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29 Cards in this Set
- Front
- Back
Powders |
are solid particles (generally heterogenous systems) They are widely used in pharmaceuticals: 1) as powders 2) as intermediates leading to other dosage forms |
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clarification |
process aiming at removing or separating a solid from a fluid -remove any unwanted particles from a liquid product -to collect the solid as the product itself |
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Mechanisms of filtration |
-straining/sieving -impingement -attractive forces -auto filtration |
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Key parameters of filtering elements |
-capacity -efficiency -degree of filtration -porosity |
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Filtration: Buchner funnel |
image |
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Rateof filtration: the Darcy’s equation |
V/t = K A ^ P / u L V/t - rate of filtration K-proportionality constant A-area ^P - pressure difference u- viscosity L - thickness |
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Waysto increase the filtration rate |
• Increase the area • Increase ΔP • Decrease the viscosity ofthe liquid • Decrease the thickness of thefilter cake • Increase the permeability of thecake |
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Centrifugation |
Particleof mass m Spinningin a centrifuge of radius r Ata velocity v Thecentrifugal force F = mv2/r C=F/G |
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Milling |
is a mechanical method that gives you a reduction in particle size |
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Cutter mill |
the material is cut by one or more blades Feed -> stationary knives outside -> rotating knives inside -> screen -> product Screen has pores that only allow certain size material through Good for fibrous material |
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Compressionmethods compression mills |
A pressure is applied e.g. roller mills, mortar pestle 1 roll is driven directly 1 roll rotates as consequence of the friction Good for crystalline material |
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Impactmethods |
Principle a) Particles are hit by a moving surface b) moving particles hit a surface |
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Hammer mill |
Same as cutter mill but instead of blades, it has hammers Feed -> stationary hammers outside -> rotating hammers inside -> screen -> product |
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Attrition methods |
Principle: Pressure and friction |
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Attrition methods Roller mills |
The rollers rotate at different speeds Each cylinder is attached to anengine and rotate independently of the others, all at different speeds. Semi solid material e.g. ointment |
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CombinedImpact and Attrition methods: Ball mill |
Chamber filled with balls, then addpowders that you want to reduces in size. The particles get hit by the ball andbroken by attrition,2 factors that affect theefficiency1.Amountof material – too much material the probability of the material hitting theballs decreases, it will hit itself (cushioning effect). Too little the ballswill hit themselves and premature wear of the equipment.2.Speed– the ball are subjected to gravity. As the mill pushes them up, gravity pushesthem down |
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Combined Impact and Attrition methods: Ball mill (2) |
Centrifugaleffect push the balls outwards Lowspeed the balls will be at the bottom of the mill (not desirable) Highspeed (above critical angular velocity- the speed at which the centrfugalforces over come gravity) ball on the outside of the mill (not desirable) Thebest milling speed is a 2/3’s of the angular forces Fluidenergy mill is a solid sample is inserted under high pressure into a liquid |
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Combined Impact and Attrition methods: Fluid energy mill |
Solid inlet -> fluid inlet jets -> centrifuging action throws coarser particles outwards -> classifier remove fine particles and fluid |
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Selectionof the appropriate type of mill -Look at the size that you want toobtain -Look at characteristics ofthe material |
-particle size to obtain -characteristics of the material: Cutter mills -> elastic, fibrous materials Attrition methods -> ointments, solid in suspensions and pastes Impact methods -> brittle materials -other factors (cost, time, stability of the ingredients) |
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Size analysis Projected area diameter (da) Projected perimeter diameter (dp) |
We approximate that particles havea diameter equivalent to a sphere of a for a different material We draw a circle that has an areathat is the same at the particle Wedraw a circle that has the same perimeter as the particle and measure thediameter |
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Statisticaldiameters: Feret’s (dF) Martin’s (dM) |
Feret’s –first you chose a direction and then draw to tangent that are parallel to thechosen direction, and then measure the distance between the two particles. Thenrepeated but chose a different direction. You repeat this a few time andcalculate the average Martin’s – dived the particle intotwo identical ideas. You repeat many times but with the particle orientateddifferently and then calculate the average. |
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Methods to measure the particle size |
Direct methods: 1) sieving 2) microscopy Indirect methods: they determine a parameter correlated with size examples: - sedimentation rate -permeability |
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Sieve methods |
Sievesare classified based on the sieve aperture diameter (expressed in µm). |
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Sieveaperture diameter |
distancebetween two consecutive wires |
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light microscopy |
size range 1-1000 um Adrop of a very dilutedsuspensionis placed on a microscope slide and particles are measured in relation to the equivalent diameter chosen (dp, da, dF or dM).Theimages are 2D |
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Electron microscopy |
Size range (as low as 0.001um) Scanningelectron microscopy (SEM): Advantages-3D images, gives information on shape Disadvantages– expensive, high level of operator expertise |
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Coulter counter |
Measure the volume of particles - 0.1-1000um Container with a small hole. Insidethe container you have a electron and outside you have another electrodeThe suspension is suspended in theelectrolyte solution. The vacuum line sucks some of the particles of the hole,it will displace an equivalent volume of the electrolyte solution and changesin the resistance between the two electrodes |
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Laserlight scattering methods |
Principle:laser light interacts with particlesLightis diffracted by particles by an angle that is inversely proportional to thevolume of the particles.Adetector analyses the radiation diffracted by the particles Heand Nelaser are the most widely used laser |
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Stokes equation |
Vst = h/t =d2 0s-0f Fg / 18n h-distance t-time 0s-density of particle 0f-density of the fluid n-viscosity Fg -gravity acceleration |