Why are no concentrated solutions used for the titration?



The Titer or Normal factorf in analytical chemistry is a factor that indicates the deviation of the actual concentration of a standard solution from the nominal concentration of the solution.

This results in the titration with the set solution

The titer is a specific value for the respective standard solution. Depending on the determination method, a slightly different titer can be determined for one and the same standard solution. A known method for determining titer is acid-base titration or redox titration, but gravimetric or argentometric determinations are also possible.

It makes sense to use the same method for the measurement and the titer determination, since the end point determination is different for each method and thus differences arise. Depending on the measuring method used to determine the titer, a suitable primary titer substance must be selected.

For this purpose, the standard solution to be determined is titrated with a basic titer substance, the exact amount of which is known. Using the reaction equation, the exact concentration of the standard solution can be determined from the volume of the standard solution added and the amount of substance used.

Example for the determination of the titer of a hydrochloric acid solution (c ~ 0.1mol / L)

Sodium carbonate is chosen as the primary substance, which reacts with hydrochloric acid as follows:

The reaction equation shows that half the amount of substance sodium carbonate corresponds to the amount of substance used in hydrochloric acid.

A certain amount of sodium carbonate, which has been precipitated from a saturated solution with carbon dioxide, washed and dried to constant mass, is weighed as precisely as possible, dissolved in water and an indicator such as methyl orange is added. Now it is titrated up to the point of transition. The concentration of the hydrochloric acid solution can be determined from the consumption of standard solution and the amount of sodium carbonate used.

Template: m (Na2CO3) = 0.4000g (~ 3.77 mmol); c (HCL) = approx. 0.1 mol / l
Consumption of volumetric solution: 75.0 ml

Stoichiometry:

(according to example: 2 * 3.77 mmol = 7.5 mmol)

The hydrochloric acid solution has an actual concentration of 0.1006 mol / l. In order to calculate the titer, one now divides the measured actual concentration by the nominal concentration and obtains:

Some standard solutions are so unstable that the titer has to be determined each time the standard solution is used. This applies particularly to low-concentration solutions of iodine, hydrochloric acid, sodium hydroxide solution and potassium permanganate; Iodine and hydrogen chloride evaporate from the solution, sodium hydroxide forms sodium carbonate with the carbon dioxide in the air and potassium permanganate decomposes to manganese dioxide in an autocatalytic process. Highly concentrated standard solutions of hydrochloric and sulfuric acid, sodium thiosulphate, cerium (IV), potassium dichromate, silver nitrate (with exclusion of light) and of course standard solutions are fairly stable. But these custom-made solutions must also be checked at least once a month.

The pharmacopoeia, the legal regulation for analysis in the pharmacy and the pharmaceutical industry, stipulates that the titer may deviate by a maximum of 10%, i.e. must have a value between 0.9 and 1.1, otherwise problems can arise when determining the content.

Category: Analytical Chemistry