Mass-transfer measurements limiting-current technique

Richard S. H. Mah and Mordechai Shacham, Pipeline Network Design and Synthesis J. Robert Selman and Charles W. Tobias, Mass-Transfer Measurements by the Limiting-Current Technique... 

MASS-TRANSFER MEASUREMENTS BY THE LIMITING-CURRENT TECHNIQUE... 

Electrochemical measurements of mass-transfer rates by the limiting-current technique have been employed with increasing frequency in the last 20 years. This chapter offers a discussion of the underlying principles, conditions of validity, and selected applications. 

Since current can be measured with ease and precision, the limiting-current technique provides a convenient and, under certain conditions, accurate method for measuring mass-transfer rates. The conditions for valid measurement and correct interpretation of limiting currents are discussed in the following sections. 

For chemists, the second important application of electrochemistry (beyond potentiometry) is the measurement of species-specific [e.g., iron(III) and iron(II)] concentrations. This is accomplished by an experiment in which the electrolysis current for a specific species is independent of applied potential (within narrow limits) and controlled by mass transfer across a concentration gradient, such that it is directly proportional to concentration (/ = kC). Although the contemporary methodology of choice is cyclic voltammetry, the foundation for all voltammetric techniques is polarography (discovered in 1922 by Professor Jaroslov Heyrovsky awarded the Nobel Prize for Chemistry in 1959). Hence, we have adopted a historical approach with a recognition that cyclic voltammetry will be the primary methodology for most chemists. 

Mass transfer to liquids in turbulent pipe flow has been studied by using tubes made from a slightly soluble solid and measuring the rate of dissolution of the solid for various liquid flow rates. An alternate technique is to make a portion of the tube wall an electrode and carry out an electrochemical reduction under conditions where the current is limited by the rate of mass transfer of the reacting ion to the wall. 

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