Table Of ContentCOMMISSION ON EQUILIBRIUM DATA
1977-1979
Titular Members
G. H. Nancollas (Chairman)
S. Arhland (Secretary)
G. Anderegg, M. T. Beck, A. S. Kertes
H. Ohtaki, D. D. Perrin, J. Stary
Associate Members
R. Battino, H. L. Clever, E. Högfeldt
D. N. Hume, Y. Marcus, L D. Pettit
M. Salomon, C. L. Young
National Representatives
A. F. M. Barton (Australia), A. Bylicki (Poland)
H. M. N. H. Irving (UK), A. E. Martell (USA)
I. N. Marov (USSR)
International Union of Pure and Applied Chemistry
lUPAC Secretariat: Bank Court Chambers, 2-3 Pound Way
Cowley Centre, Oxford 0X4 3YF, UK.
INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY
(ANALYTICAL CHEMISTRY DIVISION, COMMISSION ON
EQUILIBRIUM DATA)
CRITICAL SURVEY OF STABILITY
CONSTANTS AND RELATED
THERMODYNAMIC DATA OF FLUORIDE
COMPLEXES IN AQUEOUS SOLUTION
Prepared for publication by
A. M. BOND
Deakin University, Waurn Ponds, Victoria, Australia
and
G. T. HEFTER
University of Malaya, Kuala Lumpur, Malaysia
IUPAC Chemical Data Series, No. 27
Critical Evaluation of Equilibrium Constants in Solution
Part A: Stability Constants of Metal Complexes
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Copyright © 1980 International Union of Pure and
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First edition 1980
British Library Cataloguing in Publication Data
Critical survey of stability constants and
related thermodynamic data of fluoride
complexes in aqueous solution.
- (International Union of Pure and Applied
Chemistry. Chemical data series; vol. 27).
1. Solution (Chemistry) - Handbooks, manuals,
etc.
2. Fluorides - Thermal properties - Handbooks,
manuals, etc.
3. Coordination compounds - Handbooks, manuals,
etc.
I. Bond, A M II. Hefter, G T III. Series
54V.3422 QD544.3 80-40413
ISBN 0-08-022377-X
In order to make this volume available as economically
and as rapidly as possible the author's typescript has
been reproduced in its original form. This method has
its typographical limitations but it is hoped that they in
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Printed in Great Britain by A. Wheaton & Co., Ltd., Exeter
CHEMICAL DATA SERIES
No. 1 Y. MARCUS Critical Evaluation of Some Equilibrium
Constants Involving Organophosphorus Ex-
tradants (a volume in Critical Evaluation of
Equilibrium Constants in Solution, Part B)
No. 2 A. S. KERTES Critical Evaluation of Some Equilibrium
Constants Involving Alkylammonium Ex-
tradants (a volume in Critical Evaluation of
Equilibrium Constants in Solution, Part B)
No. 3 Y. MARCUS et al. Organophosphorus Extradants—
Equilibrium Constants of Liquid-Liquid
Distribution Reactions, Part I
No. 4 Y. MARCUS et al. Alkylammonium Salt Extradants—
Equilibrium Constants of Liquid-Liquid
Distribution Reactions, Part II
No. 5 S. ANGUS et al. Argon — International Thermodynamic
Tables of the Fluid State—1
No. 6 S. ANGUS et al. Ethylene— International Thermodynamic
Tables of the Fluid State-2
No. 7 S. ANGUS et al. Carbon Dioxide—International Ther-
modynamic Tables of the Fluid State—3
No. 8 S. ANGUS et al. Helium — International Thermodynamic
Tables of the Fluid State—4
No. 9 A. R. H. COLE Tables of Wavenumbers for the Calibration
of Infrared Spectrometers, 2nd edition
No. 10 Y. MARCUS and Ion Exchange Equilibrium Constants
D. G. HOWERY
No. 11 R. TAMAMUSHI Kinetic Parameters of Electrode Reactions
of Metallic Compounds
No. 12 D. D. PERRIN Dissociation Constants of Organic Bases in
Aqueous Solution, Supplement
No. 13 G. CHARLOT et al. Selected Constants: Oxidation-Reduction
Potentials of Inorganic Substances in
Aqueous Solution
No. 14 C. ANDEREGG Critical Survey of Stability Constants of
EDTA Complexes (a volume in Critical
Evaluation of Equilibrium Constants in
Solution, Part A)
v
No. 15 Y. MARCUS étal. Compound Forming Extractants, Solvating
Solvents and Inert Solvents—Equilibrium
Constants of Liquid-Liquid Distribution
Reactions—Part III
No. 16 S. ANGUS étal. Methane— International Thermodynamic
Tables of the Fluid State—5
No. 17 W. A. E. McBRYDE A Critical Review of Equilibrium Data for
Proton- and Metal Complexes of
1,10-Phenanthroline, 2,2'-Bipyridyl and
Related Compounds (a Volume in Critical
Evaluation of Equilibrium Constants in
Solution, Part A)
No. 18 J. STARYand Chelating Extractants—Equilibrium Con-
H. FREISER stants of Liquid-Liquid Distribution Reac-
tions—Part IV
No. 19 D. D. PERRIN Dissociation Constants of Inorganic Acids
and Bases in Aqueous Solution
No. 20 S. ANGUS et al. Nitrogen — International Thermodynamic
Tables of the Fluid State —6
No. 21 E. HÖGFELDT Stability Constants of Metal-Ion Complexes
Part A—Inorganic Ligands
No. 22 D. D. PERRIN Stability Constants of Metal-Ion Complexes
Part B —Organic Ligands
No. 23 E. P. SERJEANT Ionisation Constants of Organic Acids in
and B. DEMPSEY Aqueous Solution
No. 24 J. STARYef al. Critical Evaluation of Equilibrium Constants
Involving 8-Hydroxyquinoline and Its Metal
