Notes on the Classification of Powders – Dispensing Pharmacy

Powders are subdivided solids which are classified in the BP according to the size of their constituent particles of range from 1.25 (µm to 1.7 mm in diameter. Another classification of powders is based on the manner of their dispensing.

1. Bulk powders for external use:

(a) Dusting powders (b) Snuffs (c) Dental powder (d) Insufflations

2. Bulk powders for internal use.

3. Simple and compound powders for internal use.

4. Effervescent granules

5. Cachets

1. Bulk powders for external use

External bulk powders contain non-potent substances for external applications. These powders are dispensed in glass, plastic wide mouth bottles and also in cardboard with specific method of application. Bulk powders for external used are of four types.

(a) Dusting powders (b) Snuffs (c) Douche powders (d) Dental powder (e) Insufflation

(a) Dusting powders

These are used externally for local application not intended for systemic action. The desired characteristics of powders include- (a) homogeneity, (b) non-irritability, (c) free flow, (d) good spreadability and covering capability, (e) adsorption and absorption capacity, (f) very fine state of subdivision, and (g) capacity to protect the skin against irritation caused by friction, moisture or chemical irritants.

Dusting powders usually contain substances such as zinc oxide, starch and boric acid or natural mineral substances such as kaolin or talc.

Talc may be contaminated with pathogenic microorganisms such as – Clostridium tetani etc., and hence it should be sterilized by dry heat. Dusting powders should not be applied to broken skin. If desired, powders should be micronised or passed through a sieve # 80 or 100. Dusting powders should preferably be dispensed in sifter-top containers. Such containers provide the protection from air, moisture and contamination as well as convenience of application. Currently some foot powders and talcum powders have been marketed as pressure aerosols.

Dusting powders are employed chiefly as lubricants, protectives, absorbents, antiseptics, antipruritics, astringents and antiperspirants. Zinc oxide 20 parts

Salicylic acid 2 parts

Starch powder 78 parts

(b) Snuffs

These are finely divided solid dosage forms of medicaments dispensed in flat metal boxes with hinged lid. These powders are inhaled into nostrils for decongestion, antiseptic, and bronchodilator action.

(c) Douche powder

These powders are intended to be used as antiseptics or cleansing agents for a body cavity; most commonly for vaginal use although they may be formulated for nasal, otic or ophthalmic use also. As douche powder formulation often include aromatic oils, it becomes necessary to pass them through a # 40 or 60 sieve to eliminate agglomeration and to ensure complete mixing. They can be dispensed either in wide mouth glass bottles or in powder boxes but the former are preferred because of protection afforded against air and moisture.

Zinc sulphate

Magnesium sulphate

Boric acid

Oil of lemon

Water

(d) Dental powders

Dental powders are rarely prescribed. However this class of powders is interesting from the compounding angle. This preparation is a type of dentifrice meant for cleaning the teeth.

As such, dental powders contain detergents, abrasives, antiseptics and colouring and flavouring agents incorporated in a suitable base. Generally the base is calcium carbonate. The detergent is in the form of soap and the abrasive action is provided by finely powdered pumice stone.

Essential oils are added to provide flavour and freshness to the mouth as well as antiseptic action. Essential oils, if present in smaller quantity, are easily absorbed by calcium carbonate and pumice. This makes the uniform distribution of the oil difficult. Best results are obtained if the oil is triturated in the solids taking considerable care to distribute it uniformly.

(e) Insufflation

Insufflations are a class of powders meant for application to the body cavities e.g., ear, nose, vagina etc. The powder has to be extremely fine and must find an entry to the cavity deep enough to bring about its action at the site. It is delivered to the affected part in a stream with the help of a device called an insufflator, which blows the powder to the site.

Some of the insufflations contain volatile liquid ingredients which may require uniform distribution in the powder. If these liquid ingredients are present in large quantity, the liquid portion may have to be evaporated. Generally evaporation is brought about slowly in a china dish which is heated on a water bath. The resultant product is re-powdered and sifted through a sieve of a suitable size.

However, active volatile liquids present in small portions should not be removed by evaporation but only incorporated by trituration in the powder.

The pharmaceutical industry packages the insufflations in pressurized form i.e., aerosols. Aerosols contain the medication in a stout container with a suitable valve, the delivery of the powder being accomplished by a liquefied or compressed gas propellant of very low boiling point. On pressing the actuator of the valve the propellant delivers the medication in a stream.

2. Bulk powders for internal use

Bulk powders contain many doses in a wide-mouth container that is suitable to remove the powder by a teaspoon. The non-potent substances are used in bulk powder form such as antacid, laxative, purgative, etc.

Rhubarb powder

Light magnesium carbonate

Heavy magnesium carbonate

Ginger powder

Make a powder.

3. Simple and compound powders for internal use.

These are unit dose powders normally packed in properly folded papers and dispensed in envelopes, metal foil, small heat-sealed plastic bags or other containers.

Usually for the preparation of simple powders, the ingredients are weighed correctly and blended by geometrical mixing in ascending order of weights. The mixture is then either divided into blocks of equal size, numbers of blocks representing the number of powders to be dispensed or each dose is weighed separately and placed on a powder paper. The paper is then folded according to the pharmaceutical art and placed in either an envelope or a powder box.

