Thiourea
Formula: CH₄N₂S — Thiocarbamide, sulfourea
Appearance: White crystalline powder
Hazard: Irritant · Suspected carcinogen · Handle with care
Properties
White crystalline solid, freely soluble in water. Structurally analogous to urea but with sulfur replacing oxygen. The sulfur atom is a soft Lewis base that forms strong complexes with soft metal ions — particularly silver, gold, and platinum. Solutions are mildly acidic. In acidic solution, thiourea dissolves silver and silver sulfide readily, making it an effective silver cleaner. It can also reduce metal ions: thiourea solutions reduce Au³⁺ to Au⁰, making it useful for gold deposition from solution.
Historical Context
Thiourea’s chemistry was first systematically explored in the late 19th century as chemists examined structural analogues of urea. Its ability to complex with heavy metals attracted industrial interest for hydrometallurgy — extracting precious metals from ores.
In photography, thiourea found a niche as a toning agent and chemical intensifier. Unlike sodium thiosulfate (“hypo”), which forms water-soluble silver complexes, thiourea’s silver complex is soluble in acidic solution, which enabled new fixing and toning processes. In the mid-20th century, thiourea baths became standard for stripping silver deposits from mirrors and glassware cleanly without mechanical abrasion.
The compound also appeared in one of the stranger episodes in reaction chemistry history: it is one of several sulfur compounds known to form spontaneous oscillating reactions in certain complex chemical systems.
Experiments
Silver Cleaning Bath: Dissolve thiourea (5–10 g) in dilute hydrochloric acid (50 mL of 5% solution). Immerse tarnished silver or silver-coated objects — the silver sulfide and metallic silver dissolve rapidly, leaving the substrate clean. Demonstrates metal complexation chemistry: the thiourea sulfur atom binds strongly to Ag⁺, removing it from the surface.
Removing Silver Mirror Deposits: After making a Silver Mirror, clean the glassware by rinsing with the thiourea/HCl bath. The silver film dissolves within seconds. Compare with the slow, incomplete removal achieved by dilute acid or abrasion.
Experiments using this chemical:
- Silver Mirror Reaction — Thiourea bath for cleaning silver from glassware afterward
Safety
Moderate hazard — suspected carcinogen; avoid repeated skin contact.
Wear gloves and work in a ventilated area. Do not ingest. Avoid contact with strong oxidizers, which can liberate toxic sulfur compounds. Dispose of solutions containing dissolved silver by precipitation with sodium chloride (AgCl is insoluble) rather than pouring down the drain.
Incompatible with: Strong oxidizers (can oxidize sulfur to sulfate with heat and fume release); strong acids at high concentration (decomposition to CS₂ and NH₃); heavy metal salts (forms insoluble complexes — often the intended reaction)