The chemical speciation of both metals and non-metals, the use of polarographic techniques, and application to the study of the chemistry of anoxic waters are considered. In the first part of the paper unfamiliar terminology is explained and then an example of simple lake chemistry is presented to illustrate why the concept of speciation is necessary

Davison, W.

English

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REPORT OF THE DIREGTOR 53 
STUDIES OF CHEMICAL SPECIATION IN NATURALLY ANOXIC BASINS 
W. DAVISON 
Introduction 
This article attempts to convey the flavour of some of my work at the 
FBA. It is concerned with chemical speciation of both metals and non-
metals, the use of polarographic techniques, and applications to the study of 
the chemistry of anoxic waters. The first part of the paper tries to explain 
unfamiliar terminology and to give the reader a feeling for the subject. An 
example of simple lake chemistry is then presented to illustrate why the 
concept of speciation is necessary. 
Cations and anions cannot exist in water as single entities. They are 
complexed, either by water molecules or inorganic/organic ligands. The 
aquo complex for a metal ion, M, may be represented as M(H20)z+ where z is 
the charge of the molecular complex and y is the number of co-ordinated 
water molecules (usually six). Sometimes a convention which effectively 
discounts water as a complexing ligand is adopted and then the aquo 
complex is referred to as the free metal ion. The distribution of a metal ion 
among its various water-soluble complexed states is referred to as its 
speciation, and each molecular configuration which can exist in true 
solution is called a soluble species. Constants which define the strength of 
complexation of each particular ligand are variously called stability, 
equilibrium, dissociation, or association constants. They refer to an 
equilibrium state and are therefore thermodynamic quantities. Most 
natural systems are not at true equilibrium and so kinetic factors play an 
important role in determining species distribution at any given time. The 
interconversions between species may be controlled by chemical kinetics, 
but often the driving force which changes the equilibrium in natural systems 
originates from living processes. The rates themselves may also be 
controlled by biological factors because enzymes and 'microbes' may be 
operationally regarded as catalysts. An extreme case is that of nitrate and 
ammonia interconversion which takes place readily in a natural and 
microbially influenced environment. If the interconversion was solely 
dependent on chemical kinetics, the rates would be so slow that the two 
forms would be virtually stable. 
In natural waters it is essential to try to define, as unambiguously as 
possible, the concentrations of all individual chemical species because it is at 
this molecular level that chemical reactions occur and living organisms take 
up both nutrient and toxic elements. Therefore only by a thorough 
understanding of chemical speciation can we hope to understand the 
detailed mechanisms of the chemical and biological processes. 
Recognition of the above facts prompted me to embark on a study of 
chemical speciation with particular reference to the study of the hypolimnia 
of local lakes. This involved a good deal of basic work in theory and 
54 FORTY-EIGHTH ANNUAL REPORT 
laboratory experimentation as well as observations on natural waters. 
There is an inevitable difficulty in trying to explain such work in simple 
terms because it involves the use of some jargon and chemical and 
mathematical formulae. I hope, nevertheless, to overcome this problem 
and to show the relevance of the research to wider aspects of limnology. 
Methods of measurement 
Most analytical methods are incapable of distinguishing the individual 
chemical species which are present in a solution at equilibrium. Indeed, 
only ion-selective electrodes can truly sense the activity of an individual 
species in solution. Ion-selective electrodes operate by measuring the 
electrical potential between two electrodes in solution. The measuring 
instrument has a very high impedance and therefore the equilibrium 
conditions of the solution are not perturbed. In contrast other methods 
usually disturb or destroy the individual species. Unfortunately, with the 
notable exception of the glass pH electrode, most ion-selective electrodes are 
limited in their application to natural waters by sensitivity and interference 
problems. 
