509
Anal. Chem. 1985, 57, 509-513
Interaction between Amines and Carboxylic Acids in Acetonitrile by the Anodic Wave in Polarography Masashi Hojo* and Yoshihiko Imai Department of Chemistry, Faculty of Science, Kochi University, Kochi 780, Japan A good correlation was found between the half-wave potential (€,,,) of the anodic wave and the basicity of the amine from aniline (pKd,,+ = 10.6 in MeCN, E,,, = +0.21 V vs. AgiO.1 M AgCi0,-NleCN) to diethylamine (18.8, -0.145 V ) In acetonitrile with 0.1 M Et,NCiO, as the supporting electrolyte. The addition of a large excess of benzoic acid (HA) to a stronger base (>18) produced the species of BH'A-(HA),. The presence of the A-( HA), ion was confirmed by the E,,, shift of the anodic wave, changing the concentration of HA. Other carboxylic acids from acetic acid (pK, = 22.3) to dichioroacetic acid (15.8) gave the similar species, and the linearity was present between E,,, of the A-(HA), type species and the acidity in MeCN. Cryptand[2.2.2] formed a species with two of A-(HA),, while triethylenediamine formed a species with one A-(HA), and an unidentified anion in the presence of a large excess of benzoic or acetic acid. On the other hand, the conjugated acid of an amine gave a cathodic wave at a negative potential, e.g., E,,, = -2.37 V for the ( n BU),NH+ Ion.
Carboxylic acids are weak acids in aqueous solutions. The acidity is extremely weakened in a protophobic solvent, such as acetone, acetonitrile, nitromethane, or propylene carbonate. Homo- and heteroconjugation reactions (formation of A-(HA), and A-(HR),, respectively) of the carboxylate ions (A-) occur easily in these solvents because the protophobic aprotic solvents are very poor hydrogen bond acceptors and donors. In a previous paper (I), we confirmed the fomation of the homoconjugated species, A-(HA)* for the acetate and benzoate ions in acetonitrile, by observing the potential shift of the anodic wave with changing concentration of the corresponding acid. The conclusion in the paper also indicated the limitation of the solubility method for obtaining the formation constants of the homoconjugation reactions of the acetate and benzoate ions. In addition, the identification of the [Li2A]+and [LA,]ions made it possible to understand the inconsistency between experimental and calculated values of conductivity of lithium trifluoroacetate in propylene carbonate (2). The interaction between the acetate ion and alkaline-metal cations was examined polarographically in acetic anhydride as the solvent (3). Very recently, one of the authors suggested the formation of a few unknown species, such as S3N33-,by the analysis of the potential shift for the ion-pair formation in acetonitrile (4). Amines usually behave like weak bases in water; however, the basicity increases in aprotic solvents. Therefore, a great number of titrations for amines with strong acids have been carried out in aprotic solvents. Coetzee and Cunningham ( 5 ) examined the conductometric titration of amines with orthsubstituted benzoic acids in acetonitrile. They interpreted almost all titration curves quantitatively, considering several equilibria such as the homoconjugation reactions. However, the interactions between n-butylamine, dibutyl- or tributylamine, and benzoic acid are not yet understood. In the present paper, we have examined the interaction between several amines and carboxylic acids by the anodic waves in polarography. Amines are rather strong bases, though 0003-2700/85/0357-0509$0 1.50/0
