it consists of notes for complexometric titration principle, edta, procedure, applications. colorimetry and spectrophotometry principle, introduction, instrumentation and applications

1. COMPLEXOMETRIC TITRATION
Complexometry is the type of volumetric analysis involving the
formation of complexes which are slightly ionized in solution, like
weak electrolyte and sparingly soluble salt.
 Here metal ion solutions are titrated against solutions of polydentate
ligands.
PRINCIPLE:
 EDTA is the most commonly used polydentate ligand
 EDTA  Ethylenediaminetetraacetic acid
 Its salt is preferred because of its adequate solubility in aqueous solutions.
 EDTA forms stable, water-soluble, 1:1 complexes with most of the
polyvalent metal ions.
 It has 6 donoratoms  two nitrogen and four oxygen atoms of the
four carboxylic groups
 The complex ion has a cage-like structure in which the metal ion iis
so well surrounded by the donorgroups that it remains almost
completely isolated from the solvent molecules
 As a result the complex ion is highly stable.
 EDTA is a tetraprotic acid , denoted as H4Y
 The dissociation constants for the COOHgroups:
1. K1=1.0*10-2
2. K2=2.15*10-3
3. K3=6.2*10-7
4. K4=5.5*10-11
 From these dissociation constants,, it can be easily shown that at
pH>12, most of the EDTA exists as the tetra anion, Y4-
 Around pH 8  predominant species is HY3-
 Around pH 5  predominant species is H2Y2-
 Titration between Mn+ and EDTA:
Mn+ + H2Y2-  MY(n-4)+ + 2H+
Mn+ + HY3-  MY(n-4)+ + H+

2.  The protons liberated in these reactions would considerably alter the
pH of the solution and would thus cause the back reaction, i.e., the
dissociation of the metal-EDTA complex.
 This is prevented by adding an excess of an innocuous buffer to the
titration system.
INDICATORS FOR EDTATITRATIONS:
 Several organic dyes are used as indicators for EDTA titrations which
forms complexes with metal ions which are intensely coloured.
 The most widely used indicator is Eriochrome Black T.
 EDTA is denoted by, H3In
 Indicator exhibit different colours in various pH ranges.
 At pH less than 6  H2In- (red)
 At pH 7 or above  HIn2- (blue)
 More alkaline  In3- (orange)
EDTA titration:
During an EDTA titration two complexes are formed
i) Metal-EDTA complex
ii) Metal ion-indicator complex
 Suitable amount of indicator is added to the solution of Metal ions in the
titration flask.
 The two species react together to form a metal ion-indicator complex.
 If metal ion is bivalent M2+ , the complex formed in aqueous solution
would be MIn- (red).
 A buffer solution of appropriate pH is added to the titration flask.
 Standard solution of sodium salt of EDTA in the burette is added to
metal ion-indicator complex to form metal-EDTA complex.
 Because, Metal ion –indicator complex is less stable than the metal-
EDTA complex.

3.  Thus a change in colour from red to blue marks the approach of end
point.
 EDTA combines with metal ions in the ratio of 1:1
 Irrespective of their valencies to form almost exactly the same type of
complexes.
 The stability of EDTA complexes depends considerably on pH and
 It makes the EDTA titrations adequately selective.
 Eg: Around pH-1 divalent ions doesn’tform complexes with EDTA, but
only the trivalent ions do.
 Cd2+, Zn2+ forms more complex than Mg2+.
 In pH-7 EBT doesn’tform complex with Mg2+ thus the former cations
forms complexes easily with EDTA.
MASKING OF IONS:
 Selectivity is provided by the masking of interfering cations.
 Eg: Mg2+ and Ca2+ can be titrated in the presence of interfering ions such
as Co2+, Cu2+, Ni2+ and Pd by adding cyanide ions to the reaction mixture
as the MaskingAgent.
 The later ions forms stable cyanide complexes such as [M(CN)4]2-
 Demasking agent:reagent released the metal ion from a masking
reagent.
TYPES OF TITRATIONS:
 Direct titration
 Back titration
 Displacement method
BACK TITRATION DIRECT TITRATION