Voltammetric techniques, such as polarography, measure a current 
which flows when individual species are oxidised or reduced at an elec-
trode. This electrode reaction changes the concentration of a particular 
species in solution and so perturbs the equilibrium between the other labile 
species in solution. Therefore, while the polarographic measurement is 
being made a kinetic exchange between the solution species takes place. In 
1978 I tried to provide a conceptual definition of the measured species: 'the 
analytical concentration measured by a polarographic or voltammetric 
technique corresponds to the concentration of the electro-reducible species 
(usually the aquo ion) and those species which are in sufficiently labile 
equilibrium during the time scale of the measurement, that they are 
indistinguishable from the electro-reducible species'. At the same time 
(Davison 1978) I developed equations which enabled quantitative limits to 
be placed on the exchange kinetics of the species being measured when 
simple electrode reactions apply. These equations have now been extended 
by Van Leeuwen (1979) to more general cases. Therefore, although the 
polarographic signal does not necessarily correspond to the concentration of 
a single species, it does measure a restricted and well defined set of species 
without disturbing, except at the electrode surface, the equilibrium of the 
measured solution. This fact has been used by many workers to define 
operationally the labile species present by comparing direct voltammetric 
measurements of a metal ion with the total concentration of that metal 
present in solution (Davison 1980, Davison & Whitfield 1977). 
Synthetic solutions permit the solution composition to be varied and this 
allows the measurement of various kinetic and thermodynamic prop-
erties. In these circumstances polarography can unambiguously identify 
REPORT OF THE DIRECTOR 55 
the measured species. In a natural system it is necessary to couple the 
polarographic measurement with independently measured stability 
constants and kinetic data to calculate the equilibrium concentration of 
individual species. 
Application to natural systems 
Whitfield and I (Davison & Whitfield 1977) have critically reviewed the 
application of voltammetric techniques to the study of natural water chem-
istry. Most work has been performed on well-oxygenated waters. 
However, it is necessary when using polarography to remove oxygen from 
the system because it can participate in electrode processes and so obscure 
the signal of interest. Deoxygenation necessarily imposes some changes on 
the solution and so the polarographic measurement is made on a somewhat 
transformed system rather than the real one. In the summer the waters at 
the bottom of a productive lake can become naturally anoxic. This process 
involves an input of electrons into the system which leads to numerous redox 
reactions and the appearance in solution of reduced chemical states. This 
natural system is therefore ideally suited to the application of the 
polarographic method, which is capable of quantitatively exchanging 
electrons with the solution in a controlled manner under oxygen free 
conditions. I have performed such polarographic measurements (Davison 
1976, 1977a, 1977b) and moreover they were applied directly to lake water 
without it receiving any form of chemical pre-treatment. To do this I had 
to develop sampling and handling procedures to facilitate polarographic 
measurement in the laboratory and collection of anoxic water from the lake, 
without exposure to air. The system used (Davison 1977c) introduced less 
than 15 ug l-1 of oxygen, the limit for its measurement by polarography. 
Fig. 1 shows a typical differential pulse polarogram of hypolimnetic 
FIG. I . Differential pulse polarogram of anoxic lake water. Current is plotted against 
potential and each peak corresponds to an electrode reaction involving the indicated 
species. The solid line is expanded by a factor of 10 over the indicated current scale. 
56 FORTY-EIGHTH ANNUAL REPORT 
water. Current is plotted against imposed potential. Each peak 
corresponds to a particular electrode reaction and the peak height is 
proportional to the concentration of the measured species. The peaks at 
-1350 and -1550 mV correspond to the reduction of ferrous and manganese 
ions respectively. By modelling these reduction processes using synthetic 
solutions I was able to determine their reaction mechanisms (Davison 
1976). The sharp peak at c.-600 mV may be ascribed to mercuric 
sulphide formation at the mercury electrode. The height of the peak is 
proportional to the total concentration of soluble sulphide species in solution 
(Davison 1979a; Davison & Gabbutt 1979). Although I have shown that 
the peak at -1100mV is due to an electrode process dependent on both 
ferrous and sulphide ions (Davison 1977b), I have not yet worked out the 
details of this process. 
Heaney and I (Heaney & Davison 1977) used the measurement or labile 
Fell species by polarography to test and verify the use of 2,2'bipyridyl as a 
specific complexing agent for ferrous species. The sulphide measurement 
provides an illustration of how polarography can effectively measure several 
species simultaneously. The dissociation and association reactions of the 
various sulphide species are very rapid processes and so in the time scale of 
the measurement, typically 100 ms, the species are indistinguishable and the 
resultant signal is proportional to the sum of the concentrations of H2S, HS 
and S2-. 
