A LABORATORY PROTOCOL ON THE GEOLOGY EXPERIMENT CARRIED OUT BY SPANISH  STUDENTS OF ESCUELA PROFESIONAL “LA SALLE” PATERNA (VALENCIA)

Practice Crystallization

Introduction:

Crystallization is the simplest and most effective technique for purifying solid compounds. It consists of dissolving an impure solid in the smallest possible amount of the appropriate solvent and hot. Under these conditions, a saturated solution is generated which, on cooling, becomes supersaturated and crystallization occurs. The crystallization process is a dynamic process, so that the molecules that are in solution are in equilibrium whit those that are part of the crystal lattice. The high degree of arrangement of a crystal lattice excludes the participation of impurities in it. Therefore, it is desirable that the cooling process take places slowly so that crystals form slowly and the slow growth of the crystal lattice excludes impurities. If the cooling of the solution is very fast, the impurities can be trapped in the crystal lattice.

Objective:

Obtain crystals of ADP (ammonium dihydrogen phosphate), also called monoammonium phosphate (MAP).

ADP is highly soluble in water and crystallizes in the tetragonal system, forming square-based prisms terminated in pyramids.

Process

Step 1:

Make an assembly like the one in the image, using a tripod , a grid and a heating plate 

Step 2: 

In a cylinder, measure 250ml of distilled water and placed it in a 500 ml beaker. 

Weigh 175 g of ADP.

Step 3: 

Heat the water in the baker to about 40-60 ºC. The ADP should be added once the water is hot, stirring constantly the rod Heat

Step 4: 

Add the food  colour  and  stir  with the  rod

Step 5: 

Wait until the  temperature  reaches  80º-90ºC  so  that  all  the  ADP  dissolves. 

Step 6: 

Subsequently,  place  the solution  in  a  plastic container with a  lid  and  put  it  into  the  crystallizer.

Step 7: 

Finally,  leave  the crystallizer    in  a  place  where  nobody can  disturb  it  (if  not  the  whole  process  is  destabilized).

After  2-3  days  when  you  remove  the  excess   liquid  you will  find  a  beautiful  crystal.

Calculations

Weight  70gr  of  ADP  for  every  100 ml  of  distilled  water.

7ogr———-100ml 

 x gr———-250

x = 175gr  of  ADP

Results

Different  coloured  crystals

Questions

1. What does the success of the experiemnt depend on? On the crystalitzation temperature
2. What  is  the  supersaturated  zone?  The  one where  the  ADP  begins  to  crystallize