The objective of this experiment is to determine quantity of calcium carbonate

The objective of this experiment is to determine quantity of calcium carbonate(CaCO3) present in toothpaste by using back titration. 0.1503g of toothpaste is added to 10 cm3 of hydrochloric acid(HCl) with a molarity of 0.1672M via a glass rod. Since calcium carbonate is insoluble in hydrochloric acid, reaction was allowed to take place under heating in a fume hood to prevent inhalation of hydrochloric acid fumes. Filter funnel was rinsed with small amounts of deionised water to wash down any acid vapour that have condensed on the funnel. After conical flask is cooled, methyl orange indicator is added into conical flask and sodium hydroxide(NaOH) of 0.0797M is used to neutralise excess acid to determine amount of calcium carbonate used. End point is determined by solution turning orange. 2 Amount of acid used is determined by the amount of sodium hydroxide used. Amount of alkali used is 12.90 cm3.The experiment conducted determined that about 21.4% of toothpaste contains calcium carbonate. 3 In conclusion, this experiment was partially successful in determining the amount of calcium carbonate present. This is so because there are possible other side reactions with other alkali substances present in the toothpaste because toothpaste is a mixture of many substances. Hence, since the accuracy is slightly affected, causing the results to be slightly unreliable.
Titration is usually used to determine amount of analyte in an acid or base by directly adding a titrant into a vessel containing the analyte. However, very often the analyte is volatile or insoluble in water. Hence, excess amount of acid/base is used to determine the amount of acid/base used by reacting the excess quantity of the acid/base with another titrant. 4 This process is known as “back titration”.
Chemicals used in this experiment are hydrochloric acid, sodium hydroxide, methyl orange and calcium carbonate (in toothpaste). Hydrochloric acid is commonly used for carbon dioxide generator in labs through acid and carbonate reactions. Application of hydrochloric acid is commonly used as production of organic and inorganic substances, pH control and neutralization. Sodium hydroxide is commonly used to test for the presence on common cations like Zn2+, Pb2+, Cu2+, NH4+ ions, adhesives and sealant chemicals. Methyl orange is used to determine whether a substance is an acid or alkali by the colour of the solution due to methyl orange.
The purpose of this experiment is to establish the quantity of calcium carbonate present in toothpaste through back titration. Simultaneously, allowing students to be familiar with back titration technique.
Calcium carbonate is white and colourless powder or crystal that is insoluble in water. 7 Since calcium carbonate is insoluble in water, excess volume of hydrochloric acid is required to completely react with calcium carbonate to form calcium chloride, water and carbon dioxide.
This reaction is heated to increase rate of reaction between hydrochloric acid and calcium carbonate and preventing carbon dioxide from dissolving into water.
However, in this experiment, excess hydrochloric acid is used. Hence the reaction is as follows:
Methyl orange is added as an indicator of the end point of the reaction between sodium hydroxide and hydrochloric acid. 8 Sodium hydroxide reacts with the excess hydrochloric acid to form sodium chloride and water.
The quantity of sodium hydroxide used can be determine and hence determine the amount of excess hydrochloric acid present by mole ratio:
This enable us to calculate the amount of hydrochloric acid used:
To determine the amount of calcium carbonate present in toothpaste. we need to use mole ratio between hydrochloric acid and calcium carbonate:
By mole ratio:
Once finding the mole ratio, we can determine the mass of CaCO3:
Finally, we can determine the percentage of CaCO3 in toothpaste:
Based on the results used from the practical, the result of the average percentage toothpaste of CaCO3 in toothpaste is in range within the acceptable range of 18%~22%. However, in practicality, there may be other alkaline substances present in the toothpaste that will react with hydrochloric acid during back titration. Common active ingredients found in toothpaste are anti-carcinogen agents to produce fluoride from sodium fluoride or sodium monofluorophosphate. Anti-hypersensitivity agents such as potassium salts or amorphous calcium phosphate and antimicrobial agents like stannous fluoride or triclosan. Other components of toothpaste are abrasive agents like calcium carbonate, dehydrated silica gels (Silicon dioxide) or hydrated aluminium oxides. Detergents in form of sodium lauryl sulfate, flavouring agents like saccharin and peroxides like hydrogen peroxide.
Stannous fluoride’s pH in a 0.4%(V/V) solution is 2.8 to 3.5. Saccharin’s pH in a 0.35%(V/V) solution is 2.0. 7 Hydrochloric acid will react with aluminium oxide to form aluminium chloride and water. Hence, during the back-titration process, aluminium oxide is amphoteric which will react with hydrochloric acid. Moreover, the presence of stannous fluoride or saccharin means that there is a higher amount of H+ ions in the solution. This means that more sodium hydroxide is required to reach end point. Adding more sodium hydroxide means higher value of Vexcess. 11 This will affect the mole ratio of sodium hydroxide and hydrochloric acid. 12 Eventually, this would affect the results of average (%) of calcium carbonate in toothpaste. Therefore, the accuracy of the results is decreased.
As listed above, those ingredients are commonly present in toothpaste. However, different toothpaste brands have different compositions of each ingredients and certain ingredients may not be present in the toothpaste. Therefore, the above scenario is based on every toothpaste brands having those ingredients listed above and how each ingredient and component will affect the results and accuracy of the experiment.
In conclusion, the average (%) of calcium carbonate present in toothpaste is 21.4% by using 10cm3 of hydrochloric acid with a molarity of 0.1672M and using an average of 12.90cm3 of sodium hydroxide with a molarity of 0.0797M.
In general, the experiment was a partial successful because there are side reaction occurring which can affect the pH level of the system which will affect the end point of the reaction. However, we can still find the amount of calcium carbonate present to a certain degree as it is one of the main components present in larger quantity in the toothpaste. Hence, the other side reactions will affect the results slightly.