Complexation of zinc(II) and cadmium(II) by hydroxyethyl- and bis(hydroxyethyl)-1,4,7-triazacyclononane in water

Article abstract of DOI:10.1071/CH02202

The complexation of Zn²⁺ by hydroxyethyl- and bis(hydroxyethyl)-1,4,7-triazacyclononane ((2) and (3)) in aqueous solution at 298.2 K and I = 0.10 mol dm^-3 (NaNO₃) is characterized by log(K/dm³ mol^-1) = 10.45 ± 0.05 and 11.32 ± 0.02 and a deprotonation assigned to coordinated water is characterized by pK_a = 8.87 ± 0.11 and 8.50 ± 0.03, respectively. For the analogous Cd²⁺ complexes log(K/dm³ mol^-1) = 8.74 ± 0.02 and 9.79 ± 0.02, respectively, and no deprotonation of coordinated water is detected. The synthesis of (2) is described.

Full text of DOI:10.1071/CH02202

CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2003, 56, 61–64
Complexation of Zinc(ii) and Cadmium(ii) by Hydroxyethyl- and
Bis(hydroxyethyl)-1,4,7-triazacyclononane in Water
A
A
A,B
Steffen P. Creaser, Simon M. Pyke and Stephen F. Lincoln
A
Department of Chemistry, The University of Adelaide, Adelaide, 5000, Australia.
Author to whom correspondence should be addressed (e-mail: stephen.lincoln@adelaide.edu.au).
B
The complexation of Zn2 by hydroxyethyl- and bis(hydroxyethyl)-1,4,7-triazacyclononane ((2) and (3)) in aque-
+
−
3
3
−1
ous solution at 298.2 K and I = 0.10 mol dm (NaNO3) is characterized by log(K/dm mol ) = 10.45 ± 0.05
and 11.32 ± 0.02 and a deprotonation assigned to coordinated water is characterized by pKa = 8.87 ± 0.11 and
2
+
3
−1
8
.50 ± 0.03, respectively. For the analogous Cd complexes log(K/dm mol ) = 8.74 ± 0.02 and 9.79 ± 0.02,
respectively, and no deprotonation of coordinated water is detected. The synthesis of (2) is described.
Manuscript received: 30 September 2002.
Final version: 6 November 2002.
[
13]
Introduction
(2).2 HBr. ) This synthesis, an adaptation of a literature
method for the syntheses of bis-N-substituted 1,4,7-tri-
1
,4,7-Triazacyclononane, (1) in Scheme 1, has been the sub-
[
15]
commenced with stirring 1,4,7-triaza-
azacyclononanes,
[
1–4]
ject of several metal complexation studies
homo trisubstituted derivatives.
as have its
tricyclo[5.2.1.04 ]decane, (5), in 3 mol dm hydrochloric
.10
−3
[5–12]
The robustness of (1)
makes it an attractive platform for controlled sequential sub-
stitution at each of the secondary amines as exemplified
by hydroxyethyl-, bis(hydroxyethyl)-, and tris(hydroxyethyl)-
O
H
3
mol dmϪ3
hydrochloric acid,
1
,4,7-triazacyclononane ((2), (3), and (4), respectively). As
N
N
o
25 C, 12h, 31%
this sequential substitution proceeds there is an interplay
between the changing basicity of the amine nitrogens and
increasing ligand denticity, which is here explored through
the protonation of ligands (2) and (3) and their complexation
N
N
HN
NH
(5)
(
6)
2
+
2+
of Zn and Cd , and a comparison with their unsubstituted
and fully substituted analogues, (1) and (4), respectively.
2
O
in ethanol,
5oC, 72h, 100%
Results and Discussion
O
H
2
Ligand Syntheses
N
Syntheses of the mono- and bis-substituted derivatives
of (1) proceed through the bridged macrocycle 1,4,7-
triazatricyclo[5.2.1.0 ]decane, (5), as shown for (3).2 HBr
in Scheme 2. (We earlier described the synthesis of
OH
N
N
4
.10
3 mol dmϪ3
OH
R1
N
hydrobromic acid,
(7)
o
1
10 C, 3h, 58%
N
N
NH
R3
R2
OH
1
2
3
(
1); R = R = R = H
N
N
1
2
3
(
2); R = R = H, R = CH CH OH
2
2
.2HBr
1
2
3
(
3); R = H, R = R = CH CH OH
2 2
OH
1
2
3
(
4); R = R = R = CH CH OH
2
2
(3)
Scheme 1.