not as strong as the acetate or benzoate ion in acetonitrile. Some amines have enough complexing ability to give anodic waves, as shown by Coetzee and Kolfhoff (6). Peter et al. (7, 8) described an anodic wave, due to the complex formation between ions of mercury and the macrobicyclic ligand of cryptand[2.2.2] (see structure 1) in propylene carbonate. Kolthoff asserted in a review (9) that the process is caused by the mercuric complex as follows: L + Hg 2 LHg2++ 2e-. For the present work, we have included cryptand[2.2.2], 1, and triethylenediamine, 2, as the diamines and aniline and pyridine as the nonaliphatic monoamines. N-CH2CH2-N /c H2-CH2\
\C H Z - C/ H~ 1
2
EXPERIMENTAL SECTION Diethyl-, triethyl-, n-butyl-, dibutyl-, and tributylamines, pyridine, aniline, and diphenyl- and triphenylamines (Wako Pure Chemicals, GR grade) were used without further purification. Triethylenediamine from Nakarai Chemicals (EP grade) and cryptand[2.2.2] (4,7,13,16,21,24-hexaoxa-l,l0-diazabicyclo[8.8.8]hexacosane) from Merck (99%) were used as received. Benzoic acid (Wako,GR grade) was dried over P205under vacuum at room temperature. p-Toluenesulfonic acid (Nakarai, GR grade) was dried under vacuum at 80 "C for 2 days. Acetic, monochloroacetic, m-chlorobenzoic, and salicylic acids and phenol of GR grade and dichloroacetic and 3,5-dinitrobenzoic acids of EP grade were used as received or after being dried over P205. Perchloric acid (Wako, GR grade 60%) was titrated by a standard NaOH solution, then diluted by acetonitrile to be 0.05 or 0.1 M solution. Tetraethylammonium benzoate was prepared, as described previously ( I ) . The methods of purifying acetonitrile ( I O ) and of preparing Et4NC104( 2 1 ) as the supporting electrolyte have been mentioned elsewhere. Polarograms were recorded with a Yanagimoto polarograph, Model P-1o00, and a Watanabe X-Y recorder Model WX-4401-LO. The rate of the potential sweep was 2 mV/s. The dropping mercury electrode had the following open-circuit characteristics: m = 1.88 mg/s and 7 = 2.7 s in a 0.1 M Et,NClO,-MeCN solution at h = 40 cm. All polarographic measurements were carried out in an H-type cell at 25 k 0.2 "C. The reference electrode was a silver-silver perchlorate electrode, Ag/O.l M AgC10,-MeCN. RESULTS AND DISCUSSION Anodic Waves of Amines. The polarogram of 0.4 mM n-butylamine on DME in acetonitrile containing 0.1 M Et4NC10, is shown in Figure 1. The wave was found to be diffusion controlled and was linear to the concentration of the amine over the range of 0.14.8mM. By the analysis of E vs. log i/(id - i),, we obtained n = 3, and slope of ca. 30 mV. Therefore, it seems that the following anodic reaction occurs on the electrode (cf. ref. 12): 3n-BuNH2
+ Hg z [ H ~ ( ~ - B u N H ~+)2e~ ] ~ (1) +
Other monoamines and triethylenediamine gave waves similar to those listed in Table I. The similar analyses suggested the same anodic reactions as the case of n-butylamine. Sometimes, the drop time was enlarged to more than 6 s over the range 0 1985 American Chemical Society
510
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
"0
100 Added volumeiyl
200
Flgure 3. Amperometric titration curves for 0.35 mM tri-n-butylamine of 10 mL by 0.05 M p-toluenesulfonic acid: ( 0 )anodic wave at -0.09 V; (0)cathodic wave at -2.37 V; (A) cathodic wave of ca. -1.56 V.
