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Back Titration Essay

1. SYNOPSIS

The purpose of this experiment is to use the back titration method to determine the percentage of calcium carbonate in toothpaste. Instead of using standard titration methods where an acid is titrated directly using a standard solution of a base, back titration is used because the sample to be analysed, CaCO3 does not dissolve in water. Hydrochloric acid and sodium hydroxide are the reagents used in this experiment. The precise measured amount of HCl is added to the weighted portion of toothpaste which is CaCO3. When the reaction between HCl and CaCO3 is completed, the excess HCl is back-titrated against NaOH until the reaction reaches the end point and the indicator changes from pink to orange. Hence, the amount of HCl that has reacted with CaCO3 can be calculated by using the initial amount of HCl in excess to subtract the amount of HCl left after the reaction. The average percentage of CaCO3 in toothpaste is found to be 19.97%, which is quite close to the expected value of 20%, so most of the error should have been avoided therefore this is a successful experiment.

2. INTRODUCTION

Titration is a laboratory method used to determine the end point of a reaction between acid and base in order to find the concentration of an unknown solution. One solution of a known concentration or a titrant is slowly added to a known volume of another solution of unknown concentration or analyte until the reaction is neutralized which is usually indicated by a colour change. Back titration is used in this experiment because the sample, toothpaste is insoluble in water. Back titration is also used when the sample is volatile such as ammonia or when solution being titrated reacts very slowly with the analyte and when the exact end point of a forward titration is difficult to identify. Toothpaste is a gel-like dentifrice used to clean our teeth and serves as an abrasive that helps to remove dental plaque and food remains on our teeth.

Its major ingredients consist of fluorine, silica and calcium carbonate. There is 20% of calcium carbonate in toothpaste. The products of the reaction between calcium carbonate and hydrochloric acid are calcium chloride, water and carbon dioxide. Calcium carbonate in toothpaste helps to clean the dirty stains on our teeth to promote oral hygiene and prevent the forming of plaques on our teeth. The rational reason for conducting this experiment is to make sure that the percentage of CaCO3 in toothpaste is about 20%. A percentage higher than 20% may damage the tooth enamel and lead to tooth sensitivity and the yellowing of the teeth.

3. THEORY

Back titration technique is used to determine the percentage of calcium carbonate in toothpaste because calcium carbonate is not soluble in water. A measured amount of toothpaste which contains calcium carbonate is reacted with excess hydrochloric acid to give the products of the reaction which are calcium chloride, carbon dioxide and water according to the balanced equation: CaCO3 + 2HCl  CaCl2 + H2O + CO2. Back titration is done between excess hydrochloric acid and sodium hydroxide solution to know the volume of excess hydrochloric acid because some of the excess hydrochloric acid still remains and did not react with calcium carbonate. The volume of excess hydrochloric acid can be calculated with the formula: = Since the volume of excess hydrochloric acid is known, the volume of hydrochloric acid that has reacted with CaCO3 can be calculated by using the original volume of hydrochloric acid to minus the volume of excess hydrochloric acid.

Using the following formula, = , the number of moles of hydrochloric acid that has reacted with CaCO3 is calculated. Based on the stoichiometric equation of the reaction between CaCO3 and hydrochloric acid, the mole ratio of CaCO3 to hydrochloric acid is 2:1. By dividing the number of moles of hydrochloric acid by 2, we can find the number of moles of Ca CO3. The number of moles of CaCO3 is multiplied with its molecular weight which is to get the mass of CaCO3, then calculate the percentage of CaCO3 in toothpaste by dividing the mass of CaCO3 by the mass of toothpaste and multiply by 100. The percentage of CaCO3 toothpaste obtained should have a value close to the theoretical value of 20%. The experiment was repeated two times in total so as to obtain the average value of the percentage of CaCO3 in toothpaste.

4. PROCEDURE

A 25mL burette was clamped to a retort stand. 10 mL of 0.16 M standard hydrochloric acid was pipetted in to a clean conical flask. Approximately 0.1 – 0.2 g of toothpaste is was removed with a glass rod from the container and the weight of the toothpaste was recorded on the datasheet. The glass rod with the weighed toothpaste was placed in the conical flask which contained the 0.16 M hydrochloric acid and the toothpaste was dislodged completely from the glass rod. About 10 mL of deionised water was added to wash down the acid from the glass rod and the solution was observed to be cloudy. A funnel was inserted in the flask and the flask was gently heated over a hot plate until the reaction was completed which was indicated by the complete dissolution of the calcium carbonate.

