Equivalence Point

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Equivalence Point

 
The Equivalence point is also called as the stoichiometric point. In an acid base titration it is a point at which chemically equal quantities of acids and bases are mixed. In simple words it’s the point where the number of moles of acid is equal to the number of moles of the base. So when the acids and bases are mixed in the right proportions they can neutralize each other.

Understanding Titration:

The qualitative chemical analysis titration is a laboratory method which is used to find the concentration of an unknown quantity. In this method the titrant which is the known concentration of a solution is added to the analyte the solution with an unknown concentration until the reaction is completed. The equipment used for carrying out a titration is burette, clamp, conical flask, pipette and pH indicator (this depends on the type of reactants used.) 
equivalence point
Alkalimetry and Acidimetry:  An acid-base titration is when quantitative analysis is used to find the concentration of the unknown acid or base. Mentioned below are the two types of acid base titrations.
·         The acid-base titration which is used to determine the concentration of a basic substance is called as alkalimetry.
·         The acid-base titration which is used to determine the concentration of an acidic substance is called as Aciditmetry.
Depending on the type of acid base titration the pH indicator needs to be chosen.
 
Mentioned below are the different methods used in determining the equivalence point.
Use of pH indicators in a titration: The pH indicators are used to determine the equivalence point in a titration. This implies at equivalence point there is a change in color observed. A drop of the indicator is added in the beginning of the titration at the end point observing the color change the equivalence point can be approximated.
Conductance:  The ions present in the solution determine the conductivity of a given solution. There is a certain amount of conductivity during the starting of the reaction. After the titration occurs and the neutralization takes place there is a change in the conductivity. Therefore, by calculating the change in the conductivity the equivalence point can be approximated.
Color change: During few reactions there is a noticeable change in the color of the solution at the equivalence point. This change can be observed even without adding any indicator.
Precipitation: During a chemical reaction sometimes there is a precipitation occurring at the equivalence point. Precipitate is a solid residue formed as a result the titration.
Calorimeter: The isothermal titration calorimeter is a used to determine the equivalence point of a titration. This instrument detects the heat produced or consumed during a reaction and the change in the heat can be used to approximate the equivalence point.
Thermometric Titrimetry: As the name suggests, thermometric indicates the thermal change of a given chemical reaction. In this method the equivalence point is estimated by observing the rate of change of the temperature (that is the rise or fall of the temperature.)
Spectroscopy: This is a very interesting method to estimate the equivalence point of a reaction. In this method the equivalence point is determined by the amount of light absorbed by the reactants and the products.
Amperometry: This method is used for generally the redox reactions. In this method the equivalence point is estimated according the rate of change in the current.
Equivalence point Vs end point in a titration: Though the names sound similar both equivalence point and the end point are different. The equivalence point is determined by the theoretical calculations by equating the number of moles of the titrant and the analyte in a reaction. The end point is whereas determined in the laboratory by observing the color change or the instruments. The ideal situation is when the equivalence point is very close to the end point in a titration.
 

The most commonly used pH indicators are:

The table below list the type of indicator and the color it holds in the acidic side and the basic side.
Indicator Acidic side Basic side
Methyl violet Yellow Violet
Methyl Orange Red Yellow
Methyl Red Red Yellow
Litmus Red Blue
Phenolphthalein Colorless Pink
Bromothymol Blue Colorless Blue
 

The pH rules for the formation of different kinds of solutions:

·         A neutral solution is formed by the reaction of a strong acid and a strong base. (pH = 7)
·         An acidic solution is formed by the reaction of strong acid and a weak base. (pH < 7)
·         A basic solution is formed by the reaction of weak acid and a strong base. (pH > 7)
 

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