Alkalinity measurements within natural waters: towards a standardised approach
Introduction
In water quality studies of river systems, a fundamental diagnostic of stream acidity is the measure alkalinity. Positive alkalinity represent alkaline to circumneutral conditions where bicarbonate ions often represent the main acid buffering component, while negative alkalinities represent acidic conditions with little acid buffering capacity and where the dominant negative charges come from strong acid anions such as chloride, sulfate and nitrate (Mackereth et al., 1978). The cross-over point between positive and negative alkalinities broadly represents the divide between healthy and acid impacted systems (UKAWRG, 1988). As has been illustrated in the companion papers in this volume (Evans et al., 2001, Jarvie et al., 2001, Helliwell et al., 2001) these acid impacted sites and their potential for change are widespread across much of Scotland.
Within agencies such as the Environment Agency of England and Wales (EA) and the Scottish Environment Protection Agency (SEPA) as well as within various research organizations, a mixture of methods are used. Indeed, in some cases where the pH of water is at 4.5 or lower, the alkalinity is taken as zero and no titration at all is undertaken. A plethora of values and approaches are presented within the literature often without explanation and there is confusion over what is often measured and what assumptions have been made. While for alkaline systems such a confusion is hardly significant as the bicarbonate buffer dominates, for more acidic systems, crucial difficulties arise as discussed later in this paper. Clearly, there is a need for a standardised approach to alkalinity measurement within the environmental sciences, for acidic systems at least. This issue is of particular importance for Scotland and Wales as well as many other upland areas of the UK and Europe which are regions of outstanding natural beauty. Within these regions there are significant areas underlain by base poor geology and soils which are acid sensitive and are vulnerable to atmospheric pollution and land use change (Langan and Soulsby, 2001).
Within this paper, the range of methods used and the associated potential errors are broadly described in relation to acidic and bicarbonate bearing waters. The work utilizes data from calcium bicarbonate rich waters from the Thames basin, and acidic to neutral waters from Wales and eastern Scotland (the Dee basin). The work provides a firm base for the development of a standardized procedure for both environmental management and research purposes at a national and international level.
Section snippets
Sample sites
The data presented in this paper come from three types of site. Firstly, acidic to neutral waters from acidic soils with base poor Ordovician to Silurian slate and shale bedrock. These have been collected for over 16 years for a range of stream waters, rainfall and mist in the Plynlimon area and for sites across Wales (Neal et al., 1997, Neal et al., 1998a, Neal et al., 1998b). Secondly, alkaline waters from calcium carbonate rich Cretaceous chalk aquifers in the Thames basin have been sampled
Results
The results of this study comprise two types of issue. Firstly, there is a summary of information over the definitions of alkalinity and the assumptions made over what has and what has not been measured. This section is required to place the alkalinity issues dealt with later within the context of what needs to be measured. Secondly, there are both theoretical and practical issues which are covered and illustrated with practical examples. These aspects are dealt with separately below.
Definition of alkalinity
Alkalinity is usually expressed as the difference between two components (Mackereth et al., 1978).
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The acid buffers in solution which is usually dominated by bicarbonate, carbonate and hydroxide ions under very alkaline conditions, organic anions such as humates and fulvates particularly under acidic conditions plus less common buffers such as sulfide which occasionally provide additional buffering capacity;
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The hydrogen ion concentration.
In its complete form, alkalinity may be defined by
Alkalinity, bicarbonate alkalinity and the inorganic carbon system
In many classic water quality studies on circumneutral to alkaline systems (pH 6–10), the alkalinity is considered to be made up of overridingly bicarbonate. From this it is concluded that only a single acidimetric titration to the bicarbonate endpoint at approximately pH 4.5 is needed to determine what might be termed the bicarbonate alkalinity. Thus,
Indeed, for such studies, bicarbonate alkalinity is even often quoted in terms of bicarbonate (mg-HCO3−/l) or calcium
Alkalinity, bicarbonate alkalinity and the inorganic and organic carbon system
For acidic waters, buffers other than bicarbonate, organic acids in particular, become important and not all the buffering components are titrated out. For example, for many organic anions present in natural waters, protonation to an endpoint requires at least a titration endpoint one to two units lower than standard (Reynolds and Neal, 1987, Cantrell et al., 1990, Neal et al., 1999). Furthermore, a titration endpoint has no real meaning when mixed buffers are present in the solution. For
Standardisation of units
As stated above, at least three types of units are presented: μEq./l, mg-HCO3−/l and mg-CaCO3/l. Only the former unit is generally applicable and it is appropriate as a first step to convert the data, which use the other units. This involves a two-step procedure. Firstly, to convert mg-HCO3−/l and mg-CaCO3/l to μEq./l, the former values must be multiplied by 1000/61 and the latter must be multiplied by 1000/50. Secondly, depending upon methodology, hydrogen ion correction terms using , , , ,
Alkalinity measurement: practical considerations
The errors associated with alkalinity measurements fall into five categories.
