ANTIOXIDANTS

Antioxidants are reducing agents which are added to the drugs or other pharmaceutical preparations to prevent their oxidation.

Requirements of an Ideal Antioxidant

• It should be chemically and pharmacologically inert.
• Effective in low concentration.
• It should not be toxic.
• It should be easily soluble.

Official Antioxidant

1. HYPOPHOSPHORUS ACID
2. Sulphuer Compound
   • SULPHUR DIOXIDE
   • SODIUM META BISULPHITE
   • SODIUM THIOSULPHATE
   • SODIUM BISULPHITE
3. SODIUM NITRITE
4. NITROGEN

HYPOPHOSPHORUS ACID

M.F.: H₃PO₂

Preparation: It is prepared by mixing calcium hypophosphite with sulphuric acid or oxalic acid. The insoluble calcium salt is filtered and collected.

Ca(H₂PO2)₂ (Calcium hypophosphite)+ H₂SO₄ → CaSO₄ + 2H₃PO₂

Ca(H₂PO₂)₂ +  H₂C₂O₄ (oxalic acid)→ CaSO₄ + 2H₃PO₂

Properties (Physical & Chemical)

1.  Clear yellowish liquid with slight acidic odour.

2.  Soluble in water and alcohol.

3.  It acts as monoprotic acid and ionizes to give

H₃PO₂ + H₂O H₃O+ + H₂PO₂

4.  It acts as a powerful reducing agent. With iodine it forms iodide.

H₃PO₂ + 2I₂ + 2H₂O → 4HI + H₃PO₄ (phosphoric acid)

5.  It decolurise the KMnO4 solution.

Identification test: Upon heating with copper sulphate solution gives reddish brown precipitate.

Assay (Titremetric method)

Hypophosphorous acid is first diluted with water. Then it is titrated with sodium hydroxide using methyl orange as indicator.

Incompatability: Since it is a reducing agent, it is incomptabale with oxidizing agents.

Storage: It should be stored in well closed container.

Use : As an antioxidant in pharmaceutical preparations.

SULPHUR DIOXIDE

M.F : SO₂

Preparation:

Laboratory Method

1. Burning sulphur in presence of air (or) oxgen.
   2S + 2O₂ → 2SO₂
2. Decomposition of sodium sulphite with H₂SO₄ acid.
   NaHSO₃ + H₂SO₄ → NaHSO₄ + SO₂  + H₂O
   

Industrial Method

Roasting of metallic sulphides such as
Cu₂S + 2O₂ → 2CuO + SO₂
(Copper sulphide)

2ZnS + 3O₂ → 2ZnO + 2SO₂  etc.
(Zinc sulphide)

Properties (Physical & Chemical)

1. Colourless, non-inflammable gas with pungent odour.
2. Easily liquified.
3. Aqueous solution is acidic to litmus.
4. It is a very good reducing agent.
5. With iodine forms hydroiodic acid.
6. It decolurise the KMnO4 solution
   2KMnO4 + 2H₂O + 5SO₂ → 2MnSO₄ + 2H₂SO₄ + K₂SO₄
   

Assay
The method based upon the absorption of SO₂ into NaOH solution to form sodium bisulphite. Then bisulphite so formed is titrated with iodine solution using starch mucilage as an indicator to a blue colour end-point.

Storage
Should be stored in well-closed container.

Incompatibility
Generally incompatible with oxidizing agents (since it is a reducing agent).

Use :

1. It act as an antioxidant.
2. Used for the manufacture of sulphuric acid

SODIUM METABISULPHITE

M.F : Na₂S₂O₅

Preparation : It involves two steps
1^st Step
Passing SO₂ gas through a hot strong solution of sodium hydroxide until the solution is saturated. Sodium bisulphite is formed.
NaOH + SO₂ →NaHSO₃
II^nd Step
Sodium bisulphite loses water and gives sodium metabisulphite on cooling.
2 NaHSO₃ →Na₂S₂O₅ + H₂O

Properties:

1. Colourless crystals having sulphurous odour with saline taste.
2. Freely soluble in water.
3. Aqueous solution is acidic.

