Thus, we concluded, that the main method of phase separation in quantity , especially weight method of analysis, is the formation of a solid phase from a solution , ie sedimentation.
Many poorly soluble compounds, that are used in qualitative analysis, are unsuitable for quantitative analysis. So, you can not apply for the quantitative analysis of reaction " golden rain ", because of lead iodide markedly soluble even in cold water : SPPbI , equal to 1,1 ·10-9, which corresponds to the solubility of Pb2 +:
This solubility is unacceptably large and leads to significant errors due to the loss of lead in mother solution
Other precipitates, for example , Fe4 [Fe (CN) 6] 3 i Fe2 [Fe (CN) 6] 3 , although very insoluble , determined the disadvantage, that no permanent composition. These compounds are double salts with varying amounts of potassium and iron. So by mass of these precipitate is impossible to calculate the amount of iron . In addition, these and some other precipitates often form colloidal solutions , which also unacceptable for quantitative analysis.
The example shows, that in quantitative analysis the precipitate must satisfy the following requirements.
a) The precipitate must be practically insoluble .
To determine the numerical value of the permissible boundary must take into account the basic conditions of measurements. The sensitivity of conventional analytical balance equals 0.0001 g ( 10 -4 g ), and order of magnitude of the molecular weight of most sediment is 100. Thus , the maximum error in moles is :
The volume of liquid during the sedimentation is usually 200- 400ml , for washing the precipitate spent another 100 - 150ml of solution. So in a pinch you can take the maximum volume of liquid, that is in equilibrium with sediment, equal to 1 liter . Then we obtain, that under normal conditions of the weight analysis necessary and sufficient solubility limit is equal to 10-6 mol / l. This general requirement for sediment brought Tananayev , and it is known as a rule it.
b) The composition of the precipitate after drying or calcination must satisfy certain formula, so to be constant. Otherwise, the calculation becomes impossible.
c) The molecular weight of the substance of sediment should be large as possible . This increases the accuracy of the analysis.
d) The precipitate must be prepared in a form, suitable for its separation from the solution by filtration.
Implementation requirements b) and d ) is achieved by complying with certain conditions of sedimentation and heat treatment , as discussed in the following sections .