The quantitative determination of the substance, by precipitation, followed by isolation and the precipitate weighing, is known as gravimetric analysis. And, quantitative analysis can be used to determine the percentage of a specific ion or element in a sample.
About Gravimetric Analysis
In the case of typical gravimetric analysis, the ion of interest percentage present in a solid compound is determined. This process involves dissolving a substance of unknown composition in water and also allowing the ion of interest to react with a counter ion to produce a precipitate. Then, the precipitate is isolated, dried and weighed.
By knowing the chemical formula and mass of the precipitate, the ion of interest mass can be determined from the precipitate percent mass composition. From the masses of the original compound and the ion of interest, the percentage of the ion of interest present in the original compound can be calculated.
Quantitative Analysis of Organic Compounds
The percentage composition estimation of the different elements present in a compound is the next step in determining the compound's formula. Various methods that are employed for the estimation of different elements can be described as follows:
1. Estimation of Carbon, Hydrogen
Always, both these two elements are estimated together by the method of Liebig's combustion. A weighted amount of the compound can be strongly heated with excess copper oxide in an atmosphere of oxygen and air. Thus, the hydrogen and carbon constituents are oxidized to carbon dioxide and water, which are collected and weighed separately. The percentage of both hydrogen and carbon present in the compound is calculated as follows:
% C = 12/44 x Formed CO2 Mass / Mass of the substance x 100
% H = 1/18 x Formed H2 Mass / Mass of the substance x 100
2. Estimation of Nitrogen
This particular method can be applied to all the organic compounds that contain nitrogen. The method can be based on the principle that if an organic compound containing nitrogen is heated with copper oxide, and the free nitrogen and oxides of the nitrogen are produced along with other products (such as carbon dioxide, water vapour, and more). On passing over the heated copper, the nitrogen oxides are decreased to free nitrogen, and the full nitrogen is collected over the KOH solution. Then, the volume of nitrogen that is collected is measured, and from this, we can calculate the percentage present in the compound.
% N = 28/22400 x Volume of the nitrogen at NTP/ Mass of the compound x 100
This specific method is commonly employed for estimating the nitrogen in fertilizers and food materials. Though this method is easier to implement than the Dumas method, it is not applicable to the entire nitrogenous organic compounds.
2. Estimation of Halogen
The fuming nitric acid, along with a few silver nitrate crystals in a tube that is sealed, heats a known mass of the organic material containing halogen. The silver halide is produced, which o=is separated, washed, dried and also weighed. From the mass of silver halide, we can calculate the obtained percentage of halogen.
% Halogen = Mass of the silver halide that is obtained x The atomic mass of halogen x 100 / Molar mass of the silver halide x Mass of the compound, that is taken.
The carius method does not produce satisfactory results with iodine, because silver iodide is slightly soluble in nitric acid and some amount iodine is also formed even in the presence of excess silver nitrate. However, the results are not accurate for highly halogenated aromatic compounds.
3. Estimation of Sulphur
By carius method, sulphur is also estimated. The organic compound is only heated with nitric acid in this case. Thus, the sulphur that is present in the compound is oxidized to sulphuric acid, which can be treated with barium chloride to precipitate the barium sulphate. Then, the barium sulphate precipitate is washed, dried and also weighed. The percentage of sulphur can be calculated using the below formula from the obtained weighted mass of barium sulphate.
% S = 32/133 x Mass of BaSO4/ Mass of compound x 100
4. Estimation of Phosphorus
Phosphorus can be estimated, such as sulphur, where an organic compound is heated with the fuming nitric acid. Thus, the phosphorus present in the compound is oxidized to phosphoric acid, which is precipitated when the magnesia mixture is added. The magnesium ammonium phosphate precipitate is ignited to obtain magnesium pyrophosphate.
2MgNH4PO4 (Magnesium ammonium phosphate) → Mg2P2O7 + 2NH3 + H2O
The magnesium pyrophosphate is weighed, and the phosphorus percentage calculation is given as follows:
%P = 62/222 x Mass of the Mg2P2O7/Mass of compound x 100
Determination of Compound's Molecular Formula
The compound's molecular formula expresses the actual number of atoms of different elements present in the compound's molecule.
Molecular formulas can be either similar either as the empirical formula or a simple multiple of it.
Molecular formula = (Empirical formula) x n
Where n is a whole number, and its value is obtained by dividing the molecular mass using empirical formula mass.
n = Molecular mass/ Empirical formula mass.