Question

Explain how coulometry differs from amperometry. Why does coulometry give better precision, while amperometry is more suitable for sensors?

Answer 1

Coulorometry differs from Amperometry in the following aspects:

Coulorometry:

is the name given to a group of techniques in analytical chemistry that determine the amount of matter transformed during a reaction by eletrolysis measuring the amount of elecricity (in coulombs) consumed or produced.

There are two basic categories of coulometric techniques. Potentiostatic coulometry involves holding the electric potential constant during the reaction using a potential stat. The other, called coulometric titration or amperostatic coulometry, keeps the current (measured in amperes) constant using an amperostat.

Amperometry:

chemistry and biochemistry is detection of ions in a solution based on electric current or changes in electric current.

Amperometry is used in electrophysiology to study vesicle release events using a carbon fibre electrode. Unlike patch clamp techniques, the elctrode used for amperometry is not inserted into or attached to the cell, but brought in close proximity of the cell. The measurements from the electrode originate from an oxidising of a vesicle cargo released into the medium. Another technique used to measure vesicle release is capacitive measurements.

Coulorometry precision:using controlled-current coulometry accuracy is determined by the current efficiency, by the accuracy with which we can measure current and time, and by the accuracy of the end point. The maximum measurement errors for current and time are about ±0.01% and ±0.1%, respectively. The maximum end point error for a coulometric titration is at least as good as that for a conventional titration, and is often better when using small quantities of reagents. Together, these measurement errors suggest that an accuracy of 0.1%–0.3% is feasible. The limiting factor in many analyses, therefore, is current efficiency. A current efficiency of greater than 99.5% is fairly routine, and it often exceeds 99.9%.

Amperometry is more suitable for sensors:Amperometric biosensors function by the production of a current when a potential is applied between two electrodes. They generally have response times, dynamic ranges and sensitivities similar to the potentiometric biosensors. The simplest amperometric biosensors in common usage involve the Clark oxygen electrode. This consists of a platinum cathode at which oxygen is reduced and a silver/silver chloride reference electrode. When a potential of -0.6 V, relative to the Ag/AgCl electrode is applied to the platinum cathode, a current proportional to the oxygen concentration is produced. Normally both electrodes are bathed in a solution of saturated potassium chloride and separated from the bulk solution by an oxygen-permeable plastic membrane.