0.7 Titration  (Page 39/4)

Titration is an analytical laboratory method to determine concentration of a solution. It is based on volumetric stoichiometric analysis involving two or more solutions. In most of the cases, we shall refer aqueous solutions. Clearly, titration is limited to substances, which are soluble in some solvent (may be at a higher temperature). Further, scope of titration is wider than acid-base neutralization reactions. The redox reactions and other complex forming reactions are also studied using titration.

Completion of reaction (equivalence point) is determined using some technique that includes chemical indicators, potentiometer, pH meter and many other techniques. Sometimes, even the change in the color of reacting solution signals the end of titration. In the case of acid base reaction, chemical indicators like methyl orange or phenolphthalein are used to determine end point. These indicators changes color with the change in the pH of the solution.

Some important terms or descriptions used in titration are :

Titrant : It is the standard solution whose concentration is known. It is placed in a thin cylindrical pipe called burette with fine measuring divisions to determine volume accurately. The titrant is streamed drop by drop into the flask containing reactant/ analyte.

Reactant/ analyte : It is the solution whose concentration is not known. Titration method is used to analyze this solution. An accurate volume of the reactant is kept in conical flask. The pH measuring techniques or devices are associated with reactant volume. If chemical indicator is used to identify end point, then very small amount of indicator (few drops) is mixed with the reactant.

Acid base titration indicators

Acid base titration makes use of pH chemical indicators, which changes color in a band of pH values – not at a particular pH value. Typically, we use methyl orange and phenolphthalein to determine end points of acid-base titration. The selection of a particular indicator for the titration is decided on the basis of plot known as pH curve, which is typical of a particular reaction. The indicator is always added to the reactant in the flask.

Methyl orange

Methyl orange is a weak base. It gives end points between pH range between 3.1 and 4.4. In weak acidic or basic environment (pH>4.4), its color is yellow. In high acidic environment (pH<3.1), it is red in color. In the detection range (pH : 3.1 to 4.4), its color is orange. The color changes from yellow to orange (when solution pH falls towards detection range) or red to orange (when solution pH rises towards detection range), indicating equivalence point.

Phenolphthalein

Phenolphthalein is a weak acid. It gives end points between pH range between 8.3 and 10. In strong acidic and weak basic environment (pH<8.3), it is colorless. In strong basic environment (pH>10), it is pink in color. In the detection range (pH : 8.8 to 10), its color is pale pink. The color changes from colorless to pale pink (when solution pH rises towards detection range) or from pink to pale pink (when solution pH falls towards detection range), indicating equivalence point.