Chemistry

Conductivity of a substance is defined as the ability or power to conduct or transmit heat, electricity, or sound›. Its units are Siemens per meter [S/m] in SI and milliohms per centimeter [m mho/cm] in U.S. customary units.

12.2. Measurement of conductivity of solutions

The conductivity is the reciprocal of the resistance (1/R) and is measured in Siemens or mhos.Conductivity measurements are used routinely in many industrial and environmental applications as a fast, inexpensive and reliable way of measuring the ionic content in a solution. For example, the measurement of conductivity is a typical way to monitor and continuously trend the performance of water purification systems.

Electrical conductivity meter

Principle of the measurement

The electrical conductivity of a solution of an electrolyte is measured by determining the resistance of the solution between two flat or cylindrical electrodes separated by a fixed distance. An alternating voltage is used in order to avoid electrolysis. The resistance is measured by a conductivity meter. Typical frequencies used are in the range 1–3kHz. The dependence on the frequency is usually small, but may become appreciable at very high frequencies.

A wide variety of instrumentation is commercially available. There are two types of cell, the classical type with flat or cylindrical electrodes and a second type based on induction. Many commercial systems offer automatic temperature correction. Tables of reference conductivities are available for many common solutions.

The conductivity of an electrolyte is the conductance of a volume of solution containing one mole of dissolved electrolyte placed between two parallel electrodes 1dm apart and large enough to contain between them all the solution; the conductivity is affected by temperature.

Checking up 12.2

Describe the functioning of conductivity meter and derive the formula of calculation of conductivity

12.3. Specific conductivity of solutions

Activity 12.3:

1. Define resistivity

2. Establish a relation between conductivity and resistivity and among the following substances, which ones are conductors and non-conductors, for each you have to explain why they are or not conductors: pure water, sugar, iron plate, clothes, plastic bags, ammonia solution, salt solution, etc...

Specific Conductivity (better known as specific conductance) is the measure of the ability of that material to conduct electricity. It is represented by the symbol “К”. Hence, by definition, the specific conductance (specific conductivity), κ (kappa) is the reciprocal of the specific resistance. The SI unit of conductivity is Siemens per meter (S/m).

12.4. Molar conductivity of solutions

The molar conductivity of a solution at any given concentration is the conductance of the volume of solution containing one mole of electrolyte kept between two electrodes with the unit area of cross section and distance of unit length. In general terms, it is defined as the ratio of specific conductivity and the concentration of the electrolyte.

12.4.1. Strong electrolytes

For strong electrolyte, molar conductivity increases steadily with dilution until it reaches the maximum value at infinite dilution (at high concentration, the lower conductivity values are due to ionic interference. The formation of ionic pairs or triplet and symmetrical spheres greatly reduces the mobility of ions however as the dilution increases, there is reduced ionic interference as result of many solvent molecules surrounding the oppositely charged ions thus an increase in molar conductivity.

At infinite, there is independent migration of ions that is ions experience negligible ionic interference and move independent of each other.

The molar conductivities of strong electrolytes are high. This is because, by nature, strong electrolytes are highly dissociated when molten or when in solution into large number of ions. These ions are mobile, hence they migrate to the electrodes, resulting in the high conduction of electricity: the higher the number of ions are free in solution, the higher the conductivity.

This graph can be obtained by extrapolation of the graph to zero concentration.

12.4.2. Weak electrolytes

Weak electrolytes show partial dissociation in solution, producing few ions, which results in low conduction of electricity.

A weak electrolyte dissociates to a much lesser extent so its conductance is lower than that of a strong electrolyte at the same concentration.

Summary:

• The higher the number of ions per unit volume in solution, the greater the conductivity of the electrolytic solution. This means that the conductivity increases with concentration of ions in solution up to an optimum level over which it starts decreasing.

• On the other hand when the conductivity has decreased due to very high concentration of ions, it can be increased with dilution (i.e. lower concentrations) up to its optimum, beyond which further dilution will decrease conductivity.

• The decrease or increase of conductivity by concentrating or diluting the solution is sharp in strong electrolytes while it is gradual in weak electrolytes. The following graph shows

The table below shows the trend in conductivity with dilution for a strong and a weak acid.

Explanation of Increase in Conductivity with Dilution:

With increase in dilution (decrease in concentration), ions become farther apart, and inter-ionic forces (i.e. forces of attraction between unlike ions and forces of repulsion between like ions) decrease considerably, so that greater number of ions are able to migrate to the electrodes. In addition, due to change in equilibrium, the electrolyte undergoes further ionization from the same mass in solution (in order to balance the effect). Hence, more ions (conducting species) are introduced into the solution

12.5. Molar conductivity at infinite dilution

Kohlrausch’s law of independent migration of ions states that “at infinite dilution, where ionization of all electrolytes is complete and where all interionic effects are absent, the molar conductivity of an electrolyte is the sum of the molar conductivities of its constituent ions at constant temperature”

12.5. Factors that affect molar conductivity of solutions

12.6. Kohlrausch’s law of individual molar conductivity

12.7. Use of conductivity measurement in titration and solu-bility product

12.7.1. Using conductivity to find the end point of a titration

The end-point in titration experiment can be determined using conductivity. The procedure of the technique is:At the start of this titration the conical flask contains a strong alkali that is fully ionized in water. If electrodes are placed inside the conical flask the ions in the water will conduct electricity and a current will flow.The more ions there are the better the conductivity and the higher the current will be. The current can be measured using an ammeter. As acid is added to the alkali hydrogen ions and hydroxide ions react together to form water molecules.The number of ions in the conical flask starts to decrease and the current flowing through the solution will decrease. At neutralization all of the hydrogen ions and hydroxide ions have reacted together to form water molecules.The neutral solution contains only salt ions dissolved in water molecules. The solution will still conduct electricity because of the salt ions but the current will be at a minimum.As more acid is added the current will start to increase because there will now be unreactedhydrogen ions in the solution as well as the salt ions. The solution is now no longer neutral but has become acidic.

If you draw a graph of current against the amount of acid added you can see where the minimum is. This is the end point of the titration at neutralization.

12.7.2. Determination of solubility product by conductivity measure-ment.

Solubility product, Ksp, is the mathematical product of its dissolved ion concentrations raised to the power of their stoichiometric coefficients. Solubility products are relevant when a sparingly soluble ionic compound releases ions into solution.That is the product of the concentration of ions in the solution which are in equilibrium with the solid ion. These concentrations can be determined via conductivity measurements, consider the following examples :

The measurement of conductivity will depend on the value of Ksp for the sparingly soluble substances. The measurement of the specific conductivity, K of the saturated solution leads to a value of the concentration.

12.8. Difference between metallic conductivity and electrolyt-ic conductivity

The substances, which allow the passage of electric current, are called conductors. The best metal conductors are such as copper, silver, tin, etc. On the other hand, the substances, which do not allow the passage of electric current through them, are called non-conductors or insulators. Some common examples of insulators are rubber, wood, wax, etc.

The conductors are broadly classified into two types, Metallic and electrolytic conductors.

The electrolyte may, therefore, be defined as the substance whose aqueous solution or fused state conducts electricity accompanied by chemical decomposition. The conduction of current through electrolyte is due to the movement of ions.On the contrary, substances, which in the form of their solutions or in their molten state do not conduct electricity, are called non-electrolytes.