The main aim of the experiment is to find out the amount of calcium carbonate in toothpaste through back titration since calcium carbonate does not dissolve in water. A roughly weighed amount of calcium carbonate is mixed with hydrochloric acid and then titrated against sodium hydroxide. When the indicator turns from pink to orange, the volume of sodium hydroxide used is taken down. After doing some calculations, the average percentage of calcium carbonate in toothpaste is 19.2%. This average percentage is relatively close to the expected percentage of 20% thus this experiment is a success. Generally after performing the experiment twice, the smaller the mass of toothpaste used, the more volume of sodium hydroxide is needed to neutralise the excess hydrochloric acid. Introduction:
Usually, the acid or (base) can be titrated straight against a primary standard base (acid). However, when the substance is volatile for example ammonia or is an insoluble salt for example calcium carbonate, back titration is used. Back titration is performed by reacting that insoluble or volatile substance of unknown concentration with excess acid or (base) of known concentration and then followed by a titration of the unreacted acid or (base) with a base or (acid). Toothpaste usually contains around 20% of calcium carbonate. Calcium carbonate acts an abrasive in toothpaste and helps to remove dental plague as well as food remains stuck in between the teeth. The rationale of conducting this experiment of finding out the amount of calcium carbonate in toothpaste is to make sure that the percentage of calcium carbonate is around 20%. If the percentage is more than 20%, the enamel protecting our teeth will wear off and become thinner resulting in decolouration and teeth sensitivity.
The experiment is performed using back titration as calcium carbonate is an insoluble salt and does not dissolves in water. To find out the percentage of calcium carbonate in toothpaste, a known amount of toothpaste (calcium carbonate) is reacted with hydrochloric in excess to give calcium chloride, water and Carbon dioxide. CaCO3 + 2HCL CaCL2 + H2O + CO2 However, since hydrochloric acid is in excess, only some hydrochloric is reacted with CaCO3. To find out the amount of hydrochloric acid that is in excess, the excess hydrochloric acid is back titrated with NaOH. And to find out the amount of hydrochloric acid that is reacted, we can use the original amount of hydrochloric acid which is 10ml and minus it off the amount of hydrochloric acid that was in excess. After obtaining the volume of hydrochloric acid that is reacted, we can use to find out the number of moles of hydrochloric acid that is reacted.
From the above stoichiometric equation, the mole ratio between CaCO3 and HCL is 2:1. Therefore to find the number of moles of CaCO3, we can divide the number of moles of HCL by 2. After getting the number of moles, we can find the mass of CaCO3 by multiplying the moles by molar mass which is 100. Lastly to find out the percentage of CaCO3 in toothpaste, we can use the mass of CaCO3 divided by the mass of toothpaste used and then multiply by 100. The answer should be relatively close to the literature value which is 20%. The experiment was repeated twice and the average was taken for more accurate results. It is also important to ensure that no toothpaste is smeared onto the walls of the flask as it would affect the amount of calcium carbonate used and hence the amount of NaOH needed to neutralise the excess HCl resulting in inaccurate results.
10ml of 0.16M standard hydrochloric acid was pipetted into a clean and dry conical flask. About 0.1g to 0.2g of toothpaste was removed using a glass rod from the container and placed into the conical flask. While doing so, it is important to make sure that the toothpaste do not smear on the sides of the conical. After placing the glass rod into the conical flask, the toothpaste was completely dislodged from the glass rod. 10ml of deionised water was added to wash down the hydrochloric acid from the glass rod and then a funnel was inserted into the flask. The flask is then placed on a hot plate and allowed to be heated gently for 3 to 5 minutes. After 3 to 5 minutes, the sides of the conical flask and the funnel was rinsed with minimal amount of deionised water. The conical flask was then allowed to cool to room temperature before adding two drops of methyl orange indicator. The excess hydrochloric acid was then back titrated with 0.8M of sodium hydroxide until the indicator changed from pink to orange. This procedure was then repeated again for better accuracy of the results.
From the results, generally, the lesser the mass of toothpaste used the greater the volume of sodium hydroxide needed to neutralise the hydrochloric acid before the indicator turns orange. This is because the lesser the mass of toothpaste used, the lesser the mass of calcium carbonate present. According to this equation: CaCO3 + 2HCL CaCL2 + H2O + CO2, the lesser the amount of calcium carbonate (CaCO3) present, the lesser the amount of hydrochloric acid reacted with calcium carbonate. As a result, there will be more excess hydrochloric acid which will then need more sodium hydroxide to neutralise it. The percentage of calcium carbonate in toothpaste which is 19.2% is relatively close to the literature value of 20%.
The percentage being not exactly 20 may be due to personal errors such as the personal judgement of the end point of titration. The adding of deionised water while rinsing the sides of the funnel and conical flask may also dilute the indicator making it harder for us to notice the colour change. The percentage should be around 20 as it should be high enough to remove dental plague and food remains stuck in between the teeth. But it should also not be too high as it will cause the enamel which protects our teeth to become thinner which would in turn result in the decolouration of our teeth and tooth sensitivity problems.
The percentage of calcium carbonate in toothpaste is around 20% and that the smaller the mass of toothpaste used, the more volume of sodium hydroxide is needed to neutralise the excess hydrochloric acid. This experiment is a success as the found percentage of calcium carbonate in toothpaste of 19.2% is relatively close to the literature value of 20%. However there is a slight difference of 4.5% between the percentage of calcium hydroxide in toothpaste from the first try which is 21.5% and the second try which is 17.0%. This may be due to the difference in the addition of deionised water when rinsing the sides of the funnel and the conical flask.
1. Janice, T. (2000) Back (Indirect) Titration Calculations http://www.ausetute.com.au/backtitration.html [accessed 1/7/2014] 2. Benjamin Quek (11 August 2013) Determination of CaCO3 in toothpaste http://prezi.com/z_i3hla00s6b/determination-of-caco3-in-toothpaste/ 3. Sunil Kumar Singh (27 August 2009) Back Titration http://cnx.org/content/m17190/latest/ [accessed 1/7/2014] Books: