Nernst Slope for Potassium

Fixed T and [K]ᵢ. Measure E_K at several [K]ₒ and recover the decade slope of E versus log₁₀[K]ₒ.

University / research· 22 min·Related simulator: Biophysics, Fluids & GeoscienceNernst Potentials & Membrane

Goal

Determine the Nernst slope (mV/decade) from E_K = (RT/F) ln([K]ₒ/[K]ᵢ). A plot of E versus log₁₀[K]ₒ has slope (RT/F) ln 10.

Equipment

K⁺-selective electrode
[K]ₒ bath
Known [K]ᵢ / T

Experiment

K⁺-selective electrode in the bath. No live E on the bench — the reading appears only after you record.

Known temperature T = 20 °C and intracellular [K]ᵢ = 140 mM. The electrode reading appears only after you record.

Extracellular [K]ₒ[K]ₒ = 5.0 mM

Electrode: no reading

Record at least 6 concentrations from 2–40 mM, then fit E versus log₁₀[K]ₒ.

Measurements

№	[K]ₒ mM	E mV	log₁₀ [K]ₒ
No measurements yet — take your first reading.

Data processing

Add at least 2 measurements to compute the fit.

Uncertainty

Lab report

Opens the system print dialog — choose “Save as PDF” or your printer. Header and footer are hidden when printing.

One click opens the print dialog — choose “Save as PDF”.

Nernst Slope for Potassium

Generated: 24 Aug 2026, 22:30

Goal

Determine the Nernst slope (mV/decade) from E_K = (RT/F) ln([K]ₒ/[K]ᵢ). A plot of E versus log₁₀[K]ₒ has slope (RT/F) ln 10.

Measurement table

#	[K]ₒ (mM)	E (mV)	log₁₀ [K]ₒ
No measurements yet — take your first reading.

Fit and derived value

Add at least 2 measurements to compute the fit.

Conclusion

The fitted decade slope agrees with (RT/F) ln 10 at 20 °C. Main uncertainties: electrode noise and treating the electrode as ideally K⁺-selective.

PhysSandbox virtual lab — values come from your session; add your own discussion of error sources.

Theory

At 20 °C the ideal K⁺ slope is about 58 mV per tenfold change in [K]ₒ. [K]ᵢ and T are known; E_K is hidden. The bench is a K⁺-selective electrode, not a full GHK rest-potential calculator.

Procedure

Temperature (20 °C) and intracellular [K]ᵢ are fixed and known. You only change extracellular [K]ₒ.
Record. The electrode logs E with small noise. There is no live E_K or “58 mV/decade” readout.
The notebook computes log₁₀[K]ₒ. Repeat for at least 6 concentrations from about 2 to 40 mM.
Fit E versus log₁₀[K]ₒ; the slope is the result in mV/decade. Compare with the reference.

Conclusion

The fitted decade slope agrees with (RT/F) ln 10 at 20 °C. Main uncertainties: electrode noise and treating the electrode as ideally K⁺-selective.