Pharmaceutical Analysis Unit-V (Potentiometry)

Electrochemical cell

An electrochemical cell is consist two electrolyte solutions which are connected to each other by electrodes and a salt bridge. Each solution with electrode acts as a half cell therefore we can say that an electrochemical cell is consist of two half cells which are connected to each other to form an electrical circuit

Electrodes used in Potentiometry

For determination of potential of a given solution, two electrodes are used. one electrode is reference electrode, whose potential is known and is constant. Second electrode is indicator electrode which determines the potential of the solution under examination. 

Electrode used in Potentiometry are:- 

• Reference Electrode
• Indicator Electrode

i)Reference Electrode

1. The potential of reference electrode is known and has constant value i.e. it is independent of analyte composition. 

Electrode used are reference electrodes are of two types:-

1. Primary Electrodes : a)Hydrogen Electrodes
2. Secondary  Electrodes : a)Saturated calomel Electrodes

b)Sliver-silver Chloride Electrode        

a)Standard Hydrogen Electrode

 Standard Hydrogen Electrode is used as a reference electrode when calculating the standard electrode potential of a half cell.

Construction

The parts that make up a Standard Hydrogen Electrode are listed below.

• A platinum electrode which is covered in finely powdered platinum black (platinized platinum electrode).
• A hydrogen Blow.
• A solution of acid having a H⁺ molarity of 1 mole per cubic decimeter.
• The SHE also contains a hydro seal which is used to prevent the interference of oxygen.
• The other half-cell of the entire Galvanic cell must be attached to the Standard Hydrogen Electrode through a reservoir in order to create an ironically conductive path. This can be done through a direct connection, through a narrow tube, or even through the use of a salt bridge.

 

Diagram

      

 

 

Advantages 

1.It gives reproducible result.

2.It has no salt error.

3.It can be used over entire pH range.

4.It is used as a fundamental electrode.

5.It can be used as both indicator electrode as well as reference electrode.  

 Disadvantages

• It is not useful in solutions containing strong oxidizing or reducing agent.

• It is not useful in solution having metal ions that are below Hydrogen in potential Series.

Use of Platinum in the Standard Hydrogen Electrode

Platinum is used in the Standard Hydrogen Electrode due to the following reasons:

• Platinum is a relatively inert metal which does not corrode easily.
• Platinum has catalytic qualities which promotes the proton reduction reaction.
• The surface of platinum can be covered with platinum black, a fine powder of platinum. This type of platinum electrode is called a platinized platinum electrode.
• Platinum also improves the reaction kinetics by adsorbing hydrogen at the interface.

b)Silver-Silver Chloride Electrode

The silver/silver chloride reference electrode is a widely used reference electrode because it is simple, inexpensive, very stable and non-toxic. As a laboratory electrode such as described in the following Figure, it is mainly used with saturated potassium chloride (KCl) electrolyte, but can be used with lower concentrations such as 1 M KCl and even directly in seawater.

Construction

• It is simply a silver wire coated electrolytically with silver chloride and dipped into potassium chloride

• The potential of this half cell also depends upon temperature as well concentration of KCl used

Diagram 

                    

Advantage

• Easy to use

Disadvantage

• Difficult of prepare

  c)Saturated calomel electrode

The Saturated calomel electrode (SCE) is a reference electrode based on the reaction between elemental mercury and mercury(I) chloride. The aqueous phase in contact with the mercury and the mercury(I) chloride (Hg₂Cl₂, "calomel") is a saturated solution of potassium chloride in water. The electrode is normally linked via a porous frit to the solution in which the other electrode is immersed. This porous frit is a salt bridge. 

Construction

• It consist of an inner jacket and outer sleeve
• The inner tube(jacket)has has wire which is connected with mercury and plugged with a mixture of calomel and KCl
• The outer tube is surrounded by an outer sleeve and the tip is sealed with crystal or KCl and poroces plug of asbestos.
• The space between the inner jacket and outer sleeve is filled with either saturated KCl or 1N KCl or 0.1Kcl on which the potential of the electrode depends 
• The potential of this half cell depends upon concentration of KCl used and upon concentration of KCl used.

