Page 287 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

STANOARO SOLUTIONS 10.3

(2CN- + Ag+e[Ag(CN),]-) or of chloride ion with mercury(I1) nitrate
solution (2C1- + Hg2+=HgC12).
Ethylenediaminetetra-acetic acid, largely as the disodium salt of EDTA, is a
very important reagent for complex formation titrations and has become one
of the most important reagents used in titrimetric analysis. Equivalence point
detection by the use of metal-ion indicators has greatly enhanced its value in
titrimetry.

3. Precipitation reactions. These depend upon the combination of ions to form
a simple precipitate as in the titration of silver ion with a solution of a chloride
(Section 10.74). No change in oxidation state occurs.

4.Oxidation-reduction reactions. Under this heading are included al1 reactions
involving change of oxidation number or transfer of electrons among the
reacting substances. The standard solutions are either oxidising or reducing
agents. The principal oxidising agents are potassium permanganate, potassium
dichromate, cerium(1V) sulphate, iodine, potassium iodate, and potassium
bromate. Frequently used reducing agents are iron(I1) and tin(I1) compounds,
sodium thiosulphate, arsenic(II1) oxide, mercury(1) nitrate, vanadium(I1)
chloride or sulphate, chromium(I1) chloride or sulphate, and titanium(II1)
chloride or sulphate.

10.3 STANDARD SOLUTIONS
The word 'concentration' is frequently used as a general term referring to a
quantity of substance in a defined volume of solution. But for quantitative
titrimetric analysis use is made of standard solutions in which the base unit of
quantity employed is the mole. This follows the definition given by the
International Union of Pure and Applied Chemistry' in which:

'The mole is the amount of substance which contains as many elementary
units as there are atoms in 0.012 kilogram of carbon-12. The elementary unit
must be specified and may be an atom, a molecule, an ion, a radical, an
electron or other particle or a specified group of such particles.'

As a result standard solutions are now commonly expressed in terms of molar
concentrations or molarity (M). Such standard solutions are specified in terms
of the number of moles of solute dissolved in 1 litre of solution; for any solution,

Moles of solute
Molarity (M) =
Volume of solution in litres

As the term 'mole' refers to an amount of substance with reference to the
specified mass of carbon-12, it is possible to express the relative molecular mass
(the basis for the mole) for any substance as the additive sum of the relative
atomic masses (R.A.M.s) of its component elements, for example:
The relative molecular mass for sulphuric acid, H2S0,, is calculated from
the relative atomic masses as follows: