Titration Solutions #7: pH Value, Conductivity and Titration in Water and Soil Analysis
by Alfred Steinbach, Metrohm International Headquarters, Herisau, Switzerland
The rapid growth in the world population has led to sharp increases in the consumption of energy and resources and in the production of consumer goods and chemicals. It is estimated that there are a total of 17 million chemical compounds on the market, including as many as 100,000 that are produced on a large industrial scale. A lot of these compounds enter the environment and have to be analysed.
The Necessity of Environmental Analysis
Substances introduced into the environment are distributed among the environmental compartments water (hydrosphere), soil (pedosphere), rock (lithosphere), and air (atmosphere), as well as among the organisms living on them (biosphere).
The effects of human activities on the environment are complex and call for sensitive analytical methods and powerful analytical instruments. Only if we know the type and quantity of these contaminants can we protect the environment and its inhabitants. This requires internationally accepted standards, in which limits and test methods are defined.
In this context, the measurement of the pH value and the conductivity as well as titration play an important role. Both pH value and conductivity are parameters that can also be regarded as sum parameters as they characterize similar chemical, physical, or physicochemical features of different components. The pH value is a measure of the acidity or basicity of an aqueous solution and provides important information about chemical and biological processes in the sample. Notably, the pH value allows conclusions about the bioavailability, mobility and toxicity of pollutants (e.g., heavy metals in soil). The electrical conductivity indicates the amount of dissolved inorganic salts in water (i.e., the salinity) and is thus a fast and inexpensive indicator for estimating the extent of contamination.
Total hardness, alkalinity, acidity and chemical oxidizability of water components are further important sum parameters that are required for a first risk assessment. In contrast to the directly measured physical parameters, pH value and conductivity, they are mostly determined by potentiometric titration. While the determination of the total hardness and alkalinity implies a complexometric and an acid-base titration, respectively, the determination of the chemical oxidizability of dissolved organic material requires a redox titration of the previously added oxidant. Beyond these examples, titration also includes the analysis of individual components such as ammonia, chloride, fluoride or sulphide (Table 1). Despite the advantages of many instrumental analytical methods, titration has lost none of its appeal. It is a direct, easy, fast and versatile method with a favourable price that is ideally suited for automation. The reproducible and correct results easily cope with those of the sophisticated methods, for which reason numerous titration methods are cited in international standards.
Just to pick up a few sum parameters, this article looks at the fully automated and standard-compliant determination of the oxygen demand in water samples. Additionally, it describes the direct measurement of the pH value and the electrical conductivity in soil samples.
1) Water — There is More in It Than We’d Like
As its physical state changes, water passes through all spheres. It is the most frequently analysed environmental compartment and is also the easiest, because — unlike air or soil — it already exists in the liquid phase.
Table 1: Selection of important international water standards for determining physical and overall parameters as well as cation and anion concentrations.
As a source of food and energy, during use in irrigation, as a solvent, cleaning agent or coolant, and also as a means of transportation and discharge system for effluents, water becomes contaminated with all types of substances. The World Health Organization (WHO) has issued guideline values for about 200 substances found in water. This is why water is the subject of a host of laws, regulations and standards in most countries (Table 1). If drinking water samples are to be analysed, sample preparation is usually not necessary; however, it is generally unavoidable in the case of polluted wastewater samples.
The quality of a water sample can be effectively estimated by measuring its organic pollution load. Several overall parameters are available to this end, and all involve the oxidation of organic matter. According to the oxidizing power of the oxidizing agents used, a distinction is made between the permanganate index and the chemical oxygen demand (COD). While the permanganate index is the more informational valuable parameter for samples with minimal or low contamination, COD is suitable for severely contaminated samples.
Permanganate Index According to DIN EN ISO 8467
The permanganate index determines the easily oxidizable fraction of the organic constituents in water and is used, in a broader sense, as a measure for evaluating the organic chemical contamination in waters with minimal or low contamination, such as drinking water samples. For determination, the water sample is heated for ten minutes with sulphuric acid and an excess of permanganate solution of known concentration in a boiling water bath. After that, the permanganate consumption is determined by adding an excess of sodium oxalate solution and back-titrating the consumed oxalate with permanganate solution (Figure 2). The permanganate index is expressed as the quantity of oxygen in mg/L that would be needed for oxidation.
Figure 2: Titrimetric determination of the permanganate index: consumption of oxalate volume versus measured electrode potential.
