Risk Assessment

Risk Assessment 0.05 M EDTA , 3M HCl, 3M NH3, buffer solutions are harmful, so we must handle them with great care. Three chemicals used: 3M HCl, 3M NH3, and NH4Cl/NH3 buffer solutions are corrosive. We must avoid direct contact of chemicals with skin ;otherwise, we may get skin burn and feel sting. Furthermore, we need to wear safety glasses to protect our eyes and wear protective gloves to protect our hands. The buffer solution has a very strong smell of ammonia, so keep the buffer solution bottle closed to avoid inhale to much of the ammonia smell. If any chemicals are splashed onto our skin or clothing, we should wash the affected area immediately with slow running water. As the chemicals are corrosive, we should dispose of chemical waste with our teacher’s instruction.

P.3

Procedure 1. Set-up I. Take out a stand and clamp. II. Clamp the burette III. Add 50cm3 of the EDTA solution by using a filter funnel.

2. Preparation of sample solution I. Grind the tablet into powder with mortar and pestle.

II.

Weigh 0.01g of the powder on the analytical balance. Pour it into a conical flask

P.4 III. Measure 5cm3 of 3M HCl solutions with measuring cylinder. Take the reading at eye level.

IV. Add the 3M HCl solutions slowly to dissolve the powder. 3. Titration with EDTA solution

I.

Use a pipette to hold 50cm3 of distilled water. Measure 5cm3 of 3M NH3 solutions with measuring cylinder.

P.5 II.

Add the distilled water and the NH3 solution to neutralize the final sample solution.

III. Pour a certain amount of buffer solution into a beaker.

IV. Measure 1cm3 of buffer solution with measuring cylinder.

V. Add this to obtain a pH value of about10 VI. Add 3 drops of Calmagite indicator solution. The solution should be wine red.

P.6 VII. Titrate with standard EDTA solution with swirling till the solution colour changes to blue.

P.7

Results and calculations Sample information: Amount of calcium per tablet (labeled): 200 mg (0.2g) Weight of tablet: 0.97g Weight of powder used: 0.01g Concentration of EDTA solution: 0.05M Trial 1 Trial 2 Trail 3 Initial reading of burette (cm3) Final reading of burette (cm3) Volume of EDTA solution used (dm3) Average(dm3)

Trial 4

0.00

0.00

0.00

0.00

5.10

4.00

3.70

3.50

5.10/1000=

4.00/1000=

3.70/1000=

3.50/1000=

0.0051

0.0040

0.0037

0.0035

0.0051+0.0040+0.0037+0.0035)/4 ＝

0.004075

Molarity of EDTA solution = No. of moles of EDTA / volume of solution 0.05 = No. of moles of EDTA/0.004075 No. of moles of EDTA =2.0375 x 10 -4mol

Mass of calcium in 0.01g of powder = 0.00020375 x 40.1 = 8.170375 x 10 –3 g Mass of calcium per tablet = [8.170375 x 10 –3 (0.97/0.01)]g = 0.7925 =0.793g (cor. to 3 sig. fig) Percentage by mass of calcium per tablet: (0.7925g/ 0.970g) x 100% = 81.7% P.8

Discussion After the experiment, we have noticed that the mass of calcium per tablet was much bigger than the labeled (0.2g). We have founded several sources of errors. First, the calcium tablet contains calcium carbonate but also magnesium oxide. The Ca2+ will combine with EDTA 4- at a pH of about 10 and Mg 2+ will do this at a pH greater than 12. When we had measured the hydrochloric acid in a measuring cylinder, we had left it exposed to air for a few minutes. As concentrated hydrochloric acid gives acid mist, loss in mass might be resulted .So, the pH value of the sample solution might be greater than 12, as a result, the amount of calcium calculated might have included that of magnesium. We should control the pH value of the sample solution strictly by using the pH meter to measure and we should add the hydrochloric acid to the solution just after measuring. Furthermore, we probably had added excess EDTA solution, which means we couldn’t stop titration immediately when the end-point of the titration had reached. The volume of EDTA solution used might be greater than expected amount. Next, we hadn’t cleaned the measuring cylinder and dropper completely before next measurement. The solution might be contaminated with the last chemical used. Therefore, we should clean the measuring cylinder and dropper through before each measurement. The contamination of the solutions can be reduced.

Conclusion After calculation, 0.793 g of calcium can be founded in per tablet. The percentage by mass of calcium per tablet is 81.7%. Thus, this calcium tablet can really supplement the dietary intake.

P.9

References CHEMISTRY 221 LABORATORY—QUANTITATIVE ANALYSIS http://www.cofc.edu/~kinard/221LCHEM/CHEM221L%20Calcium%20Determi nation%20by%20EDTA%20Titration.htm Calcium Determination Using EDTA http://home.snu.edu/dept/chemistry/syllabi/ARCHIVES/CHEM3024/Labs/LAB 4%20-%20Ca_edta.htm Testing Calcium Carbonate http://www.wcaslab.com/tech/Calcium_Carbonate.htm

P.10