1
EA – AC
Winter Chemistry Lecture 6
February 12, 2003
To finish up Chapter 13, let’s spend a few minutes looking at indicators for EDTA
titrations.
There are over 200 organic compounds that have been used as indicators for metal ions in
EDTA titrations. In general these indicators are organic dyes that form colored complexes
with metal ions in a pM range that is characteristic of the particular cation and dye.
The complexes are often intensely colored and are discernible to the eye at concentrations
in the range of 10-6 to 10-7 M.
XXXXXX
Eriochrome Black T is a typical metal-ion indicator that is used in the titration of several
common cations. As shown in the transparency, this compound contains a sulfonic acid
group, which is completely dissociated in water, and two phenolic groups that only partially
dissociate.
XXXXX
Its behavior as a weak acid is described by the equations
H2In- <= =>
red
HIn2- + H+
blue
K1 = 5 x 10-7
HIn2- <= =>
blue
In3- + H+
orange
K2 = 2.8 x 10-12
Note that the acids and their conjugate bases have different colors. Thus, Eriochrome
Black T behaves as an acid/base indicator as well as a metal ion indicator.
The metal complexes of Eriochrome Black T are generally red. So is H2In- as we can see.
So a transition from MIn- to H2In- does not change color. However if we adjust the pH to 7
or above so that the indicator is in the blue form of the HIn2-, then we can detect an end
point which is red to blue.
Until the end point in a titration, the indicator complexes the excess metal ion, so the
solution is red. EDTA has much higher metal complex formation constants than the
indicators, so as EDTA is added to the solution it binds the available metal ions complexes.
Just before the equilvalence point EDTA as bound all the free metal ions and takens ions
away from the indicator complexes. Thus when EDTA has complexed all the metal ions,
the solution turns blue as the indicator is converted from MIn- to H2In-.
Complexometric titrations with EDTA have been applied to the determination of virtually
every metal cation with the exception of the alkali metal ions.
2
XXXXXX
Let’s return for a moment to an EDTA - calcium titration. This titration curve was
calculated for the conditions of the problem we solved last week. The equivalence point is
at pCa = 6.36. A titration curve for magnesium is also shown under the same pH 10
conditions.
Notice several things.
The calcium titration is over before little of the Mg starts. The indicator regions shown
show that Eriochrome black T is a good indicator for Mg as it changes color right at the
equivalence point, but that it is not a good indicator for calcium under these conditions.
Now a quick review of some of the characteristics of EDTA titrations.
Because EDTA forms complexes with most cations, the reagent might appear at first
glance to be totally lacking in selectivity. In fact, however, considerable control
over interferences can be realized by pH regulation.
For example, trivalent cations can usually be titrated without interference from divalent
species by maintaining the solution at a pH of about 1.
At this pH, the less stable divalent complexes do not form to any significant extent, but the
trivalent ions are quantitatively complexed.
Similarly, ions such as cadmium and zinc, which form more stable EDTA complexes than
does magnesium, can be determined in the presence of the latter ion by buffering the
mixture to pH 7 before titration.
Eriochrome Black T serves as an indicator for the cadmium or zinc end points at pH 7
without interference from magnesium because the indicator complex with magnesium is
not formed at this pH.
Finally, interference from a particular cation can sometimes be eliminated by adding a
suitable masking agent, an auxiliary ligand that preferentially forms highly stable
complexes with the potential interferant.
For example, cyanide ion is often employed as a masking agent to permit the titration of
magnesium and calcium ions in the presence of ions such as cadmium, cobalt, copper,
nickel, zinc and palladium.
All of the latter form sufficiently stable cyanide complexes to prevent reaction with EDTA.
Finally a word about different types of EDTA titrations.
There are cations of 40 elements that can be determined by direct titration with EDTA
using metal ion indicators.
3
The direct method is not appplicable to all cations, however, either because no good
indicators have been developed, or because the reaction between the metal ion and EDTA
is to slow to make titration practical. Such cations are analyzed by alternative methods.
It is often convenient to introduce a small amount of a cation that forms an EDTA complex
that is less stable than the analyte complex and for which a good indicator exists.
For example, indicators for calcium are generally less satisfactory than those for
magnesium ion.
