah Ricer Laboratory, E. I. du Pont de Nemours & Co., Aiken, S. C. 29801 Kalousek polarography is a variation of the squarewave and pulse techniques in which the recorded current results in part from the oxidation of the reduced species produced at the dropping mercury electrode. Two of the three Kalousek techniques were examined for applicability as analytical methods. The Kalousek current was proportional to concentration for a given reversible species, and the ratio of the Kalousek to the diffusion limited current provides an immediate qualitative measure of the kinetics of the reoxidation process. The reproducibility and analytical range is essentially equivalent to that reported for square-wave polarography, with the added advantage that reversible systems can be determined in the presence of hydronium ion discharge. Demonstrated analytical applications are based on the capability of Kalousek polarography to distinguish readily between reversible and irreversible reactions at the DME.
KALOUSEK POLAROGRAPHY (1) is a large amplitude, squarewave technique that has gone largely unnoticed in the recent literature (2). This technique, in which the recorded current results in part from the reoxidation of the reduction products formed at the dropping mercury electrode (DME), is po(1) M. Kalousek, Chem. Listy., 40, 149 (1946); Collection Czech. Chem. Commun., 13,105 (1948). (2) W. H. Reinrnuth, ANAL.CHEM., 36, 231R (1964). 1556
e
ANALYTICAL CHEMISTRY
tentially capable of resolving reversible and irreversible processes and may be applicable to systems that cannot be resolved by other relaxation techniques. In addition, the data presentation is such that kinetic complications are immediately apparent from the polarogram. Kalousek originally developed the method to study the reversibility of electrode reactions, that is, the formation of polarographically oxidizable products at the DME. Vlcek (3) has employed Kalousek polarography to demonstrate reversibility of inorganic complexes, Masek ( 4 ) has made use of this method to distinguish between a reversibly reduced cationic species and the irreversible hydronium ion reduction, and Philp, et nl. (5) chose this technique to study diketone reductions at the DME. Kambara (6) and Matsuda (7) have treated the subject mathematically. The only analytical applications reported in the literature, however, are those by 1
Present address,University of South Carolina, Columbia, S . C.
(3) . , A. A. Vlcek. ‘‘Progress in Polarography,” Vol. I, Interscience,
New York, 1962, p. 269. (41 M. Masek. Collection Czech. Chem. Commun.. 26, 195 (1959). ( 5 ) R.H. Philp, R. L. Flurry, and R. A. Day, J . Electrochem. Soc., 111, 328 (1964). (6) T. Kambara, Bull. Chem. SOC.Japan, 27,529 (1954). (7) H. Matsuda, Z . Elektrochem., 62,977 (1958).
REVE RSI BLE
I
0.096 mM Cd2+ IM HCI
= . l
.-
c.
'
T
'
Q)
t
5 cps Between Limits
0
w Q)
(+I
Time
Elect rode Potential Figure 1. Ramp voltage functions and illustrative polarograms
Kalousek and Ralek (8), who reported a linear currentconcentration relationship for cadmium, thallium, and iron. The original Kalousek polarograms ( I ) were recorded with a rotating commutator that switched the potential of the dropping mercury electrode between a potential at which the reductant was formed and a potential at which the reductant was reoxidized. The switching rate was five cycles per second, and the electrolysis current during the more anodic portion of each cycle was recorded by placing the galvanometer in series with the lead to the more positive of the two potentials. Utilizing his rotating Commutator, Kalousek devised three methods for programming the voltages applied to the DME. These methods resulted in polarograms that have been designated Types I, 11, and 111. Types I and 11, illustrated in Figure 1, are investigated in this study. Type I polarograms are recorded by superimposing a lowamplitude square wave (20-50 millivolts) onto the ramp voltage of conventional polarography. Thus, the electrode potential is cycled between the limits shown in Figure l at 5 cycles per second. Since the electrolysis current is monitored during the positive half-cycles only, the oxidation of a reduced species that was produced on the negative half-cycles will result in an anodic current. This effect is seen in the polarogram as a pronounced minimum near the half-wave potential of the reversible species. As the ramp voltage approaches the limiting current region, the anodic current decreases, and the observed current rises to correspond to the diffusion limited cathodic current. In Type I1 polarograms, the cathodic limit of the square wave is maintained at a fixed potential that is well into the limiting current region. The potential of the DME is now
alternated at 5 cycles per second between this fixed potential and the ramp potential. Polarograms recorded by this technique, Figure 1, have the familiar S shape, with a large anodic current at the start due to the oxidation of the species produced on the negative half-cycles. In this study, the electrolysis currents were measured late in the life of each drop, and the recorded current represented the instantaneous current at the end of each anodic halfcycle. The equations derived by Matsuda (7) based on the average current flowing during the anodic cycles of the square wave were, therefore, not applicable to the polarograms recorded in this study. Thus, the equation for Kalousek 2 X em polarography of reversible systems (ksh sec-I) must be derived as it applies to this operating mode. The following derivation is based on the work of Kambara (6) and Barker (9). Under conditions of semi-infinite linear diffusion, if we assume a plane electrode in an electrolyte solution that initially contains only the oxidized species that undergoes reversible reduction, then the concentration of the reduced species in a direction normal to the electrode surface at time T = t - mr after the application of a square-wave potential is given by
(8) M. Kalousek and M. Ralek, Collection Czech. Chem. Commun., 19, 1099 (1954).
