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Amperometry

There are several techniques are available for the estimation and identification of different compounds and elements for example gas and liquid chromatography, refractometry or spectrophotometry etc.

These techniques can be classified in two types.

  1. Quantitative analysis
  2. Qualitative analysis
Quantitative analysis is based on the estimation of given compound by using gravimetric analysis while qualitative analysis is based on the volume. In the same way, for the detection as well as purification of each element there is a certain chemical process.

Amperometry is also one of this technique which require complicated and expensive specialized equipment handled by a qualified personnel and are usually labour- and time-consuming. But at the same time this technique gives accurate results with many additional updates.

 

Amperometry Definition

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The detection of ions presence on solution on the basis of electric current or change in electric current is called as Amperometry. For example, for the detection of chlorine can be carried out by using amperomerty. No doubt there are other methods also available for the detection of chlorine like calorimeter which measures the chlorine level optically.

When chlorine is added to water it hydrolyzes and forms HOCl which further dissociates into respective ions.

Cl2 + H2O HOCl + H+ + Cl-
HOCl (aq) → OCl- (aq) + H+(aq)

In an amperometric method for the detection of chlorine a fixed voltage applied between two electrodes whereas the reaction takes place at the working electrode that is cathode and reduction of chlorine (HOCl) back to chloride (Cl-) takes place and chlorine generate at the anode.

Since, the current flow in amperometric sensor is due to chlorine, hence the amount of current flow is directly proportional of chlorine presented to the sensor.

Detection of Chlorine

Amperometry Principle

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Amperometry analysis is a type of voltammetric analysis which is widely used in food processing industries.

A voltammetric analyses is done by using a certain setup of a measuring chamber which contains three electrodes.
  • Working electrode (microelectrode)
  • Reference electrode
  • Auxiliary electrode
  1. Apart from these electrodes, there is a voltage source and a devices for measuring current and voltage – voltmeters and ammeter.
  2. The method is based on the principle that the measurements of changes in time (τ) in the current (I) flowing through the system of electrodes in relation to potential (E) applied to the working electrode.
  3. The change registered in the current allow drawing the I(τ) = f[E(τ)] relationship which is known the voltammogram.
  4. If the same measurements of the current are carried out at constant potential, then it becomes a classical amperometric system.
  5. Amperometric measurements completed in stirred solution or with rotating electrode which is also termed as hydrodynamic amperometry.
  6. The best use of Amperometry apparatus is to detected dissolved oxygen which is reduced at the measuring electrode.
  7. By using a micro- potentiotat the potential of the measuring electrode is kept at a constant which is main characteristic for the oxygen reduction in Amperometry technique.
  8. Hence, the diffusion limited current is directly proportional to the oxygen concentration.

Amperometric Titration

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Amperometric titration is a type of titration in which the determination of the equivalence point is done by measuring the electric current which produced by the titration reaction. For the estimation of equivalence point and end point in any titration, apart from titration indicators, amperometry can also be used.

If we take a solution containing the analyte, X with some conductive buffer and applied an electrolytic potential to the solution by using a working electrode, then the value of measured current will depend on the concentration of the analyte. This magnitude of measured current can be used to determine the concentration of the analyte and this form is called as amperometry.

However, the measurement of current depends on several other variables, and it is not always possible to control all of these variables at the same time, hence this limits the precision of direct amperometry. Now the applied potential on working electrodes will decide the mode of reaction.

If the applied potential is sufficient to reduce the given analyte, then the concentration of analyte which is close to the working electrode will decrease. Hence more analyte will diffuse slowly in the solution which is close to the working electrode and restoring the concentration.

On the other hand, If the applied potential is more, the concentration of analyte which is close to the working electrode will depend on the rate of diffusion. In this a case, the current is called as diffusion limited. The concentration of the analyte will decrease in solution due to reduction of it on working electron.

This reduction in concentration of analyte takes place with a very slow speed which depends on the size of the working electrode. If there is any other ion of molecule present in solution the concentration of the analyte will decreases due to reaction with other substance as well as due to applied potential.

After equivalence point when enough titrant has been added to react with the analyte, the excess titrant reduced at the working electrode. Since each species has a certain diffusion characteristics and different half-reaction, after equivalence point the slope of current versus added titrant will have a different slope. This inflection in slope marks the equivalence point as the change in pH marks the equivalence point in an acid-base titration.

Amperometric Titration

Example of amperometric titration is the titration if cerium (IV) ions against iron(II) which carried out at 1.8 V while the amperometric titration of KMnO4 with Fe (II) was performed at 1.2 V. The inflection point in curve denotes the equivalence point in titration.

Amperometric Graph

Advantage of Amperometry titration

  1. In amperometry titration, a few current measurements at constant applied voltage before and after the end point are sufficient to estimate the equivalence point and end point.
  2. Amperometry titration is useful for such case in which the solubility relations like potentiometric are unsatisfactory. For example, in precipitation titration or in acid-base titration.
  3. A amperometric titration can be carried out at low dilutions also at which many other titration are no longer yield accurate results.
  4. 'Foreign' salts do not create any interference in amprometric titration. They are usually added as the supporting electrolyte for the elimination of the migration current.
  5. No change in temperature is observed during titration and this titration is independent of the characteristics of the capillary.

Amperometric Biosensor

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  • Amperometric bio-sensors are based on Amperometric technique. In acts as a sensor which function by the production of a current in the presence of applied potential between two electrodes.
  • The well known Amperometric bio-sensors are the Clark oxygen electrode which consist a platinum cathode at which oxygen is reduced and a reference electrode of silver/silver chloride.
  • When a potential of -0.6 V is applied to the platinum cathode, a current produced which is proportional to the oxygen concentration.
Both electrodes of bio-sensors are dipped in a solution of saturated potassium chloride which is separated from the solution by an oxygen-permeable plastic membrane like Teflon or polytetrafluoroethylene.

The reactions occur in bio-sensor is,
Ag anode 4Ag + 4Cl- 4AgCl + 4e-
Pt cathode O2 + 4H+ + 4e-
2H2O

The rate of electrochemical reduction depends on the rate of diffusion of the oxygen from the bulk solution, which is directly related to the concentration gradient and the bulk oxygen concentration.
A very small of the oxygen present in the bulk is consumed in this process and the oxygen electrode measure the rate of a process which is the reaction is far from equilibrium. This bio-sensor is used for the determination of glucose concentrations by using of an immobilized glucose oxidize membrane.

Amperometric Biosensor

Other common Amperometric bio-sensor is a cholesterol sensor prototype, which was connected to a potentiotat throughout a wire connector. Like other bio-sensor, in this bio-sensor also electrochemical measurements are performed on three electrodes. After the stabilization of background currents, the glucose standard solution is added to the phosphate buffer solution and their steady-state current measure.
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