0
e03
0
-0.5
E
-05
/ V vs AglOl M AgC1O4
"0
Figure 1. Polarograms of amines, and the products from amines and benzoic acid in acetonitrile containing 0.1 M EtJIO., at 25 'C: (1) base current, (2) pyridine, (3) n-butylamine, (4) triethylenediamine, (5) cryptand[2.2.2]; concentrations, 0.4 mM each; solid line, without benzoic acid; dotted line, with 100 mM benzoic acid. I
1
-I
-021
2 -0.1
-
P F
0-
>
10
12
14
The p&value
16
18
20
of an Amine
Figure 2. The relation between the half-wave potential of the anodic
wave of an amine and the basicity of the amine in acetonitrile: (1) n-butylamine, (2)di-n-butylamine, (3)tri-n-butylamine, (4) diethylamine, (5)triethylamine, (6) triethylenediamine, (7) pyridine, (8)aniline. The E,,, value is 0.4 mM for each amine. of +0.1 to -0.5 V instead that the open-circuit drop time was 2.7 s. This may be caused by the strong interaction between an amine and mercury. In some cases, the calibration curves were not linear without correction for drop time. As for cryptand[2.2.2], the analysis of the anodic wave, E vs. log i/(id - i)" gave n = 1, and slope of 35 mv. The results strongly suggested the following reaction:
L
+ H g 2 [HgLI2+ + 2e-
(2)
The formation constant for the reaction Hg2+
K +LS
[HgLI2+
was estimated as log K = 20.9 in acetonitrile from the potential difference between the E,,? of the anodic wave and the potential of mercury dissolution with 0.1 M Et4NC104(+0.28 V in this case), considering the standard potential difference between Hg2+and Hg22+. A good correlation between the Ellz of the anodic wave of an amine and the basicity of the amine in acetonitrile was obtained, as shown in Figure 2. That is, the El12shifted to negative potential with increasing basicity of the amine. This tendency seems quite natural since the strong basicity of a ligand usually makes the large formation constant of a metal complex. Aniline and pyridine are the weak bases, pKdBH+
100 Added volume/A I
200
Figure 4. Amperometric titration curves for 0.39 mM pyridine of 10 mL by 0.05 M p-toluenesulfonic acid: ( 0 )anodic wave at +0.14 V (the wave height at +0.1 V); (0)cathodic wave at -1.65 V; ( A ) cathodic wave at ca. -1.53 V; (0) cathodic wave at -3.01 V.
values of 10.6 and 12.3, respectively, while the value of nbutylamine, a stronger base, is 18.3. The distinct limiting current was not observed for the aniline wave because the wave was so close to the potential limit for mercury dissolution without a complex forming agent. However, the El,* was estimated, assuming the same wave height as that of pyridine. Diphenylamine and triphenylamine, much weaker bases, than aniline, did not give an anodic wave. Cathodic Wave of the BH+ Ions. Figure 3 shows the amperometric titration curve of tri-n-butylamine with p toluenesulfonic acid. The anodic wave of the amine consumed 1 equiv of p-toluenesulfonic acid. When a small amount of the strong acid was added, a cathodic wave appeared at -2.37 V. The wave height increased up to the equivalence point and remained constant after that. The half-wave potential of the wave shifted negatively after the equivalence point (e.g., -2.43 V at 150 mL of the strong acid). Just after the equivalence point, a new wave at -1.56 V appeared, and the wave height increased linearly. By the above results, it is suggested that the ( ~ - B u ) ~ N H + , formed from ( ~ - B U ) and ~ N H+ is reduced a t -2.37 V. The p-toluenesulfonic acid solution gave an irreversible cathodic wave a t ca. -1.56 V. The cathodic wave that appeared after the equivalence point is caused by the reduction of unreacted p-toluenesulfonic acid. The titration curves for pyridine gave the behavior similar to that of tri-n-butylamine (Figure 4). The anodic wave disappeared at the equivalence point, producing pyridinium ion. The pyridinium ion was reduced (the slope of the wave of less than 59 mV) at -1.65 V; however, before the equivalence point, the half-wave potential shifted positively from -1.68 to -1.65 V. After the equivalence point, a new wave at ca. -1.53 V appeared. The reversibility of the wave was low. In the case of pyridine, the cathodic wave of pyridine itself was able to be oberved at -3.01 V. This wave disappeared after the addition of 2 equiv of p-toluenesulfonic acid. The direct observation for the consumption of two equivalent acid could be explained by the addition of two hydrogen to a pyridine molecule. A similar reaction was suggested for quinolines by Fujinaga et al. (16). The 1,2-position may be more favorable than the 1,4-position, according to the claim by Mann and Barnes (17). Several acids in pyridine as the solvent were