Scheme 2.
10.1071/CH02202
©
CSIRO 2003
0004-9425/03/010061

6
2
S. P. Creaser, S. M. Pyke and S. F. Lincoln
◦
3+
acid at 25 C for 12 h, adjusting the pH to 9 and extracting 1-
formyl-1,4,7-triazacyclononane, (6), into dichloromethane.
Potentiometric Determination of the pKas of (2).H
3
and (3).H33
+
The reaction of (6) with ethylene oxide in ethanol at
5 C for 72 h gave 1-formyl-4,7-bis(2-hydroxyethyl)-1,4,7-
trazacyclononane, (7), as a viscous oil, which was refluxed
at 110 C for 3 h in 3 mol dm hydrobromic acid to give
,4-bis(2-hydroxyethyl)-1,4,7-triazacyclononane dihydro-
bromide, (3).2 HBr, as white needles.
In a second method two equivalents of sodium ethoxide
were added to 1,4,7-triazacyclononane trihydrobromide,
1).3 HBr, in dry ethanol, followed by reaction with two
equivalents of ethylene oxide for 72 h at 25 C. The resulting
The titration of (2).H3 and (3).H3 with NaOH results in
titration curves (Fig. 1) characterizing three acid dissoci-
ations whose pKa values appear in Table 1 together with
+
3+
◦
3
2
◦
−3
those of (1).H2 and (4).H2
+
2+ [2,5]
.
The pKa1 values vary in
2
1
+
+
+
+
the sequence (1).H > (2).H < (3).H < (4).H within an
order of magnitude variation in acidity, suggesting that the
lowering of amine nitrogen basicity caused by the electron
withdrawing effect of a single hydroxyethyl pendant arm
(
+ +
in (2).H is offset by a compensating effect in (3).H and
◦
+
+
−
3
(4).H . It is probable that protonation in (1).H is stabilized
through hydrogen bonding with secondary amine groups,
while (2).H is less stable because its tertiary amine is
less basic, and the extent of intra-amine hydrogen bonding
is correspondingly decreased. This decrease in intra-amine
hydrogen bonding is likely to further decrease in (3).H
viscous oil was treated with 6 mol dm hydrobromic acid
and recrystallized from ethanol to give (3).2 HBr as white
needles. However, this method can give difficult to separate
mixtures of (2).2 HBr and (3).2 HBr.
+
+
+
and (4).H as the number of hydroxyethyl pendant arms
(
a) 12
increases but intramolecular hydrogen bonding of the pro-
tonated amine and the hydroxy groups of the pendant arms
becomes increasingly possible. This appears to offset the
decreased basicity of the tertiary amine nitrogens and be
1
0
+
2+
H /(2)/Cd
+
8
6
4
2
the origin of the increase in the pKa1 values of (3).H and
+
H /(2)
+
(
4).H . The much lower pKa1 (Table 1) of the more flex-
pH
+
+
ible (8).H linear analogue of (1).H , where (8) is linear
diethylenetriamine, suggests that the relatively rigid macro-
cyclic structures of (1)–(4) greatly increases their basicity, so
that (1).H –(4).H are substantially less acidic because of
the greater extent of intramolecular hydrogen bonding.
