Topical agents are the compound which are act locally on the body surface like skin and mucous membrane.

• They act mainly by physical and mechanical manners.
• Topical application of these drugs may extend to such body cavities that are open to the out side which includes oral, vaginal and colonic cavities.
• Topical agents may produce variety of effects such as adsorbent, astringent, demulcent, emollient and protective.
• Some topical agents may exhibit antimicrobial activity.

Topical agents are categorized according to the mechanism of action

A. PROTECTIVES

They are group of insoluble substances, applied to the skin to protect from irritation. They are non toxic and biologically inactive.

TALC I.P

M.F. Mg₆ (Si2O₅)₄ (OH)₄

It is a purified natural hydrated magnesium silicate, and may contain small amounts of aluminum silicate.

Preparation

The native talc is finely powdered and boiled with dilute HCl (to remove impurities like iron, Cao, iron etc.). The insoluble talc is completely washed with water until it becomes free from acid. The residue is then dried.

Properties

i. Very fine, white or grayish-white powder.

ii. It is smooth to touch.
iii. Odourless and tasteless.
iv. Its solution is neutral to litmus.
v. Practically insoluble in water, dilute acids and alkalies.

Storage

It should be stored in well closed container.

Use

1. As a base for dusting powders.
2. As a lubricant for tablet making.
3. As a filtering and distributing agent

ZINC OXIDE

M.F: ZnO

Preparation :

1. Large Scale: Heating metallic zinc, in a current of air to a high temperature. The metal vapour burns to form the oxide, collected as a fine white powder.
   2Zn + O₂ → 2 ZnO
2. Medicinal Grade : Zinc oxide is obtained by adding zinc sulphate to a boiling solution of sodium carbonate. The precipitated zinc carbonate is washed until it becomes free from sulphate. Now it is dried and ignited, loses CO2 and water, giving the oxide.
   ZnSO₄ + Na₂ CO₃ → ZnCO₃ + Na₂SO₄
   ZnCO₃ → ZnO + CO₂

Properties (Physical & Chemical)

1. White or faintly white very fine powder.
2. It is odourless and tasteless.
3. It is insoluble in water and alcohol.
4. It slowly absorbs CO2 from the air and forms basic zinc carbonate.
5. It reacts with acids forming zinc salts.
   ZnO + 2 NaOH → ZnCl₂ + H₂O
6. It reacts with base forms zincates.
   ZnO + 2NaOH → Na₂ ZnO₂ + H₂O

Assay: Back Titration Method

Weighed amount of the sample is dissolved in excess of std H2SO4 acid, small amount of ammonium chloride is added and heated if necessary. The excess of sulphuric acid is back titrated with std NaOH using methyl orange as an indicator.
(Ammonium chloride prevents the precipitation of zinc hydroxide by the NaOH near the end point).
ZnO + H₂SO₄ → ZnSO₄ + H₂O
Unreacted H₂SO₄ + NaOH → Na₂SO₄ + H₂O

Storage
It should be stored in well-closed container.

Use:

1. as a mild antiseptic and an astringent.
2. for making dental cement.
3. used in various skin diseases.
4. manufacture of adhesive tapes and bandages.

CALAMINE

Calamine is zinc oxide having a small amount of ferric oxide.

Preparation (Large Scale)

1^st Step: Zinc oxide is prepared by heating zinc carbonate.
ZnCO₃ → ZnO + CO₂
2_nd Step:
The calamine is then prepared by mixing zinc oxide with ferric oxide (up to 1.0 percent) thoroughly.

Properties

1. It is pink powder.
2. Almost odourless and tasteless.
3. Almost insoluble in water but dissolved completely in mineral acids

Assay

It is dissolved in excess of std. sulphuric acid and filtered then a small amount of ammonium chloride is added. The excess acid is back titrated with NaOH solution using methyl orange as an indicator.
NH₄Cl is added to prevent the precipitation of zinc hydroxide during titration.

Storage

It should be stored in a well-closed container.

Use

Astringent, antiseptic and protectant for the skin

ZINC STEARATE

M.F. (C₁₇H₃₅COO)₂ Zn

Zinc stearate contains mainly zinc stearate with variable proportion of zinc palmitate. It also contains 13% of zinc oxide (ZnO).

Preparation

It is prepared by adding zinc sulphate to a solution of sodium stearate.
2C₁₇H₃₅ COONa + ZnSO₄ → (C₁₇ H₃₅ COO)₂ Zn + Na₂SO₄
The precipitate is washed and dried.