Diagram

                           

Theory of operation

The electrode is based on the redox reaction

The Nernst equation for this reaction is

where E⁰ is the standard electrode potential for the reaction and a_Hg is the activity for the mercury cation (the activity for a liquid is 1). This activity can be found from the solubility product of the reaction

By replacing the activity in the Nernst equation with the value in the solubility equation, we get

The only variable in this equation is the activity (or concentration) of the chloride anion. But since the inner solution is satured with potassium chloride, this activity is fixed by the solubility of potassium chloride. At standard conditions, the potential of the saturated calomel electrode should be +0.241 V versus the SHE. 

Application

The SCE is used in pH measurement, cyclic voltammetry and general aqueous electrochemistry.

This electrode and the silver/silver chloride reference electrode work in the same way. In both electrodes, the activity of the metal ion is fixed by the solubility of the metal salt.

The calomel electrode contains mercury, which poses much greater health hazards than the silver metal used in the Ag/AgCl electrode

Advantages

• Eare of Constraction

• Stability of potential 

Disadvantages

• Unstable at Temparature a bore 80c

• Not Suitable when chloride ions show incompatibility
  

 

ii) Indicator electrodes        

a)Glass electrode 

A glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion. The most common application of ion-selective glass electrodes is for the measurement of ph. The pH electrode is an example of a glass electrode that is sensitive to hydrogen ions. Glass electrodes play an important part in the instrumentation for chemical analysis and physico-chemical studies. The voltage of the glass electrode, relative to some reference value, is sensitive to changes in the activity of certain type of ions. 

 

Construction

• Glass electrode most widely used indicator electrode 
• It is  selective and responsive to change in concentration of H+ ions
• glass electrode consists of a glass tube with a thin pH sensitive glass bulb at its tip 
• It has a silver-silver chloride wire at the center of the tube and the lower tip of the wire is dipped into the 0.1N HCl filled in the glass tube .This assembly acts as an indicator electrode and is dipped into a solution whose pH or potential is to be unknown.
• glass bulb acts like a semipermeable membrane for ions .the H+ ions egarily enter the glass

Diagram

                     

Advantages

• It gives a rapid response.

• It is simple to operate.

• It can be used in viscous colored solution 

• It is no salt error

 

Disadvantage 

• It is fragile, So should be handled carefully 

• Minute scratches make glass electrodes useless

• It is must be hydrated all the time 

• It is not useful in highly alkaline solution as there occurs partial exchange of cations than H+ ions

 Applications

• Glass electrodes are commonly used for pH measurements. 
•  There are also specialized ion sensitive glass electrodes used for determination of the concentration of lithium, sodium, ammonium, and other ions. 
• Glass electrodes have been utilized in a wide range of applications including pure research, control of industrial processes, analysis of foods and cosmetics, measurement of environmental indicators, and microelectrode measurements such as cell membrane electrical potential and soil acidity.

Methods of Determine end point

• There are various methods to locate the end point. 
• The main purpose to to know the end-point at which the quantities of reaching  species are present in equal amount.
• The equivalent point is generally found graphically
• There are following methods for calculating end point:-

1.Normal Titration: 

• Value of EMF or pH are recorded after addition of each volume of titrant.
• Then a graph is plotted between volume of titrant added v/s values of pH or EMF.
• At the equivalent  point there is a sharp increase in potential or there is maximum slop of curve.
• Generally S-shaped curve is obtained.

2.First derivative curve :

• Difference of EMF or pH for the volume of titrant added is plotted against(V) added .
• A bell shaped graph is obtained and end point there is maximum charge in EMF or pH .
• The end point can be recorded by drawing perpendicular from peak of graph on volume excess.

3.Second derivative curve:

• ∆²E/∆v² against the volume of titrant added is pointed .
• The end point can be obtained ass a zero point where the slope curve of ∆²E/∆v² is maximum

 Application of Potentiometry:

With the use of Potentiometry various type of  titrations can be performed

1.Acid-Base titration: 

2.Redox titration

3.Preceitation titration

7.Determination of pH

8.Determination of amines , amino acids, salts of acids using non-aqueous titration

9.Determination of fluorides

10.Determination of phenols and sulpher by using non-aqueous titration