Gravimetric analysis
Analytical balance
|
|
| Classification | Gravimetric |
|---|---|
| Analytes |
Solids Liquids |
| Other techniques | |
| Related |
Precipitation Titration |
→
Gravimetric analysis describes a set of methods used in analytical chemistry for the quantitative determination of an analyte (the ion being analyzed) based on its mass. The principle behind this type of analysis is that once an ion's mass has been determined as a unique compound, that known measurement can then be used to determine the same analyte's mass in a mixture, as long as the relative quantities of the other constituents are known.
The four main types of this method of analysis are precipitation, volitilization, electro-analytical and miscellaneous physical method Both involve changing the phase of the analyte in order to separate it in its pure form from the original mixture and both are quantitative measurements.
The precipitation method is the one used for the determination of the amount of calcium in water. Using this method, an excess of oxalic acid, H2C2O4, is added to a measured, known volume of water. By adding a reagent, here ammonia, the calcium will precipitate as calcium oxalate. The proper reagent, when added to aqueous solution, will produce highly insoluble precipitates from the positive and negative ions that would otherwise be soluble with their counterparts (fig 1.).
The reaction is
fig 1. Formation of calcium oxalate
Ca2+(aq) + C2O42 -→ CaC2O4
The precipitate is collected, dried and ignited to high (red) heat which converts it entirely to calcium oxide (fig 2.).
The reaction is
fig 2. Pure calcium oxide formed
CaC2O4 → CaO(s) + CO(g)+ CO2(g)
The pure precipitate is cooled, then measured by weighing, and the difference in weights before and after reveals the mass of analyte lost, in this case calcium oxide. That number can then be used to calculate the amount, or the percent concentration, of it in the original mix.
In volatilization methods, removal of the analyte involves separation by heating or chemically decomposing a volatile sample at a suitable temperature. In other words, thermal or chemical energy is used to precipitate a volatile species. For example, to determine the water content of a compound by vaporizing the water using thermal energy (heat). Heat can also be used, if oxygen is present, for combustion to isolate the suspect species and obtain the desired results.
The two most common gravimetric methods using volatilization are those for water and carbon dioxide. An example of this method is the isolation of sodium hydrogen bicarbonate (the main ingredient in most antacid tablets) from a mixture of carbonate and bicarbonate. The total amount of this analyte, in whatever form, is obtained by addition of an excess of dilute sulfuric acid to the analyte in solution.
...
Wikipedia