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The wrong definition of alkalinity is used. This aspect has been covered within the previous section and the relevant equations presented to standardise the measurements to a common alkalinity term.
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Colourimetric titrations may have been used which:
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measure bicarbonate alkalinity rather than alkalinity;
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do not fully compensate for hydrogen ion concentration terms within the definition of alkalinity (, , );
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do not allow for
Hydrogen ion measurement error
Most modern alkalinity titrations assess hydrogen ion concentrations indirectly from pH measurement using standard electrode techniques. However, there are three types of errors associated with the assessment of hydrogen ion concentrations using pH measurements. Firstly, the pH electrode measures, in logarithmic form, chemical activity rather than the concentration of hydrogen ions and a correction may be needed to allow for the difference. Secondly, there are pH measurement errors associated
Chemical activity–concentration effects
The pH of a solution is defined as minus the logarithm (to the base 10) of the hydrogen ion activity. The hydrogen ion activity (aH+) is a measure of the ‘reactivity’ of hydrogen ions in solution, an ‘effective concentration’. For very dilute solutions, the hydrogen ion activity and the hydrogen ion concentration are almost identical as the mobility of hydrogen ions are hardly affected by the presence of other ions in solution. However, as the salt concentration increases, the presence of other
pH measurement errors
While the concentration–activity relationship can lead to errors in the estimation of hydrogen ion concentrations from pH, there can also be systematic measurement errors associated with the pH system used. There are two types of measurement error:
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Electrode response. This type of error is associated with either a liquid junction potential or contamination of the glass electrode. Typically the error is constant with respect to the pH scale with standard offset. For example, comparison of pH
[Δbuffer]and [Δremnant]
As the pH endpoint progresses to lower values, the intermediate and strong acid anions become more protonated. Indeed, even strong acid anions such as chloride, sulfate and nitrate ultimately become protonated at extremely low pH values. The standard titrations used to determine alkalinity do not lead to full protonation of the intermediate and strong acid anions, nor do they fully protonate what are traditionally classified as the weak organic anions such as humates and fulvates found in
Field measurement
The overall value of specific alkalinity measurement types can only be finally gauged on the basis of field measurements. Table 2, Table 3 provides a summary of the field data for the Thames and Dee study. For both studies there is virtually no difference between the activity-corrected and uncorrected Gran values. Indeed, for the Dee, there is a good 1:1 correspondence between GranAlk4.5−4.0, GranAlk4.0−3.0, Alk4.5, Alk4.0 and Alk3.0 as shown both by their very similar mean values and
Discussion
While alkalinity represents an extremely important water quality parameter in reflecting both the bicarbonate bearing and acidity levels within streams, it can only be viewed as an operationally defined term. In some cases, it may well reflect bicarbonate buffering in water, but in other cases it may reflect a composite set of buffering elements or in other cases simply minus the acid strength of the water. For this reason, it should be described in terms of the endpoint pH or, for the Gran
Conclusions: towards a standardised approach
Within this paper, many aspects of alkalinity measurement have been covered and there is a need to draw together the themes developed in order to move towards a standardized approach to alkalinity measurement and presentation within an environmental database. These aspects are covered below as a set of bullet points.
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Alkalinity is an operational term in relation to the buffering capacity and acidity of a water.
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Alkalinity may be defined to a fixed pH or a pH range.
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The correct expression of
Acknowledgements
The author thanks Richard Smart of the Soil Science department, Aberdeen University, for the use of the Dee data and the analytical chemists at the Centre of Ecology and Hydrology (Wallingford) for the analysis of waters from Wales and the Thames.
References (23)
- et al.
Evaluation of acid neutralizing capacity data for solutions containing natural organic acids
Geochim Cosmochim Acta
(1990) - et al.
Long-term variability in the deposition of marine ions at west coast sites in the UK acid waters monitoring network: impacts on surface water chemistry and significance for trend determination
Sci Total Environ
(2001) - et al.
Interaction of nitrogen deposition and land use on soil and water quality in Scotland. Issues of spatial variability and scale
Sci Total Environ
(2001) - et al.
Use of continuous water quality records for hydrograph separation and to assess short term variability and extremes in acidity and dissolved carbon dioxide for the River Dee, Scotland
Sci Total Environ
(2001) - et al.
The environmental context for water quality variation in Scotland
Sci Total Environ
(2001) Determination of dissolved CO2 in upland stream water
J Hydrol
(1988)- et al.
Field and laboratory measurement of pH in low conductivity natural waters
J Hydrol
(1985) - et al.
Acid neutralization capacity measurements within natural waters: towards a standardized approach
Sci Total Environ
(1999) - et al.
A comment on the use of acidimetric titrations for the estimation of the alkalinity and bicarbonate content of acid upland surface waters
Sci Total Environ
(1987) - et al.
Factors regulating the spatial and temporal distribution of solutes concentrations in a major river system in NE Scotland
Sci Total Environ
(1998)
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