+
2+
H /(2)/Zn
+
+
[16]
The pKa2 values are smaller because of intra-protonic
0
.0
0.2
0.4
0.6
0.8
1.0
charge repulsion. There is also a systematic decrease in mag-
3
2+
2+
Volume of NaOH added/cm
nitude from (1).H2 to (4).H2 corresponding to a change
of greater than three orders of magnitude in acidity which is
consistent with a decreasing effectiveness of intramolecular
hydrogen bonding with an increase in pendant arm substi-
tution. However, the increase in pKa1 relative to pKa2 for
(
b) 12
1
0
8
6
4
2
+
2+
H /(3)/Cd
+
Table 1. Acid dissociation constants and complexation constants
H /(3)
2+
2+
for Zn and Cd complexes in aqueous solution at 298.2 K and
I = 0.10 mol dm (NaNO3)A
−
3
pH
Ligand
pKa1
pKa2
pKa3
(1).H3 10.42B
+
6.82B
–
+
2+
H /(3)/Zn
3
3+
C
C
C
(
(
(
2).H
10.24 ± 0.02
6.17 ± 0.02
4.97 ± 0.07
3.97 ± 0.06
1.97 ± 0.05C
2.25 ± 0.14
–
3
3).H3 11.02 ± 0.05
+
C
C
C
C
3
4).H3 11.50 ± 0.01
+
3
0
.0
0.2
0.4
0.6
0.8
1.0
D
D
1
1.52
3.42
Volume of NaOH added/cm3
3+
9.84E
9.02E
4.23E
pKaZn
–
(
8).H
3
3
−1
3
−1
Fig. 1. (a) Typical titration curves at 298.2 K for a solution
of 0.005 mol dm
.005 mol dm
ZnSO4; and a solution of 0.005 mol dm HNO3, 0.001 mol dm (2),
and 0.001 mol dm CdSO4 as annotated. (b) Typical titration curves at
98.2 K for a solution of 0.005 mol dm HNO3 and 0.001 mol dm
3); a solution of 0.005 mol dm
log(KZn/dm mol
)
log(KCd/dm mol )
−
3
−3
HNO3 and 0.001 mol dm
HClO4, 0.001 mol dm
(2); a solution of
−
3
−3
−3
11.62F
9.5G
0
(2), and 0.001 mol dm
(1)
(2)
(3)
−
3
−3
C
C
C
C
C
10.45 ± 0.05
8.74 ± 0.02
8.87 ± 0.11
−
3
C
11.32 ± 0.02
9.79 ± 0.02
8.50 ± 0.03
−
3
−3
(3), and
HNO3,
2
(
0
0
D
H
(
4)
12.07
10.59
–
−
3
−3
HNO3, 0.001 mol dm
−
−
3
3
−3
A
.001 mol dm
ZnSO4; and a solution of 0.005 mol dm
The supporting electrolyte in this work was NaNO3. In the literature
−3
B
C
.001 mol dm (3), and 0.001 mol dm CdSO4 as annotated. The
titrant was 0.100 mol dm
.10 mol dm (NaNO3).
studies other 1 : 1 electrolytes were used.
Ref. [2].
Ref. [3].
This
Ref. [1].
−
3
D
E
F
G
NaOH. For all titration solutions I =
work.
Ref. [5].
Ref. [16].
−
3
H
0
Ref. [10].

ii
ii
Complexation of Zn and Cd by Hydroxyethyl- and Bis(hydroxyethyl)-tacn in H2O
63
+
(
8).H2 is much less. This is consistent with protonation
is six-coordinate. The latter interpretation raises the ques-
tion as to why no deprotonation was observed for [Zn.(1)]
2
2
+
of the primary amines ensuring less charge repulsion and a
higher value for pKa2. The decrease in (8).H3 pKa3 is much
larger because of increased charge proximity, but its magni-
3
+
[3]
which should have at least one water coordinated.
A
2
+
comparison with the five-coordinate Zn
complexes of
3
+
3+
ꢀ
ꢀꢀ
ꢀ
ꢀꢀ
tude is still much greater than those of (2).H , (3).H3 , and
tripodal2,2 ,2 -triaminotetraethylamineand2,2 ,2 -tri(N,N-
dimethylamine)triaminotetraethylamine, in which the pKas
3
3
+
4).H (where in the latter case pKa3 was too small to be
3
(
[17]
detected by our method) which have a closer charge proximity
because of their macrocyclic structure.
of coordinated water are 10.26 and 9.00, respectively,
suggests that coordinated water becomes less acidic as its
environment becomes less constrained and less hydrophobic.
On this basis it is possible that the pKa is ≥10 for coor-
Potentiometric Determinations of Zn2 and Cd
Complexation by (2) and (3)
+
2+
2+
dinated water in [Zn.(1)] . It is of interest that for the
more hydrophobic 1,5,9-triazacyclododecane analogues of
2
+
2+
2+
The titration curves for the formation of [Cd.(2)] and
Cd.(3)] are shifted to a lower pH by comparison with
those for the deprotonation of (2).H and (3).H3 , respec-
tively, consistent with Cd competing strongly with H for
tetradentate (2) and pentadentate (3) (Fig. 1). The titration
[Zn.(1)] and [Zn.(2)] , their pKa values of 7.3 and 7.4
2
+
[
are attributed to deprotonation of coordinated water and the
3
3
+
3+
[18,19]
pendant arm hydroxy group, respectively.
