Principle of proportionings

Doser (or to titrate ) a chemical Espèce (Molécule or Ion) in a solution is to determine the molar concentration of this species in the solution considered, or, which is equivalent, the quantity of matter of this species in a given volume of the solution.

It will be said for example that one proportioned the ions iron (II) in a ferrous sulfate solution (II) and that the value obtained is: = 0.015 mol. L -1

In biological analysis, the proportioning of the Cholesterol in led blood, for example with a value of 6.50 mmol. L -1 (often to express in the form of a mass concentration: 2.52g.L -1 .

Methods of proportioning

Nondestructive methods

It frequently happens that the value ρ of a measurable physical size on the solution is directly related on the concentration of a species X present in the solution or to the concentration C of the species put in solution: ρ = F () or ρ = F ( C )

As follows:

the index of refraction N of a saccharose solution depends on the concentration of the aqueous solution N = F ( C )
the absorption of the light by a potassium permanganate solution depends on the molar concentration on this ion in the solution.
the conductance G of an ionic solution depends on the concentration C of ionic solid MX ( S ) put in an aqueous solution: G = F ( C ). Measurement G makes it possible to obtain the value C .

These methods of proportioning do not modify of anything the solution subjected to the proportioning which can be recovered without modification after measurements. For this reason, these methods of proportioning are qualified nondestructive analyzes.

Direct or destructive proportionings

When there does not exist nondestructive method of analysis, one must carry out a chemical conversion between the solution to be analyzed and another solution, but this reaction destroys the solution to be analyzed.

In general, the solution containing the species to be proportioned is placed in a beaker; it contains the titrated reagent. The solution which reacts with this first solution is contained in a graduated oil-can; it conient the titrating reagent.

With the oil-can one gradually pours the reagent titrating in the titrated reagent.

Reaction of proportioning and equivalence

Reaction of proportioning

During proportioning, it is carried out a chemical reaction between the titrated reagent and the titrating reagent. This reaction bears the name of reaction of proportioning; it can be a acido-basic Réaction a Réaction of oxydoreduction (redox).

So that a chemical reaction can be used like a reaction of proportioning, it is necessary that it is:

univocal
total
fast

Equivalence

It is the study of this reaction which will allow of given the sought concentration.

The question which installation is the following one: until which moment is necessary it to pour the solution containing the titrating reagent?

One pours the solution of reagent titrating in the solution containing the reagent titrated until this one completely reacted. It is said whereas one is with equivalence.

With equivalence, the titrated reagent and the titrating reagent were entirely consumed.

Advance X reaches its noted maximum value then X .

Note: For the reaction of proportioning, the réctif one limiting is:

before equivalence: the titrating reagent;
after equivalence: the titrated reagent.

How to locate equivalence?

During a proportioning, the main aim is to locate equivalence. There exist many possibilities. Equivalence can be located by:

a change of color of the reactional medium or an Indicator that one introduced into the reactional medium (variation in the absorption spectrum of the solution)
variations abrupt of the pH

The point of equivalence is to be determined with measurements precise of physical sizes directly related to the composition of the solution. When these data are in the forms of graphs, equivalence corresponds to a point of inflection of the curve (the second when the ion is diprotic, for the ions triprotic there exist two points of equivalence).

See too

Chemical reaction

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