More than a Liquid Junction: Effect of Stirring, Flow Rate, and Inward and Outward Electrolyte Diffusion on Reference Electrodes with Salt Bridges Contained in Nanoporous Glass

The Henderson equation
is usually used to calculate liquid-junction
potentials between miscible electrolyte solutions. However, the potentials
of reference electrodes that comprise an electrolyte-filled nanoporous
glass frit may also be affected by charge screening. As reported previously,
when the Debye length approaches or surpasses the glass pore diameter,
reference potentials depend on the composition of the bridge electrolyte,
the pore size of the frit, and the concentration of electrolyte in
the sample. We report here that stirring of samples may alter the
reference potential as it affects the electrolyte concentration in
the section of the nanoporous glass frit that is facing the sample
solution. When the flow rate of bridge electrolyte into the sample
is small, convective mass transport of sample into the nanoporous
frit occurs. The depth of penetration into the frit is only a few
nanometers but, despite the use of concentrated salt bridges, this
is enough to affect the extent of electrostatic screening when samples
of low ionic strength are measured. Mixing of sample and salt bridge
solutionsand in particular penetration of sample components
into the fritwas optically monitored by observation of a deeply
colored Fe­[(SCN)­(H2O)5]2+ complex
that formed in situ exclusively in the region where the sample and
salt bridge mixed. Importantly, because flow through nanoporous frits
is very slow, mass transport through these frits is dominated by diffusion.
Consequently, over as little as 1 h, reference electrode frits with
low flow rates become contaminated with sample components and undergo
depletion of electrolyte within the frit to a depth of several millimeters,
which can negatively affect subsequent experiments.