Ion-Selective Electrodes: Selectivity Coefficients for Interfering Ions of the Opposite Charge Sign

Description:
The upper detection limit of a polymeric-membrane ion-selective electrode (ISE) is limited by Donnan failure, that is, the transfer of primary ions along with interfering ions of the opposite charge sign (commonly referred to as counter ions) from the sample into the sensing membrane. While Donnan failure is well understood, it is often difficult to compare upper detection limits for ISEs described by different sources. The majority of publications on ISEs describe Donnan failure for one type of counter ion only, making it impossible for end users to predict the interference of different counter ions from such data. Moreover, linear ranges for ISEs based on different ionophores cannot be compared to one another when Donnan failure was reported for different counter ions. To this end, we introduce here selectivity coefficients, K(I,X)^potX, for interfering ions of the opposite charge sign. Using this concept, the primary ion activity at which Donnan failure occurs can be readily predicted from measured K(I,X)^potX values using the uncomplicated expression (aX^(zI⁄zx ) )⁄(K(I,X)^potX ). Consistent with the intuition that ISE users have developed over decades for conventional selectivity coefficients, large K(I,X)^potX values are characteristic for counter ions that interfere strongly. We also show that for ISEs with an ionophore that bind the primary ion with a n:1 stoichiometry, K(I,X)^potX is proportional to the stability constant of the primary ion complexes, and for both ionophore-based and ionophore-free ion-exchanger ISEs, K(I,X)^potX is proportional to K(In Xm)^(-1 ⁄zX ), where zX is the charge of the counter ion and K_(I_n X_m)^ is the equilibrium constant describing the distribution of the primary ion and counter ion between the sample and the sensing membrane.

Speaker: Philippe Buhlmann - University of Minnesota
Philippe Buhlmann

Education
• PhD in Analytical Chemistry (Dr. sc. nat. ETH), Laboratory of Organic Chemistry, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland, 1989–1993 (Advisors: Drs. Wilhelm Simon and Ernö Pretsch)

Employment
• Professor of Chemistry, Dept. of Chemistry, Univ. of Minnesota, Minneapolis, MN

Publications
• 159 publications, 15 patents granted or pending, 1 book (translated into German, Spanish, Russian, Japanese, and Chinese; English edition chapters downloaded >98,000 times from 2010 to 2020)

Professional Awards
• Fellow of the American Chemical Society
• CSE Distinguished Professor
• Janet Tarino Volunteer Award, American Chemical Society Minnesota Section
• 3M Alumni Professorship (2017–2021)
• Award for Outstanding Contributions to Postbaccalaureate, Graduate, and Professional Education, University of Minnesota Alumni Association