The electrode-electrolyte interface
What happens where metal meets salty skin. Half-cell potentials, why silver chloride is stable, why impedance depends on frequency, and why gel takes half an hour and dry electrodes are a compromise.
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You made electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry and measured their impedance, and you found that a dry electrodeDry electrode An electrode used without gel, faster to apply but with higher, less stable impedance and more motion noise; the consumer-headset compromise. Related: Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Active electrodeActive electrode An electrode with a small amplifier or buffer built into it, so the high-impedance signal travels only millimetres before being made robust. . Glossary entry looks fine at a kilohertz and terrible at ten hertz, and that a bare silver wire drifts while a chlorided one holds still. Both facts come from the same few millimetres: the boundary where electrons in metal must become ions in fluid. Let’s look at that boundary.
Electrons do not swim
Current in a wire is electrons. Current in tissue is ions: sodium, chloride, potassium. At the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel., Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies., Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal.. Glossary entry surface one has to become the other, and that happens by chemistry. Either a metal atom gives up an electron and enters solution as an ion, or an ion in solution takes an electron and plates onto the metal, or, if neither reaction is available, charge simply piles up on both sides of the boundary like a capacitor and nothing crosses.
Every metal-electrolyte boundary settles at some voltage, the half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel., ElectrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with.. Glossary entry, where the forward and reverse reactions balance. For silver in chloride solution it is about 0.22 volts. Two electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry of the same metal in the same solution have the same half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , ElectrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. . Glossary entry and the difference is zero, which is the only reason you can measure microvolts between them. Let one electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry’s chemistry wander and the difference wanders with it, by millivolts, which is what you saw as drift.
Why silver chloride
Bare silver in saline has a slow, poorly defined reaction; its potential depends on trace impurities and drifts. Silver coated with silver chloride has a fast, reversible reaction (silver plus chloride to silver chloride and back) with a potential set almost entirely by the chloride concentration, which in gel and tissue is stable. So Ag/AgClSilver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. Related: ElectrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with., Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal.. Glossary entry electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry have a stable half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , ElectrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. . Glossary entry, low noise, and low impedance at low frequency, because current can actually cross the boundary by the reaction rather than only charging a capacitor. This is why they have been the standard for a century and why you chlorided the wire.
Gold and stainless steel have no convenient reversible reaction. They are “polarizable”: charge piles up and the boundary behaves as a capacitor. That is fine at high frequency and bad at low frequency, which is exactly the shape of the dry-electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry curve in Figure 1.
Why impedance falls with frequency
The boundary is, electrically, a capacitor (the double layer of charge) in parallel with a resistor (the reaction path), in series with the resistance of the gel and the fluid. Skin adds its own layer: the stratum corneum, the dead outer layer, is a poor conductor and behaves as a large resistor in parallel with a capacitor. At high frequency the capacitors carry the current and the impedance is low; at low frequency the current has to go through the resistors and the impedance is high. EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry lives at low frequency. That is why gel, which soaks the stratum corneum and shorts out part of that resistance, matters so much, and why abrading the skin, which removes the dead layer, drops the impedance by a factor of ten.
Two electrodes: one with gel on unprepared skin, one dry. Which is noisier, and by how much?
The dry one, by a lot, and worst at low frequency. Thermal noise is proportional to the square root of the resistance, and the dry electrodeDry electrode An electrode used without gel, faster to apply but with higher, less stable impedance and more motion noise; the consumer-headset compromise. Related: Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Active electrodeActive electrode An electrode with a small amplifier or buffer built into it, so the high-impedance signal travels only millimetres before being made robust. . Glossary entry’s low-frequency resistance is ten to a hundred times higher. On top of that the dry contact is unstable, so pressure and motion modulate the impedance and turn into voltage noise. The noise explainer puts numbers on it.
Why the gel takes half an hour
Fresh gel on skin needs time for the electrolyte to penetrate the stratum corneum and for the half-cell potentialsHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , ElectrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. . Glossary entry to settle. A clinical technologist applies electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry, checks impedances, waits, re-checks. Recordings that start too early show slow drifts for the first minutes as the chemistry equilibrates. That drift is the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry, not the brain, and now you know why.
Deep dive The double layer, and why the capacitance is enormous 3 min
When metal meets electrolyte, ions of the opposite charge crowd against the surface within a nanometre or so, forming a capacitor whose plates are a few atoms apart. That gives a capacitance of tens of microfarads per square centimetre, enormous for its size. It is what makes even a polarizable electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry usable at frequencies above a few hertz, and it is what stimulation engineers exploit and fear: charge injected through this capacitor is safe until the voltage across it exceeds the point where water itself starts to electrolyze. Phase 6’s stimulation safety is about that point.
Deep dive Motion artifact is an impedance problem 3 min
Move an electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry and two things change: the double layer is disturbed (the charge distribution reshuffles, producing a transient voltage) and the skin’s stratum corneum is stretched (changing its potential, the “skin potential artifactArtifact Anything in the recording that did not come from the brain: blinks, muscle, heartbeat, movement, sweat, mains hum, electrode pops. Related: Blink artifactBlink artifact The large, slow deflection an eye blink produces in frontal channels, often ten times the size of the brain signal. , Independent component analysis (ICA)Independent component analysis (ICA) A method that unmixes multichannel data into statistically independent sources, so blinks and heartbeat can be identified and removed. , Artifact subspace reconstruction (ASR)Artifact subspace reconstruction (ASR) An automatic method that detects short high-variance bursts and reconstructs them from clean data; common in real-time pipelines. . Glossary entry”). Both produce large, slow deflections. Gel reduces the first by stabilizing the chemistry; abrasion reduces the second by removing the layer that generates it. Dry electrodesDry electrode An electrode used without gel, faster to apply but with higher, less stable impedance and more motion noise; the consumer-headset compromise. Related: Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Active electrodeActive electrode An electrode with a small amplifier or buffer built into it, so the high-impedance signal travels only millimetres before being made robust. . Glossary entry suffer from both, which is why consumer headsets are so sensitive to movement and why the active electrodeActive electrode An electrode with a small amplifier or buffer built into it, so the high-impedance signal travels only millimetres before being made robust. Related: Dry electrodeDry electrode An electrode used without gel, faster to apply but with higher, less stable impedance and more motion noise; the consumer-headset compromise. , Input impedanceInput impedance How hard the amplifier's inputs are to push current into; it must be far higher than the electrode impedance or the signal is lost in the divider. . Glossary entry helps only with the cable, not the contact.
Explain what this page was about to your roommate in three sentences. No jargon they would not know.
We measure microvolts between two matching silver chloride contacts because each sits near 220 millivolts and the shared part cancels, while bare metal drifts by millivolts as its chemistry wanders. We see higher resistance at 10 hertz than at 1 kilohertz because the outer skin and the charged layer pass fast changes more easily, so gel that soaks the skin plus a half hour of settling time lowers the slow drift. We prefer gel for brain signals because a dry contact can be ten to one hundred times more resistive at low frequencies and turns small movements into large slow deflections.
Yours does not have to match. Check that the three ideas are there.