Ignition Of The Precipitate

IGNITION OF THE PRECIPITATE: THERMOGRAVIMETRIC METHOD OF ANALYSIS

Water in precipitate : (a)adsorbed water, present on al1 solid surfaces in amount

dependent on the humidity of the atmosphere; (b)occluded water, present in solid solution or in cavities within crystals; (c)sorbed water, associated with substances having a large interna1 surface development, e.g. hydrous oxides; and (d)essential water, present as water of hydration or crystallisation [e.g.CaC204,H20 or Mg(NH4)P04,6H20] or as water of constitution [the water is not present as such but is formed on heating, e.g. Ca(OH), +Ca0 + H20].

Direction of ignition :

•To bring the precipitate

to a constant composition before it can be weighed.

Treatment : • The treatment consists in drying or igniting the precipitate

• The further treatment will depend both upon the nature of the precipitate and upon that of the filtering medium;

• Drying : untill 200 -250 degree Celcius • Ignition : up to 1200 degree Celcius

Remember….. • Precipitates that are to be dried should be collected on filter paper, or in sinteredglass or porcelain filtering crucibles.

• Precipitates that are to be ignited are

collected on filter paper, porcelain filtering crucibles, or silica filtering crucibles.

Incineration of the filter paper • A silica crucible is first ignited to

constant weight (i.e. to within 0.0002 g) at the same temperature to which the precipitate is ultimately heated. • The well drained filter paper and precipitate are carefully detached from the funnel;

• The filter paper is folded to enclose the

precipitate completely • The packet is placed point-down in the weighed crucible, which is supported on a silica triangle resting on a ring stand. • The crucible is slightly inclined, and partially covered with the lid, which should rest partly on the triangle. • A very small flame is then placed under the crucible lid; drying thus proceeds quickly and without undue risk

• When the moisture has been

expelled, the flame is increased slightly so as to carbonize the paper slowly. • The paper should not be allowed to inflame, as this may cause a mechanical expulsion of fine particles of the precipitate owing to the rapid escape of the products of combustion

• If the paper does catch fire, the flame

should be extinguished by momentarily placing the cover on the mouth of the crucible with the aid of a pair of crucible tongs. • When the paper has completely carbonized and vapors are no longer evolved, the flame is moved to the back (bottom) of the crucible and the carbon slowly burned off while the flame is gradually increased.*

• After all the carbon has been burned

away, the crucible is covered completely (if desired, the crucible may be placed in a vertical position for this purpose) and heated to the required temperature by means of a Bunsen burner. • Usually it takes about 20 minutes to char the paper, and 30-60 minutes to complete the ignition

After ignition ….. • When the ignition is ended, the flame is

removed and, after 1-2 minutes, thecrucible and lid are placed in a desiccator containing a suitable desiccant and allowed to cool for 25-30 minute • The crucible and lid are then weighed. • The crucible and contents are then ignited at the same temperature for 10-20 minutes, allowed to cool in a desiccator as before, and weighed again.

• The ignition is repeated until

constant weight is attained. Crucibles should always be handled with clean crucible tongs and preferably with platinum-tipped tongs. • It is important to note that 'heating to constant weight' has no real significance unless the periods of heating, cooling of the covered crucible, and weighing are duplicated.