Conductometric Titration

Introduction

Conductometric titration is one of the types of titrations where chemical reactions are carried out with the help of a conductometer. Like in any other titration, an indicator is used to determine the endpoint; here a conductometer is used to find the endpoint of a reaction. It works on the principle of replacing one ion with another in the reaction mixture. This exchange of ions within the solution helps measure the changes in conductance. These changes help in reaching the endpoint, which tells a reaction is completed. Such a type of titration in which the endpoint is determined with the help of a conductometer is called conductometric titration.

What is Conductometric Titration?

• It is one of the qualitative chemical analysis processes to find the concentration of a sample.

• In this titration, the increase or decrease of electrolytic conductivity is due to the change in the concentration of ions in reaction mixtures.

• In such titration, the measurements of electrical conductance work as a tool to find the endpoint.

• Therefore, such titrations help in determining the end point of titration with the help of conductivity measurements known as conductometric titrations.

Terms used in Conductometric Titration

The following terms used are −

Titrant − It refers to a solution whose concentration is known. It is a standard solution which is added to another solution whose concentration is unknown.

Analyte − It refers to a solution whose concentration is unknown.

Endpoint or equivalence Point − It refers to a point when conductivity undergoes sudden change during titration. In other words, the necessary amount of reactant is added to the solution to complete the reaction.

Conductance (G) − It is defined as the reciprocal of resistance(R). In other words, the more is the conductance, the lesser the resistance, hence more ease to flow current.

$$\mathrm{G=\frac{1}{R}}$$

Principle

Conductometric titration works on the principle of −

“One ion is exchanged with another and the difference in ionic conductivity of these ions directly affects the total electrolytic conductivity of the solution”

Also, it is a volumetric process that depends on the measurement of the “conductance” of the reaction mixture or sample during the titration. The conductance taking place in titration can be explained with ohms law which states that current (i) flowing through the conductor is directly proportional to the potential difference (V) & inversely to resistance (R).

$$\mathrm{i=\frac{V}{R}}$$

Where V= potential difference

i= current

R= resistance

Therefore, all the above factors V,i and R depend on the −

• The mobility of ions.

• The number and charge (positive or negative) on the free ions.

• Temperature.

Theory

In this type of titration, the endpoint is usually measured by electrolytic conductivity. For example, when we carry out acid-base titrations (a neutralisation reaction). Initially, there is a decrease in the conductivity of the solution. Then, there will be a sudden increase in the conductivity of the solution. This increase or decrease of conductivity is due to their ionic difference in the reaction mixture, before and after. This difference helps in producing the endpoint. To calculate the end point of such reactions, a graph is plotted. Here is the graph is shown −

Explanation

This graph is based on the acid-base titration, when NaOH (titrant) is continuously added to the acid solution, it replaces the H^+ ions by the cationic part of the base. Thereby, decreasing the H^+ ions of acid solution continuously, which also decreases the conductivity. A point is reached when all ions are exchanged and only titrant ions (OH^-) are available for conduction. This increase in the concentration of titrant ions increases the conductivity. The sudden change in conductivity helps determine the endpoint. When the observed values of conductance and titrant are plotted in a graph of Conductance vs Volume of titrant, it will give two straight lines with opposite slopes. These two lines further meet at a point known as an endpoint.

Process

To conduct this titration, the following steps are required to be followed −

• Collect all the apparatuses such as the burette, burette stand, beaker, and conductivity cell connected to the conductometer.

• First, fill the burette with the base and note its initial volume.

• Then, prepare an analyte, such as acid diluted with distilled water.

• Now, insert a conductivity cell (that should be properly dipped) into this analyte or diluted acid solution. This cell is then connected to a conductometer and is ready to record the readings.

• Now, add the base dropwise into the analyte or acid solution.

• The change in conductance must be noted along with recordings of the base. It shows a decrease in conductance initially.

• After adding more few drops it shows a sudden increase in the conductance of the reaction mixture or solution as there is an increase in an ion concentration.

• It is preferred to take a few more readings to find the correct endpoint.

• At last, draw a graph for the observed values. This plotted graph helps in finding the equivalence point or end point, from the point of intersection between the two lines.

Advantages and Disadvantages of Conductometric Titration

Some of the advantages are −

• It does not need any indicators since the change in conduction is measured by a conductometer.

• It is suitable for various types such as coloured solutions, diluted solutions and turbid solutions.

• Its endpoint is obtained graphically, so it gives better results with minimum errors.

• It has no temperature issue, as it is kept constant throughout the titration.

Some of the disadvantages are −

• It does not promise to provide 100 percent accuracy as the process does not allow for the addition of a small titrant increase.

• It is not performing so well with the precipitation reaction due to the slow separation of precipitates formed during titration and also the absorption of ions on the precipitation.

• It is also limited to redox reactions as a high concentration of hydronium ions(H_3 O⁺) in the solution tends to mask changes in conductance.

What are the Applications of Conductometric Titration?

Various applications of conductometric titration are −

• Used in checking water pollution in lakes and rivers.

• Used in tracing microorganisms in food.

• Used in estimating ash content in sugar juices.

• Used in determining the purity of water and deionisation of water.

• Used in detecting deuterium ion concentration in water.

• Used in tracing antibiotics.

Conclusion

Conductometric titration is a type of titration that determines the concentration of substances with the help of a device called a conductometer. This device contains a conductometric cell that is supposed to be inserted into the solution (unknown strength or concentration). It measures the change in ionic concentration in the solution. Differences in the ionic concentration show increase or decrease in the conductivity. These overall changes in conductance are recorded by a conductometer. Hence, these recorded values are then plotted on a graph, which helps in determining the endpoint or equivalence point. This endpoint in the titration indicates the completion of the reaction.

FAQs

1. Which electrode is used in Conductometry?

The electrode used in conductometry is a conductivity cell which is made of quartz or pyrex. It consists of two electrodes made up of platinum and must be kept in a water container to maintain its constant temperature.

2. Why is DC not used in Conductometry?

If DC is used, it leads to electrolysis and changes the concentration of the solution with time. Therefore, it is preferred to use AC instead of DC.

3. When does conductivity decrease in conductometric titrations?

During acid-base titrations, the base replaces the H₊ ions of the acid solution with its cationic part, hence decreasing the concentration of H₊ions. This decrease in concentration decreases the conductivity of the solution.

4. Does pH affect conductivity in conductometric titrations?

As pH decides the acidity or basicity of the substance by its hydrogen concentration and conductivity depends on the presence of free electrons. There is no relationship between pH and conductivity.

5. Why is the conductivity cell used in conductometric titration?

A Conductive cell connected to the conductometer, immersed in the solution, helps measure the continuous change in concentration of ions in the reaction mixture. Therefore, gives various values that help in finding the endpoint of the reaction.