what is desiccator and centrifugation.pptx

Desiccators are sealable enclosures containing desiccants used for preserving moisture-sensitive items such as cobalt chloride paper for another use. A common use for desiccators is to protect chemicals which are hygroscopic or which react with water from humidity.

The contents of desiccators are exposed to atmospheric moisture whenever the desiccators are opened. It also requires some time to achieve a low humidity. Hence they are not appropriate for storing chemicals which react quickly or violently with atmospheric moisture such as the alkali metals; a glovebox or Schlenk-type apparatus may be more suitable for these purposes.

Desiccators are sometimes used to remove traces of water from an almost-dry sample. Where a desiccator alone is unsatisfactory, the sample may be dried at elevated temperature using Abderhalden's drying pistol.
Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed.[1] The denser components of the mixture migrate away from the axis of the centrifuge, while the less dense components of the mixture migrate towards the axis. Chemists and biologists may increase the effective gravitational force of the test tube so that the precipitate (pellet) will travel quickly and fully to the bottom of the tube. The remaining liquid that lies above the precipitate is called a supernatant or supernate.

There is a correlation between the size and density of a particle and the rate that the particle separates from a heterogeneous mixture, when the only force applied is that of gravity. The larger the size and the larger the density of the particles, the faster they separate from the mixture. By applying a larger effective gravitational force to the mixture, like a centrifuge does, the separation of the particles is accelerated. This is ideal in industrial and lab settings because particles that would naturally separate over a long period of time can be separated in much less time.[2]

The rate of centrifugation is specified by the angular velocity usually expressed as revolutions per minute (RPM), or acceleration expressed as g. The conversion factor between RPM and g depends on the radius of the centrifuge rotor. The particles' settling velocity in centrifugation is a function of their size and shape, centrifugal acceleration, the volume fraction of solids present, the density difference between the particle and the liquid, and the viscosity. The most common application is the separation of solid from highly concentrated suspensions, which is used in the treatment of sewage sludges for dewatering where less consistent sediment is produced.[3]

The centrifugation method has a wide variety of industrial and laboratorial applications; not only is this process used to separate two miscible substances, but also to analyze the hydrodynamic properties of macromolecules.[4] It is one of the most import

1. Experiment No. 8 & 9 PHYSICAL PHARMACY LAB

2. Desiccator

3. Why silica gel is used in desiccator ?.. Since their micro-porous makeup has cavities that are interlocking . Silica gel is a synthetically modified silicon dioxide that has been purified and processed into a hard, smooth bead.  Silica   gel  has an extremely high area of surface. It makes for a perfect  desiccant  of high capacity. Because it has a lower vapor pressure than the air that surrounds it, molecules of water can easily adhere to its surface.

4. Why does blue silica gel turn pink ?.. As the  blue silica gel begins to adsorb moisture from the air, they contain cobalt chloride crystals ( inorganic compound of cobalt and chlorine) which indicates   the   light blue of this gel. Pink   color appears when the gel has absorbed approximately 8% of its weight in moisture . This gives an easy visual indicator of when the gel has started to become  saturated with moisture . Blue  silica gel  has cobalt chloride, which allows the blue silica gel  change its color to  pink  when it reached its maximized adsorption capacity. PINK BLUE again (it's ready to use) heat to dry out the moisture  

5. How do I know if white silica gel is saturated? Indicating and non-indicating silica gel can be mixed to help determining when the silica becomes saturated. Silica gel displays its full color change when it is saturated by holding 8-10% of its weight in moisture; at that point the silica gel should be changed or regenerated.

6. Does heat destroy silica?? The adsorption capacity of silica gel can be destroyed by contamination with dust, grease or petroleum products, and also  by heating the material above 120°C . If the silica gel has been subjected to overheating  the adsorption capacity will have been adversely effected .

7. Why use nitrogen to purge moisture?... Nitrogen  is the standard medium for contamination-free storage because it is relatively inert. It neither reacts with stored materials nor carries moisture and because it can be isolated and purified relatively inexpensively . In a  nitrogen  purge, ultra dry  nitrogen  with a dew point of -94°F (-70°C) is introduced under pressure into an enclosure or cavity in order to  remove moisture  and  water vapor , creating a much drier internal environment than standard desiccant can achieve. Purging with dry nitrogen will also reduce the oxygen level in the desiccator . When you open the desiccator doors, outside air enters that has to be purged by the gas flow.

8. CENTRIFUGATION Centrifugation  is a mechanical process which involves the use of the centrifugal force  to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed T he larger the size and the larger the density of the particles , the faster they separate from the mixture. By applying a larger effective gravitational force to the mixture, like a centrifuge does, the separation of the particles is accelerated. This is ideal in industrial and lab settings because particles that would naturally separate over a long period of time can be separated in much less time

9. Who is Theodor Svedberg ?... Theodor Svedberg was a Swedish chemist and Nobel laureate for his research on colloids and proteins using the ultracentrifuge . Svedberg  found that the size and weight of the particles determined their rate of settling out, or sedimentation, and he used this fact to measure their size. With an ultracentrifuge ,  Svedberg  went on to determine precisely the molecular weights of highly complex proteins such as hemoglobin.

10. CENTRIFUGATION

11. How is the principle of centrifugation helpful in blood banks