So we add a small amount of magnesium chloride to the EDTA solution to be used in a
calcium determination. We use Eriochrome Black T as the indicator.
In the initial stages of the titration, magnesium ions are displaced from the EDTA complex
by calcium ions and are free to combine with the indicator, thus imparting a red color to the
solution.
When all of the calcium have been complexed, however, the magnesium dissociate from
the indicator complex and bind to the first excess of EDTA. The result is a blue end point
when the free indicator is produced.
For reactions that are kinetically slow, we use the back-titration method. We add an excess
of EDTA to an aliquot of the unknown. We wait for them to react or as Jim said last week
we can heat the reaction and shorten the time to equilibrium. Then we determine the
amount of unreacted EDTA, by back-titrating the EDTA with a standard metal ion solution.
EPA approved methods and Standard Methods of Water Analysis have pages and pages of
EDTA analysis methods. They should made sense if you remember the points made in this
chapter.
We mentioned last week injecting condors in Arizona with an EDTA solution to counteract
lead poisoning.
Kf = 1018 =
[PbY2-]
[Pb2+][Y4-]
with a 10-6 M concentration of EDTA in blood stream you get
[PbY2-] = 1012 i.e. virtually all the free lead ions are removed so that can not poison
[Pb2+]
metabolism. The lead-EDTA complex is eliminated in the urine.
Now you can see why EDTA is an effective treatment form mercury cation and other metal
poisoning as well.
4
Chapter 18 is an introduction to spectroscopy and the start of a sereies of short chapters
about optical analytical instrumentation.
I assume that most of you have covered this material at least once by now and some of you
must have gone over this three or four times by now. The material is in preparation of
talking about uv-vis spectrophotometers as well as AA and ICP. So you should have a fairly
easy time for the next few weeks as you already have a working knowledge of the
instruments.
The measurement of the absorption and emission of light by materials is called
Spectrophotometry which is often shortened to spectrometry.
The terms absorption and emission have the same meaning they have in everyday use:
Absorption means to take up, and emission means to give off.
XXXXXX
When we use the word light, we usually mean light visible to our eyes. However,visible light
is only a small part of the electromagnetic spectrum, which includes radio, microwave,
infrared, visible, ultraviolet, X-ray, and gamma-ray radiation.
We will use the word light as a general term for electromagnetic radiation simply because it is
less cumbersome than saying electromagnetic radiation from radiowaves to -radiation.
Specific instruments used in spectrometry are referred to as spectrophotometers,
spectroradiometers, or spectrometers, depending on their architectures. The term
spectrometers will be used here to refer to all types.
Indeed, current usage extends the meaning of spectroscopy to include techniques that do not
even involve electromagnetic radiation. Examples of the last include acoustic, mass, and
electron spectroscopy.
Chapter 18 contains introductory material dealing with the properties of electromagnetic
radiation and its various interactions with matter. The next three chapters are then devoted to
spectrochemical methods that are based upon emission and absorption of ultraviolet, visible,
and infrared radiation by both atomic and molecular species.
Production and detection of radiation require different techniques in different regions of the
electromagnetic spectrum. However, two fundamental variables are measured in all
spectrometric methods:
1. The wavelength (or energy) of the radiation
2. The amount (or intensity) of the radiation at that wavelength
When light energy is absorbed by a specific material, it is absorbed only at certain
wavelengths. At other wavelengths, the material may be transparent. The actual
wavelengths absorbed are a property of the material.
5
Similarly, when light is emitted by an atom or molecule, it does so only at its characteristic
emission wavelengths. At other wavelengths, no emission occurs. Again emission
wavelengths are a property of the material.
In sum, the wavelengths that are absorbed and emitted depend on the identity of the
compound – its chemical structure and energy states. These wavelengths remain the same
regardless of the quantity of the analyte present. However, the amount of light absorbed or
emitted does depend on the amount of the compound present in the light path (in
uncomplicated cases).
This relationship between amount of light and amount of compound provides us with
analytical methods for quantifying analytes.
Spectrometry can be used to determine both the elemental and the molecular content of
materials. However, just like everything else we have examined, it is important to realize
that each spectrometric method has its own benefits, drawbacks, specificities, and
interferences.
XXXXXX
Expanding on the last of these, possible interferences in spectrometry can be classified into
three types.
