Gravimetric analysis

[lologeo92]

Gravimetric analysis is a technique through which the amount of an analyte (the ion being analyzed) can be determined through the measurement of mass.
Requirements for Precipitates.
- Choice of Precipitant


The precipitated form is the name given to the compound precipitated from solution by the action of the appropriate reagent,



and the weighed form is the compound which is weighed for determination of the final result






For example






, in determinations of Fe+++ and AI+++ the precipitated forms are usually the hydroxides Fe(OH)3 and Al(OH)3 .




The weighed forms are the anhydrous oxides Fe203 and A1203
,





2Fe(OH)3 = Fe203 + 3H20


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In some cases the precipitated and weighed forms may both be the same compound.
For example,


Ba++ and SO 4 - ions are precipitated and weighed as barium sulphate which is not changed chemically when ignited.
In just the same way in determination of Ag+ ions (or Cl- ions) the precipitated and weighed forms are both silver chloride AgCl, etc
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Requirements for the Precipitated Form.
.1
SP ( solubility product) > 10-8
It is the multiplication of ion concentrations
10-4M x 10-4 M= 10-8
2
It is also desirable that the structure of the precipitate should be such as to allow of rapid filtration and washing. (Large crystal form
precipitates of relatively large crystals are very convenient, because they hardly clog the filter pores, and, as their specific surface is not extensive they do not readily adsorb impurities from solution and are easily washed free from the latter.
Precipitates consisting of very small crystals, such as BaSO4 or CaC2O4, are less convenient in this respect.
Such precipitates readily pass through the filter pores; of course, this is quite inadmissible in gravimetric analysis.
.
Amorphous precipitates, such as Al(Cl)3, have extensive specific surfaces and therefore adsorb considerable amounts of impurities which are difficult to wash off. Filtration is very slow in such cases.
In such cases the analyst tries to create conditions in which the disadvantages of amorphous precipitates are reduced to a minimum
3
. The precipitated form must be converted fairly easily and completely into the weighed form.
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Requirements for the Weighed Form
1. The most important requirement is that its composition should correspond exactly to its chemical formula.
2. The weighed form must have adequate chemical stability. Analysis obviously becomes more difficult if the weighed form readily changes its composition, for example, by absorption of water vapour or CO2 from the air, by oxidation or reduction, by decomposition at higher temperatures, and similar processes.
3. Finally, it is convenient if the content of the element being determined in the precipitate should be as low as possible, because errors in the determination



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Error in weighing BaCrO4 and Cr203
Weighed form Cr2O3
152 mg Cr203 contains 104 mg Cr
1 mg Cr2O3 contains x mg Cr
x= 104/152 x 1=0.7 mg Cr
Weighed form BaCrO4
253.3 mg BaCrO4 contains 52 mg Cr
1 mg BaCrO4 contains x mg Cr
X = 52 / 253 x 1 = 0.2 mg Cr



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Amount of Precipitant
For lead sulphate solution saturated at 25oC we can write:
[Pb++] [S04 --] = SPPbSO4 = 2.2 x 10-8
if
[Pb++][SO4--] < 2.2 x 10-8
then the solution is not saturated, and some more lead sulphate can be dissolved in it.
If the solubility product is "exceeded", i.e.,
[Pb++][S04 --] > 2.2 x 10-8 (at 25o C)
Then the solution is supersaturated and a certain amount of PdSO4 must be precipitated.