Assay

Oxidation-reduction reaction: Weighed amount of sample is dissolved in water. Then the excess of iodine solution is added. (Which oxidizes the sodium metabisulphite (reducing agent) to sodium meta sulphate.
Excess of iodine is titrated with sodium thiosulphate using starch mucilage as an indicator.

Identification Test

(i) Decolurises the iodine solution.

Incompatibility
Generally incompatible with oxidizing agents.

Storage
It should be stored in well-closed container.

Use:

1. As an antioxidant in injections.
2. Preservation of food materials

SODIUM THIOSULPHATE

M.F. Na₂S₂O₃
Syn : Sodium hyposulphate or antichlor.

Preparation: Aqueous solution of sodium sulphite is heated with sulphur. The solution is concentrated, then the crystals are separated.
Na₂SO₃ + S → Na₂S₂O₃

Properties

1. Colourless crystals or coarse crystalline powder, odourless, with alkaline taste.
2. It melts at 50^oC while decomposes on being heated at 100^oC.
3. It effloresces in dry air and deliquesces in moist air.
4. Soluble in water.
5. It reduces halogens.
   2Na₂S₂O₃ + I₂ → 2NaI + Na₂S₄O₆ (sodium tetrathionate)
6. Upon treating with HCl, It liberates, sulphur, sulphur dioxide

Assay

The weighed amount is dissolved in water and titrated with iodine solution using starch mucilage as an indicator.
2Na₂ S₂O₂ + I₂ → Na₂S₄O₆ + 2NaI (soda. Tetra thionate)

Storage: Well closed container

Use :

i. With sod. nitrite as an Antidote in the treatment of cyanide poisonings (for mingthiocyanate)
ii. Useful in skin diseases.

NITROGEN

M.F : N₂
Atmospheric air contains nearly 78% of nitrogen. It also occurs as nitrate deposits.

Preparation:

1. Distillation of liquid air.
2. Decomposition of NH₃. (Haber Synthesis)
   

When burning the phosphorous in a closed container, phosphorous taken up the oxygen and is converted in phosphorous pentoxide (P₂O₅), leaving the nitrogen in the container.

Properties

1. Colourless, odourless, tasteless gas.
2. It is soluble in alcohol, slightly in water.
3. It can be liquified.
4. It is inert gas (inactive)

Storage

Stored in metal cylinder under pressure. The cylinder is painted grey with black on
the neck and shoulders. The name should be stencilled or painted on the body of the
cylinder.

Use :

1. For the packaging of pharmaceuticals (as it is inert gas).
2. Diluent for oxygen
3. Liquid nitrogen is used in surgery to remove some tumours.

SODIUM NITRITE

M.F. : NaNO₂

Preparation:
Most common suitable method
Absorbing of nitrogen oxide gas (NO) by sodium carbonate solution. The solution is concentrated to crystallize out the product.
2Na₂CO₃ + 4NO + O₂→ 4NaNO₂ + 2CO₂

Properties:

1. Colourless to slightly yellow crystals, odourless, saline taste.
2. It is deliquescent. Absorb moisture and slowly gets oxidized to sodium nitrate.
3. Soluble in water.
4. It is a reducing agent.

Assay

The method is based upon the oxidation of nitrite to nitrate. weighed sample is dissolved in water and mixed with excess volume of potassium permanganate solution.
Then 5ml of sulphuric acid was added. Then excess of oxalic acid is added, the mixture is heated to 80 and the excess oxalic acid is back titrated with the std KMnO4 solution.
NaNO₂ + H₂SO₄ → NaHSO₄ + HNO₂
HNO₂ + O → HNO₃

Storage:
It should be preserved in tightly closed container.

Use

1. Antidote for cyanide poisoning.
2. Food preservative.