Chelates (a volume in Critical Evaluation of
Equilibrium Constants in Solution, Part B)
No. 25 S. ANGUS et al. Propylene (Propene) — International
Thermodynamic Tables of the Fluid State-7
No. 26 Z. KOLARIK Critical Evaluation of Equilibrium Constants
Involving Acidic Organophosphorus Extrac-
tants (a volume in Critical Evaluation of
Equilibrium Constants in Solution, Part B)
No. 27 A. M. BOND and Critical Survey of Stability Constants and
G. T. HEFTER Related Thermodynamic Data of Fluoride
Complexes in Aqueous Solution (a volume
in Critical Evaluation of Equilibrium Con-
stants in Solution, Part A)
No. 28 P. FRANZOSINI Thermodynamic and Transport Properties
of Organic Salts
vi
I. Introductory Comments on the Survey
A. INTRODUCTION
The major problems associated with the measurement of the stability
constants of fluoride complexes in aqueous solution have been overcome in
the last decade or so and the determination of such constants is now more
straightforward than for most ligands. Indeed, since 1969 the fluoride
complexes of nearly every metal ion in the periodic table have been studied
quantitatively.
The major difficulty facing the intending investigator of fluoride
complexes is the necessity of working in acidic solution in order to
suppress metal ion hydrolysis. Hydrofluoric acid is a weak acid (pK - 3)
and, therefore, at low pH levels most fluoride is present as undissociated
HF which attacks conventional laboratory apparatus and especially glassware.
This problem is overcome by constructing or coating apparatus with HF
resistant materials (1). The fluorocarbon polymers polytetrafluoroethylene
(Teflon) and polymonochlorotrifluoroethylene (Kel-F) are particularly suited
to this purpose. Where contact with silaceous materials cannot be avoided
it should be kept to a minimum by careful experimental design.
The low pK of hydrofluoric acid also means that in acidic solutions
a
there is competition for fluoride ions between H and the metal ion of
interest. Thus, to obtain the stability constants of the metal ion-fluoride
complexes it is necessary to have accurate K values of hydrofluoric acid
a
and/or to monitor free H concentrations. This situation pertains, of
course, to the study of all complexes involving anions of weak acids and it
is not usually regarded as a problem. Indeed, it is advantageous in
applying certain techniques of stability constant measurement (2). The
difficulty in the case of fluoride complexes is that the glass electrode
which is the most convenient method for determining K values or for
measuring H concentrations can not normally be used in acidic fluoride
solutions because of HF attack. This problem can only be circumvented by the
use of alternative techniques and electrodes.
Although reasonably good stability constant data exist for most
fluoride complexes in aqueous solution, much work remains to be done on
establishing reliable values for the corresponding enthalpy and entropy
data. With the use of modern calorimetric techniques this would not seem to
present any especial problems, and there are already signs in the literature
of increasing interest in this area.
3
4 Introductory Comments
B. TECHNIQUES FOR MEASURING FLUORIDE STABILITY CONSTANTS
It is an unfortunate, but only too well known, fact of solution
chemistry that different techniques may yield different values of stability
constants. Intending investigators therefore need to recognise and
understand the known limitations of any method they are considering for use
and to bear in mind the possibility of unknown limitations. In this section
only the two methods unique to the study of fluoride systems will be
discussed, viz. the "ferri" and the fluoride ion-selective electrode
Λ
methods. Other than these two methods, the most commonly employed techniques
have been polarography, liquid-liquid distribution, potentiometry (using
metal-ion and/or hydroquinone electrodes) and ion exchange. The prospective
investigator is advised to refer to standard texts on the usage of these
techniques for the determination of stability constants. It is interesting
to note in passing the unusual importance of electrometric methods in the
determination of fluoride stability complexes.
1. The general characteristics of fluoride as a ligand.
The fluoride ion has been of considerable interest to coordination
chemists chiefly as a model for electrostatic interactions in solution (3,4).
With the important exceptions mentioned above, the fluoride ion is a "well-
behaved" ligand. It is neither reducible nor oxidisable in aqueous solution
(5), does not hydrolize under normal conditions (5) and shows little tendency
to form outer sphere complexes (6). Another feature of the fluoride ion which
is of importance in the polarographic determination of stability constants
(7) is that it does not appreciably adsorb at electrode surfaces.
