Standard Electrochemical Methods applied to analysis, detection and control - STEMS

Introduction: the development of trade, technology and research, and environmental and public health protection strategies, mostly depend on chemical analysis. The need for mutual recognition of measurements urgently asks to drag metrological issues in chemistry out from their technical framework of formation, into the fields where chemical analysis is routinely applied, as the present lack of comparability is difficult to be sustained. Among areas of priority, electrochemistry represents a key to strengthen the horizon for international traceability. To ensure industry’s competitiveness and quality of life, integrated efforts for the harmonization of electrochemical testing procedures are necessary, aiming to the establishment of standard methods of analysis traceable to the SI. These have to be integrated with a solid theoretical background and technically implemented for affordable field-measurement techniques.

NEED and RELEVANCE: chemistry is novel among the metrological disciplines, and its measures not yet diffusely standardised, even after the repeated resolutions of the CGPM¹ addressing National Metrology Institutes (NMIs) to initiate and coordinate activities in the field. An important effort is under development in the scientific community, aimed to establish leading fields of action for metrology in chemistry - as electrochemical analysis - and to cooperative operations for the integration and dissemination of the existing or emerging measurement capacities. The assembling of a critical mass of expertise, at both national and international level, represents an essential issue.

Otherwise, electrochemical analysis is a developed and well-understood discipline. Although in several cases most of the experimental and theoretical work dates to the first half of the XX century, it maintains its excellence among the experimental techniques available to characterize chemical properties, thanks also to the access of a careful control of the electrical parameters applied. Tricky room remains for the study of devices and sensors based on electrochemical phenomena, whose progresses in the number of applications and sensitivity endlessly develop. The possibility to supply remote and accurate process control and the versatility of the techniques, along with the valid theoretical models, make electrochemical alternatives competitive and pretty attractive in almost any scientific field. Research is required mainly to enhance electrochemical strategies towards accuracy, sensitivity and selectivity in chemical and physical analysis.

The main purpose of this network will be to strengthen and develop scientific and technological quality in electroanalytical chemistry, a fundamental branch among the methodologies applied to analysis, detection and control of food quality and safety, as well as to monitor environment, industrial processes, health care, and innovative materials production. The proposed research will focus on the development, validation and harmonization of the electrochemical analytical technologies and methodologies, to ensure traceability of measurements to the SI and convenient trackability of goods, throughout the productive and distribution chain.

Nevertheless the establishment of traceability chains cannot be intended as the direct mean to solve specific problems or to assure quality and safety, e.g. for food and beverage. It remains a fundamental tool at disposal for the integrated advancement of research in any of the investigative or control fields where measurements have to be applied and compared in space and time. Thus, being electrochemical analysis so widely requested, its metrological assessment will result in a general scientific and technological advancement, with effective cost reduction expected to ensue, after standardization of the measurement approaches.

Primary methods of measurement are essential for the link that they provide in the chain of traceability from the definition of units in the SI to their practical use. Secondary standards have to be developed and are useful for the extension of traceability and for dissemination of the units. The long-term multidisciplinary activity will be mainly devoted to those methodologies that, in the extended electrochemical domain, appear most widely applied and metrologically sound. The ensemble of technical resources and acknowledged competence is to be gathered in the network exactly to focus onto standardisation of electroanalytical techniques and to guarantee, when predictable, traceability to the SI.

Within the priority thematic areas, indicated in the FP6 of the European Community for RTD, specific room for the proposed research has been kept among the topics for Food Quality and Safety (1.1.5), in particular when relating to Methods of analysis, detection and control. Additional matching is foreseeable with the Complementary research indicated for Global Change and Ecosystem (1.1.6.3), focussing on "development of advanced methods for risk assessment and for appraising environmental quality, including relevant pre-normative research on measurements and testing for these purposes". Finally, Citizens and Governance in a Knowledge-based society (1.1.7) addresses to support research through "the collection and analysis of better and more genuinely comparable data".

In fact, measurements in chemistry cannot straightforwardly move from the metrological context, still under development, onto the field where they are implemented. The environmental parameters under which they are carried out can widely differ, and have to be clearly stated and defined to be useful and to provide the necessary knowledge. In solution chemistry, in particular, additional information is necessary to relate results with equilibrium conditions. A peculiar requirement imposed on chemical measurements is that they should not only be specific to a substance, frequently information on its speciation (i.e. the oxidised or reduced, combined or free ionic form), must be provided.

Considering that electrochemical techniques can furnish information specific to the ionic form of the examined substance, it is immediate to appreciate the importance that their standardisation will gain. To exemplify, the acid content in solutions is ubiquitously evaluated and expressed through pH units. It rules transport and storage of metal ions in living organisms, distribution and mobility of metal compounds in environment and food, metal-ion sequestration from river, lake or sea sediments, and protection ability from bacterial assault in beverages. Additionally, accurate determination of chemical stability constants involves pH-metric measurements, and the thermodynamic data obtained may allow the modelling of the formation of chemical species. However, two mutually inconsistent protocols have been endorsed by IUPAC² for pH metrology, based or not on thermodynamics. The values measured for identical buffers by the two methods can differ. Only recently an international pH Working Party was charged to generate a new recommended protocol, reflecting present thinking on good metrological practice.

