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PCT 411 > Micromeritics > Flashcards

Micromeritics Flashcards

1
Q

What is Micromeritics?

A

It is the science and technology of small particles as given by scientist Dalla Calle 1948.
It entails knowledge and control of size and size range of particles

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2
Q

What are the units of measurement?

A

Micron(micrometer) 10^-6m 10^-4cm 10^-3mm.

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3
Q

When particle size decreases _____

A

Surface area increases

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4
Q

Knowledge and control of the size and size range of particles are significant in pharmacy because :

A

It is related to the physical, chemical and pharmacological properties of a drug

It affects drug release administered orally, rectally, topically and parenterally

It is essential in achieving the flow properties as well as mixing of granules and powders during tablet or capsule manufacture

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5
Q

What are the Applications of Micromeritics?

A

Physical stability of formulations : suspensions, emulsions

Absorption and therapeutic activity : eg griseofulvin using too small a particle size would increase blood concentration of the drug which can have adverse effects

Release and dissolution: tablets, capsules
solid and

Dose uniformity : solid, semisolid and liquid preparations

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6
Q

Pharmaceutical powders do not contain spherical particles but are collection of particles of different shapes and size also known as

A

Heterogeneous polydispersed system

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7
Q

There is need to determine the powder fundamental attributes and composition in terms of___

A

particle size, particle size distribution , particle shape and surface area

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8
Q

Fundamental properties of a powder includes?

A

Particle size, shape and surface

Particle size distribution /range - particle size range (number or weight) = Total surface area

Size of sphere is related to its diameter but as particle asymmetry increase, expression of it’s diameter becomes a challenge.

Involves sedimentation rate that relates to stability of the formulation.

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9
Q

What’s the Equivalent Spherical Diameter?

A

This concept has been adopted because of the difficulty to express the particle size in terms of a meaningful diameter.

It relates the size of particle in question to the diameter of a sphere having respectively the same surface area, volume or stokes diameter (ds dv/dp dst)

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10
Q

ds dv/dp dst are also known as?

A

Projected diameters

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11
Q

What is ds?

A

Surface diameter
It is the diameter of a sphere having the same surface area as the particle in question.

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12
Q

What is dv?

A

Volume diameter is the diameter of a sphere having the same volume diameter as the particle in question

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13
Q

What is dp?

A

Projected particle diameter
The diameter of a sphere having the same
particle when viewed normal to its most stable plane.

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14
Q

What is dst?

A

The diameter of an equivalent sphere undergoing sedimentation at the same rate as the assymmetric particle.

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15
Q

Important parameters for consideration during powder analysis:

A

Average particle size of the powder sample

The number or weight of each fraction of each particle size (particle size distribution) = Particle size range

Surface area of the powder sample eg activated charcoal used for antidote to food poisoning is required to be finely powdered for maximum adsorption as well as therapeutic value.
Reduced particle size increases surface area

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16
Q

How do you obtain a frequency distribution curve?

A

When the number or weight of particles lying within a certain size range is plotted against the size range or mean particle size.

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17
Q

What’s the significance of the frequency distribution curve?

A

It helps to differentiate two or more powder batches that have the same average diameter

It shows the particle size that occurs most frequently within the sample (mode).

Can also plot cumulative % over or under a particular size versus particle size

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18
Q

List 3 Other useful particle size plots

A

Particle Size Distribution : Log normal distribution curve

Particle size Distribution : Log probability curve

Size Number (N) of particles per unit weight

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19
Q

List the most common methods for particle size analysis

A

Microscopy (optical microscopy)
Sieving
Sedimentation (Andreasen Pipette method)

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20
Q

Other methods of particle size analysis includes:

A

Coulter Counter - Volume displacement by particle

Beckman Coulter-submicron particles (Brownian motion of particles)

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21
Q

Describe the microscopy method of particle size analysis.

A

Emulsion or suspension (diluted or undiluted) used is mounted on a slide.

The eye piece micrometer is used to estimate the size.

The particle field can be projected on a screen for ease of reading

Can also use electronic scanners to measure particle size

22
Q

Advantage of microscopic method of particle size analysis.

A

Helps to detect the presence of agglomerates and particles of more than one component

23
Q

Disadvantages of microscopic method of particle size analysis.

A

Diameter obtained from only two (length and breadth) particle dimensions. The depth (thickness) cannot be estimated

It is slow and tedious because it entails the counting of 300 to 500 particles to obtain a good estimate of the distribution

24
Q

Describe the sieving method of particle size analysis.

A

It is a screening process that separates a powder batch into different particle sizes

Mostly applicable for coarser particles, can be used for fine (44micrometer) particles.
Particles are passed through mechanical agitation over a stack of sieves arranged from top to bottom in descending order of mesh sieves with the receptor at the base

Powder to be sieved is placed on topmost sieve , covered and shaken for a predetermined time.

Powder retained on each sieve is weighed.
A plot of cumulative % by weight of powder retained on the sieves on the probability scale against the log of the arithmetic mean mesh size of each of two successive screen gives the geometric mean weight diameter (dg)

The geometric standard deviation can be obtained from the straight line.

