What is Coulometry ? Its Concept ,Working Mechanism , Types and Applications
What is Coulometry ?
Coulometry is an analytical method for measuring an unknown concentration of an analyte in solution by completely converting the analyte from one oxidation state to another. Coulometry is an absolute measurement similar to gravimetry or titration and requires no chemical standards or calibration.
It is therefore valuable for making absolute concentration determinations of standards. Coumetry uses constant current source to deliver a measured amount of charge.One mole of electrons is equal to 96,485 coulombs of charge, and is called a faraday.
Basic Principle :
The main principle involved in the
coulometry is the measurement of the quantity of the electricity which is
directly proportional to the chemical reaction at the electrode. This is given
by the Faraday's first law:where Q is the consumed current; Mr is the relative
molecular weight.
Concept Of Coulometry :
In coulometry, the current, created
by analyte exhaustive oxidization or reduction, is measured . This technique
can be used for the quantitative analysis of various inorganic and organic
compounds. There are two types of coulometry: controlled-potential and
controlled-current coulometries .
The controlled-potential coulometric analyses
are involved in determination of inorganic ions (e.g., trace metals and
halides) and alloys analysis.
The controlled-current coulometry is
a more fruitful analytical method compared to controlled-potential coulometry,
due to the use of mediators. The analysis of proteins can be done conveniently
using controlled-current coulometry.
Controlled-current coulometric
techniques have been developed for a range of analytes that may be determined
by conventional redox titrimetry (e.g., acid-base, precipitation, or
complexation).
Instrumentation :
In the
instrumentation of the coulometry, mainly two types of electrodes are used: one
is the reference electrode and another is the working electrode.
Generally saturated calomel electrode is used as the reference electrode. It consists of porous disc at the base of the electrode which is clogged. Above it, the glass tube is filled with the potassium chloride crystals.
And above that it is filled with the calomel paste which is prepared by grinding of mercury chloride with pure mercury and minute milli litre of the saturated potassium chloride solution. Then pure mercury is placed in the electrode vessel. The advantages are the following: the easy to construct and highly stable.
Types of Coulometry
Ø Potentiostatic coulometry
Ø Coulometric tritration
Potentiostatic coulometry is
a technique most commonly referred to as "bulk electrolysis".
The working electrode is
kept at a constant potential and the current that flows through the circuit is
measured. This constant potential is applied long enough to fully reduce or
oxidize all of the electroactive species in a given solution.
As the
electroactive molecules are consumed, the current also decreases, approaching
zero when the conversion is complete. The sample mass, molecular mass, number
of electrons in the electrode reaction, and number of electrons passed during
the experiment are all related by Faraday's laws.
It follows that, if three of the values are known, then the fourth can be
calculated.
Bulk electrolysis is often used to unambiguously
assign the number of electrons consumed in a reaction observed through voltammetry. It also has the added benefit of producing a
solution of a species (oxidation state) which may not be accessible through
chemical routes. This species can then be isolated or further characterized
while in solution.
The rate of such reactions is not determined by the
concentration of the solution, but rather the mass
transfer of the electroactive
species in the solution to the electrode surface.
Rates will increase when the volume of the solution is
decreased, the solution is stirred more rapidly, or the area of the working
electrode is increased. Since mass transfer is so important the solution is
stirred during a bulk electrolysis.
However, this
technique is generally not considered a hydrodynamic technique, since a laminar flow of solution against the
electrode is neither the objective nor outcome of the stirring.
The extent to which a reaction goes to completion is
also related to how much greater the applied potential is than the reduction
potential of interest.
In the case where multiple reduction potentials are of
interest, it is often difficult to set an electrolysis potential a
"safe" distance (such as 200 mV) past a redox event. The result is
incomplete conversion of the substrate, or else conversion of some of the
substrate to the more reduced form. This factor must be considered when
analyzing the current passed and when attempting to do further
analysis/isolation/experiments with the substrate solution.
An advantage to this kind of analysis over electrogravimetry is
that it does not require that the product of the reaction be weighed. This is
useful for reactions where the product does not deposit as a solid, such as the
determination of the amount of arsenic in a sample from the electrolysis of arsenous
acid (H3AsO3) to arsenic
acid (H3AsO4).
Coulometric titrations
:
Coulometric
titrations use a constant current system
to accurately quantify the concentration of a species. In this experiment, the
applied current is equivalent to a titrant.
Current
is applied to the unknown solution until all of the unknown species is either
oxidized or reduced to a new state, at which point the potential of the working
electrode shifts dramatically. This potential shift indicates
the endpoint.
