Back Titration

Custom Student Mr. Teacher ENG 1001-04 8 April 2016

Back Titration

The objective of this experiment is to determine the percentage by mass of calcium carbonate,CaCO3 in toothpaste using back titration technique. A known weight portion of toothpaste is obtained to react with known volume and concentration of standard acid solution. After completing the reaction, the resulting solution containing excess acid is back titrated with known volume and concentration of standard base solution. Determination of excess acid after reaction allow us to calculate the amount of acid react with CaCO3 in toothpaste sample. My result for this experiment shows that 19.6% of calcium carbonate presents in toothpaste sample. In conclusion,CaCO3 only made up approximately one fifth of toothpaste sample, toothpaste derived from a variety of component such as fluoride, water and detergent. Since the result obtained approaching 20%, saying that most of the error is avoided, the chemical reaction in this experiment is rapid and complete with no side reaction, determination of concentration of reactant is accurate, it is a successful experiment.

The purpose of this experiment is to improve the understanding of practical application of back titration. Back titration is designed to resolve the problems encountered with forward titration. Back titration can be used for many reasons: when the analyte is volatile or insoluble in water; when the analyte contains impurities that interfere with forward titration; when the end point is difficult to identify in forward titration; when the analyte react slowly with titrant in forward titration.In this experiment, a weighted portion of toothpaste is analysed to determine the percentage by mass of CaCO3 present in the toothpaste sample. Back titration technique is applied in this experiment instead of forward titration.This is because the substance being analysed, toothpaste is insoluble in water but soluble in acid.

Titration is the scientific techinique of volumetric analysis used to determine the concentration of unknown solution which involves the direct and stepwise addition of standard titrant to the analyte until the reaction reach neutralization.Back titration is also a kind of titration.It is called titration done in reverse because it is not carried out with the solution whose concentration of substance being analysed is required to be known as in the case of normal titration.In figuring out the concentration of an analyte(toothpaste,CaCO3),back titration works by reacting the analyte with a known number of moles of excess intermediate reactant(hydrochloric acid,HCl). The reaction goes past the equivalence point. The amount of intermediate reactant is in excess in the reaction with analyte.The resulting mixture containing excess of intermediate reactant is then titrated back with known volume and concentration of titrant(sodium hydroxide,NaOH). Knowledge of stoichiometry of the reaction allow the concetration of the analyte in the original solution related to the amount of reagent used.

Materials was prepared and equipment was cleaned with distilled water if necessary.10 ml of 0.16M of standard HCl was measured and pipetted into a clean conical flask. Approximately 0.1-0.2g of toothpaste was removed using a glass rod from the container and the exact weight of toothpaste sample was recorded. The glass rod with weighted toothpaste was placed in the conical flask which contained 10ml of 0.16M of standard HCl. Toothpaste was dislodged completely from the glass rod into the conical flask. About 10ml of deionised water was added to wash down the toothpaste from the glass rod.A funnel was inserted in the flask and the flask was gently heated to boil over a hot plate for 3-5 minutes until the reaction was complete. The funnel and the side wall of the flask was rinsed with small amount of deionised water. The mixture in the flask was allowed to cool to room temperature.A clean graduated burette was prepared and the inside of burette was rinsed with NaOH solution.

Rinsing procedure was repeated.The graduated burette was filled with excess amount of 0.08M NaOH solution. Some solution was allowed to drain out through stopcock to a waste beaker.The burette was clamped securely to a retord stand.1-2 drops of methyl orange indicator was added to conical flask and it was swirled. The conical flask was placed under the burette.The initial burette reading was read and recorded.To begin titrating the excess HCl with NaOH, The stopcock was opened all the way and the flask was swirled.As the end point was approached, the rate of addition of HCl into the NaOH was decreased to drop by drop.The end point reached when the colour of mixture changed from red or pink to orange.The final burette reading of NaOH solution was read and recorded.The titration was repeated twice.


  • Subject:

  • University/College: University of Arkansas System

  • Type of paper: Thesis/Dissertation Chapter

  • Date: 8 April 2016

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