This process took about 5 minutes. The funnel and the side wall of the flask was rinsed with a small amount of deionised water and the flask was allowed to cool to room temperature. 2 drops of methyl orange indicator was added to the flask upon cooling to room temperature. The excess hydrochloric acid was then back titrated with 0.08 M sodium hydroxide solution. The colour of the indicator changed from pink to orange indicating the end point of the reaction between hydrochloric acid and sodium hydroxide solution. The experiment was repeated for three times in total.

6. DISCUSSION

Toothpaste contains 20% of calcium carbonate as an abrasive to prevent plaque from forming on our teeth. From the results, we can see that the two results obtained from the experiment to be relatively close. This indicates that the results obtained did not have a big fluctuation and were constant. The average value of the percentage of calcium carbonate in toothpaste is found to be 19.97% which is considered to be relatively close to the literature value of 20%. If the percentage is higher than 20%, there will be too much abrasive in the toothpaste as calcium carbonate functions as an abrasive in toothpaste. Excess abrasive in toothpaste can cause the thinning of tooth enamel and tooth sensitivity. It can also lead to discolouration such as the yellowing of the teeth. From the results, it is also found that the bigger the amount of toothpaste used, the lesser the amount of sodium hydroxide solution is used to neutralize the excess hydrochloric acid.

The percentage of calcium carbonate in toothpaste cannot be determined by forward titration so back titration is the most efficient alternative. Other than that, back titration is often required when the rate of reaction between analyte and reagent is slow or when the standard solution lacks stability. Back titration is also used when the analyte contains certain impurities resulting in lack of efficiency in forward titration due to interferences. The advantage of back titration is that it is more efficient as compared to direct titration method when the end point of back titration is easier to determine than the end point of direct titration. The disadvantage of back titration is that it is time-consuming if it is done manually compared to direct titration and more practices and skills are required to perform back titration in order to achieve effective results.

There were a few precautions taken while performing the experiment. Extra caution was taken while transferring the glass rod with the toothpaste into the conical flask so that no toothpaste was accidentally smeared at the neck of the conical flask that may affect the accuracy of the results as the mass of toothpaste decreases. A homogenous toothpaste was also used to ensure the content of calcium carbonate in toothpaste is uniform. Apart from that, the conical flask containing the hydrochloric acid and toothpaste was heated before titration in order to increase the rate of reaction between hydrochloric acid and calcium carbonate. A filter funnel was inserted into the conical flask during heating to allow carbon dioxide to be released and prevent the spillage of hydrochloric acid while placing the flask into the fume hood.

7. CONCLUSION

The primary objective of this experiment is to determine the percentage of calcium carbonate in toothpaste by using back titration instead of forward titration and standard titration techniques because calcium carbonate is insoluble in water. The average percentage of calcium carbonate in toothpaste is 19.97%. The result is relatively close to the theoretical value of 20%. This experiment is considered to be a success as the results are constant and are close to the theoretical value so most of the errors should have been avoided.

8. REFERENCES

1. Douglas A.Skoog, Donald M. West, F. James Holler, Stanley R.Crouch, Fundamentals of Analytical Chemistry Eighth Edition. Brooks/Cole; 2004. 2. N.N. Greenwood, A.Earnshaw, Chemistry of the Elements. 2nd ed. Oxford, Boston; Butterworth-Heinemann, 1998, c1997. 3. H. Stephen Stoker, Introduction to Chemical Principles. 11th ed. Boston; Pearson, c2014 4. Sean Choon. (2014) Determination of Calcium Carbonate in Toothpaste https://prezi.com/7zeb641ahats/copy-of-determination-of-calcium-carbonate-in-toothpaste/ [accessed 09/06/2015] 5. Benjamin Quek (2014) Determination of CaCO3 in Toothpaste https://prezi.com/z_i3hla00s6b/determination-of-caco3-in-toothpaste/ [accessed 09/06/2015] 6. Nattanit (Nam), Thaksaporn (May), Paradee Unchaleevilawan (Pop), Punyawee Lertworawut (Earl) (2015) Acid Base Lab: Determination of CaCO3 in toothpaste http://nattanitnamt.weebly.com/uploads/3/8/9/8/38987657/acid-base_lab_report.pdf [accessed 09/06/2015] 7. Songqianxx (2015) Pchem Formal Report http://www.studymode.com/essays/Pchem-Formal-Report-65284747.html [accessed 08/06/2015]


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