Volumetric analysis: Analysis by measurement of volume.
Volumetric analysis involves titration. In titration one reactant, A (the analyte), is in solution in a flask (usually a conical flask) and a second reactant, B (the titrant), in solution is added from a burette (a graduated glass tube with a tap at the bottom).
Equivalence point or stoichiometric point: The stage of a titration where the volume added from the burette (the titre) contains the exact amount of B required to react with all of A according to the chemical equation for the reaction.
This stoichiometric relationship is at the centre of all volumetric analysis calculations.
In titration there are commonly two scenarios.
ct(B) is known, (i.e. B is a standard solution), and nt(A) is to be determined. Here the subscript t has been used to emphasise these are quantities in the actual titration step. The amount of A in the conical flask is determined from the titre, the volume of solution B, Vt(B), delivered from the burette:
From nt(A), the amount of A in the flask, information such as the concentration of a solution of A, the amount of A in a mixture, or the percentage purity of a substance may be determined.
- If nt(A) is known, and c(B) is determined. This is called standardising the solution of B.
The amount of A may be obtained
(a) from the mass of a very pure substance A (known as a primary standard) determined by weighing and calculating n(A) from, or,
(b) if A does not have the properties of necessary for a primary standard, weighing out some other primary standard substance, X, that gives A in a stoichiometric reaction, xX aA
- By weighing a larger mass of primary standard A into a
volumetric flask (a stoppered flask with a graduation mark on the neck) of fixed volume Vf, dissolving it in the solvent (usually water), then adding more solvent until the level of the solution is on the graduation mark, and then shaking thoroughly. An aliquot of the solution is transferred from the volumetric flask to the conical (titration) flask using a pipette of volume Vp. In this case the concentration of A in the volumetric flask is
Again m(A) and Vt(B) are measured in the experiment, the molar mass of A and the volumes of the flask and pipette being known.
Household chlorine bleaches are aqueous solutions of sodium hypochlorite in sodium chloride.
When acidified, chlorine is produced:
The concentration of the bleach is often given as grams of "available chlorine" per litre.
The determination of "available chlorine" is a nice example of chemical analysis