Pharmaceutical Aids And Necessities - Antioxidants

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Antioxidants may be described as compounds which have the capability of functioning chemically as reducing agent.They are used in pharmaceutical formulations containing easily oxidized substances in order to keep the substance in their oxidized form.

The Mechanism of antioxidant action may be seen in two ways,both achieving the same net result :

• The active component is oxidized,the antioxidant reduces it back to its normal oxidation state.
• The antioxidant is oxidized in the place of active constituent.

THE THEORY OF ANTIOXIDANTS 

Antioxidant action is same as that involved in any oxidation-reduction or Redox reaction.For example,the usual way of examining a Redox reaction is to separate it into half-cell reaction of the oxidizing and reducing agents.Since in Redox reaction,there is a transfer of electron from one compound to other,the loss of gain of electron is used to balance the oxidation states on both sides of the half - cell reaction.

Consider a reaction between the oxidized form of substance(1) , and reduced form of substance(2).Oxidized form of substance (1) is given by Ox.(1) and Reduced form of substance (2) is given by Red.(2).Therefore,

Ox.(1) + e ⇄ Red.(1)

Red.(2) ⇄ Ox.(2) + e

The above reaction if explained is given that - The OXIDIZED State of substance(1) takes up an electron and converts into REDUCED State of substance(1).And, the REDUCED State of substance(2) gives up an electron and gets converted in to Oxidized state.

The Total Redox reaction is completed by adding both the reaction,we get,

Ox.(1) + e ⇄ Red.(1)

Red.(2) ⇄ Ox.(2) +  e

一一一一一一一一一一一

Ox.(1) + Red.(2) ⇄ Red.(1) + Ox.(2)

The tendency for the chemical substance to undergo oxidation and reduction can be determined by electro-chemical cells where the electron transfer takes place through/with help of electrode.The potential developed is measured by voltmeter or potentiometer.

The properties of electrochemical cells are expressed by NERST EQUATION :

E_cell  = E⁰_cell — {RT/nF} ✖ lnQ

Where,E_cell  = the potential of cell in VOLTS.

E⁰_cell = Standard Potential.[when all the reactant and product are at unit activity]

R = Gas Constant.

T = Temperature [°K - absolute temperature]

F = Farady Constant.

n = Number of Electron transferred in the Reaction.

Q = Activity Quotient of product over reactant.

At 25℃ and converting to logarithms to the base 10,the above equation can be written as,

E_cell  = E⁰_cell — {0.059/n} ✖ logQ

E⁰_cell is the sum of the E⁰ for oxidation and reduction half cell reaction.That is,

E⁰_cell  = E_1/2⁰_Oxid. + E_1/2⁰_Red.

As an example,to evaluate antioxidants,consider Dilute Hydriodic Acid that contain 10% HI stabilized with 0.8% Hypophosphorus Acid (HPH₂O₂).The Hypophosphorus Acid prevents the oxidation of iodide to molecular iodine by the following reaction :

HPH₂O₂ + I₂ + 3H₂O ⇄ H₂PHO₃+ 2I^- + 2H₃0⁺

The reaction can be factored in to the corresponding Half-cell reaction with the E_1/2⁰ values ;

1) HPH₂O₂ + 3H₂O =  H₂PHO₃ +  2H₃0⁺ + 2e     {E_1/2⁰ =+0.59}

2) I₂ + 2e = 2I^-                                                        {E_1/2⁰ =+0.54}

            一一一一一一一一一一一一一一一一一一一一一一一一一一一一一一一一

  HPH₂O₂ + I₂ + 3H₂O ⇄ H₂PHO₃+ 2I^- + 2H₃0⁺   {E_1/2⁰ = +1.13}

The positive value of e indicates spontaneous reaction,and is supported when NERST EQUATION for the redox reaction is solved :

E_cell = 1.13 一 0.059/2 log{[H₂PHO₃][I^-] / [HPH₂O₂][I₂]}

On substituting the value of HI = 0.78 M and HPH₂O₂ = 0.12 M,we get,

E_cell = 1.13 一 0.03 Log{[H₂PHO₃][0.78]² / [0.12][I₂]}

= 1.13 一 0.03 (0.705)

