Back Titration
EXPERIMENT 4:
BACK TITRATION
CONTENT
NO.
CONTENT
PAGE
1.
Synopsis
3
2.
Introduction
3
3.
Theory
4
4.
Procedure
4
5.
Results and Calculations
5 – 6
6.
Discussion
7
7.
Conclusion
7
8.
References
8
1. SYNOPSIS The purpose of this experiment is to use the back titration method to determine the percentage of calcium carbonate in toothpaste. Instead of using standard titration methods where an acid is titrated directly using a standard solution of a base, back titration is used because the sample to be analysed, CaCO3 does not dissolve in water. Hydrochloric acid and sodium hydroxide are the reagents used in this experiment. The precise measured amount of HCl is added to the weighted portion of toothpaste which is CaCO3. When the reaction between HCl and CaCO3 is completed, the excess HCl is back-titrated against NaOH until the reaction reaches the end point and the indicator changes from pink to orange. Hence, the amount of HCl that has reacted with CaCO3 can be calculated by using the initial amount of HCl in excess to subtract the amount of HCl left after the reaction. The average percentage of CaCO3 in toothpaste is found to be 19.97%, which is quite close to the expected value of 20%, so most of the error should have been avoided therefore this is a successful experiment.
2. INTRODUCTION Titration is a laboratory method used to determine the end point of a reaction between acid and base in order to find the concentration of an unknown solution. One solution of a known concentration or a titrant is slowly added to a known volume of another solution of unknown concentration or analyte until the reaction is neutralized which is usually indicated by a colour change. Back titration is used in this experiment because the sample, toothpaste is insoluble in water. Back titration is also used when the sample is volatile such as ammonia or when solution being titrated reacts very slowly with the analyte and when the exact end point of a
BACK TITRATION
CONTENT
NO.
CONTENT
PAGE
1.
Synopsis
3
2.
Introduction
3
3.
Theory
4
4.
Procedure
4
5.
Results and Calculations
5 – 6
6.
Discussion
7
7.
Conclusion
7
8.
References
8
1. SYNOPSIS The purpose of this experiment is to use the back titration method to determine the percentage of calcium carbonate in toothpaste. Instead of using standard titration methods where an acid is titrated directly using a standard solution of a base, back titration is used because the sample to be analysed, CaCO3 does not dissolve in water. Hydrochloric acid and sodium hydroxide are the reagents used in this experiment. The precise measured amount of HCl is added to the weighted portion of toothpaste which is CaCO3. When the reaction between HCl and CaCO3 is completed, the excess HCl is back-titrated against NaOH until the reaction reaches the end point and the indicator changes from pink to orange. Hence, the amount of HCl that has reacted with CaCO3 can be calculated by using the initial amount of HCl in excess to subtract the amount of HCl left after the reaction. The average percentage of CaCO3 in toothpaste is found to be 19.97%, which is quite close to the expected value of 20%, so most of the error should have been avoided therefore this is a successful experiment.
2. INTRODUCTION Titration is a laboratory method used to determine the end point of a reaction between acid and base in order to find the concentration of an unknown solution. One solution of a known concentration or a titrant is slowly added to a known volume of another solution of unknown concentration or analyte until the reaction is neutralized which is usually indicated by a colour change. Back titration is used in this experiment because the sample, toothpaste is insoluble in water. Back titration is also used when the sample is volatile such as ammonia or when solution being titrated reacts very slowly with the analyte and when the exact end point of a
References: 2. N.N. Greenwood, A.Earnshaw, Chemistry of the Elements. 2nd ed. Oxford, Boston; Butterworth-Heinemann, 1998, c1997.