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
511
Table I. Polarographic D a t a a n d t h e Results from Amperometric Titration for Amines
amines diethylamine triethylamine n-butylamine di-n-butylamine tri-n-butylamine pyridine triethylenediamine
concn/mM 0.40 0.41 0.41 0.41 0.41 0.42 0.42 0.42 0.40 0.40 0.39 0.40 0.43
EIIza -0.145 -0.12 -0.14
l,/gA
slope/mVb
2.05 2.15 2.45
(3) 28 (3) 34f ( 3 ) 28
0.40 0.40 0.40
1 1 1 1 1 1 1
Bz 1.6 Ac 1.8 -0.125
2.82
(3) 44f
Bz 1.35 Ac 2.02 -0.09
PKdB"+ in MeCN' 18.8 18.5 18.3 18.3 18.1
Bz 1.35 +0.14 -0.095
lg
1 1+1
Bz (500 mM) 1.52, 1.33 Ac (500 mM) 1.65, 1.67
0.43 cryptand[2.2.2]
eq value by amperometric titration with p-TOsd
I,l!JA in the presence of weak acids'
4.29
4.70
12.3 18.29, 10.16
1+1 28
(1) 35
Bz 3.1
9.60qh7.28h
2 2
Ac 4.0
Ovs. Ag/O.l M AgC104-MeCN electrode. * E vs. log i / ( i d - i), (1);log i/(& - i)', (2); log i/(& - i)3, (3). 100 mM benzoic acid, Bz; acetic acid, Ac. dp-Toluenesulfonic acid. e From ref 13; for triethylenediamine, ref 14. !Another wave at more positive potential was combined, especially a t the higher concentrations. g With both p-TOs and HC104. The value in the aqueous solution, ref 15.
examined electrochemically by Spritzer et al. (18),and they suggested a dimer from pyridine as the product. However, this problem is not settled (17). Perchloric acid (0.1 M in MeCN) as the titrant gave the same results as those obtained for p-toluenesulfonic acid. The El12values of pyridinium ion and HC104 itself were -1.64 V and ca. -1.2 V, respectively. By addition of a small amount of benzoic acid to the pyridine solution, a cathodic wave at -2.16 V was given. The wave height increased with the increasing concentration of benzoic acid. This wave seems to be also due to the reduction of the pyridinium ion. The difference of the El/?of PyrH' between HC104 (-1.64 V) or p-toluenesulfonic acid (-1.65 V) and benzoic acid (-2.16 V) may be caused by the ion-pair formation between the pyridinium ion and the benzoate ion. Benzoic acid gave the cathodic wave at -2.48 V (0.4 mM) in acetonitrile. Hickey et al. (19) reported the different El12values for pyridinium ion, produced from different acids in pyridine solutions containing Et4NC104as the supporting electrolyte. When benzoic acid was added to n-butylamine solution, the cathodic wave appeared at ca. -2.47 V, which could not be distinguished from the cathodic wave of benzoic acid itself. When cryptand[2.2.2] solutions were amperometrically titrated by 0.1 M HClO., and 0.1 M p-toluenesulfonic acid, the anodic wave of the cryptand disappeared at 2 equiv of the strong acid in the both cases. These results seem to be good evidence that cryptand[2.2.2] takes two protons almost at the same time. On the other hand, the anodic wave of triethylenediamine at ca. -0.1 V disappeared a t 1 equiv of ptoluenesulfonic acid. Formation of BH+A-(HA)2by the Addition of Carboxylic Acids to Monoamine Solutions. When benzoic acid was added to 0.4 mM tri-n-butylamine solution, the anodic wave of the amine was markedly influenced (see Figure 5). The anodic wave of tri-n-butylamine was observed a t -0.09 V with a wave height of 2.62 FA. The addition of 0.1 mM benzoic acid deformed the wave. The half-wave potential became -0.13 V (Il = 1.25 FA) in the presence of 0.2 mM benzoic acid. The E l I 2value shifted negatively up to -0.175 V (Il= 1.35 FA) in 1.5 mM benzoic acid. In turn, a large excess of benzoic acid made the wave shift positively, e.g., from -0.16 V (10 mM acid) to -0.10 V (50 mM acid). In the presence of
*0.2
0
0 0 E / V vs Ag/01M &C104
-0.2
0
-05
Figure 5. Change of the anodic wave from tri-n-butylamine by addition of benzoic acid, [tri-n-butylamine] = 0.40 mM: (1) acid free, (2) 0.1 mM, (3) 0.2 mM, (4) 0.4 mM, (5)0.8 mM, (6) 10 mM, (7) 20 mM, (8) 100 mM benzoic acid.