Chemical Oxygen Demand According to DIN 38409-44 and ASTM D 1252
The chemical oxygen demand (COD) is a measured value for the sum of the substances in a certain volume of water that can be oxidized by chromate. Chromate is a much stronger oxidizing agent than permanganate, which is why it also oxidizes most organic compounds practically completely into CO2. In effluent treatment plants, COD is regarded as a valuable indicative parameter for evaluating treatment performance.
For volumetric determination of the COD, the sample of water is heated over a defined period with potassium dichromate. The remaining quantity of potassium dichromate is then back-titrated with ammonium iron(II) sulphate.
The water constituents are oxidized either in a special COD heating device with reflux condensing or in a closed reaction vessel with an external heat source. Titration takes place directly in the reaction vessels without the need for the content to be transferred to other vessels. This prevents any sample losses and saves valuable time, especially when there is a high sample throughput.
2) Soil — The Complex Multiphase System
The pedosphere is the name given to a highly complex boundary area where hydrosphere, atmosphere, lithosphere and biosphere coexist. It is a complex multiphase system that consists predominantly of soil mineral matter, approximately equal portions of soil air and soil water and a small amount or organic matter. It serves as a source of water and nutrients for plants, is a habitat for a large number of organisms and is an important carbon sink. Harmful substances contained in it come mainly from weathering, cultivation or the air. Harmful soil pollutants can easily enter the human body through plants and animals. The constituents of soil are very difficult to access and difficult to mobilize. Sample preparation usually involves extraction and digestion procedures (Table 2).
Soil pH Value — Soil Reaction According to ISO 10390
The pH value of a soil sample is the most frequently determined parameter in soil analysis. It is the characteristic value of what is known as “soil reaction,” and allows soils to be classified according to their acidity and alkalinity.
Determination of the pH provides information about the acid or base action of the soil solution. This in turn is enormously important for the nutrient supply and microbial activity of the soil. For example, many metals (trace elements) are considerably more mobile in acid soils. If the soil pH values are very low, toxic levels can soon be reached and these can damage the roots of plants. Moreover, excessively high pH values quickly lead to a shortage of trace elements because of immobilization.
A simple method for determining the pH value is described in ISO 10390. It describes the slurrying of an air-dried and sieved (max. 2 mm screen) soil sample with distilled water and subsequent measurement. Alternatively, extraction can be performed using 0.01 molar CaCl2 or 1 molar KCl. The cations in these solutions displace the protons that are absorbed on the ion exchangers of the soil as quantitatively as possible.
Determination of Electrical Conductivity in Soil Samples According to ISO 11265
The determination of conductivity provides qualitative information about the amount of dissolved salts in the soil. It enables conclusions to be drawn about the ability of the soil water to mobilize mineral substances in the soil.
To determine electrical conductivity, a weighed-out quantity of a dried soil sample is shaken up with a defined volume of distilled water and then measured directly.
Every water and soil quality monitoring programme consists in determining physical and chemical properties, with the pH value and conductivity being the most important. Besides these direct measurements, titration is a strong tool for the fast and accurate determination of important sum parameters such as hardness, alkalinity, acidity or oxygen demand. Particularly in complex water and soil samples, the straightforward determination of a single sum parameter often spares the time-consuming analysis of numerous single compounds. However, some cases require the concentration of individual compounds. Also here, titration is the method of choice, from the analysis of ammonium, over the analysis of halides and cyanide to the determination of sulphide. This is the case all the more as titration is ideally suited for automation.
(1) Environmental analysis – Quality control of water, soil, and air, Metrohm brochure, Article no. 8.000.5044EN (can be downloaded under: www.metrohm.com/com/Applications/Industry.html, search for 8.000.5044EN).
(2) R. Reeve, Introduction to Environmental Analysis, John Wiley & Sons, Chichester, UK (2002).
(3) C. Zhang, Fundamentals of environmental sampling and analysis, John Wiley & Sons, Hoboken, USA (2007).
(4) W. Kleiböhmer, Environmental Analysis, Volume 3 (Handbook of Analytical Separations), Elsevier Science B.V., Amsterdam, The Netherlands.
Alfred Steinbach is scientific technical writer and editor in the Marketing Department of Metrohm International Headquarters, based in Herisau, Switzerland. He obtained his Masters in nuclear chemistry from the University of Cologne, Germany, and his PhD in environmental and analytical biogeochemistry from the University of Hamburg, Germany. He joined Metrohm AG in 2006.