Stannous Chloride
Formula: SnCl₂·2H₂O — Tin(II) chloride dihydrate, tin salt
Appearance: White crystalline solid
Hazard: Irritant · Harmful if ingested
Properties
White crystalline solid, soluble in water but prone to hydrolysis — add a small amount of hydrochloric acid to prevent the formation of insoluble Sn(OH)Cl precipitate. Powerful reducing agent: Sn²⁺ is readily oxidized to Sn⁴⁺, making it useful for reducing metal ions. Reduces gold(III) to gold(0) (the basis of the “purple of Cassius” test), ferric to ferrous, and mercury(II) to mercury(0). In alkaline solution, forms Sn(OH)₂ which further reduces to tin metal. Related to stannic chloride (SnCl₄, tin(IV) chloride), which is a different compound entirely.
Historical Context
Tin plating is one of the oldest metal-finishing processes — tinned iron (“tinplate”) for food containers dates to 17th-century Bohemia. The Romans used tin vessels and tin-lead alloys extensively. Stannous chloride became important industrially as an electrolyte for tin electroplating and as a mordant in the textile dyeing industry, where it fixes dyes to cloth.
The “purple of Cassius” reaction — adding stannous chloride to gold chloride solution produces a beautiful deep purple colloidal gold — was described by Andreas Cassius in 1685 and was the first reproducible test for gold. The purple is caused by gold nanoparticles of specific size; the reaction is now understood as a controlled colloidal synthesis. This same principle underpins the ruby red color of cranberry glass, which contains colloidal gold.
In photography, stannous chloride found use as a chemical intensifier, strengthening weak negatives by reducing silver ions to metallic silver in the image areas.
Experiments
Prussian Blue (Stannous Method): Add a few drops of stannous chloride solution to ferric chloride solution, then add potassium ferricyanide. The Sn²⁺ reduces some Fe³⁺ to Fe²⁺, and the resulting ferrous ions immediately react with ferricyanide to form Prussian blue. This two-step route illustrates sequential redox and precipitation chemistry.
Purple of Cassius (Gold Nanoparticles): Add a small amount of stannous chloride solution to a few mL of dilute gold chloride solution (made by dissolving a small piece of gold in aqua regia, or using commercial gold chloride). A deep purple color develops as gold nanoparticles form — the color depending on particle size. One of the oldest-known synthesis routes to controlled metal nanoparticles.
Reduction Demonstration: Add stannous chloride to ferric chloride solution — the yellow Fe³⁺ color fades to the pale green of Fe²⁺. A simple, clean demonstration of a two-electron reduction: Sn²⁺ + 2Fe³⁺ → Sn⁴⁺ + 2Fe²⁺.
Experiments using this chemical:
- Prussian Blue Synthesis — Reduction route via Fe³⁺ → Fe²⁺
- The Many Colors of Iron — Reduction of ferric to ferrous
Safety
Moderate hazard — irritant; add HCl to solutions to prevent hydrolysis.
Irritant to skin and eyes; wear gloves. Solutions prepared without acid will form a cloudy precipitate — dissolve in dilute hydrochloric acid first. Harmful if ingested in quantity. Tin compounds are generally less toxic than comparable heavy metals. Dispose of solutions by neutralization and dilution.
Incompatible with: Strong oxidizers (rapidly oxidize Sn²⁺ to Sn⁴⁺); hydrogen sulfide and sulfide salts (SnS precipitate); alkalis (Sn(OH)₂ precipitate, which can be further reduced to tin metal); nitric acid