250 mM of the acid, the wave became reversible and diffusion shifted positively 120 mV by the controlled, and the 10-fold increase of the acid concentration. The wave height became constant with more than 10 mM benzoic acid (1.4 pA, cf. 1.35 pA a t 100 mM). The change of the El/*of the anodic wave is clearly displayed in Figure 6. Other monoamines, n-butylamine and di-n-butylamine (shown in Figure 71, gave the same change in E l I 2in the presence of benzoic acid (50-300, or 500 mM). These half-wave potentials were identical with the values of tetraethylammonium benzoate (Et4N+C6H5COO-)in the presence of benzoic acid (the corresponding concentrations) within the experimental error. In the previous paper ( I ) , we have confirmed the formatin of the A-(HA), species with the formation constant of 1 x IO7 for the homoconjugation reaction
+
K
A- 2HA S A-(HA), Therefore, it is apparent that the following reaction occurs and that the formed A-(HA), gives the new anodic wave instead of the wave of (n-Bu),N. (n-Bu),N 3HA e ( ~ - B u ) ~ N H + A - ( H A ) ~(3)
+
512
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
/
1
VI 5
-.. N
W 1 ' 1
,
,
1
15
10
20
50
100 200
pK,
500
[Benzoic acid1 / rnM
Figure 6. Effect of benzoic acid on the half-wave potential of the anodic wave from various amines. The concentration of amine is 0.4 mM each: (0)n-butylamine, (0)tri-n-butylamine, (A)cryptand[2.2.2], (V)the first wave from triethylenediimine, ('(I)the second wave from triethylenediamine, ( 0 )pyridine . The right-hand scale is used for pyridine and the second wave from triethylenediamine.
+02' 10
20
50 100 Concn of acidlrnM
I lo00
Figure 7. Effect of various acids on the half-wave potential of the anodic wave from di-n-butylamine. [di-n-butylamine] = 0.4 mM: (1) phenol, (2) acetic, (3) benzoic, (4) m-chlorobenzoic, (5) monochloroacetic, (6) salicylic, (7) 3,5dinitrobenzoic,(8) dichloroacetic acid. After the addition of a large amount of the acid, tri-n-butylamine cannot give the anodic wave any more as the amine is converted to the (n-Bu)3NH+ ion. The conjugated acid (BH+) of the amine gave the cathodic wave at -2.37 V, as described before. The similar reactions were confirmed for the primary and secondary amines. The ion-pair formation between the conjugate acid (e.g., (n-Bu),NH+) and A-(HA), was suggested to be negligible because the Ell2values were identical for Et4N+A-(HA)2,(n-Bu)3NH+A-(HA)2,etc. Effect of Various Acids on the Anodic Wave from Di-n -butylamine. The effect of various acids on the anodic wave from di-n-butylamine is shown in Figure 7. The concentration increase of acetic acid also shifted the anodic wave 120 mV positively, as benzoic acid did. Each point for acetic acid in Figure 7 is identical with the El12 values (1) for Et4N+CH3COO-(0.35 mM) in the presence of the same concentration of acetic acid. (The homoconjugation constant of A-(HA)2for the acetate ion has been given by K = ca. 6 X lo7, cf. ref 1.) The effect of acetic acid identified the formation of the BH+A-(HA)2species again. h e n monochloroacetic, 3,5-dinitrobenzoic, and dichloroacetic acids formed the BH+A-(HA)2species when more than 100 mM concentration of the acids was present in the solution. In lower concentrations, however, BH+HA2-would be predominant, judging from the slope of 60-90 mV. For these stronger acids, the anodic waves showed good reversibility in the presence of as low as 10 mM of the acids. The effect of phenol (pK, = 27.2 in MeCN (21)) was not so clear, as the E l l z shifted gradually. The slope of E l I 2vs. concentration of the acid was 90 mV over the range of 20-150 mM for rn-chlorobenzoic acid. (The solubility of chlorosubstituted benzoic acid is rather low in MeCN, cf. ref 20.) For salicylic acid, the slope changed from 59 to 98 mV, which
