Conductimetry, Chemistry tutorial

Introduction 

Conductimetry analysis involves the application of electrolytic cell to calculate the conductance of a following solution. This is depends on the electrical properties of ions present in the solution to be analyzed. When a current is passed through a solution enclosing negative and positive ions, these ions shift towards the electrodes carrying the current into the solution. Positively charged ions migrate towards the cathode and negative ions towards the anode. But, the rate at which the ions move is influenced via a lot of factors these as the degree of the solvation of the ions, temperature and viscosity of the medium. Because the movement of ions is responsible for the conduction of electricity through the solution, the level of flow of current is dependent on the number of ions per unit volume of solution (concentration), the ionic charge and the rate of migration towards the electrodes. As well, the extent of conduction based on the potential dissimilarity across the solution.

Definition

Conductimetry is an electrolytic technique that is utilized to determine the conductance of example solution depends on the electrical properties of the constituent ions.

Basic Concepts of Conductance Analysis

The current passing between the 2 electrodes is carried through the ions in the solution and is related to the number of ions present. This is as a consequence of both the molar concentration of the compound and its extent of ionization. The anions donate electrons to the anode whilst the cations accepts electron from the cathode. It is this convey of electrons that verifies the amount of current flowing and the contribution of each ionic specie is computed through its ionic mobility.

The current flowing through a conductor is described via Ohm's law that situations that the electric current flowing through a conductor is directly proportional to the applied voltage and inversely proportional to the resistance of the conductor.

For example Current (I) = Voltage (v)/Resistance(R)

But in words of conductance (C),

   I = C X V

Specific and Molar Conductance

The precise conductance (K) of a given solution is described as the conductance per centimeter of a solution that has a cross-sectional area of 1 cm2, and is measured in Scm-1(or in non SI unit of ?-1cm-1).

The molar conductance (Λ) is the precise conductance of a solution accurate for the concentration of ions in the solution. That is, Molar Conductance = Conductance x Volume of solution that encloses 1 gram mole as we know: The value of conductance reduces as the concentration of the solution reduces, but the value for molar conductance will enhance. This is due to enhanced dissociation of molecules in dilute solutions.

Conductimetric measurements

The basic instrument utilized for conductimetric measurements is termed conductivity meter. This instrument has an essential arrangement of a Wheatstone bridge. In this case, the conductance of solution is computed using an alternating current rather than a direct one. This is to prevent the accumulation of charges at the electrodes that would change the composition of the solution at the electrodes and initiate substantial errors in the measurement.

The essential circuit diagram of the instrument is specified below:

560_A Wheatstone.jpg

Fig: A wheatstone

With fixed resistance, R1 and R2, the variable resistance RV is adjusted until current just flows through the galvanometer. Under these conditions,

  R1 x RV = Rcell x R2

This provides a value for resistance of the cell that can be transferred to conductance via calculating the reciprocal. The electrodes conductivity cells are generally made of platinum coated through platinum black through a identified area. Although that in many cells the distance between the electrodes is adjustable, but in any experiment it must be held steady and for many estimates the accurate value is needed. The cells must be thermostatically controlled since any transforms in temperature will reason importance alteration of conductivity values.

Experimental precautions

  • Solvent purity

The presence of ions in the water utilized in conductivity measurement can guide to serious errors mainly in the analysis of a extremely dilute solution where the computed conductance might be of almost the similar magnitude as the conductance of the dissolved impurities. Therefore, in conductivity measurement de-ionized water has a precise conductance (conductivity) of 5.5 x 10-8 ?-1cm-1

  • Titrant Volume

During conductimetric titrations, the concentrations of the conducting species should not change significantly; or else errors will be introduced into the measurements. Therefore, the concentration of the titrants should be about 10 times that of the solution in the conductivity cell so that the raise in volume at, and beyond, the end point will be tiny.

  • Temperature

The precise conductance of most ions generally amplifies via about 2% for each degree rise in temperature. It is therefore extremely significant to control the temperature accurately in such measurements. For example, for titrations and other routine analysis, the temperature may be controlled for ± 0.250C that ensure a precision of ± 0.5%. But, for other purposes, these as the determination of dissociation constants, more accurate temperature control is required.

  • Extraneous ions

The presence of huge concentrations of extraneous ions in solution to be analyzed is generally not needed. This is since any transform in conductance is being masked via these ions. This occurs particularly if the limiting equivalent conductance of the foreign ions is similar to those of the ions to be analyzed. Such problem might be overcome via using elevated precision conductivity meters or bridges.

Applications

Conductimetry is an extremely helpful phenomenon for the determination of diverse physical constants, these as dissociation and solubility constants. But its main application is for monitoring titrations.

  • Solubility determination

Conductimetry is applied in the determination of solubilities of sparingly soluble salts. It engages the measurement of the conductivity (k) for the saturated solution and then followed through the computation of the concentration (C) from the equation: Λ=1000K/C

Where Λ is the equivalent conductance or specific conductance of 1 cm3 solution enclosing 1 gram-equivalent of solute (molar conductance)

  • Dissociation constant

This is computed through calculating the degree of dissociation from the equivalent conductance of the analyte. For a weak acid HA,

i.e.     Kc = [H+][A-]/[HA]= α2C/1- α

Where α is the degree of dissociation given by:

       α = Λ c/ Λd where ΛC is the equivalent conductance before dissociation while Λd is equivalent conductance after full dissociation.

  • Precipitation titrations

In such titrations, the conductance of a solution is determined as function of the volume of a titrant. The figure of the titration curve based on the conductance of the ion in the cell (iC) and that of the ion in the titrant (it). If iC is greater than it, the conductance reduces at 1st, then enhances after the endpoint as the concentration of the titrant ion amplifies. The end point is the intersection of the 2 lines.

  • Acid-base titration

As an acid is titrated through an alkali, the ionic composition of the mixture transforms and this is reflected in the conductivity of the solution. In conductimetric titration, the conductivity can be plotted against the volume of the alkali that is being utilized. This provides increase to dissimilar titration curves as below:

827_The end point is where the two lines intersect each other.jpg

As we know: the end point is where the two lines intersect each other.

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