saturated calomel electrode
/ KAL-uh-mel /
Think of a classic, dependable yardstick that laboratories trusted for decades to set a fixed voltage. The saturated calomel electrode is one such yardstick — a small assembly of liquid mercury sitting under a paste of mercury(I) chloride (the old name for which is calomel), all bathed in a saturated potassium chloride solution. Because that chemistry settles to one reproducible state, the electrode holds a steady, known potential.
Formally, the saturated calomel electrode (SCE) is a reference electrode based on the equilibrium between mercury, mercury(I) chloride, and chloride ions, with the chloride activity fixed by using saturated potassium chloride. Its potential is about +0.241 volts versus the standard hydrogen electrode at 25°C, stable and reproducible enough to serve as the baseline against which indicator electrodes are read.
It matters because for much of the twentieth century the SCE was the workhorse reference for pH meters and electrochemistry. The honest caveat is mercury: because of its toxicity and disposal burden, the SCE has largely been replaced in routine work by the mercury-free silver–silver chloride electrode, which fills a very similar role. Saturated KCl also has a notable temperature dependence, so the SCE's potential shifts a little as temperature changes.
In an older textbook voltammetry setup, potentials are reported 'versus SCE': the saturated calomel electrode is the fixed reference, so a peak listed at −0.5 V vs SCE means half a volt more negative than that calomel baseline.
A mercury-based reference holding about +0.241 V vs the hydrogen electrode.
Because electrode potentials are reported relative to a reference, the same reaction can be quoted 'vs SCE' or 'vs standard hydrogen electrode' or 'vs Ag/AgCl' — always check which reference a number is against before comparing values from different sources.