,
I
1
,
I
I
20 in MeCN
Flgure 8. The relation between the half-wave potential of A-(HA), and the pK, value in acetonitrile: (1) acetic, (2) benzoic, (3)monochloroacetic, (4) 3,5dinitrobenzoic, (5) dichloroacetic acid. The BH+A-(HA), ions were produced by 0.4 mM di-n-butylamine and 100 mM of each acid. means that BH'HA; is changing to BH+A-(HA)2,but not completely even at 500 mM of the acid. The Ellzof A-(HA)2depended on both the concentration and the strength of an acid. In Figure 8, the Ellz value of A-(HA), was plotted against the pKa value in acetonitrile (pKa values from ref 21). Here, the value at 100 mM HA was chosen as the El12of A-(HA),. A good linearity was obtained between the Ell2and the acidity. Since the measurable potential is limited to ca. +0.25 V in acetonitrile-0.1 M Et4NC104,it is suggested by extrapolation that an acid which is stronger than pKa = 14 cannot display the anodic wave due to the homoconjugated species as following reaction: A-(HA)2
+ Hg e 1/2Hg2(A-)2+ 2HA + e-
(4)
In fact, the addition of 2,6-dihydroxybenzoic acid (pK, = 12.6) to a di-n-butylamine solution caused the anodic wave to disappear completely. Interaction between Benzoic Acid and Diamines. When a small amount of benzoic acid was added to a 0.4 mM triethylenediamine solution, the anodic wave of triethylenediamine ( E l j 2= -0.105 V) shifted negatively up to -0.16 V (0.8-1.8 mM benzoic acid). The effect of a large excess of the acid on the anodic wave was identical with the effect on a monoamine, as shown in Figure 6. However, a second wave appeared at a potential very close to the positive potential limit (see Figure 1). The limiting current for the wave was difficult to observe at the lower concentration of the acid. At more than 200 mM, the limiting current was given. The change of the E l j 2of the second wave was also shown in Figure 6. The E l j 2value of this wave shifted negatively with increasing concentration of the acid, as pyridine. The effect of benzoic acid on cryptand[2.2.2] was more ideal than triethylenediamine and was easily explained. The wave height of the anodic wave in the presence of 100 mM benzoic acid was twice (2.9 pA) as high as that of a monoamine ( 1.4 FA) or the first wave of triethylenediamine (1.37 pA) for the same concentration of amines (0.4 mM). The twice wave height suggested the formation of twice the concentration of A-(HA),. Therefore, 0.8 mM Et4N+C6H5COO-was examined in 100 mM benzoic acid solution, and an anodic wave with 2.8 pA (Ell2= -0.75 V) was obtained to confirm the following reaction. N
Prbl
N x f i O x N
b u o J
+
6 HA
( H A ) 2 A - ' Hr N0xQ O /-CIO \ N H
A- (HA)z
C0,Od
Amperometric titrations also supported the above reaction. Ten milliliters of a 0.4 mM cryptand[2.2.2] solution containing 100 mM benzoic acid was titrated by 0.05 M p-toluenesulfonic acid (Figure 9). The anodic wave decreased linearly with the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
in Figure 6 suggested that the basicity of the second step is too weak to have a stable interaction with the weak acids. Registry No. Diethylamine, 109-89-7;triethylamine, 121-44-8; butylamine, 109-73-9; dibutylamine, 111-92-2; tributylamine, 102-82-9;pyridine, 110-86-1;triethylenediamine, 280-57-9; cryptand[2.2.2], 23978-09-8; phenol, 108-95-2;acetic acid, 64-19-7; benzoic acid, 65-85-0; m-chlorobenzoic acid, 535-80-8; monochloroacetic acid, 79-11-8;salicylic acid, 69-72-7;3,5-dinitrobenzoic acid, 99-34-3;dichloroacetic acid, 79-43-6;p-toluenesulfonic acid, 104-15-4;perchloric acid, 7601-90-3.