The ferrous sulphide system as an example 
Fig. 1 shows that it is possible to measure simultaneously the ferrous and 
sulphide components of anoxic water from a single polarographic scan. 
This technique has been used to follow the development of these two 
components with respect to depth and time in the seasonally anoxic 
hypolimnion of Esthwaite Water. The sulphide profile with depth is 
particularly informative because initially the measured concentration 
increases with depth, which is consistent with production of this reduced 
form of sulphur in the sediment and subsequent transport through the water 
column by turbulent diffusion. The concentration of ferrous iron also 
increases with depth. However, as the season progresses the peaked 
sulphide profile shown in Fig. 2 develops (Davison & Heaney 1978). This 
peak is due to the combined concentrations of ferrous iron and sulphide in 
the bottom waters exceeding the solubility product of ferrous sulphide with 
consequent near stoichiometric removal of Fe2+ and S2- from solution. At 
depths less than that of the sulphide peak the solubility product is not 
exceeded and no precipitation occurs. This is demonstrated by the plot on 
a logarithmic scale of the product of [Fe2+] and [S2-] against depth (Fig. 
2). From the oxic layer to the sulphide peak the product increases and 
below this it remains constant. 
For Esthwaite Water, with its characteristic peaked sulphide profile, the 
shape of the iron profile remains relatively unchanged because there is 
nearly two orders of magnitude more iron in solution than 
sulphide. Where the reverse is true, for example in the Black Sea where 
sulphide is dominant (Spencer & Brewer 1971), a peak is observed in the 
iron profile. 
The constant concentration product plotted in Fig. 2 should provide a 
measure of the solubility of ferrous sulphide which operates under these 
natural conditions. In order to use this value to study the formation of iron 
sulphide minerals in nature, the solubility must be expressed in a standard 
form which permits comparison with other measurements. The procedure 
for doing this is outlined below. It illustrates the necessity of being able to 
define the measured species from a knowledge of the technique and solution 
chemistry. 
The solubility product for ferrous sulphide may be defined as 
( 1 ) 
where a represents the activity of the individual aquo Fe2+ and S2_ 
species. K is related to the concentration product cKso by equation 3 
(2) 
(3) 
where [] denotes concentration of an individual aquo species and y is a single 
ion activity coefficient. Therefore to calculate Kso, the concentrations of 
REPORT OF THE DIRECTOR 57 
58 FORTY-EIGHTH ANNUAL REPORT 
the S2- and Fe2+ species need to be known. For Fig. 2 the concentration of 
S2 - was calculated using established values for 
(4) 
(5) 
where (H+)* = 10-pH; pH is measured on the defined NBS (National Bureau 
of Standards) scale. Unfortunately there is some doubt as to the 
theoretical possibility of measuring K'2 (Giggenbach 1971), and to avoid 
this problem it is necessary to define a new solubility product which does not 
need a value for K'2. If [S2-] is eliminated between equations (2) and (5), 
K' may be defined: 
sp 
(6) 
To calculate K'sp values for three variables are needed. (H+)* is calculated 
directly from the measured pH using the ion-selective glass electrode. 
[HS~] is calculated from the polarographically measured value of 
[H2S] + [HS-] + [S2-] using equation (4) and assuming that [S2_] is 
negligibly small. Polarographic measurement gives the concentration of 
labile electroreducible forms of iron. Only a fraction of this value will be 
represented by the aquo ion and this fraction may be calculated using 
existing data for stability constants (Davison 1979b). K'sp is not a unique 
constant, however, because it applies to a particular solution. KNBS applies 
to zero ionic strength and may be calculated from K'sp if the single ion 
activity coefficients, yFe2+ and yHS-, are known: 
(7) 
Heaney and I (Davison & Heaney 1978) used Debye-Hiickel theory to 
calculate activity coefficients for fresh water with low ionic strength, and 
then we calculated KNBS directly from equation (7). 
The value obtained for Esthwaite Water was in good agreement with the 
laboratory-determined value for synthetic solutions and the measured value 
for a seasonally anoxic marine basin (Davison & Heaney 1980). This 
indicates that the formation of ferrous sulphide minerals in natural 
sedimentary sites is controlled by simple chemical processes. 