The magnitude of the current (in amperes)
and the duration of the current (seconds)
can be used to determine the moles of
the unknown species in solution.
When the volume of the solution is known, then
the molarity of
the unknown species can be determined.
Advantages
of Coulometric Titration :
Coulometric
titration has the advantage that constant current sources for the generation of
titrants are relatively easy to make.
·
The electrochemical generation of a
titrant is much more sensitive and can be much more accurately controlled than
the mechanical addition of titrant using a burette drive. For example, a
constant current flow of 10 µA for 100ms is easily generated and corresponds to
about 10 micrograms of titrant.
·
The preparation of standard
solutions and titer determination is of course no longer necessary.
·
Chemical substances that are
unstable or difficult to handle because of their high volatility or reactivity
in solution can also very easily be used as titrants. Examples are bromine, chlorine,
Ti3+, Sn2+, Cr2+, and Karl Fischer reagents (iodine).
·
Coulometric titration can also be
performed under inert atmosphere or be remotely controlled e.g. with
radioactive substance
Applications :
The Karl Fischer reaction uses a coulometric titration to determine the
amount of water in a sample. It can determine concentrations of water on the
order of milligrams per liter. It is used to find the amount of water in
substances such as butter, sugar, cheese, paper, and petroleum.
The reaction involves converting solid iodine into
hydrogen iodide in the presence of sulfur dioxide and water.
Methanol is most often used as the solvent, but ethylene glycol and diethylene glycol also
work. Pyridine is
often used to prevent the buildup of sulfuric
acid, although the use of imidazole and diethanolamine for this
role are becoming more common.
All reagents must be anhydrous for the analysis to be quantitative. The
balanced chemical equation, using methanol and pyridine
In
this reaction, a single molecule of water reacts with a molecule of iodine.
Since this technique is used to determine the water content of samples,
atmospheric humidity could alter the results. Therefore, the system is usually
isolated with drying tubes or placed in an inert gas container.
In addition, the solvent will undoubtedly have
some water in it so the solvent’s water content must be measured to compensate
for this inaccuracy.
To determine the amount of water in the sample,
analysis must first be performed using either back or direct titration. In the direct method, just enough of the reagents
will be added to completely use up all of the water. At this point in the
titration, the current approaches zero.
It is then possible to relate the amount of reagents
used to the amount of water in the system via stoichiometry.
The
back-titration method is similar, but involves the addition of an excess of the
reagent. This excess is then consumed by adding a known amount of a standard
solution with known water content.
The result reflects the water content of the sample
and the standard solution. Since the amount of water in the standard solution
is known, the difference reflects the water content of the sample.
v Determination of film thickness :
Coulometry can be used in the
determination of the thickness of metallic coatings. This is performed by
measuring the quantity of electricity needed to dissolve a
well-defined area of the coating. The film thickness is proportional to the constant current the molecular weight of the metal,
the density of the
metal, and the surface area.
The electrodes for this reaction are often platinum
electrode and an electrode that relates to the reaction. For tin coating on a
copper wire, a tin electrode is used, while a sodium chloride-zinc sulfate
electrode would be used to determine the zinc film on a piece of steel. Special
cells have been created to adhere to the surface of the metal to measure its
thickness. These are basically columns with the internal electrodes with
magnets or weights to attach to the surface.
The results obtained by this coulometric method are
similar to those achieved by other chemical and metallurgic techniques.
v Inorganic
Analysis :
Controlled potential coulometric
methods have widespread use in the determination of several metal ions. As many
as 55 elements of the periodic table can be determined by the cathodic
reduction of metal ions to metallic state.
Most of the can form amalgams with
mercury, and hence controlled potential coulometry with mercury cathode is
usually preferred.
v Micro
analysis :
Controlled potential coulometry is more popular than
the electrogravimetric methods since it avoids the final step of weighing the
product. This technique is especially useful for the determination of small
amounts of analyte (0.01 – 1 mg) with an accuracy of (± 0.5 %).
v Analysis of radioactive materials :
The technique is widely adopted for the determination
of uranium and plutonium and thus finds extensive use in the nuclear energy
field. Reduction of UO22+ to U4+ can be carried out in H2SO4 medium with a
mercury pool cathode (− 0.6 V vs. SCE).
Samples containing 7 – 75 mg of uranium have been
analyzed with an accuracy of ± 0.l %.
v Electrolytic
determination of organic compounds:
Controlled potential coulometry offers a new step for the electrolytic determination of organic compounds. Trichloroacetic acid and picric acid are quantitatively reduced at a mercury cathode.Coulometric methods permit the analysis of these compounds with an accuracy of 0.1%
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