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Electrochemistry, and What in the World is an Ussing Chamber? - Tom - 06-07-2021
Electrochemistry, as apparent from the name, is the study at the margin between electricity and chemistry. Chemists consider electrochemistry to be a branch of chemistry, while engineers don't consider electrochemistry to be a branch of electronics. The resulting asymmetry complicates the situation for those few of us approaching electrochemistry from the electronics side. Electrochemical matters are viewed differently by normal folk who don't have Ohm's Law tattooed on their cortex. So here, we will look at an Ussing Chamber as an electrical apparatus, with chemistry somewhere in the distance, and with biology hovering at the far horizon. The ultimate object of taking a step back is to simplify, given the underlying principle.
If you remember anything from a physics or chemistry course, you will know that interactions between atoms and molecules are largely driven by electric charge. Electrons are mobile, negatively-charged particles that mix and match according to relatively simple rules, which soon lead to a huge variety of complex behaviors. For example, we know that when you combine oxygen and hydrogen and a spark you get explosive energy, plus a very little bit of water. There is no electrical circuit involved, and Ohm's Law does not seem at all relevant. Yet the reverse reaction shows a clearer picture. Hydrolysis is the running of an electric current, being a stream of charged particles, through ionized water and so producing oxygen at one electrode and hydrogen at the other. It is not that hard to measure the current and count the atoms and see that there is a simple, fixed relationship. That study is called coulometry, and it stands squarely at the intersection of chemistry and physics and electronics.
An Ussing Chamber is a particular apparatus for coulometry involving a membrane. Since membranes tend to fall in the purview of the life-sciences, we now need to add biology to the mix. The various skills required to master all of the above are becoming excessive. Let's make the electronics piece as simple as possible. Modern electronics with a computer overseeing a process can do things that were inconceivable when Hans Ussing invented the Ussing Chamber in 1946. If you accept his view of its experimental capabilities, you will sell it far short. Think of the modern electronic interface as an ideal view into the chamber's electric internals, without the compromises needed for 1940's technology.
An Ussing Chamber contains a conductive solution, or electrolyte, divided into halves by a membrane. There are two pairs of electrodes, each pair with one electrode on either side of the membrane. One pair of electrodes simply measures the voltage difference across the membrane. The other pair of electrodes injects a current, where ions must cross the membrane to complete the electric circuit. The object is to quantify an electrochemical reaction in terms of the relationship between voltage and current. You may be studying the membrane itself, or properties of the electrolyte, or of substances added. In any case, you don't want the voltage and current measurements to interact. To the extent you must draw current in order to measure voltage, that constitutes an error. If you cannot provide a particular current at a particular voltage, that is a real limitation. Currents and voltage differences can be very small, so resolution is at a premium. These reactions usually proceed slowly, so faster measurement speed is generally not required. Since it can take hours for these systems to reach equilibrium, long-term stability of the electronic interface is essential.
The relationship between voltage and current will reflect the chemistry. The modern apparatus can control either one, and measure the other in any sequence desired. For older technology, you would set a voltage which would result in a current flow. The amount of current flow corresponding to the voltage would depend on series resistance. (Remember Ohms Law?) Series resistance might be largely determined by electrode geometry or aging, or by temperature, or other incompletely controlled variables. More series resistance would need to be added in order to measure small currents. The modern apparatus can eliminate series resistance as an error term. If the current is set to 1 nA, it is 1 nA with an ohm of series resistance or a megohm of series resistance. You can sweep the relationship of voltage to current, or current to voltage, over a chosen range at a chosen rate in steps or with a smooth ramp. You can reverse polarity at will. Again, you can fix the current and measure the resulting voltage, or fix the voltage, and measure the resulting current.
The Lawson Labs Ussing Chamber interface uses Excel as the intelligence, so the user has complete control without needing to know a programming language. If you are familiar with Excel, you are halfway up the learning curve, already. There is no need to add electronics or programming to the list of skills required. That is good thing, because electrochemistry is complex enough to begin with.
Tom Lawson
June 2021