Electrode Note

Explaining the Setup

1. What Is the Electrode Made Of?

In ECGs and other biopotential measurements, we use an electrode made from silver (Ag). To make it function properly in a biological system, it’s coated with silver chloride (AgCl) — a stable salt. This combination is called a silver/silver chloride (Ag/AgCl) electrode.

• The silver is a metal, so it handles electrons.

• The silver chloride lets the electrode interact with ions, which are the form of charge used in the body.

This electrode sits on the skin, but it needs help to connect well to the body's electrical system.

2. Why Do We Use Electrode Paste?

Your skin — especially the outer layer — has high electrical resistance. That makes it hard for small electrical signals from the body to reach the electrode.

To solve this, a special paste or gel is applied between the skin and the electrode. This paste contains ions, especially chloride ions (Cl⁻).

Here’s what the paste does:

• Improves electrical contact by allowing ionic current to flow between the skin and the electrode.

• Hydrates and softens the skin.

• Provides the Cl⁻ ions needed for the electrode’s chemical reactions.

3. What Happens at the Electrode Surface?

Now we come to the core electrochemical process. At the surface of the silver electrode, a chemical reaction can occur:

$\text{Ag} \rightarrow \text{Ag}^+ + e^-$

This means:

• A silver atom (Ag) on the surface loses an electron and becomes a silver ion (Ag⁺).

• That Ag⁺ ion moves into the paste, while the electron stays in the metal.

This reaction is how electronic current is produced in the wire — the electron can now move into your measuring device (e.g. ECG amplifier).

This also means that the ionic activity in the body is now connected to electronic current in a wire — a critical transition point.

4. Why Does Ag Dissolve at All?

This is where concentration gradients come in.

• When silver atoms ionize into Ag⁺, those ions enter the paste.

• Initially, there's little or no Ag⁺ in the paste, so it’s like dropping food dye into clean water — the ions spread out, creating a high-to-low concentration gradient.

• This gradient encourages more silver to dissolve, because ions "want" to spread out evenly (that’s diffusion).

But eventually, something pushes back…

5. What Stops Silver from Dissolving Forever?

As more and more Ag⁺ ions gather near the electrode, positive charges accumulate in one area.

Remember: like charges repel each other. So these excess Ag⁺ ions create an electric field that pushes back against any new Ag⁺ trying to join.

This field is the opposing electric field — it builds up naturally as a result of ion concentration.

At some point, the driving force from the concentration gradient (trying to spread ions out) is exactly balanced by the repelling force of the electric field.

When that happens, equilibrium is reached. Silver still dissolves and re-deposits, but the rates balance out.

6. What Is the Double Layer?

At this equilibrium point, a very thin structure forms at the electrode surface:

• One side: a layer of positive Ag⁺ ions very close to the electrode.

• Opposite side: a layer of negative Cl⁻ ions in the paste, attracted to the positive Ag⁺.

These two layers of opposite charge are separated by a nanometer-scale distance, creating what we call the electrical double layer.

This structure behaves like a capacitor:

• Charges are separated across a small distance.

• This allows the system to store electrical energy temporarily.

7. What Is the Voltage Drop (Half-Cell Potential)?

Because charges are separated in the double layer, this creates an electric potential difference across it — i.e., a voltage.

This voltage is called the half-cell potential, and for the Ag/AgCl system, it’s about +0.8 V.

• This is a real, measurable voltage that comes purely from the chemical and ionic arrangement at the interface.

• It’s important to be aware of it when designing systems that measure biopotentials, because it affects the baseline of the signal.

🔁 Final Summary

How Signals are Detected

Now that you understand the structure and electrochemistry of the Ag/AgCl electrode system, let’s connect it to how electrical signals from the heart are actually detected through the skin using this setup.

🔹 1. The Heart Generates Electrical Activity

The heart doesn’t just beat mechanically — it’s controlled by a coordinated wave of electrical depolarization:

• The SA node fires → atria depolarize and contract.

• Signal pauses at AV node, then passes into the ventricles → they depolarize and contract.

This movement of charge (mainly ions like Na⁺, K⁺, Ca²⁺) through heart muscle cells creates time-varying voltage fields in the tissues.

These voltages:

• Spread throughout the body (via the conductive fluids in tissues),

• Reach the surface of the skin, where they can be measured.

But there's a challenge…

🔹 2. Ionic Fields Must Be Translated into Electronic Signals

The body transmits signals using ions. But measurement devices (like an ECG amplifier) use electrons.

So we need something that:

• Sits on the skin, where voltage changes reach,

• And converts ion flow into electron flow.

That’s what the Ag/AgCl electrode system does.

🔹 3. The Electrode Picks Up Local Voltage Changes

As the heart signal passes through the body:

• A small voltage difference appears between points on the skin (typically 0.1 to 5 mV).

• When this voltage change reaches the electrode site, it alters the local electric field in the electrolyte paste.

This change affects the equilibrium at the silver/silver chloride interface.

🔹 4. Voltage Changes Disturb the Double Layer Equilibrium

Let’s say a more negative potential arrives at the skin under the electrode. What happens?

• The half-cell potential momentarily changes when the biological signal is applied, as the redox equilibrium shifts.

• But it quickly settles to a new balance point via chemical reactions, and this adjustment is what causes electrons to flow.

• The increased negativity attracts more positive Ag⁺ ions to dissolve from the electrode.

• The chemical reaction shifts:

  $\text{Ag} \rightarrow \text{Ag}^+ + e^-$

• This reaction releases electrons into the silver wire of the electrode.

If the local voltage instead became more positive:

• The reaction would reverse:

  $\text{Ag}^+ + e^- \rightarrow \text{Ag}$

• Electrons are drawn out of the wire, and Ag⁺ ions rejoin the solid metal.

So the heart signal causes the chemical balance at the electrode surface to shift back and forth, creating a tiny but measurable current of electrons in the electrode wire.

🔹 5. The Electrode Wire Carries This as an Electronic Signal

Once the electron current enters the wire:

• It’s a regular electronic current, just like in any electrical circuit.

• The wire leads to an amplifier, which boosts the signal.

• The signal is then recorded or displayed as an ECG trace.

This is how your electrode "listens" to the ionic changes caused by the heart and converts them into something that a machine can understand.

🧠 Summary Flow (Signal Pathway)

1. Heart cells depolarize → ion movement creates electrical field

2. Field reaches skin surface → voltage appears across skin

3. Electrode paste conducts ionic signal to electrode

4. Ag/AgCl interface reacts to voltage change by releasing or absorbing electrons

5. Electrons flow into wire → signal reaches amplifier

6. Amplifier outputs ECG waveform

⚡ Recap: Why It Works

Would you like this flow shown in a labeled diagram, or would a practice question help consolidate it?