Chemical speciation, fast response, remote control and on-line implementation in difficult environments are requirements to be satisfied frequently by chemical analysis, although the same are not typical features of primary or secondary methods. Fortunately, the survey of effective electro-analytical tools encompasses several dc and ac techniques for titrimetric procedures (e.g. potentiometry, coulometry), or for direct measurements (e.g. polarography, conductivity). The sensors available include Ion Selective Electrodes (ISE) and solid state devices, applicable either in aqueous or mixed solvent media. These alternative offer an essential contribute to estimate the amount of substance and characterise the relevant species in solution, although they still ask for harmonisation, validation and, overall, traceability assessment.

Thus, moving from the basic levels of traceability already established, the program will assure that interpretative models are extracted, and methodological approaches imitated, to be applied on the ordinary levels active in the analytical fields. Among others, selected scientific issues and major analytical problems will emerge, as the priority thematic areas for research below reported as keywords.

The Electrical instrumentations and Methodologies for application in chemical analysis are the best available and versatile in terms of affordability and for the attainable level of accuracy. Additionally, chemical and electrical quantities remain proportional during electrochemical transformation. This allows for the enforcement of the electric units in chemistry, a major key to set primary methods of measurement for the mole. The ability to measure electrical quantities in a consistent and repeatable manner is vital for maintaining and extending the economic and technical success of chemical methodologies. Thus, experimental and theoretical assessment of the electrical instrumentation and circuitry applied to electrochemical measurements is worthwhile. This capacity of integration, to be inserted among the main purposes of the network, fits exceptionally well with the activities traditionally carried on by the NMIs active in Electrical Metrology.

Metrology: traceability to the SI, uncertainty estimates, standardisation, accreditation, and quality assurance have to be considered as major objectives as well as selectivity and sensitivity. Innovations and developments in the field can be confidently expected. A joint program of activities will be considered, involving education and dissemination of results outside the network. The development of specific reference materials will be studied, along with the exploitation of pure substances and their protection from contamination.

The assessment and standardisation of electrochemical methods represent the focal points upon which all of the network’s efforts will concentrate. Part of the tasks will be devoted to the characterisation of non-aqueous solvents or aqueous-organic mixed media, these last are promising for the establishment of reference solutions and buffers especially intended to extend the range of traceable electrochemical standards. It will be necessary to move from measurement to theory and vice versa, to model instrumentation and solution chemistry for the valid assessment of the analytical techniques as well as to sustain primary definition of chemical quantities. Experimental development will support the affordable strategies to define standards. Parameters measured by innovative methods and conventional techniques, electrochemical or not (pH can be estimated also by way of optical methods), can be compared.

Water not only represents the principal medium subject of the research program, it is also the most important utility supplied to laboratories and the most widely used analytical reagent. Without the availability of pure water, research in analytical chemistry and in the areas of semiconductor, power generation, pharmaceuticals and biotechnology could not have reached the actual level of advancement. The possibility to define traceable ultrapure water through electrochemical units will open the path for its application as an important primary standard for physical and chemical properties of liquids.

The measurement techniques reported in the following paragraphs are appropriate for metrological exploitation and significant in chemical analysis. The proofs of their reliability based on traceability to national or international standards are required by regulatory bodies and quality assurance criteria. For example, the quality of instrumentation for pure water supply is defined³ in terms of on-line conductivity, pH and total Oxygen measurement. Primary standards and provisions for the dissemination to the working level of these quantities are therefore needed, and CCQM⁴ included pH, electrolytic conductivity, coulometry and, more recently, ion selective electrodes (ISE) in its work program.

pH Standardisation: the appraisal of acidity is the main and oldest test applied for food and beverage quality. Actually, for pH Harned cells are extensively used and studied as primary standards, and a predicting scheme for them already exists, limited to aqueous/alcoholic mixed solvents. Its extension to non-alcoholic solvents can be envisaged through research and study on aqueous-organic media, as the field of organic solvents completely lacks reference materials for pH. The characterization of new reference solutions, midway in the pH scales, will match that of secondary cells, to achieve the prescribed multi-standard system. The work will be completed by the characterisation of salt bridges in aqueous-organic solvent mixtures.