4. APPLICATIONS:
 Complexometric titration is widely used in the medical industry
because of the micro litre size sample involved. The method is
efficient in research related to the biological cell.
 Ability to titrate the amount of ions available in a living cell.
 Ability to introduce ions into a cell in case of deficiencies.
 Complexometric titration involves the treatment of complex ions
such as magnesium, calcium, copper, iron, nickel, lead and zinc
with EDTA as the complexing agent.
 Complexometric titration is an efficient method for determining
the level of hardness of water.
COLORIMETRY AND SPECTROPHOTOMETRY
Both the techniques are falls under the broad classification of photometry
which involves in the measurement of absorption and emission of the
radiation.
RANGE:
Colorimetry : visible range (4000-8000A)
Spectrophotometry: UV visible range (2000-8000A)
PRINCIPLE:
 It is based on Beer-lambert’slaw.
 It states that the amount of light absorbed by a color solution is
directly proportional to concentration of the solution and length
of a light path through the solution.
Where,
A = absorbance/optical density of solution
C = concentration of solution
Log I0/It = A = εcl

5. l = path length ε = absorption coefficient
BEER’S LAW:
This law states that the amount of light absorbed is directly
proportional to the concentration of the solute in the solution.
Log I0/It = kc
Where,
K = absorption coefficient
C = concentration of the solution
I0 = intensity of incident light
It = intensity of transmitted light
LAMBERT’S LAW:
This law states that the amount of light absorbed is directly
proportional to the length and thickness of the solution under analysis.
Log I0/It = kt
Where,
t = thickness/length of the absorbing medium (solution)
APPLICATIONS
 Detection of concentration of substances.
 Detection of impurities.
 Structure elucidation of organic compounds.
 Monitoring dissolved oxygen content in freshwater and marine
ecosystems.
 Characterization of proteins.
 Detection of functional groups.
 Molecular weight determination of compounds.
 Determination of pk value of an indicator
 Determination of molar composition of complexes
 Determination of instability constants
QUANTITATIVE ANALYSIS:
 In order to prepare calibration curves, a series of solution, each with the
known concentration, are prepared.
 The colorimeter or spectrophotometeris set at that wavelength where the
absorption is maximum.
 After this these absorbanceof each solution is measured and then plotted
against be concentration of the solution to get a calibration curve.
 If absorbanceis measured at a wavelength as used for the calibration
curve of an unknown solution, then concentration of the unknown
solution can be measured from the calibration curve by seeing the value
of concentration against the absorbanceof the solution.
 Colorimetric analysis is generally used whenever the sample is coloured.

6. Eg: dichromate, permanganate, cupricion and ferric ion.
STRUCTURE OF INORGANIC COMPOUNDS:
 Absorption spectralstudies have revealed that deep blue colour of
ferric thiocyanate is due to the presence of FeCNS2+ ions.
STRUCTURE OFF INORGANIC COMPLEXES:
 Spectrophotometryhas been used to distinguish between cis and
trans isomers of a complex.
 Equilibrium between the octahedral, tetrahedral, squareplanar, and
the five coordinate configurations have been studied by absorption
spectroscopy.
 The geometrical isomers of transition complexes can be easily
distinguished from their visible spectra.
 Eg: cis isomer of [Co(en)2F2]NO3 is violet whereas the trans isomer
is green
DETERMINATION OF MOLECULAR WEIGHT:
 When a compound reacts with a reagent to form a derivative
having a characteristic absorptionin another wavelength region,
the molar absorptivity ε of the derivative is almost the same as that
of a reagent.
 For example, amine picrates  max absorption= 380nm having
ε = 13,400 which is same as that of picric acid.
 Thus, one can determine the molecular weight of given amine
picrate by dissolving its known amount in a known volume of
solvent and then its absorbanceis measured at 380nm.
 Then on applying BEER’S LAW, we get
A = act where, a = absorptivity ; c = concentration
 Molecular weight can be calculated by applying the following
relation:
A DOUBLE BEAM UV-VISIBLE ABSORPTION SPECTROMETER
INSTRUMENTATION
If you pass white light through a coloured substance, some of the light gets absorbed.
A solution containing hydrated copper(II) ions, for example, looks pale blue because the
solution absorbs light from the red end of the spectrum. The remaining wavelengths in the
light combine in the eye and brain to give the appearance of cyan (pale blue).
ε = a × molecular weight

