Micromeretics and powder rheology: Derived properties of powders – Densities, bulkiness and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam

Density (Ƿ): Density is defined as weight per unit volume. Based on the types of volume defined, the corresponding ‘density’ may be proposed.

Table 1 – Types of densities

DENSITY DETERMINATION METHODS:

Bulk Density: Bulk density is determined by measuring the volume of the known mass of powder that has been passed through a screen into a graduated cylinder (Method I) or through a volume measuring apparatus into a cup (Method II).

Method I—Graduated cylinder method

Approximately 50 g of powder sample (M), previously passed through sieve no. 18 to break up agglomerates that may have formed during storage, is introduced into a 100-mL graduated cylinder without compacting. The apparent volume (Vb) is then read to the nearest graduated unit. The bulk density is calculated in g/cm3 by the formula

M / V_b

Method II—Scott volumeter

A known volume of a powder is allowed to flow through the scott volumeter apparatus into the sample receiving cup and the weight of the powder is determined. The bulk density is calculated in g/cm3 as described above.

Tapped Density: Tapped density is determined by mechanically tapping a graduated cylinder containing a powder sample.

Method

Approximately 50 g of powder sample (M), previously passed through sieve no. 18, is introduced into a 100-mL graduated cylinder without compacting. After observing the apparent volume(Vb), the cylinder is mechanically tapped by raising the cylinder and allowing it to drop under its own weight using a mechanical tapped density tester that provides a fixed drop of 142 mm at a nominal rate of 300 drops per minute. The cylinder is tapped 500 times initially and the tapped volume is measured (Vx). The tapping is repeated an additional 750 times and the tapped volume (Vy) is measured. If the difference between the two volumes is less than 2%, Vy is the final tapped volume. If the difference between the two volumes is more than 2%, then an additional 1250 tappings in increments is recommended until the difference between successive measurements is less than 2%. The tapped density is calculated in g/cm3 by the following formula:

M / V_y

True Density:

Gas pycnometer

Gas pycnometer is a device used to measure the density, or more accurately the volume, of the powder. In a gas pycnometer, the volume occupied by a known mass of powder is determined by measuring the volume of gas displaced by the powder. A gas pycnometer is also sometimes referred to as a helium pycnometer. Helium is a nonadsorbing gas that penetrates the smallest pores and crevices and is useful in determining true density, particularly of porous solids.

The working equation for a gas pycnometer is

V_t = V_c (U₁ – U_s / U₁ – U₂)

Where, The reading on the scale at this stage denotes Us. The difference between U1 and Us gives the volume occupied by the powder sample.

Liquid displacement

The pycnometer can be used to determine the density of a solid object using liquid in which the solid does not dissolve. It uses a working liquid with well-known density, such as benzene, ethyl alcohol and water, for estimating true density of nonporous solid particles since these liquids cannot efficiently penetrate the smallest pores and crevices of a porous material.

The density of the measured solid object is calculated as

Ƿ_s = M_s / V_s

Where, M_s is mass of powder sample and V_s is volume of measured solid object.

Granular Density:

Mercury displacement method or mercury porosimetry

The volume of granules can be measured by the mercury displacement method. The method is similar to the liquid displacement method, but here instead of a working liquid, mercury is selected as a solvent. Mercury is suitable to determine granular density because it fills the interparticulate voids but fails to penetrate intraparticulate spaces due to its large size, which is of the order 10 μm.

Mercury porosimetry involves the intrusion of mercury, a nonwetting liquid, at high pressure into a material. The pore size can be determined based on the external pressure needed to force the liquid into a pore against the opposing force of the liquid’s surface tension.

Bulkiness:

The reciprocal of bulk density is often called bulk or bulkiness. It is an important consideration in the packaging of the solid powders. For example, the bulk densities of calcium carbonate vary from 0.1 to 1.3, and the lightest or bulkiest type would require a container about 13 times larger than that needed for the heaviest type. A decrease in particle size increases the bulkiness.

Multiple choice questions (MCQs)

1.If hausner ratio of granule is in the range of 1.26-1.34 indicates ____ type of flow

a)Excellent

b)Good

c)Fair

d)Passable

2._____ technique is utilized for estimation of surface diameter

a)Air permeability

b)Vacuum

c)Filtration

d)Weighing

3.The angle of repose values are utilized in

a)Tablet manufacturing

b)Opthalmic products

c)Biphasic liquids

d)None of the above

4.____ = 100 X (tapped density – bulk density) / tapped density

a)Porosity

b)Hausner ratio

c)Compressibility index

d)None of the above

5.Flow property of powders can be enhanced by

a)Altering particle size

b)Altering particle shape

c)Altering surface area

d)All of the above

6.Particle size can be reduced by

a)Mortar and pestle

b)Sieve

c)Grinder

d)Both a and c

7.Method for determining surface area

a)Absorption method

b)Adsorption method

c)Air permeability method

d)Both b and c

8.The specific surface is

a)Surface area per unit volume

b)Surface area per unit weight

c)Both a and b

d)None of these

9.Sieving errors can arise due to

a)Loading of sieves

b)Duration of sieving

c)Intensity of agitation

d)All of the above

10.A sphere has ____ surface area

a)Maximum

b)Minimum

c)Normal

d)None of the above

11.The adsorbed layer in adsorption method is monomolecular at

a)Low pressure

b)High pressure

c)Very low pressure

d)Very high pressure

12.In sieve shaker, the particles are separated on the basis of

a)Particle shape

b)Particle size

c)Particle volume

d)Particle density

13.Sieve number indicates the number of meshes in

a)2.54mm

b)25.4mm

c)254mm

d)0.254mm

14.As per IP 1996 fine powder should pass through sieve number

a)10

b)22

c)44

d)85

15.Diameter represented by the length of line that bisects particle image is known as

a)Martin diameter

b)Feret’s diameter

c)Projected diameter

d)Stokes diameter

Solutions:

1. d) Passable
2. a) Air permeability
3. a) Tablet manufacturing
4. c) Compressibility index
5. d) All of the above
6. d) Both a and c
7. d) Both b and c
8. c) Both a and b
9. d) All of the above
10. b) Minimum
11. a) Low pressure
12. b) Particle size
13. b) 4mm
14. d) 85
15. a) Martin diameter

References:

1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 52-56.

2. Martins Physical Pharmacy, 6th edition 2011, page no. 835-842.

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