Analytical Chemistry

Table 5.10 Some examples of EDTA titration methods

Analyte and matrix Outline procedure

Ca and Mg in water (water

hardness)

Ca determination

Add 2 cm^3 of NaOH solution (0.1 mol dm–^3 ) to 50 cm^3 of sample and titrate with EDTA

using murexide indicator (Table 5.8).

Mg determination^

Destroy murexide colour with 1 cm^3 concentrated HCl, add 3 cm^3 NH 3 —NH 4 Cl buffer and

titrate with EDTA using eriochrome black T.

Pb in minerals Dissolve the sample in nitric acid. Separate Pb by extraction with CHCl 3 solution of sodium

diethyl dithiocarbonate using alkaline cyanide solution to mask interferences. Titrate Pb in

an ammonia–ammonium chloride medium at pH = 10 using eriochrome black T.

Zn in light alloys Dissolve the sample in NaOH solution and precipitate ZnS with NaS solution. Dissolve

ZnS in HCl solution, add ammonium citrate to mask Al and then titrate Zn at pH = 9 using

eriochrome black T as indicator.

Ca, Mg and Zn in biological

tissue

Ash the tissue and extract the residue with HCl solution. Separate Ca, Mg and Zn by

retention on a cation exchange column followed by elution with HCl (5 mol dm–^3 ). Titrate

Ca using murexide, Ca plus Mg using eriochrome black T, then add chloral hydrate to give

a further end point with Zn.

Au in Au–Pt–Ag alloys Dissolve sample in 'aqua regia' and filter off AgCl. Extract Au with diethyl ether, evaporate

solvent, and react Au with K 2 Ni(CN) 4. Buffer with NH 3 —NH 4 Cl, react liberated Ni with

excess EDTA. Back titrate EDTA excess with Mn(II) using eriochrome black T.

given in Table 5.11. It is however beyond the scope of this text to discuss such reagents in detail.
Inorganic ligands which have been used as complexometric titrants include the halide ions and
pseudohalides CN– and SCN–. Chloride ion may be determined by titration with mercury(II) nitrate
solution, when stoichiometric formation HgCl 2 is indicated by a consideration of the stepwise formation

constants for (K 1 = 5.5 × 106 , K 2 = 3.0 × 106 , K 3 = 7, K 4 = 10) which reflect the low stability of

and. The end point may be detected by the reaction of excess mercury ions with sodium
nitroprusside or diphenylcarbazone, when a white precipitate is produced in each case. The titration can
be used to determine chlorides in solution within the range 1– 100 μg cm–^3 and has been widely
employed for the analysis of water samples. Parallel reactions with Br–, SCN– and CN– may also be used
but iodides are more difficult to handle and quantitative analysis is not easy.

Cyanide reagents have been traditionally used in the metal-finishing industry and the reaction,