Synopsis The objective of this experiment is to determine the percentage of calcium carbonate

Synopsis
The objective of this experiment is to determine the percentage of calcium carbonate (CaCO _3) in toothpaste, by performing back titration since calcium carbonate is not soluble in water. An exact volume of hydrochloric acid will react with a weighted amount of toothpaste sample (CaCO _3). After reaction is complete, the excess hydrochloric acid is back titrated against sodium hydroxide until reaction is complete, indicated by a colour change from pink to orange. By using stoichiometric calculations, the amount of calcium carbonate can be found. The results of my experiment shows that the average percentage of calcium carbonate in toothpaste sample is 21.4%, which is between the acceptable % of 18% to 22%. Therefore, I conclude that this experiment is a success but it also has areas to improve on.

Introduction

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Titrations are chemical techniques used to determine the amount of a compound present in a mixture. Titration is a technique where a solution of known concentration (titrant) is used to determine the concentration of an unknown solution (analyte).

One of the chemicals used for this experiment is hydrochloric acid (HCl), a colourless liquid that has a strong pungent smell, is soluble in water, has low melting point and is highly corrosive. Moreover, it is a strong acid as when it is dissolved in water, all its molecules will ionise to form hydrogen and chlorine ions. Hydrochloric acid was founded by alchemist Jabir ibn Hayyan during the 800s and it was an essential and commonly used chemical. (PubChem, 2004) It has been widely used in many industries such as in the manufacturing industry which produces steel, suspension bridges or vehicles. In the food industry, it is used to process sugar and make gelatin. (Council, 2003)

Another chemical used is sodium hydroxide (NaOH), which exists as a white crystalline odourless solid or a colourless liquid. It is a strong base that is soluble in water and is very corrosive. Sodium hydroxide was discovered by Humphrey Day in England in the year 1807. Sodium hydroxide is a frequently used chemical in the industry. It is used to neutralize acids and make sodium salts. For example, it is used to refine petroleum and remove sulfuric and organic acids. It is also used to hydrolyze fats in the production of soaps and in the manufacture of rayon, textiles, and cellophane. Other uses include the manufacturing of plastics, extraction of rubber from used materials, removing the skins of vegetables or fruits making the production of food easier and purify vegetable oil. (PubChem, 2005)

The use of a strong acid and strong base is very common as they completely dissociate in water, resulting in a strong acid-strong base neutralisation reaction.
Lastly, methyl orange was an indicator used in the experiment. It is a pH indicator regularly used during titration as it has a distinct colour change at different pH values. Pink is observed in acidic medium whereas yellow is observed in a basic medium. (Wikipedia, 2018) Discovered by several workers between 1975 to 1976, methyl orange is also used for dyeing textiles.
The aim of this experiment is to better understand back titration, which is used when the substance being analysed is either volatile (evaporates easily), insoluble in water, reacts too slowly in forward titration or when the end point in forward titration is too difficult to determine.

Theory
In this experiment, back titration is used to determine the amount (percentage by mass) of calcium carbonate in a weighted portion of toothpaste. Back titration is used in this experiment as calcium carbonate in toothpaste is insoluble in water.
This was done by reacting the analyte (known concentration), toothpaste, with an excess of 0.1672M of hydrochloric acid, HCl. After the reaction is complete, the excess hydrochloric acid is back-titrated against 0.0797M of sodium hydroxide (NaOH).
Reactions involved: CaCO _3 + 2 HCl -> CaCl _2 + H_2 0 + CO _2
NaOH + HCl (excess) -> NaCl +? H?_2 0

By using stoichiometric calculations, the concentration and amount of analyte (CaCO _3) in original solution can be found.

Discussion

Toothpaste contains between 18% to 22% of calcium carbonate which ensures that plaque does not develop on our teeth. The average percentage of calcium carbonate in the toothpaste sample was calculated to be 21.4% based on my experiment. If the percentage of calcium carbonate were to be any higher, there will be excess abrasive in the toothpaste leading to damage of tooth enamel, tooth sensitivity and even discolouration of teeth. (Standring, 2008) Results of my experiment were rather accurate due to the measures taken to ensure accuracy of results. A glass funnel was inserted in the flask when the solution was being heated. This allowed carbon dioxide to escape while preventing hydrochloric acid from escaping as the funnel catches it and drips back. Any loss in hydrochloric acid will lead to inaccurate results.
Also, the toothpaste was carefully transferred into conical flask, ensuring none was smudged on the conical flask as it changes the mass of toothpaste.
Conclusion
Based on the results of the experiment I have conducted, the average percentage of calcium carbonate in the toothpaste sample was 21.4%, which falls within the 18% to 22% range. This shows that the experiment was conducted rather accurately.
However, since the actual value of calcium carbonate in toothpaste is 20%, I feel that the experiment could have been conducted more accurately. There are a few sources of error that might have affected the accuracy of my results. (Anon., 2011) Firstly, I might have mistaken the colour of the indicator at end point and may have over titrated it. This will result in inaccurate volume of NaOH used to be recorded, affecting the results. In order to prevent this, when nearing the end point, NaOH should be added drop by drop to prevent over titration.
Another possible error was not allowing solution to cool to room temperature after heating, which might affect titration results as some pH indicators are sensitive to temperature changes. In order to prevent this, a thermometer should be used to measure the temperature of solution to ensure that it has cooled down to room temperature before carrying out titration.
Thus, I feel that this experiment is a success as the results are a near the actual value, however the accuracy of this experiment can definitely be improved to get a more accurate value.