Physical and Chemical Properties:
Molecular Weight: 190.11 g/mol
Melting Point: Approximately 150°C (decomposes)
Density: 1.48 g/cm³
Solubility: Highly soluble in water (approximately 54 g/100 mL at 20°C), insoluble in ethanol
pH: Aqueous solution is acidic (pH approximately 4.0–5.5)
Chemical Properties:
Sodium metabisulfite is a strong reducing agent, reacting with oxidants to form sodium sulfate (Na₂SO₄).
It decomposes into sulfurous acid (H₂SO₃) or sulfur dioxide (SO₂) under acidic conditions.
Preparation Method:
Typically produced by reacting sulfur dioxide gas with sodium hydroxide (NaOH) or sodium carbonate (Na₂CO₃).
Safety:
Sodium metabisulfite is somewhat irritating; contact with skin or inhalation of its dust may cause discomfort.
Protective measures should be taken during handling to avoid prolonged exposure.
Uses in Electroplating
Sodium metabisulfite is primarily used as a reducing agent and auxiliary in the electroplating industry. Its specific applications include the following:
Metal Surface Treatment:
Before electroplating, sodium metabisulfite can remove oxide layers or impurities from metal surfaces. For instance, in nickel or copper plating, it reduces surface oxides, ensuring a clean substrate for better adhesion and quality of the coating.
Plating Bath Stabilizer:
In certain plating solutions (e.g., gold or silver plating baths), sodium metabisulfite acts as a reducing agent to prevent oxidation of metal ions (such as Au³⁺ or Ag⁺) or unwanted side reactions, thus maintaining the stability of the plating bath.
Neutralizing Excess Oxidants:
During electroplating, if oxidative chemicals (e.g., hydrogen peroxide or potassium permanganate) are used in pretreatment, sodium metabisulfite can neutralize residual oxidants, preventing interference with subsequent plating processes.
Special Coating Preparation:
In some electroless or electroplating processes, sodium metabisulfite serves as a reducing agent to facilitate the deposition of specific metal layers. For example, in tin or zinc plating, it helps control the reduction rate of metal ions.
