Expt5 Double Indicator Full Report

Page 1 of 6

I.

Introduction

In the acid-base titration, the equivalence point is known with the help of indicator which changes its color at the end point. In the titration of the polyacidic base or polybasic acid there are more than one endpoint. One indicator is not able to give color change at every endpoint. So, to find out each end point, more than one indicator must be used (Class 11 Chemistry Notes Stoichiometry - Double Indicator Acid Base Titration, 2012). Sodium Carbonate is a Bronsted Base that is used as a primary standard for the standardization of strong acids. It hydrolyzes in two steps:

Sodium Carbonate can be titrated to give end points corresponding to the stepwise additions of protons to form HCO3- and CO2 (Christian, 2004). When phenolphthalein is used in the above titration, it changes its color at the first end point when HCO3- is formed and with it, the second end point is not known. Similarly, with methyl orange, it changes color at second end point only and the first end point cannot is not known. It is because all indicator changes color on the basis of pH in the medium (Class 11 Chemistry Notes Stoichiometry - Double Indicator Acid Base Titration, 2012). Phenolphthalein changes color at pH 8.3-10:

While, methyl orange changes color at pH 3.1-4.4:

Calculations in double indicator titration involves determination of the volume of the volume of acid used when using phenolphthalein and methyl orange indicators, determining the concentration of salt in a given mixture and also determining the percentage composition of a given salt mixture. This experiment should be able to guide the student on how to determine qualitatively and quantitatively the components of carbonate mixture.

Page 2 of 6

II.

Methodology

This experiment is composed of three parts. The first procedure is to prepare a dilution of 1:25 from the original stock of an unknown sample using a 100 mL volumetric flask. Dilution is the process of reducing the concentration of solute usually simply by adding more solvent while, dilution factor is the ratio of the final volume /aliquot volume, where final volume = aliquot + diluent . Thus, 4 mL of the original unknown sample is needed to prepare 100 mL dilution of 1:25. The second part of the experiment is the qualitative analysis of components in which methyl orange and phenolphthalein were used separately on the titration of the 10.0 mL of diluted unknown sample with the 0.05 N standard HCl solution.Volume of HCl used for each indicator had to be compared to be able to identify the components present: if Vph>1/2 Vmo, then NaOH & Na2CO3 are the components present; if Vph