MIDLANDS STATE UNIVERSITY
DEPARTMENT OF CHEMICAL TECHNOLOGY
DEPARTMENT OF CHEMICAL TECHNOLOGY
PART ONE ANALYTICAL CHEMISTRY (CT104)
EDTA TITRATIONS
Ethylenedinitrilotetraacetic acid (EDTA:1,2 diaminoethanetetra-acetic acid) is one of a class of
aminopolycarboxylic acids which form soluable strong complexes with many metal ions. When a solution
containing asuitable divalent metal ion is titrated with EDTA , the reaction that occurs can be formulated:
M2+
H2Y2 -
+
MY2-
2H+
+
which is a special case of the general equation:
Mm+
+
HnYn-4
MYm-4
+
nH+
Where Y4- is the fully deprotonated anion of EDTA.
The free acid has the formula:
-
OOC-CH2
CH2-COOH
N-CH2-CH2-N
CH2-COO-
HOOC-CH2
It is a hexadentate ligand and one of its most interesting features is that it forms 1:1 complexes with metal ions
regardless of their charge. The first practical application of the EDTA titration was the determination of water
hardness (calcium and magnesium salts) but it can be used for many other ions as well. In this practical EDTA
titrations will be illustrated by using it to titrate zinc, calcium, magnesium and nickel.
The end-points of titrations with EDTA are usually detected by metallochromic indicators. These are organic
colourship matters that undergo a colour change when they form metal complexes. Generally speaking, not only the
dyestuff but also its metal complex are coloured, so that the end-point of the titration is not characterized by e
appearance or disappearance of a color , but rather a change in color. Two of the most common metallochromic
indicators are murexide and eriochrome black T (Erio T for short).
Murexide
Murexide , which is blue in alkaline solution , forms a pink complex with calcium ions.When a solution containing
calcium and murexide is titrated with EDTA, the original blue color of the indicator is restored when all the calcium
ions have been complexed .The anion of murexide has the following constitution.
HN
O
C
C O
C
HN C O
O C NH
N
C
C O
O C NH
Several other metal ions such as cadmium , cobalt , copper , mercury , nickel and zinc give a color with murexide.
Magnesium is without effect ; hence , at a suitable pH , calcium can be titrated in the presence of magnesium when
murexide is used as indicator.
Eriochrome Black T
This indicator is blue between pH 6.3 and 11.3.The magnesium complex is wine-red ; when magnesium is titrated
with EDTA in the presence of this indicator between these pH limits , the latter reverts to its original blue color
when all the magnesium ions have been complexed.The optimum pH is 10.
The complex formed between calcium and this dye is too unstable for indicator purposes.When a solutuion
containing both calcium and magnesium os titrated with EDTA , calcium is preferentially complexed by EDTA , but
an end point is not obtained until all the magnesium has also been complexed. Hence the sum of magnesium and
calcium is obtained .Several other metals give similar color reactions and can be titrated in the same way as
magnesium, e.g. zinc ,cadmium ,lead and manganese.
Zinc
In this practical EDTA will be standardized with a zinc solution.This titration presents no difficulties and involves
titration in an ammoniacal solution with Erio T as indicator.
Magnesium
The indicator Erio T is used in this titration.The stability of the complexes of Mg with EDTA and with the indicator
are just high enough to permit an accurate titration and in point of the fact that color change at the end-point (winered) is less sharp than in many other complexometric titrations. The titration must be continued until the last
suggestion of a reddish blue has gone.Interfering metals such as Cu,Ni,Co,Fe etc are masked by KCN - sodium
sulphide is equally effective as a precipitant.
Calcium
The titration of calcium with EDTA can be carried out in very dilute solutions with murexide as indicator.
However,the the color change from red to blue- violet which takes place place in strongly alkaline medium (Ph 12)
is not so sharp as in metallochromic indicators.
Nickel
Nickel can be titrated directly in strongly ammoniacal solution using murexide as indicator .With all nickel titrations
,, it is necessary to titrate slowly near the end-point, as the rate of formation of the Ni-complex is not very high.
Ni can also be determined by way of back-titrations, and this has the advantage that the lower rate of reaction is now
of no consequence.The back-titration can be carried out with either copper or zinc.
Procedure
Standardization of EDTA solution
Weigh accurately about 0.25g of zinc oxide and transfer into a conical flask .Add 0.5M HCl dropwise until all the
zinc oxide has dissolved .Cool and transfer to a 250ml volumetric flask with distilled water .Pipette 10ml portions
into each of three 250ml conical flasks ;add concentrated ammonia solution in excess to dissolve the precipitate
formed .Titrate with the EDTA solution using Erio T indicator .Calculate the molarity of the EDTA solution.
HARDNESS OF WATER
Hardness of water is due calcium and magnesium salts which are present as hydrogen carbonates (temporary
hardness), and sulphates, chlorides and nitrates (permanent hardness). These salts are leached out by natural water,
usually containing dissolved CO2, as it flows among rock strata underground. Hard water has poor washing
properties and when heated leaves deposits of solid CaCO 3. The determination of water hardness is, therefore,
important in many industrial processes where water is an important raw material, so that where necessary,
appropriate steps can be taken.
DETERMINATION OF Ca HARDNESS IN WATER
Pipette 100ml of bore-hole water into a 250ml conical flask, add 2ml of the ammonia buffer pH 10 and add 2 drops
of freshly prepared 1% sodium sulphide solution. After 1 minute add a tip of a spatula Erio T indicator and titrate
with the standardized EDTA solution until the last traces of red have disappeared from the solution.
Calculate the total hardness as ppm CaCO3.
DETERMINATION OF NICKEL
Pipette 100ml of the neutral sample solution into a 250ml conical flask. Add murexide indicator. Then add 10 ml of
1M NH4Cl. If the pH of the solution is below 7 the indicator will have an orange-yellow colour (due to NiH4D+) and
dilute ammonia must be added dropwise until the colour changes to yellow (due to NiH 2D-). Start the titration and
continue until the end-point is near. If the colour reverts to orange because of a fall in pH, add a few drops of
ammonia and continue the titration. Just before the end-point make the solution strongly ammonical by adding 10ml
of concentrated ammonia and titrate to a brilliant colour change from yellow to bluish-violet.
Calculate the concentration of Ni in the sample solution.