Conductometry

 Chapter 12 

Conductometry 

Introduction 

• Conductometry involves determination of conductance of an electrolyte solution using a conductometer. When solutions of electrolytes are subjected to an electric field, they conduct electric current by migration of ions. This process of conductance obey Ohm's law.
• Conductance is expressed as the reciprocal of resistance i.e., and it is denoted by unit1known as mhos or ohms.

There are different types of conductance as follows

                    Specific conductance
                    Equivalent conductance
                    Molar conductance

Specific Conductance

• This is one of the types of conductance which is defined as, "the reciprocal of resistance of 1 cm cube of liquid or solution at a specified temperature". Specific conductance is generally denoted by K. The specific conductance is depending upon the number of ions present in the electrolyte solution which in turn is temperature dependent

Equivalent Conductance

• The conductance of a solution consisting of a gram equivalent of a solute between two large parallel electrodes at 1 cm apart is referred as equivalent conductance and is denoted by a

Equivalent conductance = Specific conductance x Volume in cubic cm

Molar Conductance

• Molar conductivity is defined as, "the conductivity of all ions present in a solution of one mole/per litre of solution". Molar conductance is denoted by the following expression.

Molar conductance, p = k xV

• where V is the volume in ml containing 1 g mole of the electrolyte. If c is the concentration of the solution in g mole per liter.

Conductivity Cells 

• Conductance of a electrolyte solution can be measured by a suitable instrumentation which consists of a conductivity cell and conductometer. After connecting the conductivity cell to the instrument, measurement can be made. There are various types of conductivity cells available.

Generally, in the market, the following types of conductivity cells are available

Electrode type:     

• Conductivity cells with two electrodes: These are the conventional type of cellsused very commonly. 

• Conductivity cells with four electrodes: These types of cells are used in dirty media with high conductivity levels

2. Induction type conductivity cells:

• This type is used for highly corrosive medium to measure very high conductivity.

Electrode Type Conductivity Cells 

Two electrode conductivity cells:

• This type of cells consists of two metal electrodes or some time three electrodes. These two electrodes are usually 1 cm* metal plates and separated by 1 cm distance. This setup is equivalent to a cell constont of 1 cm

Cell constant: 

• Cell constant is the intrinsic feature of a conductivity cell. It is depending upon the cell geometry and is expressed as cm 1 No conductivity cell will have the ability to measure the full range of conductance of electrolyte solution. So, in practice, various cells are used with different cell constant. However, a cell constant C of 1 cm 1 is universally accepted one, as this allows the measurement of conductance from low level to high level.
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Inductive Type Conductivity Cells 

• These conductivity cells use inductive sensors or toroidal sensors. Inductive sensors usually contain two coils, sealed within a non-conductive housing. The first coil induces an electrical current in the water, while the second coil detects the magnitude of the induced current, which is proportional to the conductivity of the solution.

• In the (a) type, a wide mouth bottle with bark cork which has holes for passing two platinum wires of 1 sq. cm size is used.

• In the (b) type, the electrodes are firmly fixed in the Perspex lid which is provided with opening for the stirrer and the tip of the burette. The stirrer may be replaced by magnetic stirrer. 

• In the (c) or Dip type copper wires are fixed in a wide bore corning glass tube, the tip of which have two platinum plates of 1 sq. cm in size, fixed at 1 cm apart. The terminals of copper wire are taken out for connections. The position of wire is fixed in glass tube by rosin.

Conductometric Titrations 

• One of the most important applications of conductometry is the conductometric titrations. Various types of acid-base titrations, replacement titrations, precipitations and complex formation reaction involving titrations are possible.

Principle of Conductometry Titrations 

• It is known that, at infinite dilutions or in very dilute solutions, ions act independent of each other, and they contribute to the conductance of the solution. Both cations and anions have varying degree of ionic motilities (or conductance values). Thus, when a solution of one electrolyte is added (as a Titrant) to the solution of another electrolyte, the overall conductance (after addition) will depend on whether a reaction occurs or not. If no chemical reaction occurs between the electrolyte, solution and another added to it, the overall conductance of the solution will increase. All ions will contribute to the conductance of the solution. For example, addition of sodium nitrate solution to the sodium chloride solution.

Types of Conductometric Titrations 

Various types of acid-alkali titrations are carried out using Conductometry. Some typical conductometric titration graphs which are obtained are shown hereafter.

1. Strong acid with strong base
2. Strong acid with weak base
3. Weak acid with strong base
4. Weak acid with strong base
5. Very weak acid with strong base
6. Mixture of hydrochloric acid
7. Displacement titrations
8. Precipitation and complex formation titrations
9. Redox titrations

Applications of Conductometry in Analysis 

• Determination of solubility of sparingly soluble material: The specific conductance (k) of saturated solution of sparingly soluble salt is determined. The equivalent conductance (+) of sparingly soluble salt in saturated solution is considered practically equal to the limiting value (+) of conductance. This can be obtained by summation of limiting conductance of constituent ions. Thus, if S is the solubility in g/litre and k is specific conductance then,

 

• hus, from measuring specific conductance and limiting conductance of constituent ions of sparingly soluble material, solubility a can be determined.

High Frequency Method 

• In the conductivity method, we have seen that migration of ions occurs under the influence of electric field. Thus, when low frequency current is applied to any molecule in solution containing two electrodes, the electrons of the molecule are attracted towards the positive end of electrode while the positive end of molecule is attracted towards opposite end. This orientation of molecule occurs upon ionization and dissociation in polar solvents. This effect is of short duration and disappears when the field is removed.

• Some practical difficulties are encountered in the measurement of conductance for certain molecules by the conventional method. These are the effects of polarization (partial or incomplete) under the frequency of current used, poor ionizing solvent media like certain non-aqueous solvents and the effect of adsorption at the electrode surface.