Sodium Thiosulphate

The chemical formula Na₂S₂O₃ represents Sodium Thiosulfate, a crystalline compound composed of sodium (Na), sulfur (S), and oxygen (O). Let’s understand its properties and applications in detail

1.0What is Sodium Thiosulfate

Sodium thiosulfate (Na₂S₂O₃) is a salt composed of sodium (Na⁺), sulfur (S), and oxygen (O). It is a white, crystalline substance that dissolves readily in water. The compound consists of two sodium ions and one thiosulfate ion (S₂O₃^2-). The thiosulfate ion is an interesting structure with a sulfur atom at the center, bonded to three oxygen atoms and another sulfur atom. The overall charge on the thiosulfate ion is -2.Sodium thiosulphate molecular weight  is 158.11 grams per mole. The more commonly available pentahydrate form, Na₂S₂O₃·5H₂O, has a molar mass of 248.18 g/mol.

Here is a representation of the Crystals of Sodium Thiosulphate

Sodium thiosulfate is prepared by neutralizing sulfur dioxide with sodium hydroxide and is often encountered as the pentahydrate Na₂S₂O₃·5H₂O, a colorless, monoclinic crystalline substance that is readily soluble in water.

The name "thiosulfate" is derived from the presence of sulfur and oxygen in the molecule. In chemical reactions, sodium thiosulfate can undergo various transformations, including redox reactions where it can either gain or lose electrons. Understanding its chemical structure and reactivity is crucial for its diverse applications in photography, medicine, water treatment, and analytical chemistry.

2.0Structure of Sodium Thiosulphate

3.0Physical Properties of Sodium Thiosulphate 

Sodium thiosulfate (Na₂S₂O₃) has several notable physical properties:

4.0Chemical Properties of Sodium Thiosulfate

Sodium thiosulfate (Na₂S₂O₃) exhibits various chemical properties due to its composition and the nature of the thiosulfate ion. Here are some key chemical properties:

• Redox Properties: Sodium thiosulfate is involved in redox reactions. It can act as both an oxidizing agent and a reducing agent. In the presence of strong acids, it can be oxidized to form sulfur or sulfur dioxide. In other reactions, it can undergo reduction.
• Reaction with Acids: Sodium thiosulfate reacts with acids, such as hydrochloric acid (HCl), to produce sulfur dioxide, sulfur, and water. The reaction is commonly used in analytical chemistry to generate a standardized solution of iodine for titrations.

                       Na₂S₂O₃    +  2 HCl     →    2 NaCl  +  S  +  SO₂  +  H₂O

• Complex Formation: Thiosulfate ions can form complexes with metal ions. In the presence of certain metal ions, thiosulfate can act as a ligand, forming coordination complexes.
• Tetrathionate Formation: In alkaline conditions, sodium thiosulfate can undergo further reactions to form tetrathionate ions (S₄O₆^2-). This property is sometimes utilized in analytical chemistry for specific reactions.
• Dechlorination: Sodium thiosulfate is known for its ability to dechlorinate water. It reacts with chlorine to form sodium chloride and sulfate:

                      2Na₂S₂O₃   +      4Cl₂      →    4NaCl    +    2Na₂SO₄

• Photographic Fixing: In photography, sodium thiosulfate is used to fix developed silver halide crystals by converting them into a water-soluble complex, preventing further exposure to light.

5.0Applications of Sodium Thiosulphate 

Sodium thiosulphate ( Na₂S₂O₃ ) finds applications in various industries and scientific fields due to its unique properties. Some notable applications include:

1. Photography: Sodium thiosulphate solution is a key component in photographic fixing solutions. It is used to remove unexposed silver halide from developed photographic emulsions, stabilizing the final image.

2. Medical Use: Sodium thiosulfate is employed as an antidote for cyanide poisoning. It forms a complex with cyanide ions, reducing their toxicity. This application is critical in emergency medicine.

3. Water Treatment: Sodium thiosulphate solution is used to dechlorinate water. It reacts with chlorine, neutralizing its presence in tap water, swimming pools, and wastewater before discharge.

4. Analytical Chemistry: Sodium thiosulfate is widely used in analytical chemistry for titrations, particularly in the determination of iodine concentration. It is also used to standardize solutions of other substances.

5. Gold Extraction: In certain gold extraction processes, sodium thiosulfate is used as an alternative to traditional cyanide-based methods. This is especially relevant in environmentally conscious mining practices.

6. Chemical Reduction Reactions: Sodium thiosulfate is involved in redox reactions, serving as both an oxidizing and reducing agent. This versatility makes it useful in various chemical processes and experiments.

7. Hair Care Products: In the cosmetic industry, sodium thiosulfate is used in hair care products to neutralize or remove residual hair dyes, preventing discoloration.

8. Wastewater Treatment: It is utilized to neutralize excess chlorine in wastewater before discharge, contributing to environmental protection.

6.0 Preparation of Sodium Thiosulphate

(i)

(ii)     2Na₂S + Na₂CO₃ + 4SO₂       →     3Na₂S₂O₃ + CO₂

(iii)    6NaOH + 4S               →    Na₂S₂O₃ + 2Na₂S + 3H₂O

         3Ca(OH)₂ + 12 S        →    CaS₂O₃ + 2CaS₅ + 3H₂O

(iv)    Na₂SO₃ + Na₂S + I₂    →     Na₂S₂O₃ + 2NaI (spring reaction)

(v)     2Na₂S + 2O₂ + H₂O    →     Na₂S₂O₃ + 2NaOH

[Na₂S is readily oxidised in air giving rise to Na₂S₂O₃]

7.0Reactions of Sodium Thiosulphate

1. Reaction with Iodine (I₂): In this reaction, sodium thiosulfate reacts with iodine to form sodium iodide and sodium tetrathionate.

           Na₂S₂O₃    +   I₂               →   Na₂S₄O₆    + 2NaI 

2. Reaction with Chlorine Water (Cl₂ in water): In the presence of water, chlorine reacts with sodium thiosulfate to form sodium bisulfate, hydrochloric acid, and sulfuric acid.

Na₂S₂O₃       +  Cl₂ – water     →    NaHSO₄  + S + 2HCl

3. Reaction with Bromine Water (Br₂ in water): Similar to chlorine water, bromine water reacts with sodium thiosulfate in the presence of water to produce sodium bisulfate and sodium bromide.

Na₂S₂O₃       +  Br₂ – water     →   NaHSO₄   + S + 2HBr

4. Reaction with Chlorine (Cl₂):Chlorine reacts with sodium thiosulfate to produce sodium chloride and sodium tetrathionate.

Na₂S₂O₃       +  4Cl₂ + 5H₂O     →  Na₂SO₄ + H₂SO₄ + 8HCl

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2NaHSO₄