Analysis of Calcium Carbonate in Toothpaste

Synopsis

The objective of this experiment is to determine the percentage by mass of calcium carbonate, CaCO3 in toothpaste using the back titration technique. A known weight portion of toothpaste reacts with a known volume and concentration of standard acid solution. After completing the reaction, the resulting solution with excess acid undergoes back titration with a known volume and concentration of standard base solution. This determines the amount of acid that reacted with CaCO3 in the toothpaste sample. The result of this experiment indicates that 19.

6% of calcium carbonate is present in the toothpaste. In conclusion, calcium carbonate constitutes only approximately one-fifth of the toothpaste sample, as toothpaste is composed of various components, including fluoride, water, and detergent. With the result approaching 20%, signifying minimal error, and considering the rapid and complete chemical reaction observed, this experiment is deemed successful.

Objectives

The primary aim of this experiment is to enhance understanding of the practical application of back titration. Back titration serves as a valuable approach to overcoming challenges encountered in forward titration.

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This method proves useful when the analyte is volatile or insoluble in water, when impurities interfere with forward titration, when the end point is challenging to identify, or when the analyte reacts slowly with the titrant. In this context, the analysis involves a weighted portion of toothpaste to determine the percentage by mass of CaCO3 present in the sample. The application of back titration is chosen due to the insolubility of toothpaste in water but its solubility in acid.

Theory

Titration is a scientific technique of volumetric analysis used to determine the concentration of an unknown solution.

It involves the stepwise addition of standard titrant to the analyte until neutralization is reached. Back titration, a variant of titration, is conducted in reverse, not with the solution whose substance concentration is sought. In the case of back titration, the analyte (toothpaste, CaCO3) reacts with a known excess of intermediate reactant (hydrochloric acid, HCl) past the equivalence point. The resulting mixture, containing excess intermediate reactant, is then titrated back with a known volume and concentration of titrant (sodium hydroxide, NaOH). Understanding the stoichiometry of the reaction enables the determination of the analyte's concentration in the original solution concerning the amount of reagent used.

Procedure

The experimental materials were prepared, and equipment was cleaned with distilled water if necessary. A measured 10 ml of 0.16M standard HCl was pipetted into a clean conical flask. Approximately 0.1-0.2g of toothpaste was accurately weighed, and the exact weight was recorded. The toothpaste, obtained with a glass rod, was placed in the conical flask containing 10ml of 0.16M standard HCl. After dislodging the toothpaste, 10ml of deionized water was added to wash down any remaining residue. The mixture was gently heated to boiling over a hot plate for 3-5 minutes. After cooling to room temperature, a clean graduated burette was prepared with excess 0.08M NaOH solution. Methyl orange indicator was added to the conical flask, and the initial burette reading was recorded. The excess HCl was titrated with NaOH, and the end point was reached when the color changed from red or pink to orange. The final burette reading of NaOH solution was recorded. The titration was repeated twice for accuracy.