Coulometry

Coulometry is an application of Faraday’s First law of electrolysis. Coulometry methods are based on stoicheometry associated with the conversion of species from one oxidation state to another.

Suppose we want to determine amount of Cu++ present in the given solution. The solution is on electrolysis, Cu++ will be reduced at the cathode to Cu.

Cu++ + 2e- = Cu

Thus 2 moles of electrons are required to reduce 1 mole of Cu++ ions. That is 2x 96487 coulombs of charge is equivalent to 1 mole of Cu++ ions. Thus if we measure the exact amount of charge required to reduce the Cu++ present in the analyte, then amount of Cu++ present can be calculated.

The weight of the substance produced or consumed in an electrolysis involving Q coulombs is therefore given by,

W= M×Q96487n (1)

Where.

W = wt of analyte

M = molecular weight of analyte

96487 is Faraday’s constant and equal to charge on 1 mole of electron. The Faraday constant is approximately taken as equal to 96500 coulombs for ordinary work.

Analytical methods based upon the measurement of a quantity of charge and the application of the equation 1 is termed Coulometric methods. Thus coulometric analysis boils down to the measurement of Q.

How to determine total charge Q ?

If current is constant then, quantity of charge Q, is given by,

Q = It

where I is current in ampere and t is time in second.

If current is not constant, then Q is given by,

Q=∫0

t

idt

The value of Q can be determined by graphical method by plotting I against time and determining the area under the curve or by including an integrator in the circuit.

Types of Coulometry

We cannot simultaneously control both current and potential, if we choose to control the potential, then we must accept the resulting current, and we must accept the resulting potential if we choose to control the current. So Coulometry may be:

1. Controlled current Coulometry2. Controlled potential Coulometry

Controlled current Coulometry is also refer to as coulometric titration

Constant-current Coulometry is employed to generate the reagent required to react with the analyte or the electron itself reacts with the analyte and from the number of coulomb used the analyte concentration is calculated. In volumetric titration the volume of a standard solution required to reach an equivalence point is measured. In coulometric titration the charge required to reach an equivalence point is measured. Supply of constant current over a fixed time period is analogous to adding a fixed volume of standard solution. Switch is used to get moles of reagent is analogous to stopcock in burette. Thus coulometric titration electrons serve as titrant.

Instrumentation

Controlled-current coulometry normally is carried out using a galvanostat and an electrochemical cell consisting of a working electrode and a counter electrode.

Figure 1: Cell for constant current Coulometry

The working electrode is constructed from Pt, is also called the generator electrode since it is where the mediator reacts to generate the species reacting with the analyte. The counter electrode is isolated from the analytical solution by a salt bridge.

Analyte

The other necessary instrumental is an accurate clock for measuring the electrolysis time, t, and a switch for starting and stopping the electrolysis. Digital clocks provide a more accurate measurement of time. The switch must control the flow of current and the clock, so that an accurate determination of the electrolysis time is possible.

Constant current Coulometry titration may be (a) Direct Coulometry titration (b) Indirect coulometric titration

Direct Coulometry titration

In direct Coulometry titration the analyte itself is reacted at electrode. The amount of charge flowed is measured and the concentration of electro-active species in calculated. Here desired electrode reaction should be 100% efficient and should not be any undesired reaction.

Indirect coulometric titration

In indirect Coulometry titration, the reagent is electrolytically generated which in turn reacts chemically with the analyte. Switching the current on and off is analogous to opening and closing of burette.

Bromine, for example, is a useful titrant in organic reaction but it is very difficult to prepare as standard solution. However it can be generated and standardize simultaneously by Coulometry. By placing a bromide salt in solution and electrochemically oxidising it to bromine, the standard titrant is generated in situ. The amount of bromine generated is determined by measuring the current and time involved. For example, phenol can be titrated by this technique,

The sample of phenol and large excess of soluble bromide salt is placed in titration cell. The current is initiated and bromine is generated and reacts as given in equation 1. The excess bromine is determined by increase in absorbance. The time required to reach this point is measured. By careful analysis of the data amount of phenol can be calculated.

The chloride in urine can also be determined by coulometric titration of chloride by silver ion generated by anodic oxidation of silver. In a Cotlove Chloridemeter spiral silver is immersed in the test solution to which silver acts as an anode. Upon electrolysis, the generated silver (I) removes the chloride by precipitation and end point is detected potentiometrically.

Ag Ag+ + e-

Ag+ + Cl- AgCl(s)

The coulometric titration of iron (II) can be done by electrolytically generated cerium(IV) ion. Ce(IV) is generated by anodic oxidation of Ce(III) present in large excess and the the Ce(IV) ion in turn

oxidises iron (II). A potentiometric end point detector is required. Their will be four electrodes in the solution, an anode and cathode for electrolytic generation of titrant and indicator and reference electrode pair connected to digital potentiometer for potentiometric end point detection.

Figure 2 shows a coulometric titration set up. The platinum electrode is directly placed in the test solution. And auxiliary electrode is in separate compartment. The clock and current source are wired together so that one switch turns on both. For a constant current a high dc voltage in series with such a large resistor that changes in electrolyte cell resistance during operation are negligible and current remains constant. The number of coulombs involved in the electrode reaction is then calculated from the value of the current and the time required for complete reaction.

Figure 2: Coulometric titration set up.

Advantages of coulometric titration over volumetric titration

No need to prepare standard solution Tiny quantity of reagent with good accuracy can be generated by controlling current and

time electronically Unstable generated reagent such as bromine, chlorine ion can be utilised since they can be

generated and consumed immediately.

Controlled Potential Coulometry

In controlled potential Coulometry, electricity is passed through a solution in which potential is held constant through a solution. The technique is use for the determination of trace amount of metal ion in solution. The block diagram of a controlled potential Coulometry is shown in figure 3.

Analyte

Figure 3: Block diagram for controlled potential Coulometry.

The coulometric cell consists of three electrodes (cathode, anode and reference) and potentiostat to control the potential of the electrode at which desire reaction takes place. The electrode at which the desire reaction occurs is called working electrode. The second electrode in the electrolysis circuit

is called counter electrode. During a controlled potential electrolysis current will decay. Since current changes continuously, the total coulombs, Q, associated with the desire reaction is given by,

Q=∫0

t

idt

The integration represents the area under the current-time curve. Nowadays, the integration is commonly performed electronically by a integrator attached in the circuit.

Controlled current Coulometry versus controlled potential Coulometry

Controlled current Coulometry, has two advantages over controlled potential Coulometry.

First: Rapid analysis since the current does not decrease over time. Thus, a typical analysis time for controlled current Coulometry is less than 10 min, as opposed to approximately 30-60 min for controlled potential Coulometry.

Second: Total charge is simply the product of current and time. A method for integrating the current-time curve, therefore, is not necessary.

Two important experimental problems

First: As electrolysis occurs, the current due to its oxidation or reduction steadily decreases. To maintain a constant current the cell potential must change until another oxidation or reduction reaction can occur at the working electrode. Unless the system is carefully designed, these secondary reactions will produce a current efficiency of less than 100%. That introduce error I the analysis.

Second: A method of determining when the analyte has been exhaustively electrolyzed. A suitable means of determining the end-point of the reaction is needed.