SODIUM THIOSULPHATE

SODIUM THIOSULFATE

Sodium thiosulfate is an important inorganic salt with several medical uses. It is also called sodium hyposulfite or 'hypo'.

Formula and structure: The chemical formula of sodium thiosulfate is Na₂S₂O₃ and its molar mass is 158.11 g/mol. It is also available as its pentahydrate salt (Na₂S₂O₃.5H₂O), with a molar mass of 248.18 g/mol. It is an ionic compound composed of two sodium cations (Na⁺) and the negatively charged thiosulfate anion (S₂O₃^-), in which the central sulfur atom is bonded to three oxygen atoms and another sulfur atom, all through single and double bonds with resonance character. The solid exists in a monoclinic crystal structure.

Preparation: Sodium thiosulfate can be prepared by heating sulfur with either aqueous sodium sulfite solution or aqueous sodium hydroxide solution.

6 NaOH + 4 S → Na₂S₂O₃ + 2 Na₂S + 3 H₂O

Physical properties : Sodium thiosulfate is a white crystalline solid which is odorless and highly water soluble. It has a density of 1.667 g/mL and a melting point of 48.3°C.

Chemical properties: Sodium thiosulfate is a neutral salt which readily dissociates in water to give sodium and thiosulfate ions. Na₂S₂O₃ is a stable solid under normal conditions, but decomposes upon heating to give sodium sulfate and sodium polysulfide:

4 Na₂S₂O₃ → 3 Na₂SO₄ + Na₂S₅

It also decomposes when treated with dilute acids to give sulfur and sulfur dioxide (called 'clock reaction'):

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

It reacts stoichiometrically (in equimolar amounts) with aqueous solutions of iodine, and so, it is widely used in laboratories for iodine based titrations.

Uses: Sodium thiosulfate is used in several pharmaceutical preparations and also has various medical properties. It is an important antidote used for treating cyanide poisoning. Apart from its medical uses, it also has applications in water treatment, neutralizing bleach, leather tanning, gold extraction, photographic processing, and chemical heating pads.

Health effects/safety hazards: Sodium thiosulfate is not a toxic material and is used for medical purposes. However, when it decomposes, it produces toxic sulfur oxide fumes, which can cause irritation to eyes, skin and mucous membranes.

Reaction of Sodium thiosulphate and Hydrochloric Acid

The effect of concentration of the reactant on the rate of a reaction can be studied easily by the reaction between sodium thiosulphate and hydrochloric acid.

Sodium thiosulphate reacts with dilute acid to produce sulphur dioxide, sulphur and water. Sulphur dioxide is a soluble gas and dissolves completely in aqueous solution. The sulphur formed however is insoluble and exist in the mixture as a white or pale yellow precipitate or a colloid that gives a milky appearance and makes the solution opaque. Therefore the rate of the reaction can be studied by monitoring the opaqueness of the reaction. This can be easily done by measuring the time taken (t) for forming a certain amount of sulphur.

In this experiment, the reaction is carried out by mixing the reactants in a conical flask, and placing the conical flask on the top of a piece of white paper that has a cross mark on it. Before the reaction starts, the mark is clearly visible from the top of the conical flask through the solution. However, as the reaction proceeds, sulphur precipitates that make the solution more opaque and eventually the mark gets completely masked. The time taken (t) for the mark to totally disappear indicates how fast the reaction has occurred.

We can plot a graph between 1/t (in second) and the concentration of sodium thiosulphate by taking 1/t along ordinate axis and concentration of Na₂S₂O₃ along abscissa. The graph should be a straight slopping line.