The Clark Electrode

Take home messages

• The Clark electrode measures oxygen tension in a fluid sample
• It is a polarographic electrode, meaning it measures current rather than voltage
• It thus requires a 0.6V power supply

Why you need to know this

The Clark electrode is prime exam fodder at both primary and final levels, and it's one of those topics where, if you take the time to understand it, you can win a load of easy marks on exam day.

How it works

You can choose to simply memorise the above diagram if you'd prefer (or the exam is tomorrow), or we can actually understand it, and never have to memorise it again.

Let's build it up from the basics, so it actually makes sense.

Start with a reaction

We want to measure the oxygen tension, using electrical equipment, so we need some sort of reaction that turns 'change in partial pressure of oxygen' to 'change in electricity'.

• This can be a change in current or voltage
• This means we need a reaction that either causes electrons to flow, or to build up on one side of a circuit

Either way, we need an equation that contains oxygen, and a change in electrons.

In this equation, we can see that we start with oxygen and electrons on the left, neither of which are present on the right.

Assuming a steady unlimited supply of electrons and water, the rate of this reaction will depend entirely on the partial pressure of oxygen.

Perfect.

Where are we going to get our electrons?

Some sort of electrode sounds like a sensible starting point, and we're going to use platinum for the following reasons:

• It is a metal, meaning it will gladly supply electrons, if encouraged to do so*
• It's a very effective catalyst of the above reaction, making the whole process more efficient and speedy
• It's resistant to reacting with stuff itself, so it's durable and doesn't mess with the numbers

*This brings us to our next issue.

How do we encourage it to give up electrons?

Well, we could shove so many electrons into the electrode that it doesn't know what to do with them, and to do this, we need a voltage, from a power supply.

Where are we getting the electrons from?

Ah. Good point.

It's all very well having a power source driving electrons down into our platinum electrode, but we need to source these electrons from somewhere.

It would be simply magical if we could find a second reaction that doesn't involve oxygen (and thus won't mess with our measurements) that will churn out a steady supply of electrons on demand.

Enter the silver/silver chloride electrode.

If you stick a block of silver on the other end of your power supply and start ripping electrons out of it, it's actually really rather a good sport about the whole thing

As long as you have enough silver, it'll keep merrily pouring out electrons as long as there's enough voltage to drink them up.

Of course this leaves us with a rather pertinent issue of the increasing quantity of silver ions left lying around, so we'll throw in some chloride ions to mop them up shall we?

This source of chloride ions is found in the electrolyte solution (KCl) in which our electrodes are immersed.

There.

Much better.

Now we need to let oxygen in and start measuring

Fabulous, lets:

• Wrap everything up in an oxygen-permeable membrane, made of polytetrafluoroethylene or Teflon
• Throw in an ammeter as well, to measure the current
• Calibrate to a few known samples so we know what currents we're expecting
• Let our blood sample pass across the other side of the membrane

Now we can package this whole contraption into a neat pokey stick that can be dipped into a blood sample.

Behold, a machine that will generate and measure a current that will change linearly with changes in oxygen tension, for all clinically important PO2 values.

Beautiful.

Hopefully this image makes a tad more sense now.

There's a video in the members section below as well.

A spot of history

Meet the father of biosensors, and the absolute legend you have to thank for the magic you've just witnessed.

In the 1950s he developed the first ever bubble oxygenator to be used in cardiac surgery.

Frustratingly, when he came to publish his results, his work was refused by the editors as it wasn't possible to measure the oxygen tension in the blood that the device was pumping out.

So Clark made a thing to do just that, in 1954.

Not only did his device quickly become the foundation of pretty much every ABG machine from the 1970s onwards, it also paved the way for other biosensors including the glucose sensors used by millions around the world today.

Here's his diagram

Here are some of his papers

Useful Tweets

References and Further Reading

ABG and measuring gases

How your ABG machine actually works.

AnaestheasierAnaestheasier ltd.

Principles of oxygen measurement with the Clark electrode

Leland C. Clark never called his device “the Clark Oxygen Electrode”, as such a gesture would probably have been viewed by his contemporaries as mildly disgusting. The paper he published discusses the “continuous recording of blood oxygen tensions by polarography”- it was a “polarographic” electrode, and this is also how it is referred to in some of the earlier literature. The polarogram is the graphed relationship of current and voltage which is discussed at length elsewhere.

Deranged PhysiologyAlex Yartsev

Leland C. Clark biography, birth date, birth place and pictures

Leland C. Clark is credited as Biochemist, Inventor of Oxycyte, Clark Oxygen Electrode. The feasibility of biosensing was first demonstrated by Leland C. Clark in the mid-1960s, when he measured glucose concentration in solution using what

Leland C. Clark

Measurement of oxygen tension by electrolysis of dissolved oxygen

The oxygen cathode is essentially an electrolytic cell. The potential difference across the oxygen-containing electolyte allows a reduction of the dissolved oxygen; the oxygen borrows electrons from the cathode and hydrogen ions from the water molecules of the aqueous solution. A satisfactory amount of detail about these methods is available in William L. Nastuk’s 1962 textbook, “Electrophysiological Methods:Physical Techniques in Biological Research”.

Deranged PhysiologyAlex Yartsev

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