= 1.13 一 0.02 = 1.11

The large positive value of   indicates that the Hypophosphorous Acid present should effectively reduce any other molecule of iodine formed in Dilute Hydriodic Acid solution by air oxidation.(The above example is just a short part of theory).The Hypophosphorous Acid transfers two electron and reduces one molecule of iodine,which subsequently forms phosphorous acid.In turn Phosphorous Acid can transfer two more electron to reduce another molecule of iodine,which forms phosphoric Acid.Thus in the end,one molecule Hyphophosphorous Acid can reduce two molecules of iodine,forming four iodide ions and one molecule of phosphoric acid,

Selection Of Antioxidants

In selecting antioxidants,the desired Redox reaction taking place should be assessed through the use of standard electrode potentials and the NERST EQUATION.Other aspects involves physiological and chemical incompatibilities.An antioxidant in pharmaceutical formulation sholud be physiologically inert.The toxicity effect should be assessed.

Antioxidants have similar chemical problems as that of buffers.While selecting antioxidants,solubility problems arises between reducing agents and the Drug.Another very serious problems is associated with the handling of antioxidants.It is concerned with mixing of antioxidants with strong oxidizing agents.Strong reducing agent (e.g. Hypophosphorous Acid) will form explosives mixtures when combined in dry form or in concentrated solutions with strong oxidizing agent.

Official Antioxidants

SOME OF THE OFFICIAL ANTIOXIDANTS STATED BY U.S.P. AND N.F. ARE GIVEN BELOW

(1)

Hypophosphorous Acid

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Hypophosphorous Acid is a clear,colorless or slightly yellow,odorless liquid having not less 30% and not more than 32.0% HPH₂O₂ .The Acid is syrupy liquid which condenses at 17.4℃ and melts as 26.5℃ .Hypophosphorous Acid when reacts with any concentrated oxidizing agents,it can produce incompatibility.But,we get a desirable reaction with reducible substance,and so Hypophosphorous Acid can function in dilute solution as a very effective reducing agent or Antioxidant.

USES of Hypophosphorous Acid :

• Hypophosphorous Acid serves to prevent formation of iodine in Diluted Hydriodic Acid and Hydriodic Acid Syrup .
• Salts of these acid is used for its antioxidant properties.
• Sodium Hypophosphite is used as a preservative in food
• Ammonia Hypophosphite is found as preservative in many preparations.

(2)

Sulfur Dioxide

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Sulfur Dioxide is a colorless,non-flammable gases possessing a strong suffocating odor,which is a characteristic of sulfur burning.Under pressure,sulfur dioxide condenses to colorless liquid which boils at -10℃.

Uses Of Sulfur Dioxide :

• Sulfur Dioxide is classified antioxidant for pharmaceutical aids.
• It has extensive uses in industry for bleaching wood pulp,fumigating grains and houses.
• It protects many susceptible compounds from oxidation by reducing the oxidized form back again or reacting with oxygen before the susceptible compounds do.
• It is usually used in injectable preparation enclosed in single dose ampules or multiple dose vials

(4)

Sodium Thiosulfate

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Sodium Thiosulfate is considered as an official antidote and an antioxidant.Its reducing properties are shown by its application as a titrating reagent in iodine determination.Sodium Thiosulfate U.S.P. occurs as large,colorless crystals or as a coarse crystalline powder.It is deliquescent in moist air and effloresces in dry air at temperature above 33℃.

Uses of Sodium Thiosulfate :

• Sodium Thiosulfate is used as an antioxidant only in the solution containing iodides.
• In the U.S.P. Sodium Thiosulfate is categorized as an antidote for Cyanide Poisoning.
• It is less frequently used with acids to treat dermatological problems.

(5)

Sodium Bisulfite

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Sodium Bisulfite is described by the U.S.P. as a mixture of Sodium Bisulfite(NaHSO₃) and Sodium Metabisulfite (Na2S2O5).Sodium Bisulfite is white or yellowish crystals or a granular powder having the odor of Sulfur Dioxide.

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