ignition of the precipitate

8/14/2019 Ignition of the Precipitate

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IGNITION OF THEPRECIPITATE:

THERMOGRAVIMETRIC

METHOD OF ANALYSIS


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Water in precipitate :

(a)adsorbed water, present on al1 solid surfaces in amountdependent on the humidity of the atmosphere;

(b)occluded water, present in solid solution or in cavitieswithin crystals;

(c)sorbed water, associated with substances having alarge interna1 surface development, e.g. hydrousoxides; and

(d)essential water, present as water of hydration orcrystallisation [e.g.CaC204,H20 or Mg(NH4)P04,6H20]or as water of constitution[the water is not present as such but is formed onheating, e.g.Ca(OH), +Ca0 + H20].


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Direction of ignition :

To bring the precipitate

to a constantcomposition before it can

be weighed.


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Treatment :

The treatment consists in drying or igniting theprecipitate

The further treatment will depend both upon thenature of the precipitate and upon that of thefiltering medium;

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


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Remember..

Precipitates that are to be dried should becollected on filter paper, or in sintered-

glass or porcelain filtering crucibles.

Precipitates that are to be ignited are

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


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Incineration of the filter paper

A silica crucible is first ignited to constantweight (i.e. to within 0.0002 g) at the

same temperature to which the precipitateis ultimately heated.

The well drained filter paper and

precipitate are carefully detached from thefunnel;


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The filter paper is folded to enclose theprecipitate completely

The packet is placed point-down in theweighed crucible, which is supported on asilica triangle resting on a ring stand.

The crucible is slightly inclined, andpartially covered with the lid, which shouldrest partly on the triangle.

A very small flameis then placed underthe crucible lid; drying thus proceedsquickly and without undue risk


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When the moisture has been

expelled, the flame is increasedslightly so as to carbonize the paperslowly.

The paper should not be allowed toinflame, as this may cause amechanical expulsion of fine particles

of the precipitate owing to the rapidescape of the products of combustion


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If the paper does catch fire, the flameshould be extinguished by momentarily

placing the cover on the mouth of thecrucible with the aid of a pair of crucibletongs.

When the paper has completelycarbonized and vapors are no longerevolved, the flame is moved to the back

(bottom) of the crucible and the carbonslowly burned off while the flame isgradually increased.*


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After all the carbon has been burnedaway, the crucible is covered completely

(if desired, the crucible may be placed in avertical position for this purpose) andheated 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


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After ignition ..

When the ignition is ended, the flame isremoved and, after 1-2 minutes, thecrucible andlid are placed in a desiccator containing a

suitable desiccant and allowed to cool for 25-30minute

The crucible and lid are then weighed. The crucible and contents are then ignited at the

same temperature for 10-20 minutes, allowed tocool in a desiccator as before, and weighedagain.


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The ignition is repeated until constantweight is attained. Crucibles should alwaysbe handled with clean crucible tongs andpreferably with platinum-tipped tongs.

It is important to note that 'heating toconstant weight' has no real significanceunless the periods of heating, cooling ofthe covered crucible, and weighing are

duplicated.

ignition of the precipitate

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