25
Q

Advantage of sieving method of powder analysis

A

Faster and less expensive method for particle size analysis

26
Q

Disadvantage of sieving method of powder analysis

A

Sieve loading, duration and intensity of agitation affect reproducibility of result

27
Q

Three commonly used sedimentation methods includes:

A

Pipette method (Andreasen Apparatus)
Characterized with ease of analysis ;accuracy and economy of equipment used.

Balance method

Hydrometer method

28
Q

Describe the pipette method(Andreasen apparatus) of powder analysis

A

Andreasen Apparatus- 550 mL vessel containing 10 mL pipette sealed into a ground - glass vessel stopper; pipette’s lower tip within the vessel is 20 cm below the surface of the suspension

Analysis undertaken using between 1-2% suspension of the particles in a medium containing a suitable deflocculating agent brought to the 550 mL mark.

Stoppered vessel is shaken to distribute the particles uniformly in the medium; pipette position is ensured by clamping securely and placed in a constant temperature bath
At 10 min intervals,. 10 mL samples are withdrawn and discharged through the 2 way stop clock

Samples are evaporated, weighed and appropriately analyzed (with correction for the deflocculating agent).

The particle diameter corresponding to the various time periods is obtained using the Stoke’s equations

h’ in the equation= height of the liquid above the lower end of the pipette at the time each sample is removed

The residue/dried sample obtained at time “t”= weight fraction having particles of sizes less than the size obtained by the law calculated for that period of settling (weight undersize)
Successive weights gives cumulative weight undersize ( expressed as weight unit or % total of final sediment)
Data obtained useful for frequency distribution or cumulative frequency distribution of powder sample.
A plot of cumulative % (by weight) undersize on a probability scale vs the particle diameter on a log scale gives the statiscal diameter

29
Q

What assumptions guide the validity of using stokes equation for sedimentation method?

A

For spheres falling freely without hindrance at constant rate

Asymmetric particles with the assumption that the diameter obtained is a relative particle size falling at the same velocity as that of the particles being considered

Deflocculating agent needed to keep particles free and separate as they fall through the medium

Laminar flow of the dispersion medium around the settling particle

30
Q

Describe the Volume displacement method(Coulter counter).

A

Coulter Counter-Operates on the principle that when a particle suspended in a circulating liquid displaces a known volume of the liquid when it is passed through a small orifice.

Suspension must be sufficiently dilute so that the particles pass through essentially one at a time.

Displaced volume causes a change in electrical resistance which is registered by a pulse height analyzer calibrated in terms of particle size.

Instrument electronically records the pulses of particles within two threshold values of the analyzer (upper and lower limits).
Variation of threshold settings and counting of particles in a constant sample size, a particle size distribution can be obtained.

Can count particles at the rate of 4000 per sec; hence useful for both gross counts and particle size distribution within a relatively short time.

31
Q

Describe the Photon correlation method (Beckmann Coulter)

A

Beckman Coulter Uses photon correlation spectroscopy that senses the brownian motion of the particles in a suspension.

The smaller the particles, the faster the brownian motion

Laser beam passes through the sample and sensor detects the light scattered by the particles undergoing the Brownian motion

Useful for submicron particles ranging (0.0033 - 0.3μm)

Particle size and size distribution, molecular weight and diffusion coefficient can be determined

Applicable in sub micron analysis as found in micro emulsions, pigments, dyes, colloids, micelles, solubilized systems, liposomes
and microparticles

32
Q

Describe the surface area of a powder

A

Surface area (SA) per unit weight or volume of powder is important in formulation studies.
Specific surface area (SSA) is the surface area per unit volume (Sv) or unit weight (Sw).

SA/SSA determinations of a powder batch is important in surface adsorption or/and dissolution studies.

It is affected by particle shape.
A powder batch with asymmetric particles will have larger SA/SSA than one composed to spherical particles.

Thus the more asymmetric the particles in a powder batch is, the larger the SA /SSA

33
Q

List the two methods for determining surface area of powder particles

A

Surface area can be determined from the knowledge of particle size distribution

Two methods are used
1. Air Adsorption method
2. Air Permeability method

34
Q

Describe the Air adsorption method

A

It is based on the principle that the amount of gas or liquid solute adsorbed to form a mono layer is a direct function of the surface area of the sample

At high pressures adsorption is multilayer

A plot of volume of gas adsorbed per gram of adsorbent vs the pressure of gas at constant temperatures

The volume is calculated by the Brauner, Emmett and Tella equation
P / V(Po - P) = 1/ Vmb + (b - 1)P/VmbPo

In the BET equation,
V m = volume of nitrogen gas in cm3
that 1 g of the powder can absorb when the monolayer is complete

V= volume of gas in cm3 adsorbed per gram of powder at pressure P

P= Saturation vapour pressure of liquefied nitrogen at experimental temperature

b= constant which expresses the difference between the heat of adsorption and heat of liquefaction of the adsorbate (nitrogen)

35
Q

Describe Air Permeability method

A

Air permeability method
Is based on the fact that the rate at which a gas or liquid permeates a bed of powder is related to the
surface area(SA) exposed to the permeant
The resistant to the flow of a liquid such as air through a plug of compacted powder is the surface area of the powder
The greater the SA per gram of powder, the greater the resistance to flow
Permeability is inversely proportional to specific surface area

It involves simple instrumentation
The speed of determination of SA using this method is fast
Thus widely used in pharmaceutical systems especially in batch to batch variation analysis

36
Q

List 3 derived properties of powder

A

Porosity, Packing arrangement and dissolution/dissolution rate

37
Q

What is vb?