1 . Spectral interference: The absorption or emission of other components of the matrix
occurs at the same wavelength(s) used for the assay.
2. Chemical interference: The material being determined is not in the same chemical form
over the determinable concentration range, or else the form varies significantly with changes
in the matrix. Thus, part of the analyte signal expected may not appear at the wavelength(s)
chosen for the analysis.
3. Instrument interference: Effects on the analytical signal due to imperfections in the
instrument.
Before we talk about specific forms of spectroscopy, let’s review the general terms that are
used to describe the intensity and wavelength of light.
Light has both wave-like and particle-like properties. When we think about light as wave,
we describe it as having oscillating electric and magnetic fields.
For a light wave moving through a vacuum, its speed c, its wavelength  (lambda), and its
frequency  (nu), are related by this equation
XXXXX
speed of light = c = 
in a vacuum
When passing through any material, the speed is slower. Call that speed v, and
speed of light = v = 
in general
6
It is the wavelength that changes.
In air the speed is slower by only 0.03% so only in calculations involving solids do we
assume that v does not equal c which is 2.9979 x 108 m/s (29970 x 108 in air).
On the other hand, when light interacts with atoms and molecules, waves no longer explain
its attributes easily. In this case, it is better to consider light as particles of energy, photons.
In this situation, we find that the frequency  of electromagnetic radiation is proportional to
the energy E of the photon. The constant of proportionality is called Planck's constant, h.
XXXXX
E = h = hc/
The photon energy also is related to the wavelength of light. In a vacuum, we get E = hc/.
Because energy, frequency and wavelength are interchangeable, and because there are a
number of different electromagnetic regions, we measure light with a whole host of units.
The SI energy term is the joule, so if frequency is in s-1, then h = 6.62 x 10-34 joule*s.
XXXXXX
Table 1 lists some of the other terms that are used and the light region in which they are used.
Commonly Used Measures of Wavelength and Energy
Name of unit
Symbol
nanometer
Angstrom
millimicron
micron, micrometer
wavenumber (reciprocal cm)
electron volt
hertz
= cycles per s
nm
Å
m
m
cm-1
eV
Hz
cps
Unit for




/c
E


Used in range of
the current term for UV, visible, near-infrared
X-ray, UV-visible (in older work)
Visible (in older work)
Infrared (common in physics literature)
Infrared, UV-visible (less common)
X-ray, -ray
Radiofrequency, microwave
Radiofrequency (older literature)
1Å = 0.1 nm, 1 m = 1 nm, 1 eV = 1.602 x 10-19 joules
Sometimes it is convenient to label the radiation with the inverse of the wavelength. An
example is cm-1. This inverse of centimeters is called the wavenumber, and the unit is used
most often in infrared spectrometry.
The symbol is nu bar, and is called the wavenumber with units of reciprocal centimeters.
It is important to realize that, regardless of the units of expression, a unique energy can be
assigned to each wavelength and frequency of light. The longer the wavelength, the lower
the energy and frequency of light.
7
Another form of energy that you know about is the temperature, which is a measure of the
average kinetic energy of an object. Energy is directly proportional to the temperature and
can be expressed as either the energy per atom or energy per mole.
E  kB T
or
E  RT
Where kB is Boltzmann’s constant and R is the ideal gas constant.
One other term is needed here. In quantitative spectrometry we measure power which equals
energy per unit time. In spectroscopy we use transducers that measure the amount of light
the is emitted from or absorbed by a sample. The light’s power is the quantity that is
measured in most forms of spectroscopy and is the product of the number of photons and
their energy divided by time.
XXXXX
This diagram illustrates the transformations of energy by an atom, molecule or ion.
The conservation of energy dictates that the sum of all the forms of energy entering the
sample must equal the sum of all the forms of energy leaving plus the energy remaining in
the material.
The energy is carried into and out from the sample as light, as heat, and as the kinetic energy
of particles such as electrons or atoms. The names of various spectroscopies depend on the
form of energy that impinges on the sample and the form that leaves.
The first item in Table 2 of the handout is emission spectroscopy. Here we put heat into the
sample and get light out.
Emission instruments are most commonly used in the spectroscopy of atoms of elements and
we generally use high temperatures such as that of flames. Generally we measure the power
of the emitted light.
