Crystallization by shock cooling

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BACKGROUND

Crystallization is the spontaneous arrangement of the particles into a repetitive orderly array,i.e., regular geometric pattern. Crystals are commonly obtained from liquid state. The formation of crystals from solution involves 3 steps.¹

1. Super saturation
   
2. Nuclei formation
   
3. Crystal growth
   

Potassium nitrate crystals can be obtained using shock cooling technique. The solid (solute) is added to a solvent continuously until the solid is dissolved. Such a solution is called as saturated solution.^2,3 The rate of dissolution process is enhanced by increasing the temperature and agitation. When some solid remained undissolved, then such a solution is called as super saturated solution. When the temperature of supersaturated solution is decreased rapidly (shock cooling), the solubility of solute decreases. As a result, the dissolved solid gets crystallized, through the processes of nucleation and crystal growth. The extent of crystallization depends on the time of contact in low temperature. The crystals are collected by filtration and weighed. Yield is expressed as percent weight of crystals obtained. A graph is plotted taking time vs. weight of crystals.^4,5

Aim: To study the crystallization behavior of potassium nitrate.

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REQUIREMENTS

Apparatus: Beaker, 250ml

Weighing balance

Water bath

Stop watch

Test tubes,10ml

Measuring cylinder,100ml

Dryer

Funnel

Filter paper

Potassium nitrate

Ice bath

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PROCEDURE

37.5 grams of potassium nitrate is accurately weighed (W₁g). 50 ml water is transferred into 250 ml beaker. Beaker containing water is placed in constant temperature water bath maintained at 50⁰c Potassium nitrate is added into water little by little, the solution is stirred with glass rod to dissolve the solute. This process is continued until saturated solution (with little excess crystals) is formed. Weight of potassium nitrate remained is weighed (W₂g). Difference in the weights Wg (W₁-W₂) gives weight of potassium nitrate added into 50 ml water. From this, 10 ml quantities of saturated solution are transferred into 5 test tubes. All the test tubes are placed in an ice bath at once. Temperature of the solution decreases suddenly due to shock cooling forming supersaturated solution .Nucleation and crystal growth takes place. After 10 mins, the solution of first test tube is filtered to collect crystals. This is repeated after every 10 minutes there after using the solutions of other test tubes. All the crystals collected on the filter paper separately are subjected to drying using dryer. Weights of each sample of crystals are recorded in table, and data is processed. A graph is plotted taking time on X-axis and percent weight of crystals on Y-axis as shown in fig.

Observations and calculations:

a= Weight of potassium nitrate present in 10 ml of water,g

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CONCLUSION

The crystal of potassium nitrate
was prepared by shock cooling method and submitted and the yield was found to be_____________________

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REFERENCES

1. Subrahmanyam CVS, Setty JT, Suresh S. Laboratory manual of pharmaceutical engineering (Unit operations). Delhi Vallabh Publications.
   
2. Pharmaceutical Engineering by K.Sanbamurty (New Age International, New Delhi)
   
3. Cooper and Gunn’s Tutorial Pharmacy Edited by S.J.Carter (CBS Publishers, Delhi)
   
4. Pharmaceutical Dosage forms by Aulton.(Churchill Livingstone, Edinburg)
   
5. Chemical Engineering by Badger and Banchero (Mc Graw Hill, New Delhi)