(9) G. C. Barker, Polarographic Theory, Part I, AERE C/R 1553, Harwell Berks (1957).
>
where
VOL. 39, NO. 13, NOVEMBER 1967
* 1557
I":Potential)
I
i (Currant)
&w/Boostrr
2P-K2X
Amp.
Figure 2. Schematic diagram of Kalousek polarograph
CR1= the concentration of the reduced species at the electrode surface during the anodic half-cycles of the square wave CR2 = the concentration of the reduced species at the electrode surface during the cathodic half-cycles of the square wave m = number of half-cycles since the instant of polarization = period for a half-cycle 7 = elapsed time since instant of polarization t = the maximum integer less than or equal to t / T . j
Barker (9) has shown that the concentration gradient at a plane electrode whose area varies with time in the same manner as that of the DME is given by C ( z , 2= ) C* erf
f(t) (4)
where f ( t ) and J ( T ) are functions of the drop area. Barker's approach can be used to correct Equation 3 for the effects of the expanding drop to give
The concentrations CRzand CR1are defined by the Nernst equation at the potentials of the cathodic and anodic halfcycles, respectively. The remaining terms and those that follow have their usual significance. Since the instantaneous electrolysis current is given by
the Kalousek current at a plane electrode is obtained by substituting the derivative of Equation 1 with respect to X, evaluated at X = 0, into Equation 2. This substitution gives n-t
m=l
Equation 3 is the solution for the Kalousek current at a plane electrode, whereas the desired solution is at the DME. 1558 *
ANALYTICAL CHEMISTRY
Equation 5 refers to linear diffusion and is therefore not a rigorous solution for the Kalousek current at the DME. However, the equation can be utilized to evaluate the electrolysis current late in the life of a drop when the radius of the drop is large compared to the thickness of the diffusion layer. The first term of Equation 5 is a modified form of the Ilkovic equation and the second term is equivalent to the Barker equation for square-wave polarography with the exception that the summation is not carried over an infinite number of cycles, thereby eliminating the effect of the expanding drop. Therefore, the measured Kalousek current is the algebraic sum of the D.C. polarographic component and a square-wave component.
While the potential-time functions in these experiments bear a superficial resemblance to those for square-wave ( I ) and pulse ( 1 0 , I I ) polarography, the differences are significant. These differences arise primarily from the mode of current measurement. In both Kalousek techniques, the current is monitored on the anodic half-cycle only; in the square-wave technique, the current is measured on both half-cycles; in the pulse technique, current is measured on cathodic cyles only. Kalousek measurements result in a presentation in which both anodic and cathodic currents can be resolved. One is thus able to observe directly the ratio of an anodic current (dependent upon the kinetics of the electrode process) to the cathodic limiting current. The sensitivity resulting from current measurement in a cyclic process is maintained, while each polarogram gives information from which kinetic changes are immediately apparent. Since the sensitivity for reversible systems in Kalousek polarography and related techniques is highly dependent on instrumental factors (Le., AE, 7, t-mT, etc.), it is difficult to make a direct comparison of the sensitivities reported for other techniques. The sensitivity obtained in this work is comparable to that of square-wave polarography and somewhat less than that reported for pulse polarography (11). For systems that give no anodic current, the lower limit of sensitivity is essentially that of Tast polarography for the instrument used in this study.