Added volurnelul
Flgure 9. Arnperometric titration curves for 0.43 mM triethyienediirnlne (in the presenceof 500 mM benzoic acid) and 0.4 rnM cryptand[2.2.2]
(100 rnM benzoic acid) by 0.05 M p-toluenesulfonk acid: (0) first wave from triithylenediarnlne, (0)second wave from triethylenediamine,(0) wave from cryptand[2.2.2].
addition of a strong acid to disappear at two equivalent point. A solution containing 100 mM acetic acid showed a similar behavior. Monoamines, such as n-butylamine or di-n-butylamine containing 100 mM benzoic acid and/or acetic acid also showed the linear decrease and consumed 1 equiv of p-toluenesulfonic acid
+
BH+A-(HA)2 H+ 3 BH'
+ 3HA
(5)
The p-toluenesulfonate ion did not show any clear anodic wave in the acetonitrile solution, just like tetraethylammonium methanesulfonate (22). Amperometric titration curves for triethylenediamine in the presence of 500 mM benzoic acid are also shown in Figure 9. Up to the first equivalent point, only the first wave (+0.015 V, due to A-(HA)2)decreased linearly. Then, the second wave (+0.155 V) consumed another equivalent of p-toluenesulfonic acid. The presence of 500 mM acetic acid made a similar result, except that the second wave was not as clear as in the case of benzoic acid. The above results gave an indication for the species formed in triethylenediamine solution with an extremely large excess of carboxylic acids. ,CHz-Cb
\ / H*
( H A l2A-+HN-CH~-CH~-NH+A-(HA
c'
H 2-c
)n
( n =0-4?)
The negative shift of
513
of triethylenediamine second wave
LITERATURE CITED (1) Hojo, M.; Imai, Y. Bull Chem. SOC.Jpn. 1983, 56, 1963-1967 (2) Jansen, M. L.; Yeager, H. L. J. Phys. Chem. 1974, 78, 1380-1382. (3) Hojo, M.; Imai, Y. Bunsekl Kagaku 1983, 3 2 , E77-E80. (4) Chivers, T.; Hojo, M. Inorg. Chem. 1984, 2 3 , 1526-1530, 2738-2742. (5) Coetzee, J. F.; Cunningham, G. P. J. Am. Chem. SOC. 1965, 87, 2534-2539. (6) Coetzee, J. F.; Kolthoff, 1. M. J. A m . Chem. SOC. 1957, 79, 61 10-6115. (7) Peter, F.; Dross, M.;Pospisil, L.; Kuta, J. J. Nectroanal. Chem. 1978, 90,239-249. (8) Pospisll, L.; Kuta, J.; Peter, F.; Gross, M. J. Electroanal. Chem. 1978,
90,251-259.
(9) Kolthoff, I.M. Anal. Chem. 1979, 5 1 . 1R-22R. (10) Fujinaga, T.; Okazaki, S.; Hojo, M. J. Electroanal. Chem. 1080, 113,
__
89-98
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RECEIVED for review September 5,1984. Accepted November 8, 1984.