The above example shows that the consideration of the solubility of even a 
simple solid phase, FeS, requires careful attention to detail. In order to 
calculate its solubility the following information was necessary: measured 
pH; measured concentrations of ferrous and sulphide species; single ion 
activity coefficients of Fe2+ and HS-; pK'1 for dissociation of hydrogen 
sulphide; and the percentage of labile soluble ferrous iron in the aquo 
form. This data was assembled by both careful measurement and the 
extensive use of established ion-activity and stability constant data. Of 
special relevance to this article is the fact that the calculation would not 
have been possible without a knowledge of the chemical speciation of both 
REPORT OF THE DIRECTOR 59 
ferrous and sulphide ions for the particular conditions of the water 
studied. In the past such knowledge has been lacking. This has led to 
considerable misunderstanding about the role of chemical processes in 
natural systems. Hopefully a greater attention to detail in future work will 
prevent misinterpretation of data and allow a more sensible comparison of 
results obtained from different environmental systems. 
REFERENCES 
Davison, W. (1976). Comparison of differential pulse and d.c. sampled 
polarography for the determination of ferrous and manganous ions in lake 
water. J. electroanal. Chem. 72, 229-37. 
Davison, W. (1977a). The polarographic measurement of 02, Fe2+, Mn2+, and 
S2- in hypolimnetic water. Limnol. Oceanogr. 22, 746-53. 
Davison, W. (1977b). The use of polarography in the investigation of 
hypolimnetic waters and the sediment-water interface. In Interactions 
between sediments and fresh waters (ed. H. L. Golterman). The Hague. 
Junk. 473 pp. 
Davison, W. (1977c). Sampling and handling procedures for the polarographic 
measurement of oxygen in hypolimnetic waters. Freshwat. Biol. 7, 393-401. 
Davison, W. (1978). Denning the electroanalytically measured species in a 
natural water sample. J. electroanal. Chem. 87, 395-404. 
Davison, W. (1979a). Differential pulse polarographic peak current and 
potential relationships applied to insoluble film formation. J. electroanal. 
Chem. 99, 371-73. 
Davison, W. (1979b). Soluble inorganic ferrous complexes in natural 
waters. Geochim. cosmochim. Acta, 43, 1693-96. 
Davison, W. (1980). Ultra-trace analysis of soluble zinc, cadmium, copper and 
lead in Windermere lakewater using anodic stripping voltammetry and atomic 
absorption spectroscopy. Freshwat. Biol, in press. 
Davison, W. & Gabbutt, C. D. (1979). Polarographic methods for measuring 
uncomplexed sulphide ions in natural waters. J. electroanal. Chem. 99, 
311-20. 
Davison, W. & Heaney, S. I. (1978). Ferrous iron-sulfide interactions in anoxic 
hypolimnetic waters. Limnol. Oceanogr. 23, 1194-1200. 
Davison, W. & Heaney, S. I. (1980). Determination of the solubility of ferrous 
sulfide in a seasonally anoxic marine basin. Limnol. Oceanogr. 25, 153-56. 
Davison, W. & Whitfield, M. (1977). Modulated polarographic and 
voltammetric techniques in the study of natural water chemistry. J. 
electroanal. Chem. 75, 763-89. 
Giggenbach, W. (1971). Optical spectra of highly alkaline sulphide solutions 
and the second dissociation constant of hydrogen sulphide. Inorg. Chem. 10, 
1333-38. 
Heaney, S. I. & Davison, W. (1977). The determination of ferrous iron in 
natural waters with 2,2' bipyridyl. Limnol. Oceanogr. 22, 753-60. 
Leeuwen, H. P. van. (1979). Kinetic classification of metal complexes in 
electroanalytical speciation. J. electroanal. Chem. 99, 93-102. 
Spencer, D. W. & Brewer, P. G. (1971). Vertical advective diffusion and redox 
potentials as controls on the distribution of manganese and other trace metals 
dissolved in waters of the Black Sea. J. geophys. Res. 76, 5877-92. 


Studies of chemical speciation in naturally anoxic basins

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Studies of chemical speciation in naturally anoxic basins

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