Gravimetric Analysis
Finding the Composition of a Mixture
Gravimetric analysis
gravimetric analysts a procedure in which the masses of solids are measured in order to deduce the concentration of a particular constituent of a mixture
Not all substances are easily dissolved in water and suitable for volumetric analysis. For many of these, we may use gravimetric analysis, a procedure where we use the masses of solids to determine the concentration of a substance (which we will call A) in a mixture. There are three major pathways to take, depending on the physical and chemical properties of A relative to those of the other components of the mixture, which of course must be known before we can proceed to design our analysis.
Case 1 "A" is the only volatile component of a mixture
If the other components are stable and non-volatile, we simply heat the mixture and determine the mass loss due to A. This gives us the percentage by mass of A in the mixture. This method is used to determine the degree of hydration of a salt and can be used to determine the moisture content of some foods
Case 2 "A" is the only component insoluble in a particular solvent
The solvent used would be selected by trial and error. When we add this solvent
to
a weighed sample of the solid, all the other components of the mixture will
dissolve.
A will be the only solid present and can be filtered off, rinsed, dried and
weighed
If the solid can be dried by in an oven or over a Bunsen burner, we would use an ashless filter paper, so that it burns away without leaving a residue to contribute to the mass. If the solid would decompose or melt on heating, we must weigh a dried filter paper, dry our solid plus filter paper in a desiccator, then weigh the final sample of solid plus paper.
Case 3 "A" is the only component soluble in a particular solvent
Again, the solvent used would be selected by trial and error. When we add this solvent to a weighed sample of the solid, all the other components of the mixture will remain as solids and can be filtered off. "A" will be the only substance present in the solvent. To extract it, we then add a substance which will precipitate A, so that we can filter it off, and rinse, dry and weigh the precipitate. From the mass of the precipitate we can calculate the mass of A using stoichiometry, and hence calculate its concentration in the original mixture.
Solubility
High Solubility Low Solubility
Compounds containing the following ions are generally soluble in water·
Na+ K+ NH4+ NO3 - CH3COO-· Cl- , Br- I- (unless combined with Ag+ or
Pg2+)· SO42- (except BaSO4 and BaSO4 . AgSO4 and CaSO4 are slightly soluble)
Compounds containing the following ions are generally insoluble unless combined
with Na+ , K+ or NH4+· CO32- , PO42- , S2-· OH- (unless combined
with Ba2+ and Sr2+ ; Ca(OH)2 is slightly soluble)
The drawbacks of gravimetric analysis
When our procedure relies on the collection of a precipitate, there are a number of possible major sources of error:
We are assuming that our constituent A is the only one that dissolves in, or does not dissolve in, the solvent. This, of course, may not be the case.
As the study of solubility curves in the previous course showed, all substances have a certain solubility in a solvent, which depends on the temperature. Usually it increases with temperature. Hence when we collect our precipitate, some of the substance will remain dissolved in solution, and be 'lost'; a little more will be lost as we rinse the precipitate clean of solution with fresh solvent. This makes our experimental result for the concentration of A in the original mixture slightly lower than the true result. We minimise this error by selecting a solvent in which the precipitate has extremely low solubility and by using a minimum amount of solvent for dissolving A or the other components of the original mixture, and for rinsing the precipitate.
This error is compounded by the fact that at each stage, little splashes, inefficient rinsing out of containers, and inefficient filtering, can cause some A to be lost.
The precipitate may not be dried properly and so residual moisture can add to the mass, making our experimental result for the concentration of A in the original mixture slightly higher than the true result. We minimise this error by a procedure known as weighing to constant mass, which means that we weigh our dried precipitate, then dry it again, weigh it again, and so on, until successive readings are the same.
Not all A may be precipitated out in Case 3. To overcome this, we test the filtrate for the presence of A. If some is still present, we add more precipitating agent and re-filter.
In a filtration, the filtrate is the liquid that passes through the filter paper. The remaining solid is called the residue.