Gravimetry by Precipitation
Gravimetry is a method of quantitative chemical analysis. It qualifies as a macroscopic quantitative method of analysis because it involves relatively important quantities of a substance to be determined compared to more recent methods, such as electroche
- PDF / 492,278 Bytes
- 16 Pages / 439 x 666 pts Page_size
- 86 Downloads / 195 Views
Gravimetry by Precipitation
Gravimetry is a method of quantitative chemical analysis. It qualifies as a macroscopic quantitative method of analysis because it involves relatively important quantities of a substance to be determined compared to more recent methods, such as electrochemical, spectroscopic and chromatographic means. From this standpoint, it should instead be compared to titrimetric methods. However, it has remained a method of choice for the analysis of standard compounds, those compounds with which the more recent instrumental methods of analysis listed above are calibrated.
38.1
Principle and Some Definitions
In gravimetric analysis, the substance to be determined (the analyte) is transformed quantitatively into an insoluble precipitate that is isolated and weighed. Most of the time, the substance to be analyzed is transformed quantitatively into another that yields the precipitate that will be weighed. The weight of the analyte, and through it its mole number, can easily be calculated from that of the precipitate if the precipitate’s composition is known. (Here we shall confuse weights and masses. This has no effect on the accuracy of the different results, provided calculations do not mix both quantities.) One distinguishes among • gravimetry by precipitation, which is characterized by the fact that the separation of the substance to be determined (or of one of its derivatives) is carried out by precipitation. This is the sole sort of gravimetry that will be studied here; • electro-gravimetry, in which the element or the compound to be determined is deposited quantitatively upon a suitable electrode. Weighing the electrode before and after the process permits the determination; • gravimetry by volatilization, in which the loss of weight by volatilization is determined by weighing. The volatilization is due to a gain in energy by the sample. Most often, it is thermal energy. An important example of such a process is provided by the determination of moisture in a sample;
J.-L. Burgot, Ionic Equilibria in Analytical Chemistry, DOI 10.1007/978-1-4419-8382-4_38, © Springer Science+Business Media, LLC 2012
705
706
38 Gravimetry by Precipitation
• particulate gravimetry, in which the analyte is already under a physical or chemical form that can be easily separated from a liquid, that is, as a gas or a solid matrix. Briefly, in this process, no prior chemical transformation of the compound to be determined is needed. Some authors distinguish two sorts of gravimetry by precipitation: • direct gravimetry, in which the analyte itself is weighed. For example, when nickel is determined by the weighing of its dimethylglyoximate, a direct gravimetry is carried out; • indirect gravimetry, in which the analyte is not weighed. For example, phosphite ions PO3 3− may be determined by weighing dichlorodimercury(I) (“calomel”) that has previously been formed from it and that has precipitated. Phosphite ions do indeed quantitatively reduce Hg(II) into dichlorodimercury(I) while simultaneously being oxidized int
Data Loading...
