pH Measurements and Ion Selective Electrodes
At NPL we carry out active research in the area of pH standards, in particular, the study of the Harned Cell as a primary method for pH measurement.
pH Measurements
electrode are pictured sitting
in measurement cells. The high
accuracy voltmeters used in the
measurements are pictured beneath.
A major contributor to the uncertainty in pH measurement arises from the reference potential of the Ag/AgCl reference electrode. We are interested in the role that the variability of manufactured Ag/AgCl reference electrodes plays in the overall uncertainty of a pH measurement. Our work is aimed at:
- Comparing Ag/AgCl electrode manufacturing techniques
- Investigating the absolute voltage response of alternative Ag/AgCl electrode designs
- Investigating the transient potential difference of alternative Ag/AgCl electrode designs
Some Ag/AgCl electrodes, and our apparatus for testing the voltage response, is pictured below.
Other pH-related work at NPL involves:
- Developing suitable methodologies for online pH and ion measurement in industrially relevant environments
- Providing input into drafting a new international standard for the performance of on-line pH measurements
- Co-operating with the RSC's Electroanalytical Group to organise periodically seminars that raise and discuss salient issues relating to current topics in pH study amongst the academic and industrial community.
- Representation on relevant electrochemistry committes such as the CCQM Electrochemical Analysis working Group, EUROMET Electrochemistry Technical Committee, and the Society of Chemical Industry's Electrochemical Technology Group Committee
Ion Selective Electrodes
Ion Selective Electrodes (ISEs) are useful analytical tools for the analysis of a range of anions and cations. They have been developing continually over the last forty years. ISEs are the generalised form of the glass electrode for pH measurements. The use of ISEs in environmental analysis offers several advantages over other methods of analysis. They are cheap instruments, which are easy to use and maintain, and can be easily miniaturised making them ideal for in-situ real time measurements that can not be achieved using other analysis techniques. More recently they have found practical applications in the analysis of environmental samples, usually where in-situ determinations are not practical with other methods. The current challenges for ISEs are to improve achievable detection limits (which range from mass fractions of 10-5 to 10-9 depending on the sample), and to improve selectivity by the tailoring of ion selective membranes for specific analytes, especially for heavy metals analytes relevant to environmental analysis.
Research at NPL focuses mainly on the development of suitable methodologies for online or in-situ ion measurement of heavy metals in environmental and industrial matrices, such as soluble extract of ambient particular matter, on-line process monitoring, and even fuel cell environments. In particular, novel designs of miniaturised ISE are of interest in order to make in-situ analysis increasingly non-invasive.
References
- Brown, R J C, Milton, M J T, 'The microporous structure of silver/silver chloride electrodes and the implications for Harned Cell operation', Accred. Qual. Assur., 2005, 10, (7), 352-355
- Brown, R J C, Milton, M J T, 'Observation of a combined diluted and salting effect in buffers under conditions of high dilution and high ionic strength', Accred. Qual. Assur., 2003, 8, (11), 505-510
- Brown, R J C, Milton, M J T, Brewer, P J, 'Improved methodology for high accuracy titration of HCl and experimental demonstration of equivalence with coulometry', NPL Report COAM 18, January 2003
- Buck, R P, Rondinini, S, Covington, A K, Baucke, F G K, Brett, C M A, Camoes, M F, Milton, M J T, Mussini, T, Naumann, R, Pratt, K W, Spitzer, P, Wilson, G S, 'Measurement of pH. Definition, standards and procedures (IUPAC Recomendations 2002)', Pure Appl. Chem., 2002, 74, (11), 2169-2200
- Brown, R J C, Milton, M J T, Brewer, P, Wielgosz, R I, 'High accuracy titrimetry with application to HCl', NPL Report COAM 5, November 2001