T Y P E I KALOUSEK P O L A R O G R U 0.14mM Cadmium in IM KCI Dk :2 . 8 6 ’ ~ cm2/sec -1.0
m = 1.75 x 10-3g/sec t = 4.64 sec r = 0.085 sec
0.2
0.1
EDMEEXPERIMENTA4L
Instrumentation. All polarograms were recorded with a multipurpose instrument (12) based on operational amplifiers. A block diagram of the circuit for Kalousek polarography is shown in Figure 2. The circuitry is conventional with the exception of the drop detector and booster amplifiers which have been described (13, 14). Polarograms recorded with this instrument differ from those described by Kalousek in that the ramp voltage is stepped incrementally with each drop, the electrolysis current is monitored after 3 seconds of drop life, the electrolysis current is monitored only after the double-layer charging current has decayed, and the recorded current is the instantaneous current at the electrode at the end of each anodic half-cycle. By stepping the ramp voltage in increments as each drop falls, the reduction and oxidation potentials remain constant throughout the life of each drop, The current-monitoring sequence is accomplished by inserting two relays in series between the current amplifier and the voltage follower, which serves to monitor the output current. The first relay is energized by the drop detector through a three-second delay network. Thus, this relay connects the voltage follower to the current amplifier during the last part of drop life. Note that this relay also serves to step the ramp voltage at the instant of detachment. The second relay serves to connect the voltage follower to the current amplifier during the anodic half-cycles and only after the double-layer charging spike had decayed. Barker (IO) has described the technique utilized to eliminate the drop charging current and the advantages of measuring the electrolysis current late in the life of each drop. The technique of sampling the drop current near the end of the anodic half-cycles results in a considerable loss in sensitivity since the drop current decays as t - l I z ; this tech~
~
~~-~
(10) G. C. Barker and A. W. Gardner, 2.AnaE. Clzem., 173, 79 ( 1960). (11) E. P. Parry and R. A. Osteryoung, ANAL.CHEM., 37, 1634 ( 1965). (12) R. C. Propst. U.S. At. Energy Comm. Rept. DP-903 (1964). (13) R. C . Propst and M. H. Goosey, ANAL. CHEM.,36, 2382 (1964). (14) R. C. Propst. Ibid.,34, 588 (1962).
-0.1
0
-0.2.
Et
Figure 3 Calculated and observed polarograms for cadmium nique, however, does permit the use of rather simple gating circuits. Apparatus. A conventional 3-electrode polarographic cell was utilized. Polarograms were recorded at room temperature (24” f 1O C). The flow rate of the DME was 1.758 mg/sec, and the drop time was 4.64 seconds as measured at -0.66 volt US. the saturated calomel electrode (SCE). All solutions were degassed with helium prior to analysis. Reagents. All solutions were prepared from reagentgrade chemicals and deionized water. The cadmium and indium standards were prepared by acid dissolution of the metals. Current Measurements from Experimental Curves. Currents werc measured from recorded curves as indicated in Figure 1. In all cases i, was measured between the background current and the maximum anodic current, and id was measured between the background current and the cathodic plateau. RESULTS AND DISCUSSION
In the discussion that follows, the term “reversible” is defined as not distinguishable from mass-transport controlled. For the Kalousek technique, this means that the heterogeneous rate constant is greater than about lop3 cm sec-I. “Irreversible” is defined as an electrode reaction partially controlled by the rate of the electrochemical reaction. Theoretical Form of Current-Potential Curves. The applicability of Equation 5 to the polarograms recorded in this study is illustrated in Figure 3, which shows a plot of Equation 5 (open circles) superimposed on a Type I polarogram of 0.14 m M cadmium in 1M potassium chloride. The values of C*,t , T , D R ,m, and AEutilized in these calculations are given in Figure 3. To illustrate the shift in Emi,with El,z, the theoretical curve was calculated as ik us. El - El/z,where El is the potential of the DME during the anodic half-periods of the square wave, and the abscissa of the polarogram was VOL. 39, NO. 13, NOVEMBER 1967
1559
3.0I
I
I Cadrnlurn
Table I. Calculated and Observed Values of Eminas a Function of AE
I
in IM KGI
E
‘1 * 2,o B
I
S I
20 30 50
100
0
50
100
I50
200
AE, Square- Wave Amplitude, rnV
where
id = the diffusion limited current it = the instantaneous Kalousek current i, = the maximum anodic current Note that all of the above currents are measured with respect to a background scan on the electrolyte, and that the value for the ratio (id - ir)/(ia ir)approaches infinity as ir approaches i,. Also note that the ratio approaches zero as ia approaches
+
id.