4. Effervescent granules

This class of preparations can be supplied either by compounding the ingredients as granules or dispensed in the form of salts. The ingredients whether in granular form or present as salts, react in presence of water evolving carbon dioxide gas.

For evolution of the gas two constituents are essential, a soluble carbonate such as sodium bicarbonate and an organic acid such as citric or tartaric acid. The preparation can be supplied either as a bulk powder or distributed in individual powders.

There are three alternative methods of dispensing depending upon the nature of prescription.

(i) If the effervescent salts are prescribed to be the dispensed in bulk form, no granulation is necessary. The ingredients are mixed uniformly and directions stated on the label to add the prescribed quantity to water, before use.

(ii) If the effervescent salt is prescribed in divided doses, the ingredients which cause effervescence on mixing with water are enclosed separately in papers of different colour. The patient is advised to take one powder of each colour and add to water, before use. Quantities of the sodium bicarbonate and the organic acid, citric or tartaric, are equimolecular in proportion.

(iii) In the third case the product contains all the ingredients mixed together in a granular form. Preparation of granular products requires pharmaceutical technique. If sodium bicarbonate and citric acid are taken in equimolecular proportion and mixed to make granules, the quantity of water of crystallization liberated from the citric acid is large enough to make the mass wet and carbon dioxide may be liberated during the preparation itself. If one tries to substitute citric acid by tartaric acid, which contains no water of crystallization; it may not be possible to form a mass necessary for granulation.

Therefore both citric and tartaric acids are taken in suitable proportions leaving a little acid in surplus than the quantity required to neutralize sodium bicarbonate. This surplus is necessary to give the final preparation an acidic taste that is more palatable. There is a certain loss in weight of such a preparation due to the loss of water in drying the granules and partial loss of carbon dioxide due to its release during preparation.

Heating is done on a water bath keeping all the ingredients thoroughly mixed in a porcelain dish. Gentle application of heat liberates the water of crystallization from citric acid and the mass tends to be coherent.

Prolonged heating may result in complete evaporation of the released water leaving the product in the form of a dry lump which can not be rendered into granules. The coherent mass is transferred from the porcelain dish to an inverted sieve of suitable aperture size kept over a glazed paper.

The mass is pressed through the sieve taking care to change the position of the sieve over the paper to prevent the formation of a lump of the sieved granules. The granules are dried in an oven taking care to regulate the temperature which should be generally kept below 80°C.

The operation requires considerable skill and experience to obtain granules of uniform size and an elegant product. If necessary, the dry granules are passed through a sieve of appropriate size to break larger granules which result due to sticking of the sieved wet granules^–.

The water of crystallization of the citric acid and the water from the reactions make the material coherent. Loss of weight occurs during granulation due to (a) evaporation from the damp mixture, and (b) loss of carbon dioxide. The losses constitute approximately one-seventh of the weight of powder used and must be allowed for when calculating the amount to be prepared.

Chemical reaction

3 NaHC0₃ + C₆H₈0₇.H₂0 = C₆H₅Na₃0₇ + 3 C0₂ + 3 H₂0 (Sodium bicarbonate) (Citric acid)

2 NaHC0₃ + C₄H₆0₆ – C₄H₄Na₂06 + 2 C0₂ + 2 H₂0 (Sodium bicarbonate) (Tartaric acid)

5. Cachets

Cachet as a unit dosage form was very popular sometime back. Presently cachets are seldom used and have been replaced by capsules. Cachets, like capsules, can be easily filled and sealed at the dispensing counter.

This dosage form holds larger quantity of the medication as compared to capsules. Since the cachets are made of flour and water they are easily damaged in handling. Further this dosage form offers little protection against light and moisture.

Due to its size and shape a cachet is difficult to swallow. The process of filling is similar to that of capsules. The drug is placed in one of the two halves of the cachet, the upper half is then placed over it and pressed with the help of a suitable device.

The flange of the upper plate is moistened carefully taking care not to wet it, with the help of a dampener. The sealing takes place due to the moisture between the flanges of the upper and the lower half and the pressure over the flanges. About 15 minutes are allowed for drying of the seal.

After this time the middle portion of the cachet is slightly pressed to ensure complete sealing. In absence of a machine a pharmacist can improvise and use two bottles the mouths of which are broad enough so that flanges of the plates – upper and lower, when kept over the mouths of the bottles, just rest over them. The drug is transferred to one of the plates resting over the mouth of the bottle kept vertically on the working bench. The flange of the empty half resting over the mouth of another bottle is moistened with the help of a damp camel hair brush.

The empty half of the cachet is then placed over the other half in which the medication is kept so that the flanges of the two halves are perfectly superimposed. The second bottle is then inverted and brought over the superimposed cachet and carefully put over the flange and pressed in position without disturbing the resting place of the cachet.

This provides a good seal. Cachets can be dry-sealed also. These cachets however are of a different shape where the cap is pressed over the body of the cachet. A protruded stud is also provided to hold the upper and lower halves together.

Like capsules, cachets are also expected to remain untouched by hand and one should use gloves while handling them. Since there are inherent losses of the drug in this operation also like that of powders and capsules, the quantities of each ingredient should be weighed for an extra powder over the number to be dispensed. The cachets are dispensed in wide-mouthed bottles of glass or plastic with a perfectly fitting cap. The patient should be instructed to keep the bottle securely closed.