Anal. Chem. 2002, 74, 2938-2942
Proton and Hydroxide Ion Mobility in Capillary Electrophoresis Angela B. Duso† and David D. Y. Chen*
Department of Chemistry, University of British Columbia, Vancouver, BC, Canada, V6T 1Z1
A capillary electrophoresis method for measuring the effective mobilities of proton and hydroxide ions was developed. Photodiode array detection coupled with pHindicating dyes enabled the detection of proton and hydroxide ions using a conventional CE instrument. The effects of ionic strength and pH of the background electrolyte on the measurement of effective mobility of the proton were first investigated. Mobility measurements were found to be independent of applied voltage. Hydroxide ion mobility was also measured and found to be in agreement with literature values. The effects of pH indicator and proton source on the ion mobility were determined and are described herein. The absolute mobility (µ0) of an ion is given by
µ0 ) q/6πηr
(1)
Equation 1 holds true for ideal spherical particles in infinitely dilute solution. The absolute mobility is a characteristic constant of a particular ion in a specific solvent, and it is related to equivalent ionic conductance (λ0) at infinite dilution.1 The equivalent conductance (Λ) of a solution is the ratio of conductivity to concentration of the an electrolyte. Conductivity is related to the currentcarrying capacity of the electrolytes in a solution. Equivalent conductance can be divided into contributions from the positive and negative ions in a solution
Λ0 ) λ0+ + λ0-
(2)
Equivalent ionic conductance at infinite dilution (λ0) relates to absolute ionic mobility (µ0) through the Faraday constant (F).
λ0 ) µ0F
(3)
Absolute mobility cannot be measured directly, but it can be extrapolated from measurements on dilute solutions.2 Many * To whom correspondence should be addressed. Tel: (604) 822-0878. Fax: (604) 822-2847. E-mail:
[email protected]. † Current address: Kwantlen University College, 13366 72 Ave., Surrey, BC, Canada V3M 2M8. (1) Robinson, R.; Stokes, R. Electrolyte Solutions, 1st ed.; Butterworth Scientific Publishers: London, 1955. (2) MacInnes, D.; Shedlovsky, T.; Longsworth, L. Chem. Rev. 1933, 33, 2946.
2938 Analytical Chemistry, Vol. 74, No. 13, July 1, 2002
physical chemists determined ion mobilities in the late 19th century using conductivity measurements.3 Kohlrausch determined empirically that, for strong electrolytes measured in dilute solution (