Ion Selective Electrodes (ISE) belong to the family of potentiometric, amperometric or voltammetric sensors. They are widely and successfully applied in various fields of electrochemical analysis. The concentration of inorganic ions is generally quite high in all the natural fluids. Some of them, e.g. alkaline ones, are endowed with a good affinity towards molecules containing oxygen-donor atoms. This can affect formation constants of complexes, with repercussions on the electrical charge and conformation in solution between the free and the bound form of the ligand species. Membrane ISE can be successfully applied in the characterization of complex in solution, even when ions in excess are present. The formation constants of alkali metal complexes obtained by the pH-metric method can be confirmed by ISE-based measurements.

rH Standardisation: rH is an index of the reducing power of red-ox systems, its value is linked with the more familiar quantities of red-ox potential and pH. Recently it has become an important factor in quality and environmental controls, although its domain has hitherto been neglected for any type of regulation. This lack of knowledge concerns the rationale for rH standardisation and rH scales, the determination of standards in water and in aqueous-organic solvents. These are susceptible of high interest for environmental, biological, hydrographic, quality-control measurement and for enology and brewing. A systematic methodological set-up and experimental programme has to be considered to ensure for rH a number of reference points uniformly distributed within its measurable range.

Electrolytic conductivity: the measurement of electrolytic conductivity is a functional analytical tool applied in various fields of science and technology. Although it is a non-specific sum parameter, through conductivity an estimate is offered for the concentration of ionized substances in a liquid sample. Under given conditions, it is a useful and accessible quantitative measure of quality, replacing laborious and expensive chemical analyses. Important examples of application are for water purity assessment, according to normative standards, and in pharmaceuticals, drinking water, food industry, health care, and environmental monitoring. The completion of a chain of traceability for low conductivity measurements, down to ultra pure water, emerges among the foreseeable actions.

Coulometric Methodologies: based on electrochemical transformation of a substance and quantitatively described by the Faraday’s Law, coulometry accounts as a primary method for the Amount of Substance whose determination is proportional with the electric charge. The number of moles is then directly linked with a basic unit of SI and does not need a reference for comparative evaluation. Additionally, the Faraday number is a fundamental constant and has been proposed as an ultimate standard in chemistry. The main use of coulometry is for the production of primary reference materials that can be subsequently used to disseminate traceability for chemical measurements. A coulometric system can act as a measurement standard and, at the same time, offer measurement capability.

The laboratories that will form the Network are reported below, along with their basic research capabilities:

INTEGRATION AND STRUCTURING EFFECT: the network under definition moves from a common interest to cooperation and adapt of research, previously expressed by a number of the participants to strengthen their complementary activities in the respective areas. Leading Institutions will be selected for any of the concerted actions envisaged, to constitute the nucleus onto which an array of collaborative scientific, educational and application-devoted activities will be defined. A steering committee will include representatives of the different typologies of the participants (NMIs, Universities and Research Institutes, SMEs). It will coordinate and recommend progressively motions towards the goals of the project, as well as connections with the research and development environment in Europe.

The participating NMIs already cooperate and commonly share experimental work within the frame of the major regional (European) and international organizations active for Metrology in Chemistry (CCQM, EUROMET and EURACHEM). They are lively involved for the maintaining of primary physical units and primary methods of chemical measurement, for the dissemination work and to ensure traceability and accreditation services in chemistry at a national and international level.

The Universities gathered in the project possess clear experience and practice in electrochemistry, sharing work and expertise in international chemical normative and standardizing organizations, as IUPAC. They can support and lead the labor to connect and harmonize the metrological and experimental actions under a common theoretical frame. Analyses of science-society issues, associated with the research carried out, could find room to nucleate and grow around specific existing academic facilities.

The SMEs and the Institutes developing and producing instrumental devices, sensors, data analysis and reference materials, will be active for exploitation and as consultative and reviser partners of the actions undertaken. Joint management and application of the new knowledge, and promotional actions will be provided, along with services in support of technological innovation.

Coordination and management of activities will include the following services in particular. Dissemination: the knowledge developed and the perspective for the feasible use of the results can be widely spread by way of the metrological organizations, functional for the communication of the achievements and the dissemination. Education and exchanges of personnel, in particular from/to the SMEs, will be valuable to train operators, technicians and researchers, and for the development and adaptation to use of joint research infrastructures. Guidance documents for the topics covered will be prepared for scientists, analysts, engineers, government regulatory agencies, academics, monitoring communities, and planning organisations. Web facilities will include the utilization of shared virtual sites for the exchange of information and communication, and the development of interactive working.

The way forward: in the last decades electroanalytical chemistry enlarged its application in the gas and solid phases and reached resolution of the order of angstroms. It is predictable that developments will permit to deposit selective polymers over micron-size electrode substrates. Gas-sensor arrays are already apt for detecting headspace samples in biological processes for food and beverage industries. Recent studies spot also on environmental problems, to control agriculture and sewage odors and prevent microbial contamination in biotechnology. Further issues to integrate the program, could relate with the selection of appropriate and metrologically sound means, electronic or electrolytic, useful to achieve in sensors selectivity and sensitivity, required as strategic keys in chemical analysis. An effort will be deserved to aggregate analytical laboratories devoted to the assurance of food quality, health care, drugs check, and environmental and process control.