Avoiding Errors in Electrochemical Measurements: Effect of Frit Material on the Performance of Reference Electrodes with Porous Frit Junctions

Abstract

In many commercially available and in-house-prepared reference electrodes, nanoporous glass frits (often of the brand named Vycor) contain the electrolyte solution that forms a salt bridge between the sample and the reference solution. Recently, we showed that in samples with low ionic strength, the half-cell potentials of reference electrodes comprising nanoporous Vycor frits are affected by the sample and can shift in response to the sample composition by more than 50 mV (which can cause up to 900% error in potentiometric measurements). It was confirmed that the large potential variations result from electrostatic screening of ion transfer through the frit due to the negatively charged surfaces of the glass nanopores. Since the commercial production of porous Vycor glass was recently discontinued, new materials have been used lately as porous frits in commercially available reference electrodes, namely frits made of Teflon, polyethylene, or one of two porous glasses sold under the brand names CoralPor and Electro-porous KT. In this work, we studied the effect of the frit characteristics on the performance of reference electrodes, and show that the unwanted changes in the reference potential are not unique to electrodes with Vycor frits. Increasing the pore size in the glass frits from the <10 nm into the 1 μm range or switching to polymeric frits with pores in the 1 to 10 μm range nearly eliminates the potential variations caused by electrostatic screening of ion transport through the frit pores. Unfortunately, bigger frit pores result in larger flow rates of the reference solution through the pores, which can result in the contamination of test solutions.

Indirect Potentiometric Determination of Polyquaternium Polymer Concentrations by Equilibrium Binding to 1-Dodecyl Sulfate

“…where E is the measured potential of the electrochemical cell, E o is the standard potential of the cell (including contributions from the reference electrode and the liquid junction to the sample), 32,33 R is the ideal gas constant, T is the temperature, z is the charge of the measured ion, F is Faraday's constant, aDS is the activity of DS, KDS/Cl is the selectivity coefficient for DS over Cl -, and aCl is the activity of chloride. All solutions used in this study contained 1.0 mM NaCl to control the ionic strength and activity of chloride.…”

Microbial Fuel Cell: An Efficient Method to Utilize Prokaryotic Potential to Engender Reliable Energy

“…The microbes for fuel cell must be developed or grown anaerobically, remembering the ultimate objective of delivering power [13]. The strategy should be anticipated towards limiting oxygen spillage, predominantly across the cathode and proton exchanger membrane/salt bridge, for minimal occurring of aerobic action [14]. Regardless of whether oxygen leaks in anode chamber it is quickly devoured by the microscopic organisms producing a low redox potential [15].…”

Critical Comparison of Reference Electrodes with Salt Bridges Contained in Nanoporous Glass with 5, 20, 50, and 100 nm Diameter Pores

“…The measurement of stable and reproducible reference potentials is an area of interest for many but is not without its complications. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Consistent and constant reference potentials are particularly difficult to measure in very dilute aqueous solutions, a problem of concern especially for accurate pH measurements of environmental importance. 9,[15][16][17][18] In many cases, salt bridges are used to provide for electrical contact between reference electrodes and samples.…”

“…22 To reduce the cross contamination of samples and the salt bridge electrolyte by convective mixing, porous glasses with pore diameters of 4 -10 nm have been used to contain the salt bridge. 4,23 Unfortunately, this can introduce complications due to electrostatic screening by electrical charges on the pore walls of these frits. 23 In particular, sample-dependent phase boundary potentials were shown to develop at the interface of nanoporous glass frits with 4 -6 nm pores due to the negatively charged surface of glass (e.g., sample cations freely enter the frit pores but anions are excluded) in all but very acidic solutions.…”

“…22,[25][26][27][28] However, frits with 500 -1000 nm pores allow for a flow rate of the bridge electrolyte into the sample that is often unacceptably large. 4 Similarly, polymeric frits with 1 and 10 μm pores were found to exhibit no charge screening but had flow rates that quickly contaminated samples and required frequent replenishing of bridge electrolyte. 4 Reference electrodes comprising polymeric frit materials with much smaller pore sizes are feasible but at this point require custom synthesis and are not easy to functionalize with a hydrophilic surface that does not bind certain ions specifically.…”