COULOMETRY: PRINCIPLE AND APPLICATIONS

INTRODUCTION:

Coulometry is a dynamic techniques in analytical chemistry which determines the amount of matter transformed during an electrolysis reaction by  measuring an unknown concentration of an analyte in solution and completely converting the analyte from one oxidationstate to another. It  is an absolute measurement similar to gravimetry or titration and requires no chemical standards or calibration.

Figure: Schematic of a coulometric cell Coulometric Titration

There are two basic categories of coulometry: -

1.      Controlled potential coulometry or Potentiostatic coulometry

2.      Controlled-current coulometry or amperostatic coulometry

These coulometric methods can be classified in following categories: -

1.      Potentiostatic coulometry:Potentiostatic coulometry technique is  most commonly referred to as bulk electrolysis. Here the working electrode is kept at a constant potential applied to the electrochemical cell during the reaction using a potentiostat.

The current efficiency is the percentage of current which leads to the analyte’s oxidation or reduction; to ensure 100% current efficiency is to hold the working electrode at a constant potential, chosen so that the analyte reacts completely without simultaneously oxidizing or reducing an interfering species.

1.1       Principles:

 

·         The working electrode will be kept at constant potential that analyte’s quantitative reduction or oxidation occurs without simultaneously reducing or oxidizing other species in the solution

·         The current flowing through the cell is proportional to the analyte’s concentration.

·         As electrolysis progresses the analyte’s concentration decreases, as well as the current will decrease. When the reaction is complete, the current is negligible.

·         The quantity of electricity is usually measured with an electronic integrator.

1.      Amperostatic coulometry:Amperostatic coulometry pass a constant current through the electrochemical cell to deliver a measured amount of charge using an amperostat.

2.1   Principles:

 

·         The current is kept constant until an indicator signals completion of the analytical reaction.

·          The quantity of charge required to attain the end point is calculated from the magnitude of the current and the time of its passage.

Charge(Q) = Current (i) x Time in coulombs(t) 

·         Here the applied current is equivalent to a titrant.

·         Electrolytic method ® external power added to system

·         The current in a coulometric titration is carefully maintained at a constant and accurately known level by means of an amperostat.

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.

Method for the external generation of oxidizing and reducing agents in coulometric titration: 

Example:

Coulometric Titration of Cl^-

Then Ag electrode is used to produce Ag⁺

Ag(s)®Ag⁺ +e^-

Ag⁺+Cl^-®AgCl (ppt)

Applications:

Coulometry is used for the quantitative analysis of both inorganic and organic analytes.

·         Karl Fischer reaction:

The most widely used coulo­metric titration is the Karl Fischer titration (KF titration) for the determination of 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.

In the Karl Fischer reaction, iodine is generated in situ from iodide ions. Iodine then reacts stoichiometrically with water in the presence of chemically bound sulfur dioxide. The reaction involved canbe shown as following,

ROH + SO₂ + RN → (RNH)SO₃R

2 RN + (RNH)SO₃R + I₂ + H₂O → (RNH)SO₄R + 2 (RNH)I

The solvent can be used here is Methanol,ethylene glycol,diethylene glycol etc. The balanced chemical equation, using pyridine, is:

py.I₂+py.SO₂+py+H₂O®2py.HI+py.SO₃

Since the concentration of pyridine is sufficiently large, I₂ and SO₂ react with pyridine (py) to form the complexes py•I₂ and py•SO₂. When added to a sample containing water, I₂ is reduced to I–and SO₂ is oxidized to SO₃. Methanol is included to prevent the further reaction of py•SO₃ with water. The titration’s end point is signaled when the solution changes from the product’s yellow color to the brown color of the Karl Fischer reagent.

The coulometric Karl Fischer titration can be used to measure a large variety of samples. They are prepared in different ways depending on the type of sample involved. Liquid and solu­ble samples are simply injected into the titration cell. With solid samples, the water content can be determined either by (external) extraction or by heating the samples in an oven and passing the moisture evolved into the titration cell using a carrier gas and a transfer tube.

Due to its extremely high sensitivity, the coulometric Karl Fischer titra­tion is an excellent method for the determination of very low quantities of water.

·         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 {\displaystyle \Delta }is proportional to the constant current {\displaystyle i}, the molecular weight {\displaystyle M}of the metal, the density {\displaystyle \rho }of the metal, and the surface area.

REFERENCES

1.      www.wikipedia.com

2.      Coulometric Methods by Siham Abdoun

3.      Coulometric Titration by A. De Agostini