By: SADIA SHAKEEL

CONTENTS: Weighing Measurement of liquids Dissolution Filtration Mixing Size Reduction Size separation

OBJECTIVES: At the end of the lecture the students will be able to: Explain the techniques employed and things to be considered in Weighing, Measurement of liquids, Dissolution, Filtration,Mixing, Size Reduction, Size separation.

WEIGHING:

WEIGHING BALANCES: Class A : maximum weighing amount is 200g and minimum weighing amount is g Class B: maximum weighing amount is 50 g and minimum weighing amount is 0.2 g

Cont…………… MINIMUM WEIGHABLE AMOUNT: 50 mg for Class A 1OO mg for Class B APPROXIMATION: Should be avoidable. If not rounded to two places of decimal in grammes,e.g; g (15.67 g)

WEIGHING TECHNIQUES FOR THE DISPENSING BALANCE: Clean the balance and pans if necessary. Select a suitable weighing vessel or paper. Check that the pointer is at the null point but it is able to move freely. Place the required weights on the left-hand pan. Close the balance drawer. Collect the medicament from the shelf. Check the label word for w ord with the formula. Record the Batch number.

Cont……………. Hold the bottle in the left hand. Keep the label uppermost so that it is visible during weighing. Remove the lid or stopper. Use a spatula to transfer medicament to the right hand pan until the pointer returns to the null point. Close the stock container. Recheck the weights and the medicament against the formula.

Cont…… A check by a second person may be made before the stock bottle is returned to the shelf. Return the stock container to the shelf. Return the weights to the drawer and carefully clean the balance.

Things to be considered: Materials are not weighed directly onto the pans of electronic balance. Check the balance is at zero. Medicament is transferred to the pan until the required weight is read on the scale. Clean the balance carefully. Tare the balance before weighing the medicament.

MEASUREMENT:

MINIMUM MEASUREABLE AMOUNT: The recommended minimum measureable quantity for a small volume is 1 ml. A graduated pipette may be used to deliver volumes of 0.1 ml.

Advantages of conical flask: Easier to fill without spilling liquid on the side. Easier to drain. Easier to rinse out. Easier to clean after use. Can read the meniscus.

MEASURING TECHNIQUE FOR THE CONICAL MEASURES: Choose the smallest measure that will hold the required volume. Collect the medicament from the shelf. Hold the bottle in the right hand. Keep the label uppermost so that it is visible during measuring. Remove the lid or stopper. Carefully pour the liquid into the centre of the measure. Hold the measure at eye level.

Cont……… Align the top of the graduation line with the lowest part of the liquid surface, the true meniscus. Close the stock container. Recheck the medicament and the volume against the formula. A check by a second person may be made before the stock bottle is returned to the shelf. Return the stock container tidily to the shelf. Pour the liquid into the vessel in which the preparation is being made.

Things to be considered: Place the bottle on the bench with the label facing you. Use of pipette teat. When reading a scale on the side of an instrument filled up with liquid, the meniscus must be taken into account in order to obtain a precise measurement.

PARALLAX ERROR: The apparent change in position of an object when viewed from two different positions.

DISSOLUTION: The transfer of a molecule of a solute into solution in a solvent involves a change in the environments of both solute and solvent. The solute is separated from other similar molecules and becomes surrounded by solvent molecules. Similar is the case with the solvent molecules.

DISSOLUTION TECHNIQUE: Select a vessel to prepare in which to prepare the solution. Powder the medicament. Weigh the medicament. Agitate the solution. Heat the solution. Heating is appropriate for substances that are: more soluble at high temperature non heat labile non volatile

Cont….. Check that the solution is complete. Examine the solution critically for traces of undissolved solid. Restore the solution to room temperature (if necessary) Transfer to a measure and adjust to final volume.

Factors contributing in the increase of dissolution rate: Fine size of the powder Agitation Raising the temperature Special technique for volatile substances: Volatile solvents Volatile solutes

FILTRATION: It may be defined as the separation of a solid from a fluid by means of a porous medium that retain the solid but allow the fluid to pass through.

Factors contributing in the increase of filtration rate : Increasing the pressure differences across the filter. Reducing the viscosity of the liquid. Increasing the surface area of the filter. Using a filtration medium of high porosity.

STRAINING: Coarse filtration is used to remove large foreign particles from: Preparations in which traces of fine particles are tolerable. Heterogeneous systems. Viscous preparations.

FINE FILTRATION: For oral solutions or solutions that are applied to mucous membrane or broken skin a high degree of clarification is necessary. This is achieved with filter papers or sintered glass filter.

CHARACTERISTICS OF WHATMAN SERIES FILTER PAPERS

SINTERED GLASS FILTERS: The most suitable grades are numbers are: 3 for filtration of small volume by gravity 4 for filtration of large or small volumes by filter or pressure These are useful for substances that attack filter paper like potassium permanganate ( oxidizes) and zinc chloride ( dissolves)

MIXING: An operation in which two or more components in a separate condition are treated so that each particle lies as nearly as possible in contact with a particle of each of the other ingredients. It is probably the most widely performed unit operation in pharmaceutical manufacturing.

Cont…… Mixtures of liquids Mixtures of liquids and solids Mixtures of solids Mixtures containing semi solid

SIZE REDUCTION: Reduction of a materials to smaller pieces,to coarse partticles or to powders.

Advantages of using fine powders: They mix more uniformly and suspend more easily. They dissolve more quickly. They are absorbed more readily from the GIT. They yield preparations that are relatively free from grittiness.

Cont……….. DRY GRINDING: Compression, Attrition WET GRINDING: Levigation

SIZE SEPARATION:

CONT…… On a small scale done by sifting. Generally wire mesh are used, the most suitable grades being those with mesh diameter 180 µm (for dusting powders) and 250 µm for external preparations. Sifting tends to separate powders of different densities.

THANX FOR ATTENTION REFERENCES: Pharmaceutical practice DM Collett Pg #