As mentioned above, because of the low pK of HF it is necessary to
monitor hydrogen ion concentrations when the pH<5. Although there is some
evidence that the glass electrode may be used in very dilute HF solutions
(8) it is probably safer to use the quinhydrone electrode or, if "poisoning"
is not a problem, the hydrogen electrode may be used.
Standard fluoride solutions up to lAf can be conveniently made up by
weight from sodium fluoride. If higher fluoride concentrations are required,
potassium or lithium fluorides must be used. As fluoride ions are adsorbed
by glass surfaces even in neutral solutions all dilute standards should be
made up daily. This is imperative at fluoride concentrations less
than 10~2M. More concentrated standards can be safely stored in flasks
for varying periods of time. A much better practice, however, is to transfer
standard solutions immediately after making up to thoroughly cleaned
polyethylene bottles.
Introductory Comments 5
Many commercial analytical reagent grade fluoride samples have been
found to have a slight pH buffering capacity around pH7 presumably due to
small amounts of fluorosilicates. Such impurities may have an appreciable
effect on stability constants obtained by measurements of [H ]. All
fluoride stock solutions should therefore be checked carefully (1).
2. The "ferri" method.
Until the development of the fluoride ion-selective electrode, the
"ferri" method of Brosset and Orring (9,10) was the most widely used
method for determining fluoride stability constants. This method utilises
the effect of fluoride ions on the potential of the FeCIII)/Fe(II) redox
couple. The considerable difference in strength of the FeCIII) fluoride
complexes relative to those of the Fe(II) provides a sensitive measure of
fluoride concentrations. Typically, a differential potentiometric
titration technique is employed (11). The prospective investigator is
directed to the original literature (9-12) for specific experimental details.
3. The fluovide ion-selective electvode method
The fluoride ion-selective electrode, invented by Frant and Ross in
19 6 6 (13) has had a profound influence on the measurement of fluoride
stability constants. Almost every common metal ion system has now been
investigated with this electrode, some on numerous occasions. Magnesium,
for example, was studied no less than seven times during a three year
period. Essentially, the electrode may be used to measure fluoride
stability constants in the same manner as a glass electrode is used to
measure acidity constants, although concentrations rather than activities
have invariably been used. For the purpose of complexation studies, the
electrode appears to exhibit a close-to-Nernstian response (59.2mV per
tenfold concentration change) over a free (although not total) fluoride
range of IM to 10"3M (14,15). In non (fluoride) buffered media the limit
of detection is about 10~SM. The only major interference comes from
hydroxide ions. For this reason pH < 6.0 is recommended for most
determinations. Since this pH is well above the levels usually necessary
to suppress metal ion hydrolysis , this restriction does not normally present
any problems. Typically, a potentiometric titration technique is employed.
The prospective investigator is referred to the original literature (13-16)
for experimental details.
6 Introductory Comments
4. Comparison of the two methods
In providing a comparison of the two methods, it is most gratifying
to note that both techniques have produced highly consistent results for
many ions as can be seen from the tables of data (Part A). From an
experimental point of view, however, the fluoride electrode must be judged
as being considerably easier to use and its simplicity, directne.ss,
reliability and ruggedness appeals to many workers. By contrast the
"ferri" method presents a somewhat tedious experimental problem (17)
requiring the precise measurement and control of the Fe (III)/Fe (II)
ratio, in addition to the standardizations common to both methods.
The "ferri" method also has a number of other restrictions. It cannot
be used to investigate systems which can either oxidize Fe (II) (e.g.,
Np02+ (17)) or reduce Fe (III) (e.g., U4+ (18)), nor can it be used (19)
to determine the first fluoride complex of a species forming a much
stronger complex than the first iron (III) complex (i.e., when Ki(MF n )
>> K!(FeF2+)). Finally, the "ferri" method has been shown (20) to have
only limited use for the detection of weak complexes (K < 10) and it can
x
only be used at low pH values where hydrolysis of Fe (III) is suppressed.
This latter requirement necessitates working under conditions where
fluoride is protonated and the equilibria
H+ + F" ^ HF
HF + F" ^ HF2
must be considered, Obviously a knowledge of the stability constants
for the equilibria
Fe3 + + nF" * FeF(3"n)+
Fe2 + + nF"" ^ Fe(2"n) +
is also required (9-11).
One suggested disadvantage (21) of the fluoride electrode method has
been that it is insensitive to the detection of higher order complexes.
Aziz and Lyle (21) report this to be the case for Eu3+, Y3+ and Sc3 +
systems. However, other workers (14, 15) do not seem to have had this
difficulty which appears to have been based on a misconception about the
limits of detection of the fluoride electrode in fluoride buffered media
(see above). Further investigation of this point would be worthwhile.
5. Comments on the potentiometric determination of fluoride stability constants.
The importance of potentiometry in the determination of fluoride
stability constants warrants discussion of two important (but frequently
overlooked) difficulties associated with potentiometric measurements.
Firstly, particular care must be exercised to minimise the variation in