2
+
+
Conclusion
2
+
2+
curves for the formation of [Zn.(2)] and [Zn.(3)] are fur-
ther shifted to a lower pH consistent with these complexes
This study of the protonation and metal ion complexation of
the new ligands (2) and (3) provides a foundation for studies
of ternary complex formation. The reliable syntheses of (2)
and (3) provide an opportunity to substitute mixed pendant
arms onto the 1,4,7-triazacyclononane platform which may
act independently or cooperatively as receptors for metal ions
and other guest species.
2
+
being more stable than their Cd analogues. In addition,
there is a flattening of the [Zn.(2)]2 and [Zn.(3)] titration
+
2+
curves in the pH range 8–10 consistent with the occurrence
of deprotonation of [Zn.(2)] and [Zn.(3)] . The complex-
ation of Zn and Cd by (2) and (3) are characterized by
complexation constants, KZn and KCd, determined from the
titration curves shown in Figure 1.These are shown inTable 1
2
+
2+
2
+
2+
[1,3,5,10]
Experimental
together with those reported for (1) and (4)
and two
2
+
pKa values pertaining to deprotonations of [Zn(2)] and
Syntheses
2
+
[
Zn(3)] . Several factors determine the magnitude of KZn
,4,7-Triazatricyclo[5.2.1.04.10]decane,[20]
1
1-formyl-1,4,7-triazacyclo-
15]
1,4,7-triazacyclononane trihydrobromide, and 1,4,7-tris(2-
[
and KCd, among which are the variations in basicity of the
amine nitrogens as they are substituted with pendant hydrox-
yethyl arms and the consequent increase in the number of
donor groups and the change from the borderline hard Lewis
nonane,
[
5]
hydroxyethyl)-1,4,7-triazacyclononane were synthesized by methods
similar to those reported in the literature. Solvents were purified by
[
21]
and other reagents (Aldrich) were used without
literature methods
further purification.
2
+
2+
acid Zn to the softer Cd . This results in the magnitudes
of KZn and KCd varying with the ligand in the sequence
1-Formyl-4,7-bis(2-hydroxyethyl)-1,4,7-trazacyclononane (7)
3
3
(
1) > (2) < (3) < (4) such that the change of ligand is accom-
panied by more than an order of magnitude change in KZn and
Cd. The competing effects of the decrease in the basicity of
Ethylene oxide (0.40 cm , 8.01 mmol) in chilled ethanol (2.5 cm ) was
mixed with 1-formyl-1,4,7-triazacyclononane (0.21 g, 1.36 mmol) in
◦
ethanol at 0 C and stirred for 72 h in a stoppered flask at ambient
K
temperature. Removal of ethanol under reduced pressure gave the title
the nitrogens with pendant arm substitution are increasingly
offset by the increase in ligand denticity. The smaller size
◦
compound as a viscous oil which was dried under high vacuum at 80 C
−1
for 6 h (0.33 g, quantitative). νmax (neat)/cm 3396 (OH), 1655 (C O);
and greater Lewis acidity of Zn2 by comparison with Cd
+
2+
H 3 2 2
δ (600 MHz, CDCl ) 2.59 (2 H, m, C(3)H ), 2.64 (2 H, m, C(4)H ),
results in KZn ≥ 500 K for each ligand. The overall effect
2.74 (2 H, t, J 5.4, C(8)H2), 2.78 (2 H, t, J 5.4, C(10)H2), 2.92 (2 H,
m, C(5)H ), 2.94 (2 H, m, C(2)H ), 3.40 (2 H, m, C(6)H ), 3.47 (2 H,
Cd
2
+
2+
2
2
2
is that [Zn.(4)] is the most stable complex and [Cd.(1)] is
2
+
m, C(1)H2), 3.56 (2 H, t, J 5.4, C(9)H2), 3.63 (2 H, t, J 5.4, C(11)H2),
.49 (2 H, br s, 2 × OH), 8.15 (1 H, s, HC(17)O); δC (300 MHz) 47.94
C1), 51.30 (C6), 52.96 (C2), 54.31 (C4), 54.90 (C3), 55.44 (C5), 59.01
the least stable. (For [Zn.(8)] , log KZn = 8.8 is significantly
4
(
[
16]
)
smaller because of the absence of a macrocyclic effect.
