Chapter 8
Complexometric (EDTA)
Titratioms
cactions involving complex ions or undissociated neutral molecules in solution fall under the class
Nof complexometric titrations. The basic requirement is that such a complex should be extremely
soluble. The complexes of metals with EDTA (disodium salt) fall into this category. In addition to
EDTA titration, titrations involving mercuric nitrate and silver cyanide are also known as complexometric
titrations.
8.1 EDTA AND THE COMPLEXONES
EDTA also called by several other names such as Versene, Complexone II, Sequesterene, Nullapon,
Trilon B, Idranat II. It has the structure as shown in Fig. 8.1.
HOOCCH, CH,COOH
N
-CH,CH,
HOOCCH, CH,COOH
Fig. 8.1 EDTA
CO
CO
H,c
CH,
Ca
H,C
CO
Fig. 8,2 Ca (NTA) complex
, 62 ANALYTICAL CHEMISTRY
It has both donor oxygen atoms and donor nitrogen atoms and it can form six (or
chelate rings simultaneously. Another agent is nitriliotriacetic acid N(CH,COOH),. five)
plex is shown (Fig. 8.2). Schwarzenbach exploited a number of amine carboxylic Its
acids as calciumemmbeCom-red
masking agents.
Flaschka reviewed its use as a masking agent. Several metals from complexes at titrating and
complexing action is proportional to the activity of the free anion y- (if acid is H,Y
constants pK, = 2.0, pK, =2.64, pK, =6.16, pK, =10.26) showing that activity of Y4- with
dif ering pH. The
1.0 to 10.0and universally changes as the square of [H"]in pH 3.0-6.0. The varies
complexes of
iofromnisatiophin
metals with
higher charge are more stable. Only Be?*; UO;" do not form complexes with EDTA. In addition
complexing agents are used in titrimetry (Fig. 8.3 and Fig. 8.4) are EGTA and CDTA. other
CH,-0-CH,CH,-N(CH,COOH),
CH,-0-Ch, -CH, -N(CH,COOH),
i.e. 1, 2-di (2-aminoethoxylethane)
NNNN (tetraacetic acid (EGTA)
Fig, 8.3 EGTA
CH,
CH, CH N(CH,COOH),.
CH, CH -N(CH,COOH),
CH,
i.e. 1, 2-di diaminocyclohexane
tetraacetic and (CDTA)
Fig. 8.4 CDTA
Similarly triethylene tramine (trien) is also used in titrations. H,Y or Na, H,Y
EDTA dissolves readily in water. It is available in high degree of are its formulat.
aprimary standard but due to an uncertain purity. The dihydrate can be used as
amount of water it is better to standardise it. Cadmium metal
is a good primary standard for such
standardisation.
8.2
COMPLEXOMETRIC TITRATION CURVES
EDTA is hexadentate, but in titration we use the
During the complexation reaction we have disodium salt hence it behaves as quadridentate,
M+R IMR'4)
IMR4)] so K,s absolutely stability constant
The fourdissociation constants of acid HR are as follows:
Case (i) H,R +H,0 H,0* +
H,R K=1.02x10
Complexometric (EDTA)
Titratioms
cactions involving complex ions or undissociated neutral molecules in solution fall under the class
Nof complexometric titrations. The basic requirement is that such a complex should be extremely
soluble. The complexes of metals with EDTA (disodium salt) fall into this category. In addition to
EDTA titration, titrations involving mercuric nitrate and silver cyanide are also known as complexometric
titrations.
8.1 EDTA AND THE COMPLEXONES
EDTA also called by several other names such as Versene, Complexone II, Sequesterene, Nullapon,
Trilon B, Idranat II. It has the structure as shown in Fig. 8.1.
HOOCCH, CH,COOH
N
-CH,CH,
HOOCCH, CH,COOH
Fig. 8.1 EDTA
CO
CO
H,c
CH,
Ca
H,C
CO
Fig. 8,2 Ca (NTA) complex
, 62 ANALYTICAL CHEMISTRY
It has both donor oxygen atoms and donor nitrogen atoms and it can form six (or
chelate rings simultaneously. Another agent is nitriliotriacetic acid N(CH,COOH),. five)
plex is shown (Fig. 8.2). Schwarzenbach exploited a number of amine carboxylic Its
acids as calciumemmbeCom-red
masking agents.
Flaschka reviewed its use as a masking agent. Several metals from complexes at titrating and
complexing action is proportional to the activity of the free anion y- (if acid is H,Y
constants pK, = 2.0, pK, =2.64, pK, =6.16, pK, =10.26) showing that activity of Y4- with
dif ering pH. The
1.0 to 10.0and universally changes as the square of [H"]in pH 3.0-6.0. The varies
complexes of
iofromnisatiophin
metals with
higher charge are more stable. Only Be?*; UO;" do not form complexes with EDTA. In addition
complexing agents are used in titrimetry (Fig. 8.3 and Fig. 8.4) are EGTA and CDTA. other
CH,-0-CH,CH,-N(CH,COOH),
CH,-0-Ch, -CH, -N(CH,COOH),
i.e. 1, 2-di (2-aminoethoxylethane)
NNNN (tetraacetic acid (EGTA)
Fig, 8.3 EGTA
CH,
CH, CH N(CH,COOH),.
CH, CH -N(CH,COOH),
CH,
i.e. 1, 2-di diaminocyclohexane
tetraacetic and (CDTA)
Fig. 8.4 CDTA
Similarly triethylene tramine (trien) is also used in titrations. H,Y or Na, H,Y
EDTA dissolves readily in water. It is available in high degree of are its formulat.
aprimary standard but due to an uncertain purity. The dihydrate can be used as
amount of water it is better to standardise it. Cadmium metal
is a good primary standard for such
standardisation.
8.2
COMPLEXOMETRIC TITRATION CURVES
EDTA is hexadentate, but in titration we use the
During the complexation reaction we have disodium salt hence it behaves as quadridentate,
M+R IMR'4)
IMR4)] so K,s absolutely stability constant
The fourdissociation constants of acid HR are as follows:
Case (i) H,R +H,0 H,0* +
H,R K=1.02x10