A

the volume occupied by a quantity of powder is bulk volume vb

38
Q

Vp is?

A

True volume of the particles
Vo = Vp + V (void volume or volume of spaces between particles)

39
Q

What is Powder Porosity?

A

Porosity or Voids (€) of a powder, expressed as a %; is defined as the ratio of the void volume to the bulk volume of the packing.

€ = 1 - Vp

40
Q

Hypotheticcase:Apowderwith thefollowing
Powder True density =3.203
Powder weight = 131.3g
Bulk volume = 82.0 cm
Calculation of porosity will be :

A

V=M/D =41.0cm
Void volume = Vb - Vp
= 82.0 cm-41.0 cm
Space Porosity = 0.5 or 50%

41
Q

Hypotheticcase:Apowderwith thefollowing
Powder True density =3.203
Powder weight = 131.3g
Bulk volume = 82.0 cm
Calculation of porosity will be :

A

V=M/D =41.0cm
Void volume = Vb - Vp
= 82.0 cm-41.0 cm
Space Porosity = 0.5 or 50%

42
Q

What are the 2 ideal particle packing arrangement types?

A

Rhombohedral (closest) = € (26%)

Cubic ( most open, loosest) = € (48%)

The particles in a powder system are neither spherical in shape nor uniform in size, their packing arrangement are intermediate between these two ideal packagings

43
Q

Most powders in practice have porosities between ___ except if the particles are of greatly different sizes, the smaller particles to be stacked in between the larger ones, such that the porosity falls below the theoretical___

Powders that contain flocculates or aggregates or bridges and arches forms, result in higher value of the theoretical porosity of____

A
  1. 30 - 50%
  2. 26 %
  3. (above 48%).
44
Q

Describe particle density as a derived property of powders

A

Particle densities must be expressed with great care because they may be hard and smooth or rough and spongy
Though density is weight per unit volume, however for particles with internal pores and microscopic cracks, these issues need to be considered when particle densities is considered
Thus, three types of densities are :True (P),Granule(dg), Bulk(db)

True density excludes the voids and intra particle pores larger than molecular or atomic dimensions in the crystal lattices

dg =Determined by the mercury displacement. It does not penetrate at ordinary pressures into pores smaller than about 10 μm
db= Determined from bulk powder and weight of a dry powder

45
Q

Differentiate between the density of non porous and porous powder systems

A

For a non porous powder system, true and granule densities are identical. Are obtained by the displacement of either helium, benzene, mercury or water

For porous material, density is determined by displacement of helium which penetrates into the smallest pores and is not by the material.

Density obtained by displacement method is approximately equal to true density. May differ from this if the liquid does not penetrate the pores.

46
Q

____ is useful for Density measurements, Pressure recorded on a____

Equipment “dead space” is determined by___

A
  1. Densitometer
  2. mercury manometer
  3. Application of Helium
47
Q

Powder sample is degassed before introduction into the tube of the equipment .
Helium is applied and the volume of same penetrating into the small cracks , crevices and pores is calculated.
The difference between the volume of helium filling the empty apparatus and the volume of helium in the presence of powder sample is______
True density is determined if the powder weight is known
Granule density determined similarly but mercury is used because______

A
  1. equivalent to the powder volume.
  2. it fills the void spaces and not the particles internal pores
48
Q

Granule volume is determined from the _____

A

Intra particulate spaces

49
Q

Describe the bulk density

A

It is ratio of powder weight to the bulk volume.
➢It is dependent primarily on particle size, size distribution,
shape and the interparticulate forces.
➢Powder systems with loosely packed particles have low bulk density and for tightly packed particles, the bulk density will be high
➢Bulkiness or specific of bulk volume : This is the reciprocal of bulk density specific bulk volume.
➢It increases with a decrease in particle size
➢It is an important criteria in packaging of powder

50
Q

Extensively describe powder follow properties

A

A bulk powder behaves like a Non-Newtonian liquid as it exhibits plastic flow and occasionally dilantancy
➢ The interpaticulate forces in a powder system contributes among other factors to its intrinsic flow property
➢ Other powder flow determinants are particle size, shape, surface texture, density, porosity and moisture.
➢ Powders should be dried to reduce moisture cohesiveness
➢ Powder flow is affected by the quantity of fines . Too small or high amount of fines in a powder system will negatively affect powder flow
➢ An optimum amount is required for good powder flow

PCT 411 (1 decks)

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