This experiment is performed to calculate the amount of Calcium Carbonate (CaCO3) in toothpaste using back titration and to show the quantitative transfers of solids and liquids. Back titration is when a volatile substance of unknown concentration is reacted with an excess of acid (or base) of known concentration and then, titrate the unreacted acid (or base) with a base (or acid). This is used instead of direct titration because CaCO3 is volatile – it is insoluble in water but soluble in acid. In this experiment, an excess of standard hydrochloric acid (HCl) is added to a weighed sample of toothpaste which contains approximately 20% of CaCO3 as an abrasive. After reaction is complete, this excess acid is back-titrated against a standard sodium hydroxide (NaOH) solution. The end point of reaction is pin-pointed by the color change in the methyl orange indicator added. Determination of the amount of excess acid aids the calculation of the amount of CaCO3 present in the toothpaste sample.
The experiment is considered a success because the average percentage of CaCO3, in the toothpaste samples, calculated fall within the acceptable range of 18-22%.
Toothpaste, also known as dentrifrice, is used on a daily basis to clean teeth by removing dental plaque which is caused by the formation of a film of bacteria on teeth. Toothpaste is usually in form of paste or gel. One of the most vital substances found in toothpaste is CaCO3, which cleans the dirty stains on teeth, reducing the chance of the formation of plaque. CaCO3 is a chemical usually found in rocks, shells and egg shells. The particle size of the calcium carbonate precipitate in toothpaste is around 4 µm and the pH is about 9, indicating its basic nature.
CaCO3 is an example of abrasives. High levels of abrasives in toothpaste is detrimental to teeth, since they weaken the enamel, thinning it. This causes the yellowing of teeth and also sensitivity issues. (Stay, n.d.)
Hence, the rationale of the experiment is to calculate the approximate amount of CaCO3 in toothpaste, to ensure that the level of CaCO3 is not above the acceptable range of 18-22%.
Back titration is used as CaCO3 is insoluble in water. Hence, a weighed sample of the toothpaste is reacted with excess hydrochloric acid to give the products, calcium chloride, water and carbon dioxide. This is according to the below reaction:
CaCO3 + 2 HCl ? CaCl2 + H2O + CO2
Back titration is done subsequently with the excess unreacted HCl and NaOH solution. The equation is as follows:
NaOH + HCl (excess) ? NaCl + H2O
The amount of NaOH used in the titration process is needed to calculate the amount of excess HCl using this Molarity equation derived from the mole ratio (Equation 1):
2374900196215MHCl VHCl excess
00MHCl VHCl excess
center1426210Moles of HClreacted
00Moles of HClreacted
With the molarity of both NaOH and HCl and also the volume of NaOH used already calculated, the volume of excess HCl can be calculated by substituting the values into the equation. Then, the amount of HCl that was reacted with CaCO3 can be calculated by deducting the volume of excess HCl from the original volume of HCl prepared. Using the volume of reacted HCl, we can then find the moles of HCl that was reacted using the following equation derived also from mole ration (Equation 2):
2336800177800MHCl VHCl reacted
00MHCl VHCl reacted
2247900680085Moles of HClreacted
00Moles of HClreacted
From the moles of HCl reacted, the moles of CaCO3 can also be calculated. From the equation, the stoichiometric mole ratio of HCl to CaCO3 is 2:1. Hence, the moles can be calculated using the equation (Equation 3):
2279650210820Moles of CaCO3
00Moles of CaCO3
2387600490855Mass of CaCO3
00Mass of CaCO3
Hence, the moles of CaCO3 is multiplied by its molecular weight to find the mass. From there, the percentage of CaCO3 in the toothpaste can be found by (Equation 4):
2082800173990Mass of toothpaste sample
00Mass of toothpaste sample
Standard HCl with concentration of 0.1652 M, measuring 10.00 mL, was pipetted into a clean conical flask. Using negative weighing, 0.1-0.2 g of toothpaste was transferred with a glass rod from the container and the weight was recorded. The toothpaste sample was carefully dislodged into the above mentioned conical flask since toothpaste can be easily smeared onto the neck of the conical flask which affects thet quantitative transfer of the toothpaste. Deionised water of about 10.0 mL was then used to wash down any toothpaste left on the glass rod.
A glass filter funnel was inserted into the conical flask and was gently heated over a hot plate until the reaction is complete. This is indicated by the complete dissolution of the CaCO3 and takes around three to five minutes. The filter funnel and side walls of the conical flask was then rinsed with small amounts of deionised water and left to cool to room temperature.
One to two drops of methyl orange indicator were added. This indicator changes color from pink to orange. If yellow color is achieved, it indicates over-titration.
Back titration is performed on 0.0792 M NaOH with the excess HCl. The burette reading is read after the HCl is added into it. The stopcock was opened almost all the way to begin titration. The flask was swirled during the titration process for proper mixing. A slight orange color was observed to persist in the unmixed parts of the mixture for a few seconds, meaning the end-point is approaching. At this point, the rate of addition was decreased until dropwise and the flask was thoroughly mixed before next drop was added. When the orange solution was observed to persist throughout the solution after swirling, it meant that the end-point was reached. The final burette reading was recorded. The difference in burette reading is the amount of HCl that was added. The first titration gave an approximate amount of HCl needed.
This process was repeated once more to find the average.
Results and Calculation
The concentration of standard HCl solution is 0.1652 M.
The concentration of standard NaOH solution is 0.0792 M.
Results 1st 2nd
Mass of toothpaste (g) 0.1175 0.1199
Initial burette reading (mL) 0.00 0.00
Final burette reading (mL) 14.20 14.50
Volume of NaOH used (mL) 14.20 14.50
Using mole ratio (Equation 1):
VHCl excess (mL) 6.81 6.95
VHCl reacted (10 mL – VHCl excess) 3.19 3.05
Using mole ratio (Equation 2):
Moles of HCl reacted (moles) 0.000527 0.000504
Using mole ratio (Equation 3):
Moles of CaCO3 (moles) 0.000264 0.000252
Mass of CaCO3 (g) 0.0264 0.0252
% CaCO3 in toothpaste (%) 22.4% 21.0%
Average % CaCO3 in samples 21.7%
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