The second item is absorption spectrometry. Here we put light in and instead of measuring
the heat out, we quantify the amount of light absorbed. Your total phosphorus lab involves
absorption spectroscopy.
We will talk about some of the other transformations in the future. Auger = oh jay.
XXXXX
Absorption spectrometry involves measuring the fraction of light of a given wavelength that
passes through a sample. This transparency illustrates the generalized geometry of such an
instrument.
8
Most light sources produce light having unwanted wavelengths in addition to the ones
desired. The exceptions to this general characteristic are radiofrequency sources and lasers.
Passing the light through either a monochromator or a filter selects the wavelength for the
assay.
The figure shows the monochromator between the light source and the sample. However, it
is possible to have other designs that still accomplish the same measurement such as
placing the monochromator between the sample and the tranducer.
For analyses, two measurements of the amount of light absorbed are made. In the first, we
measure the amount of light (at the chosen wavelength) that falls on the transducer when a
blank is in place. Call the power Po for the blank, that is, when the concentration of the
analyte in the sample is zero.
Measurements made when the analyte or calibration (standard) samples are in place are
compared with the blank measurement. Let us call the power that is measured with the
samples or standards present P.
The comparison that we always make involves the ratio P/Po, with both power
measurements made at the same instrument settings-wavelength, geometry, and so forth.
XXXX
Three different terms are used to express this ratio. The first is simply the ratio P/Po, which
is called the transmittance. The usual abbreviation for transmittance is an upper-case T.
T = P/Po also I/Io for intensity
Note P varies from 0 when all the light is absorbed, to 1 or P = Po when no light is absorbed
by the analyte.
The second term for the ratio is the percent transmittance,
%T = T x 100%
The third term is the negative logarithm (to the base 10) of T, which is called the
absorbance and is abbreviated by an upper-case A. The algebraic expression for
absorbance is
A = - log P/Po = - log T
i.e. A is p function of T
Another name, optical density, also is used for this quantity, but its use no longer is
recommended.
The absorbance of a sample is proportional to the total amount of material that absorbs the
incident light. Experimentally, we find that
9
XXXXXX
A=abc
In other words, the absorbance is directly proportional to the following:
a, a constant that is a property of the material itself as well as the wavelength of the
measurement;
b, the length of the path through which the light travels in the sample; and
c, the concentration of the material that absorbs the light.
The difference in use of c as the speed of light and c as the concentration should be obvious
from the context.
This equation is the Beer-Lambert law which is also known as Beer’s Law. When the
concentration c is in (mol L-1) and the path length in cm, then the constant a will have the
units (L mol-1 cm-1), in which case, it is given the symbol .  is called the molar extinction
coefficient, or molar absorptivity (the term currently preferred).
A (unitless) =  (L mol-1 cm-1) b(cm) c(mol L-1)
A = bc =  lc
It is important to note that  varies with the measurement wavelength, i.e. each substance
has a different value of  for each wavelength. Thus the value of  is often written
(nm) such as 530 to indicate that the measurement wavelength is 530 nm.
If no units are noted, then the use of  generally implies molar units.
Other units might be used for the absorptivity such as (g L-1)-1 cm-1.
In that case, the accompanying concentration unit c then must be in (g L-1).
It is also important to note that c is concentration and not activity.
In most cases, to overcome instrument errors, we determined the relationship between A and
c by calibration.
XXXXXX
Absorption spectroscopy is usually performed in the uv, visible, near-infrared and infrared
spectral regions. Here is a figure that displays visible spectra (A vs λ) of two acid/base
indicators and the colored compound potassium dichromate.
The spectrum is A vs wavelength. Note that for the same concentration the absorbance is
different for different wavelengths. The concentration and pathlength are constant, so the
spectrum shows varation of  with wavelength.  is zero at some wavelengths, i.e. the
material is transparent to these wavelengths.
10
So a spectrum shows you directly the variations of the molar absorptivity  with wavelength.
Unless there are interferences present, we will want to measure concentrations using an
absorbance maximum where we get the most change in absorbance for a small change in
concentration, i.e. greatest sensitivity.
We can also see that Phenolphthalein absorbs yellow and green and some blue light but
transmits red and orange so that it appears to the eye as a red solution.