The proton dissociations characterized by pKa values of
(C10), 59.20 (C11), 59.47 (C9), 59.82 (C8), 163.88 (HC(7)O); m/z (LSI)
.87 and 8.50 for [Zn.(2)]2 and [Zn.(3)] could either
+
2+
+
8
246.1808 (M + H) . C11H24N3O3 requires 246.1818.
arise from deprotonation of coordinated water or of the
pendant arm hydroxy group. (The former is possible for
1
,4-Bis(hydroxyethyl)-1,4,7-triazacyclononane Dihydrobromide
(
3).2HBr
2
+
2+
and vice versa.) Thus,
[
Zn.(1)] but not for [Zn.(4)]
Method 1. 1-Formyl-4,7-bis(2-hydroxyethyl)-1,4,7-triazacyclo-
either polarization of the hydroxyl O–H bond is decreased
−3
nonane, (7) (0.394 g, 161 mmol) was refluxed in 3 mol dm hydro-
2
+
2+
in [Zn.(4)] by comparison with those in [Zn.(2)] and
3
◦
bromic acid (25 cm ) at 110 C for 2.5 h. Removal of the solvent
2
+
[
Zn.(3)] , which is contrary to the trend predicted by
the decrease in pKa from [Zn.(2)] to [Zn.(3)] , or the
absence of coordinated water in [Zn.(4)] precludes its
under reduced pressure gave a brittle pale yellow solid which was
2
+
2+
recrystallized from water/ethanol to give (3).2 HBr as white crystals
◦
2
+
(0.351 g, 58%), mp 155–157 C. δ (300 MHz, D2O) 3.30 (4 H, t, J
H
5
.3, 2 × NCH2CH2OH pendant arm), 3.42 (4 H, s, 2 × NCH2CH2NH
ring), 3.54 (8 H, m, 2 × HNCH2CH2NCH2 ring), 3.60 (2 H, br s,
× OH), 3.98 (4 H, t, J 5.3, 2 × NCH2CH2OH pendant arm); δC
(50.32 MHz) 43.84 (2 × NHCH2 ring), 50.60 (2 × HNCH2CH2N ring),
deprotonation and the observed pKas are those of coordi-
2
+
2+
nated water in [Zn.(2)] and [Zn.(3)] with the inference
2
2
+
2+
that [Zn.(2)] is either five- or six-coordinate and [Zn.(3)]

6
4
S. P. Creaser, S. M. Pyke and S. F. Lincoln
5
5
1.26 ((CH2)2NH ring), 58.28 (2 × NCH2CH2OH pendant arm),),
was at least one mole in excess of that required to completely protonate
+
−3
9.28 (2 × NCH2CH2OH pendant arm); m/z (LSI) 218.1897 (M + H) .
the ligand) with 0.100 mol dm NaOH. The stepwise complexation
constants for the Zn2 and Cd complexes were determined at the
+
2+
C10H24N3O2 requires 218.1868. (Found: C, 31.6; H, 6.9; N, 11.1%.
C10H25Br2N3O2 requires C, 31.7; H, 6.7; N, 11.1%).
same concentrations of (2) and (3) and HNO3 in the presence of either
−
3
−3
2+
2+
Method 2. 1,4,7-Triazacyclononane trihydrobromide, (1).3HBr
1.00 × 10 mol dm [Zn ]total or [Cd ]total. Generally the pH titra-
2
+
(
5.08 g, 13.7 mmol) was added to a solution of sodium metal (0.66 g,
tion range was 2.00 to 10.5. The pKas and stepwise Zn complexation
constants were derived using the program SUPERQUAD
in Table 1.
3
[22]
2
8.7 mmol) in dry ethanol (80 cm ) and was stirred for 16 h. Sodium
and appear
bromide was removed by gravity filtration and the solution was cooled to
0
◦
3
3
C. Ethylene oxide (1.43 cm , 28.6 mmol) in chilled ethanol (5 cm )
was added and the reaction mixture was stirred at room temperature
in a stoppered flask for 96 h. The solvent was removed under reduced
pressure to give a viscous oil which was dissolved in water and acidi-
Acknowledgments
3
We thank theAustralian Research Council and the University
of Adelaide for supporting this research.
fied to pH 2 with 3 mol dm hydrobromic acid. Removal of the solvent
under reduced pressure gave a brittle white solid which was recrystal-
lized from ethanol to give (3).2 HBr as white crystals (2.37 g, 37%).
The physical and spectroscopic data of the product were identical to
those of (3).2 HBr obtained in method 1. Unfortunately, method 2 some-
times gave a mixture of (2).2 HBr and (3).2 HBr, which were difficult
to separate.