Properties

1. White, amorphous powder free from grittiness.
2. It has faint characteristic odour.
3. It is insoluble in water, alcohol and other.

Assay:By Complexometric Method

Weighed amount of sample is boiled with H2SO4 (to convert the zinc present in Zinc Stearate to zinc sulphate). Then the solution is treated with ammonia – ammonium chloride buffer to adjust the pH = 10 an titrated with std EDTA using eriochrome black T as indicator at 40^oC.

(C₁₇ H₃₅ COO)₂ Zn + H₂SO₄ → ZnSO₄ + 2C₁₇H₃₅ COOH.
EDTA + Zn₂+ → (EPTA Zn complex)

Use

• Astringent and antimicrobial agent
• As a protectant in skin disorder.

Storage

It should be stored in a well-closed container.

TITANIUM DIOXIDE

M.F. TiO₂
It is an oxide of Titanium (Ti)

Properties

1. White powder, odourless and tasteless.
2. Insoluble in water and in dilute mineral acids.
3. It dissolves slowly in hot H2SO4 and ammonium sulphate forms double salt.

Assay: By Complexometric Method

Weighed sample is dissolved in a mixture of H2SO4 and ammonium sulphate. The solution is filtered and washed several times with water. Then 50 ml of EDTA is added and the pH is adjusted to 5 and the resulting solution is titrated with ZnCl2 (zinc chloride) using xylenol orange solution as indicator.

Use: Very good topical protective.

B. ASTRINGENTS

Astringents are the substances that precipitate the protein. This action may be on mucosal membrane when taken internally or on the skin for topical use.

Astringent when applied topically, cause precipitate of protein on the surface of cell by coagulation. The action depends upon extent of penetration of agent and the type of chemical action resultingwith protein.

It reduces the cell permeability and inhibits the transcapillary movement of plasma proteins, Which help to reduce local oedema, exudation and inflammation.

The protein precipitation caused by astringent due to the presence of metallic ions having large charge or strong electrostatic fields. The metal forms complex with various polar groups present on the protein

Astringent has a variety of uses which include:
1) To treat diarrhoea.
2) To arrest haemorrhage by coagulation of the blood.
3) To reduce inflammation of the mucous membrane.
4) To decrease sweating and to make skin tougher.
5) To promote the healing process.
6) Astringent also possess deodorant properties.

Alum

Formula KAl (SO₄)₂. 12H₂O. Alum is potassium aluminium sulphate. It is a double salt containing potassium and aluminium sulphate.

Preparation

It is prepared by adding a concentrated solution of potassium sulphate to a hot solution of an equal amount aluminium sulphate.

K₂SO₄ + Al₂ (SO₄)₃ + 24H₂O → 2KAl (SO₄)₂. 12H₂O

The solution is concentrated, cooled and the crystals are separated.

Properties

1. Colourless transparent crystals having a sweet astringent taste.
2. It is soluble in water but insoluble in alcohol.
3. At 200^oC it loses its water at crystallization and becomes anhydrous

Assay

Complexometric Titration Method

Weighed amount is dissolved in water, treated with known volume of Std EDTA (Aluminium forms complex with EDTA). The unreacted EDTA is back titrated with standard lead nitrate in presence of hexamine buffer using xylenol orange as indicator.

Use

1. Used in the preparation of toxoids.
2. Antiseptic and local styptic (to stop bleeding from cuts).

ZINC SULPHATE

M.F. ZnSO₄. 7H₂O

It is prepared by two methods.

1st Method
It is prepared by heating zinc sulphide in presence of air. The heated mass is dissolved in hot water, and filtered.
ZnS + 2O₂ → ZnSO₄

Offecial Method
It is obtained by digesting metallic zinc aranules in dil. H2SO4.
Zn + H₂SO₄ → ZnSO₄ + H2 (gas)

Properties

1. Granular crystalline powder.
2. It is odourless and metallic taste.
3. It effloresces in dry air.
4. It is very soluble in water and glycerine.
5. Insoluble in alcohol.
6. It forms double salts with potassium and ammonium sulphate.
   ZnSO₄ + (NH₄)₂ SO₄ → ZnSO₄ (NH4)₂. SO₄ 6H₂

Assay: Complex metric Titration Method

It is titrated with EDTA in presence of ammonia – ammonium chloride buffer using eriochrome black T as an indicator.

Use

1. Internally acts as an emetic.
2. Externally as an astringent.
3. 0.25% solution is used as eye lotion.