The applicability of Equation 6 to Type I1 polarograms was demonstrated by plotting the data for a Type I1 polarogram of 0.14 m M cadmium in 1M potassium chloride as log (id - &)/(ia ik)us. El. The slope of the resulting plot that corresponds to RTJF was 0.030, and the half-wave potential was -0.64 V. These values agree well with the theoretical value of 0.0294 for RTjF and the published value (15) of -0.64 for the half-wave potential. Shift in Emi, with AE. Because the Kalousek current, Equation 5 , is the algebraic sum of the diffusion and squarewave currents, the minimum in the current-potential curve for Type I polarograms does not correspond to the half-wave potential. An expression for this shift in Emin with A E can be derived by combining Equation 5 with the Nernst equation and writing EZin terms of EI-AE. When the resulting equation for in is differentiated with respect to El and the result is equated to zero, one obtains
+
(15) L. Meites, “Handbook of Analytical Chemistry,” 1st ed., McGraw-Hill, New York, 1963, pp. 5-59. 1560
a
ANALYTICAL CHEMISTRY
Calcd
Found
0.016
0.015
0.015 0,019
0.019 0.019
0.027 0.052
0.045
0,030
nF RT
exp -. (Emin - E d nF b - exp-AE[--RT b-1
Figure 4. Kalousek current us. square-wave amplitude shifted to conform to this convention. We believe the results show excellent agreement between the two curves in spite of the 8 % difference in the calculated and observed values for i x at Emin. Although better agreement is desirable, a difference as large as 5 was anticipated in view of the approximate nature of Equation 5 and the uncertainties in the symmetry of the square wave and the values for f, T , and D. The square-wave frequency and symmetry were determined by means of a Tektronix Model 564 Oscilloscope. Equation 5 can also be applied to Type I1 polarograms for reversible processes. Since, in this case, CR2is approximately equivalent to the bulk concentration of the species (the potential E2 is set well into the limiting current region of the wave), Equation 5 can be combined with the Nernst expressions for CR1and c R 2 to give
Emin, V
-
AE, mV 10
- i]
rt
where 1
i
Equation 7 shows that the shift in E,,,,, is not only a function of AE but also of the parameter b. Since the parameter b is a function characteristic of the instrument, Emin values recorded with different instruments will be difficult to compare. Calfor cadmium as a funcculated and observed values of Emin tion of AE are given in Table I. The polarograms from which these experimental values were obtained were recorded by stepping the electrode potential 0.005 V with each drop. Thus, these results can be in error by as much as 0.005 V. Equation 7 does not apply to low (< 10 mV) values of AE because no minimum can exist when the instantaneous squarewave current is less than the instantaneous diffusion limited current. Although no minimum exists for Type I1 polarography, Type I1 polarograms can be treated as conventional polarograms when the wave height is measured from the residual current plateau without regard to sign. In this case, the current at the half-wave potential equals iSw/2where i, is the difference in height between the limiting and residual current plateaus. Note that the definition of i,w conforms to Equation 5. Kalousek Current as a Function of AE. Figure 4 shows the relationship between values of i,t measured at Ernznand the amplitude of the square-wave voltage (AE) for Type I polarograms. Although an equation for this relationship can be obtained by combining Equation 5, Equation 7, and the Wernst equation (noting that Ez = El AE), the resulting expression is cumbersome, Inspection of Equation 5 shows that as A E becomes larger, the current ir approaches the maximum observed for Type I1 polarography. However, in this case, the minimum in the current-potential curve becomes so broad that Emi, becomes meaningless. Sensitivity for Reversible Systems. The Kalousek technique is most sensitive with Type I1 polarograms for reversible systems. This increase in sensitivity over conventional polarography is illustrated in Table 11, which shows the ratio of is, to id for Type I1 polarography. This increase in sensitivity can be predicted from Equation 5 since, in this case,
+
T1+
Cd+2 Pb+2
.-
In+3
L
P
uos+2
2.11x I O - 5 ~ TAST POLAROGRAM 0
4 2.0 c
c
z
L
a
-
1.5
Table 11. Sensitivity for Reversible Systems (i/c values in p A/mmole) i, (Type II)i/d idType IIYc idlc 4.94 1.28 3.86 26.7 7.56 3.53 9.90 3.63 35.9 36.2 11.4 3.18 4.40 3.57 15.7
Table 111. Determination of Cadmium in 1M HCI Cadmium found, mmoles Cadmium taken, mmoles Type I1 Type 1 0,0190 0.0190 0.0193 0.0215 0.0206 0.0206 0,0245 0.0240 0.0247 0.0485 0.0483 0.0483 0,0579 0,0576 0.0582 Std dev (relative) 0.9% 2.3z
.-0UI
.n Y-
1.0
"
-0.4
-0,5 -06 -0.7 -0.0 Electrode Potential vs. S . C . E .
