Mercury(I) Chloride
Properties
| State | Solid |
| Color | White |
| Solubility | Nearly insoluble in water (2 mg/L); insoluble in alcohol |
| Melting Point | Sublimes at 383 °C |
| Boiling Point | 384 °C (sublimes) |
About Mercury(I) Chloride
Mercury(I) chloride is the white tasteless salt that 19th-century physicians spooned into patients for everything from constipation to syphilis under the name calomel — and it is also the salt that powered electrochemistry for the entire 20th century in the form of the saturated calomel electrode (SCE). Structurally, calomel is unusual among inorganic salts because it does not contain Hg+ as a monomer. Instead it contains the dimeric mercurous cation Hg2²⁺ with a real Hg-Hg covalent bond about 2.53 Å long, making it one of the few stable metal-metal bonded cations in classical inorganic chemistry. Each Hg in the dimer is formally +1, hence mercury(I), even though the chemistry behaves like nothing else with that oxidation state. The SCE — Hg | Hg2Cl2 | KCl(saturated) — gives a stable +0.2412 V vs SHE at 25°C with a temperature coefficient of about 0.65 mV/°C, and it was the workhorse reference for pH meters, ion-selective electrodes, polarography, and voltammetry from roughly 1900 through the 1990s. Polarography itself, the technique that won Jaroslav Heyrovský the 1959 Nobel in Chemistry, used a dropping mercury electrode against an SCE. Today most labs have switched to Ag/AgCl reference electrodes for routine work because they are mercury-free, smaller, and don't have the SCE's tendency to clog its frit with KCl crystals at refrigerator temperatures. Calomel survives in specialty high-precision electrochemistry where its long-term stability is hard to match.
Where you'll encounter it
If you have ever pulled an old SCE out of a 1980s-vintage electrochemistry kit — a stout glass tube with a porous frit, KCl crystals at the bottom, mercury blob in the middle, and a wire poking out the top — you have held the reference electrode that calibrated almost every pH meter in industry for decades. In a teaching lab, the SCE is still the cleanest way to demonstrate why a reference electrode matters: hook one up alongside a Pt working electrode in a beaker of Fe(CN)6³⁻/⁴⁻, sweep the potential, and watch the Nernstian wave at exactly the textbook E° vs SCE. Historically, calomel was also the disastrous medicine that killed several US presidents' physicians' patients — Andrew Jackson reportedly took it for years and showed classic chronic mercury symptoms (tremor, mood changes, gum line) by his second term.
Common Uses
- Saturated calomel electrode (SCE) reference electrode for electrochemistry and pH measurement
- High-precision specialty electrochemistry (long-term-stability voltammetry)
- Historical 19th-century medicinal use as laxative, diuretic, and antisyphilitic (now obsolete)
- Restricted-use fungicide in some agricultural and bulb-treatment applications
- Reagent in classical inorganic synthesis (Hg2(NO3)2 precipitation, Hg/HgCl reduction)
Safety Information
GHS: Acute toxicity (oral, Category 3), Specific target organ toxicity (repeated exposure - nervous system and kidneys, Category 1), Aquatic chronic hazard (Category 1). Less acutely toxic than HgCl2 because of the very low (~2 mg/L) water solubility, but in the gut Hg2Cl2 disproportionates over time to Hg(0) + HgCl2, regenerating the soluble toxic species. Chronic exposure produces mercury poisoning: tremor, erethism (mood instability), gingivitis, kidney damage. OSHA PEL 0.1 mg/m3 ceiling (as Hg). Banned or severely restricted in pharmaceutical and most agricultural uses under the Minamata Convention on Mercury (2013). Disposal as hazardous waste; never down the drain.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.