Page 40 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

1 INTRODUCTION

1.1 0 INTERFERENCES

Whatever the method finally chosen for the required determination, it should
ideally be a specific method; that is to Say, it should be capable of measuring
the amount of desired substance accurately, no matter what other substances
may be present. In practice few analytical procedures attain this ideal, but many
methods are selective; in other words, they can be used to determine any of a
small group of ions in the presence of certain specified ions. In many instances
the desired selectivity is achieved by carrying out the procedure under carefully
controlled conditions, particularly with reference to the pH of the solution.
Frequently, however, there are substances present that prevent direct
measurement of the amount of a given ion; these are referred to as interferences,
and the selection of methods for separating the interferences from the substance
to be determined are as important as the choice of the method of determination.
Typical separation procedures include the following:

(a) Selectioe precipitation. The addition of appropriate reagents may convert
interfering ions into precipitates which can be filtered off, careful pH control
is often necessary in order to achieve a clean separation, and it must be
borne in mind that precipitates tend to adsorb substances from solution
and care must be taken to ensure that as little as possible of the substance
to be determined is lost in this way.
(b) Masking. A complexing agent is added, and if the resultant complexes are
sufficiently stable they will fail to react with reagents added in a subsequent
operation: this may be a titrimetric procedure or a gravimetric precipitation
method.
(c) Selectioe oxidation (reduction). The sample is treated with a selective
oxidising or reducing agent which will react with some of the ions present:
the resultant change in oxidation state will often facilitate separation. For
example, to precipitate iron as hydroxide, the solution is always oxidised
so that iron(II1) hydroxide is precipitated: this precipitates at a lower pH
than does iron(I1) hydroxide and the latter could be contaminated with
the hydroxides of many bivalent metals.
(d) Soloent extraction. When metal ions are converted into chelate compounds
by treatment with suitable organic reagents, the resulting complexes are
soluble in organic solvents and can thus be extracted from the aqueous
solution. Many ion-association complexes containing bulky ions which are
largely organic in character (e.g. the tetraphenylarsonium ion (C, H ,),As + )
are soluble in organic solvents and can thus be utilised to extract appropriate
metals ions from aqueous solution. Such treatment may be used to isolate
the ion which is to be determined, or alternatively, to remove interfering
substances.
(e) Ion exchange. Ion exchange materials are insoluble substances containing
ions which are capable of replacement by ions from a solution containing
electrolytes. The phosphate ion is an interference encountered in many
analyses involving the determination of metals; in other than acidic solutions
the phosphates of most metals are precipitated. If, however, the solution is
passed through a column of an anion exchange resin in the chloride form,
then phosphate ions are replaced by chloride ions. Equally, the determination
of phosphates is difficult in the presence of a variety of metallic ions, but