Figure 5. Determination of cadmium in 1M nitric acid the value for the summation term was 3.64. Thus, the ratio of i, to id is given by
Note that the value of the ratio for the indium systems is significantly less than 3.64 indicating kinetic complications, and that the value of 3.57 for the uranium system indicates reversibility for the U(V1)-U(V) process. Thus, the ratio of ir to id serves as an indication of reversibility. Half Width of Minimum. Intuitively, one would expect the same type relationship between the width of the minimum 2 that applies to square-wave and pulse polarat ography to also hold for Type I polarography; however, this relationship can only be approximated in Type I polarography because the Kalousek current is the algebraic sum of the diffusion limited and square-wave components as indicated previously. Perhaps the Type I technique would be more useful if only the square-wave component is recorded. Under these conditions, the width at one half the depth of the minimum should be inversely proportional to n as in square-wave and pulse polarography. Resolution. As in square-wave and pulse polarography, the resolution with the Type I technique is related to the amplitude of the square wave. Since no minimum is observed for low amplitudes of the square wave (less than about 10 mV), the resolution with Kalousek polarography is not so good as with other relaxation techniques. Analytical Applications. Typical analytical results for reversible systems are given in Tables I11 and IV. The results by Type I polarography were obtained with a square-wave
Table IV. Determination of Uranium in 1M HCI Uranium found, mmoles Uranium Type 1 Type I1 taken, mmoles 0.00398 0.00398 0.00405 0.00642 0.00635 0.00645 0.00793 0.00790 0.00780 0.0111 0.0111 0.0114 0.0158 0.0158 0.0157 Std dev (relative) 0.9% 1.7% Table V. Determination of Cadmium in Presence of Europium in 1M KCI (Cadmium-europium ratio: 0.842) -Cadmium found, mmoles Cadmium taken, mmoles Type I Type I1 0,00824 0.00840 0.00840 0.0124 0.0127 0.0120 0.0206 0.0215 0.0214 0.0247 0.0235 0.0230 0.0618 0.0628 0.0615 Std dev (relative) 3.5z 4.0%
amplitude of 50 mV and were more precise than those obtained by the Type I1 technique. The difference in precision of the Type I and Type I1 techniques cannot be explained at this time. Application of the Kalousek techniques to the determination of a reversible species in the presence of an irreversible species that is reduced at or near the same potential was demonstrated by the determination of cadmium in 1M potassium chloride solutions containing europium(II1). Since the Eu(II1)-Eu(I1) couple is irreversible in 1M solutions of potassium chloride, the maximum anodic current for the system is a direct measure of the cadmium concentration. These results are given in Table V. Another interesting application of Kalousek polarography is the determination of a reversible species which has a reduction wave masked by hydrogen discharge. A reversible system undergoing reduction simultaneously with the highly irreversible hydronium ion can, in principle, be determined by either Type I or Type I1 polarography. The system chosen VOL. 39, NO. 13, NOVEMBER 1967
1561
Table VI. Determination of Cadmium in 1M HNOa (Type I Polarograms) Cadmium taken, mmoles Cadmium found, mmoles 0.0211 0.0210 0.05288 0.0530 0.106 0.105 0 . I58 0.158 0.211 0.214 Std dev (relative) 0.96%
to test this hypothesis was cadmium in 1M nitric acid. Figure 5 shows a comparison of conventional and Type I polarograms for the cadmium analysis; the results are given in Table VI. Although the Kalousek technique does not offer any ad-
vantages over square-wave and pulse polarography for routine applications, this technique appears to have real value in its application to electrochemically irreversible systems. For example, Matsuda (7) has shown that the anodic current observed in Type I polarography is a direct measure of the quantities 01 and kahfor the oxidation process; thus, in principle, these quantities can be directly obtained from the polarogram. As another example, anodic waves have been observed [e.g., for Eu(III), Cr(III), and Ni(1I)I at potentials widely separated from the reduction waves. Analysis can thus be based on measurement of the anodic current in those cases where the reduction waves are ill-defined or masked by the discharge of hydronium ion.
RECEIVED for review May 31, 1967. Accepted August 17, 1967. Work supported by Contract AT(07-2)-1 with the U. S. Atomic Energy Commission.
tentismetric Study of Base Strengths in the Binary Solvent, cid-p-Dioxane Orland W. Kolling and D. Allan Garber Chemistry Department, Southwestern College, Winfield, Kansas 67156 The indicator function of the glass electrode in acetic acid-p-dioxane i s identical to that in anhydrous acetic acid, and the glass-calomel pair is precisely responsive within the mixed solvent mole fraction range from 0.45 t o 1.00 in acetic acid. Increases in the apparent base strengths of strong and weak bases are observed when half-neutralization potentials in the binary solvent are compared to those in pure acetic acid. These changes appear to arise from two sources involving the p-dioxane content of the solvent: a positive shift in the standard potential for the cell; and the repression of ion pair dissociation for both the perchloric acid titrant and the base perchlorate salt with decreasing dielectric constant. Within the solvent mole fraction region considered, the value of KE for both strong and weak bases is independent of the amount of Pdioxane present in the mixed solvent.
NONAQUEOUS TITRATIONS of nitrogen bases in the presence of p-dioxane as the major solvent component or as the medium for the perchloric acid titrant have been analytically important for several years. Early methods of this type included the titration of aliphatic amines by Fritz ( I ) , and the determination of hydrohalide salts of organic bases by Pifer and Wollish (2). An equal molar mixture of p-dioxane and formic acid was found by McCurdy and Galt (3) to be a more effective medium than glacial acetic acid for the conductometric detection of the titration end point for weak bases. More recently, Puthoff and Benedict ( 4 ) demonstrated the suitability of perchloric acid in p-dioxane as a titrant for the potentiometric titration of high molecular weight amines and their salts.
(1) J. S.Fritz, ANAL.CHEM., 22, 578 (1950). (2) C . Pifer and E. Wollish, Ibid.,24, 300 (1952). (3) W. McCurdy and J. Galt, Ibid.,30, 940 (1958). (4) M. Puthoff and J. Benedict, Ibid.,36,2205 (1964). 1562
e
ANALYTICAL CHEMISTRY
The apparent strength of weak bases is increased in p dioxane media compared to that in solvents of higher dielectric constant; however, the solvent parameters responsible for this effect have not been identified. The extensive studies by R. M. Fuoss (5) on the conductance of salts have shown that ion pair association increases nearly predictably with decreasing dielectric constant in binary solvents which are largely p-dioxane. On the other hand, specific solvation of ions by p-dioxane can compete with the influence of the dielectric constant upon association equilibria (6). The intent of the investigation reported herein was to determine what solvent influences are exerted by p-dioxane upon the potentiometrically measured basicity of nitrogen bases and periodic group Ia acetates. Since reliable basicity constants are available for a wide range of compounds in anhydrous acetic acid, the addition of p-dioxane to the solvent permits a more exact evaluation of its effect upon the apparent base strength. Changes in the e.m.f. for the glass-calomel electrodes in acetic acid-p-dioxane were determined for solutions of perchloric acid, as well as for the four common reference bases used in acetic acid : potassium hydrogen phthalate; sodium acetate; sodium salicylate; and 1,3-diphenylguanidine. Additional strong and very weak bases (in acetic acid) included in this work are listed in Table I. For the two-component solvent, the dielectric constant interval extended from 3.7 to 6.24 (acetic acid) at 25" C.
EXPERIMENTAL Apparatus. All emf measurements were made with a Leeds & Northrup model 7401 pH meter equipped with the standard calomel and glass electrodes. Because of the slow response of the electrodes in p-doxane media, equilibrium ( 5 ) T. Fabry and R. Fuoss, J. Phys. Chem., 68,971 (1964). ( 6 ) J. B. Hyne, J. Am. Chem. SOC.,85, 304 (1963).