7.  Some colourless substances also absorb light - but in the ultra-violet region.
Since we can't see UV light, we don't notice this absorption.
 Different substances absorb different wavelengths of light, and this can be
used to help to identify the substance - the presence of particular metal ions,
for example, or of particular functional groups in organic compounds.
 The amount of absorption is also dependent on the concentration of the
substance if it is in solution. Measurement of the amount of absorption can be
used to find concentrations of very dilute solutions.
 An absorption spectrometer measures the way that the light absorbed by a
compound varies across the UV and visible spectrum.
A simple double beam spectrometer
The light source
 A light source which gives the entire visible spectrum plus the near ultra-violet
covering the range from about 200 nm to about 800 nm. (This extends slightly into
the near infra-red as well.)
 A combination of two lamp is used - a deuterium lamp for the UV part of the
spectrum, and a tungsten / halogen lamp for the visible part.
 The combined output of these two bulbs is focussed on to a diffraction grating.
The diffraction grating and the slit
 A prism splits light into its component colours. A diffraction grating does the same job,

8. The arrows show the way the various wavelengths of the light are sent off in different
directions. The slit only allows light of a very narrow range of wavelengths through into the
rest of the spectrometer. By gradually rotating the diffraction grating, you can allow light
from the whole spectrum (a tiny part of the range at a time) through into the rest of the
instrument.
The rotating discs:
Each disc is made up of a number of different segments.
The light coming from the diffraction grating and slit will hit the rotating disc and one of
three things can happen.
 If it hits the transparent section, it will go straight through and pass through the
cell containing the sample. It is then bounced by a mirror onto a second
rotating disc. This disc is rotating such that when the light arrives from the first
disc, it meets the mirrored section of the second disc. That bounces it onto the
detector.
 It is following the red path in the diagram:

 If the original beam of light from the slit hits the mirrored section of the first
rotating disc, it is bounced down along the green path. After the mirror, it
but more efficiently.

9. passes through a reference cell (more about that later). Finally the light gets to
the second disc which is rotating in such a way that it meets the transparent
section. It goes straight through to the detector.
1. If the light meets the first disc at the black section, it is blocked - and for a very short
while no light passes through the spectrometer. This just allows the computer to make
allowance for any current generated by the detector in the absence of any light.
The sample and reference cells:
 These are small rectangular glass or quartz containers. They are often designed so that
the light beam travels a distance of 1 cm through the contents.
 The sample cell contains a solution of the substance you are testing - usually very
dilute. The solvent is chosen so that it doesn't absorb any significant amount of light
in the wavelength range we are interested in (200 - 800 nm).
 The reference cell just contains the pure solvent.
The detector and computer
 The detector converts the incoming light into a current. The higher the current, then
greater is the intensity of the light.
 For each wavelength of light passing through the spectrometer, the intensity of the
light passing through the reference cell is measured. This is usually referred to as Io -
that's I for Intensity.
 The intensity of the light passing through the sample cell is also measured for that
wavelength - given the symbol, I.
 If I is less than Io, then obviously the sample has absorbed some of the light. A simple
bit of maths is then done in the computer to convert this into something called
the absorbance of the sample - given the symbol, A.
 For reasons which will become clearer when we do a bit of theory on another page,
the relationship between A and the two intensities is given by:
 On most of the diagrams you will come across, the absorbance ranges from 0 to 1, but
it can go higher than that.
 An absorbance of 0 at some wavelength means that no light of that particular
wavelength has been absorbed. The intensities of the sample and reference beam are
both the same, so the ratio Io/I is 1. Log10 of 1 is zero.

10.  An absorbance of 1 happens when 90% of the light at that wavelength has been
absorbed - which means that the intensity is 10% of what it would otherwise be.
 In that case, Io/I is 100/I0 (=10) and log10 of 10 is 1.
The chart recorder
Chart recorders usually plot absorbance against wavelength. The output might look like this:
This particular substance has what are known as absorbance peaks at 255 and 395 nm. How
these arise and how they are interpreted are discussed on another page.