A Brief Introduction to Potentiometry

In pharmacology, potentiometry involves measuring the concentration of a solute present in a given solution. The main aim of potentiometry is to find out the electromotive force of a solution sample. This is done using a galvanic cell. Potentiometry is very important in the pharmaceutical field, a professional space that connects medical sciences and chemistry. It is also responsible for producing, disposing of, effectively controlling and using drugs. Let us learn more about potentiometry, its principle and the instruments used. 

Principle

In 1889, Nernst, a scientist, proposed the equation that explains that an electrochemical cell’s potential is directly proportional to the concentration of the given sample. 

The Nernst equation: E = E° + (0.592n)logc

In this equation, E = the solution potential

E°= the standard potential of the electrode

n = the valency of the ion in the sample solution

c = the concentration of the solution sample.

Cremer discovered that a potential difference persisted between reference and indicator electrodes in 1906. When a pair of electrodes are put in a solution sample, it determines the difference in potential. This is recorded after adding a titrant or when there is an alteration of the concentration of ions. The potential difference of a full (complete) cell is given by the equation: Ecell = – Eref + Eindicator + Ejunction

Here, Eref = the potential difference of the reference electrode

Eindicator = the potential difference of the indicator electrode

Ejunction = the potential difference at the junction of the sample solution. The cell potential is measured in mV (millivolts) 

The mechanism that is taking place here is:

• The electrons travel from the reference electrode to the indicator electrode.
• The negative ions (anions) run towards the cathode. 
• The positive ions (cations) run towards the anode.

Instruments used in potentiometry

The instruments used in potentiometry are:

• An indicator electrode

• A reference electrode

• A potential measuring device

The principle of potentiometric titration

The endpoint is measured when there is a sudden change in the electric potential. This happens when a titrant is added to the solution sample because there is an alternation in the ionic activity and concentration of the solution sample. 

There are three elements in a basic potentiometric titration – a reference electrode, an indicator electrode and a mechanical stirrer. The endpoint is determined when there is a rapid change in the solution potential. An electronic voltmeter is used to determine the electromotive force of the solution sample. 

There are four types of potentiometric titrations in pharmaceutical analysis. 

They are: 

Acid-base titration 

There is a change in concentrations of H+ and OH- during the neutralisation of bases and acids. In this titration, a hydrogen electrode is used. The reference electrode is the N-Calomel electrode (NCE).

An acid of known volume is kept in a beaker with an automatic stirrer. The hydrogen electrode is linked to NCE through a salt bridge. The hydrogen and NCE are connected to a potentiometer which tracks the EMF (electromotive force). 

Oxidation-reduction titration

An indicator electrode is used in the oxidation-reduction titration. The indicator electrode possesses a potential directly proportional to the logarithm of the concentration of the two oxidation states (reactant or titrant). 

Precipitation titration 

An indicator electrode is made of a metal involved in the reaction. The other type of electrode that can be used is whose potential is governed by the anion concentration that is precipitated. The solubility of the substance that is being precipitated determines the endpoint of the titration. 

Complexometric titration 

This titration is achieved by an electrode of a metal whose ions are involved in complex formation. For example, silver electrodes titrate cyanide ions with a standard silver solution. The complex formation reaction is Ag+ + 2CN– ⇋ [Ag(CN)2–]

After reaching the equivalence point, Ag(CN)2 forms a solid precipitate. After this point, adding silver does not change the concentration of the complex. If more than one complex is formed, this reaction becomes complicated. In this situation, EDTA can be used to handle. 

Importance of potentiometric titration in pharmaceutical analysis 

The importance of potentiometric titrations in the pharmaceutical analysis are 

• This type of titration does not require an indicator electrode. But there are a few potentiometric titrations with both reference and indicator electrodes.  
• This titration process is accurate and gives precise results up to three digits in millimetres. 
• Depending on the determining analytes, there are several potentiometric titrations. They are – redox, acid-base, complexometric, and precipitation titrations. 
• This titration works almost as well as automated systems, along with a great capacity for processing samples. 
• The modern methods of this titration can be used to find out pH, like HPLC (high-performance liquid chromatography) 
• It is affordable and simple. Hence, it can be used widely. 

Conclusion

This article talks about what is potentiometry. Potentiometry is used to find out the electromotive force of a solution sample. The potential of a full cell is determined by Ecell = – Eref + Eindicator + Ejunction.  It is governed by the Nernst equation E = E° + (0.592n)logc. 

The series of processes like purifying, identifying, and determining a compound used to formulate pharmaceutical drugs is called pharmaceutical analysis. Potentiometric titration is a method used in pharmaceutical analysis. 

There are four types of potentiometric titrations – complexometric, acid-base, oxidation-reduction and precipitation. This article also discussed the importance of potentiometric titrations in pharmaceutical analysis.