Instant Notes: Analytical Chemistry

It is possible to titrate two cationic species in a solution by performing the

titration at different pH values. However, if a solution of high pH must be used,

this might cause precipitation of metal hydroxides or other insoluble species. In

order to prevent this, secondary complexing agentscan be added to retain the

metal ion in solution. Ammonium chloride and triethylamine are typical

reagents for this purpose.

Zinc ions, which might otherwise form insoluble Zn(OH) 2 at pH of 10, may be

converted to soluble zinc amine complexes. These are less stable than the EDTA

complex and the zinc may then be reacted quantitatively.

Secondary complexing agents may also act as masking agents. Examples

of this are the use of cyanide ions to form stronger complexes with heavy

metal ions so that magnesium can be titrated, or masking Fe^2 +and Mn^2 +using

hydroxylamine in water hardness determinations.

Standard solutions of EDTA may be prepared from the dry disodium salt

(Na 2 H 2 Y, RMM 336 or the dihydrate, RMM 372), by dissolving a known amount

in water free of heavy metals. Alternatively, the solution may be standardized

by a standard magnesium salt solution.

In complex and precipitation titrations, as in others, the end point corre-

sponds to a rapid change in the concentration of species. This may be detected

by instrumental methods, particularly potentiometry (see Topic C3) and by

visual indicators discussed below. Using suitable indicators, or potentiometric

measurements, it is possible to detect two or more end points.

Complexometric indicatorsbehave in a similar way to titrating complexing

agents such as EDTA. They generally change color with pH, but one species,for

example HIn^2 - , will react with excess metal ions Mn+:

HIn^2 - +Mn+=MIn(n-3)++H+

blue red

Some selected indicators for complexometric titrations are given in Table 2.

In order to use potentiometric methodsto study complexometric titrations,

an electrode specific to the metal ion may be used (see Topics C3 and C5), for

example, a copper ISE to follow the reaction of copper with EDTA.

Alternatively, a ‘J’-shaped electrode with a small mercury pool may be used

together with a small amount of added Hg-EDTA complex. This acts in a similar

way to the Class 2 electrodes, where the complexes determine the concentration

of ions in contact with the mercury pool:

Mn++HY^3 - =MY(n-4)++H+

Hg^2 ++HY^3 - =HgY^2 - +H+

Hg^2 ++2e-=Hg

C7 – Titrimetry II: complexation, precipitation and redox titrations 91

Table 2. Indicators for complexometric titrations

Indicator Free color Complex color* Metal ions

Eriochrome black T Blue Red Ba, Cd, Ca, Pb, Zn

Pyrocatechol violet Yellow Blue Al, Bi, Cd, Co, Cu

Fe, Mg, Mn, Ni, Zn

Xylenol orange Yellow Red Bio, Cd, Pb, Th, Zn

Calcon carboxylic acid Blue Red Ca, Cd, Mg, Mn, Zn

*varies with metal and pH