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
- HYPOPHOSPHORUS ACID
- Sulphuer Compound
- SODIUM NITRITE
1. HYPOPHOSPHORUS ACID
M.F. : H3PO2
It is prepared by mixing calcium hypophosphite with sulphuric acid or oxalic acid. The insoluble calcium salt is filtered and collected.
Ca(H2PO2)2 + H2SO4 → CaSO4 + 2H3PO2
Ca(H2PO2)2 (Calcium hypophosphite) + H2C2O4 (oxalic acid) → CaSO4 + 2H3PO2
Properties (Physical & Chemical)
- Clear yellowish liquid with slight acidic odour.
- Soluble in water and alcohol.
- It acts as monoprotic acid and ionizes to give
H3PO2 + H2O H3O+ + H2PO2
- It acts as a powerful reducing agent. With iodine it forms iodide.
H3PO2 + 2I2 + 2H2O → 4HI + H3PO4 (phosphoric acid)
- It decolurise the KMnO4 solution.
i. 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.
Incompatibility: Since it is a reducing agent, it is incompatible with oxidizing agents.
Storage: It should be stored in a well-closed container.
Use: As an antioxidant in pharmaceutical preparations.
M.F : SO2
- Burning sulphur in presence of air (or) oxgen.
2S + 2O2 → 2SO2
- Decomposition of sodium sulphite with H2SO4 acid.
NaHSO3 + H2SO4 → NaHSO4 + SO2 + H2O
Roasting of metallic sulphides such as
Cu2S + 2O2 → 2CuO + SO2
2ZnS + 3O2 → 2ZnO + 2SO2 etc.
Properties (Physical & Chemical)
- Colourless, non-inflammable gas with pungent odour.
- Easily liquified.
- Aqueous solution is acidic to litmus.
- It is a very good reducing agent.
- With iodine forms hydroiodic acid.
- It decolurise the KMnO4 solution
2KMnO4 + 2H2O + 5SO2 → 2MnSO4 + 2H2SO4 + K2SO4
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.
Should be stored in well-closed container.
Generally incompatible with oxidizing agents (since it is a reducing agent).
- It act as an antioxidant.
- Used for the manufacture of sulphuric acid
M.F : Na2S2O5
Preparation : It involves two steps
Passing SO2 gas through a hot strong solution of sodium hydroxide until the solution is saturated. Sodium bisulphite is formed.
NaOH + SO2 →NaHSO3
Sodium bisulphite loses water and gives sodium metabisulphite on cooling.
2 NaHSO3 →Na2S2O5 + H2O
- Colourless crystals having sulphurous odour with saline taste.
- Freely soluble in water.
- Aqueous solution is acidic.
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.
(i) Decolurises the iodine solution.
Generally incompatible with oxidizing agents.
It should be stored in well-closed container.
- As an antioxidant in injections.
- Preservation of food materials
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
- Colourless crystals or coarse crystalline powder, odourless, with alkaline taste.
- It melts at 50oC while decomposes on being heated at 100oC.
- It effloresces in dry air and deliquesces in moist air.
- Soluble in water.
- It reduces halogens.
2Na2S2O3 + I2 → 2NaI + Na2S4O6 (sodium tetrathionate)
- Upon treating with HCl, It liberates, sulphur, sulphur dioxide
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
i. With sod. nitrite as an Antidote in the treatment of cyanide poisonings (for mingthiocyanate)
ii. Useful in skin diseases.
M.F : N2
Atmospheric air contains nearly 78% of nitrogen. It also occurs as nitrate deposits.
- Distillation of liquid air.
- 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.
- Colourless, odourless, tasteless gas.
- It is soluble in alcohol, slightly in water.
- It can be liquified.
- It is inert gas (inactive)
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
- For the packaging of pharmaceuticals (as it is inert gas).
- Diluent for oxygen
- Liquid nitrogen is used in surgery to remove some tumours.
M.F. : NaNO2
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
- Colourless to slightly yellow crystals, odourless, saline taste.
- It is deliquescent. Absorb moisture and slowly gets oxidized to sodium nitrate.
- Soluble in water.
- It is a reducing agent.
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
It should be preserved in tightly closed container.
- Antidote for cyanide poisoning.
- Food preservative.