References
[
1] T. Arishima, K. Hamada, S. Takamoto, Bull. Chem. Soc. Jpn.
973, 1119.
1
1
,4-Bis(hydroxyethyl)-1,4,7-triazacyclononane, (3), may be
[
[
[
2] R. L. Yang, L. J. Zompa, Inorg. Chem. 1976, 15, 1499.
3] L. J. Zompa, Inorg. Chem. 1978, 17, 2531.
4] P. Chaudhuri, K. Wieghart, Y.-H. Tsai, C. Kruger, Inorg. Chem.
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+
down a column of Amberlite IRA–400 in the Na form. Removal of
water from the collected effluent under reduced pressure and extrac-
tion of the resulting white solid with chloroform gave a solution of (3)
which was isolated as a white amorphous solid after the chloroform was
1
982, 21, 3086.
5] B. A. Sayer, J. P. Michael, R. D. Hancock, Inorg. Chim. Acta
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Soc., Dalton Trans. 1994, 1619.
[
[
[
[
1
−
1
removed under reduced pressure. νmax (neat)/cm 3338 (OH); δH (200
MHz, CDCl3) 2.67 (4 H, t, J 5.1, 2 × NCH2CH2OH pendant arm) 2.68
(
4 H, s, 2 × NCH2CH2NH ring), 2.77 (8 H, m, 2 × HNCH2CH2NCH2
ring), 3.48 (2 H, br s, 2 × OH), 3.61 (4 H, t, J 5.1, 2 × NCH2CH2OH
pendant arm); δC (50.32 MHz) 47.74 (2 × NHCH2 ring), 53.42 (2 ×
HNCH2CH2N ring), 53.99 ((CH2)2NH ring), 59.54 (2 × NCH2CH2OH
pendant arm), 59.75 (2 × NCH2CH2OH pendant arm); m/z (EI) 218
+
(
M , 100%), 187 (16), 129 (77), 88 (71), 58 (37), 44(18).
[
9] J. Huskens, A. D. Sherry, J. Am. Chem. Soc. 1996, 118, 4396.
[
10] R. Luckay, R. D. Hancock, I. Cukrowski, J. H. Riebenspies,
Inorg. Chim. Acta, 1996, 246, 159.
Potentiometric Titrations
[
11] A. A. Watson, A. C. Wills, D. P. Fairlie, Inorg. Chem. 1997, 36,
752.
Titration solutions were prepared in deionised water that was further
purified using a MilliQ-Reagent system to produce water with a resis-
tance of >15 Mꢀ cm. Ionic strength was adjusted to 0.10 mol dm
[12] J. M. Weeks, M. A. Buntine, S. F. Lincoln, E. R. T. Tiekink,
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−
3
with NaNO3. The reagents NaNO3, ZnSO4, and CdSO4 were recrystal-
lized from water, and dried to constant weight and stored under vacuum
over P2O5. (CAUTION: Anhydrous nitrates are potentially explosive
and should be handled with care.) Nitric acid (70%, Ajax) and sodium
hydroxide (Convol) were used as received.
Potentiometric titrations were carried out using a Metrohm E665
Dosimat autoburette interfaced to a Laser XT/3-8086 PC in conjunction
with an Orion SA720 potentiometer and an Orion Ross Sureflow combi-
nation electrode.Values of E0 and pKw were determined by titration of a
−
3
−3
−3
solution that was 3.00 × 10 mol dm in HNO3 and 0.05 mol dm in
−
3
NaNO3 against 0.05 mol dm NaOH. Titration solutions were thermo-
statted at 298.2 ± 0.05 K in a closed water jacketed vessel which had a
small vent for the nitrogen stream which was passed through aqueous
−3
0
.10 mol dm NaNO3 prior to being bubbled through the magneti-
cally stirred titration solutions to exclude atmospheric carbon dioxide.
−
3
The instrumentation was calibrated by titration of 0.100 mol dm
3
−3
NaOH (1.00 cm ) from the autoburette against 0.004 mol dm HNO3
3
3+ 3+
10.00 cm ).ThepKa valuesof(2).H and(3).H3 weredeterminedby
3
(
3
−3
−3
titration of 10.0 cm solutions of 1.00 × 10 mol dm in either (2) or
(
3) and 5.00×10− mol dm in HNO3 (such that the acid concentration
3
−3