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.

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Procedure1. 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

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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.

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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

R esults 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 Trial 4

Initial reading

of burette

(cm3)

0.00 0.00 0.00 0.00

Final reading

of burette

(cm3)

5.10 4.00 3.70 3.50

Volume of

EDTA solution

used (dm3)

5.10/1000=

0.0051

4.00/1000=

0.0040

3.70/1000=

0.0037

3.50/1000=

0.0035

Average(dm3) 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%

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DiscussionAfter 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.

ConclusionAfter 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.

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References

CHEMISTRY 221 LABORATORY—QUANTITATIVE ANALYSIS

http://www.cofc.edu/~kinard/221LCHEM/CHEM221L%20Calcium

%20Determination%20by%20EDTA%20Titration.htm

Calcium Determination Using EDTA

http://home.snu.edu/dept/chemistry/syllabi/ARCHIVES/CHEM3024/Labs/

LAB4%20-%20Ca_edta.htm

Testing Calcium Carbonate

http://www.wcaslab.com/tech/Calcium_Carbonate.htm

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