Complexometric Titration: Determination of Water Hardness or Calcium in a Calcium Supplement
-Unknown #208-
Abstract
The experiment will explore complexometric titrations and use them to determine water hardness. Calcium chloride will be prepared and used in titration with EDTA using a buffer for a more exact end point and EBT to visually see the end point. Finally the process will be repeated with a sample of water to determine water hardness. The experiment will be considered successful if the %RSD is 0.5% or less.
Experimental
Stoichiometric Equations:
Mathematical Formulas:
% Ca =
Conversion factors:
%RSD =
Reagents:
Common Name
Formula
Molecular Weight (g/mol)
Calcium Carbonate
100.1
Calcium
Ca
40.08
EDTA
298.3
Calcium Chloride
110.98
Materials:
1 micro buret
1 100mL Volumetric Flask
1 1mL pipet
Plastic micro buret tip
1 2mL pipet
Weighing funnel
1 Pasteur pipet and bulb
~20-22mL.01 M EDTA
3 10 mL Erlenmeyer Flasks
~4 mL 6M HCl
Magnetic stirring plate
3 stir rods Apparatus:
Procedure:
1. Accurately weigh between .1300 and .1400g that has been previously dried for one hour at C
2. Transfer the solid to a 100mL volumetric flask with a funnel and rinse the funnel with 10 to 20mL deionized water.
3. Add 6M HCl drop wise until effervescence of stops.
4. Add water to the fill line and mix solutions by inverting.
5. Fill microburet with EDTA soln. (refill with each titration)
6. Pipet 1.000mL soln. into 3 Erlenmeyer flasks
7. Add 4-6 drops buffer and 3-4 drops EBT
8. Perform a titration with EDTA from wine red to blue until there are 3 precise results.
9. Repeat 6-8 with 2mL of unknown
10. Discard waste and wash glassware by rinsing with copious amounts of deionized water.
Results
Data Tables:
Trial 1
Trial 2
Trial 3
Volume Taken
1.000 mL
1.000 mL
1.000 mL
Initial EDTA Volume
0.000 mL
0.010 mL
0.000 mL
Final EDTA Volume
1.343 mL
1.352 mL
1.340 mL
EDTA Used
1.343 mL
1.342 mL
1.340 mL
Trial 1
Trial 2
Trial 3
Volume Taken
2.000 mL
2.000 mL
2.000 mL
Initial EDTA Volume
0.010 mL
0.013 mL
0.000 mL
Final EDTA Volume
1.614 mL
1.619 mL
1.602 mL
EDTA Used
1.604 mL
1.606 mL
1.602 mL
Example Calculations: →
=
Trial 1
Trial 2
Trial 3
Average
Concentration EDTA
.
Standard Deviation=1.114
%RSD= .1140%
Trial 1
Trial 2
Trial 3
Average
Moles EDTA Used
Standard Deviation=
%RSD=.1277%
→ %RSD = →
→ Average moles =
% Ca =
ppm= → = 783.8ppm
Discussion To begin the experiment a calcium chloride solution had to be prepared by adding hydrochloric acid to powdered calcium carbonate. The acid and carbonate reacted to form calcium chloride and water molecules suspended in solution as well as carbon dioxide which was visible as effervescence given off as each drop of HCl was added. Once the effervescence stopped water was added to the solution until the total volume reached 100mL. This solution was used in a complexometric titration with ethylenediaminetetraacetate.
A complexometric titration is a technique that uses a colored complex to find the end point of a titration. In this experiment the indicator used was Eriochrome Black T (EBT) at pH 10. When added to a solution containing metal cations, it forms a wine red complex. The end point is detected by the change of color of the EBT complex from a wine red to a light blue. Only a small amount of the indicator is added in order to leave most of the metal ions uncomplexed. When the end point in the EDTA titration is reached, all of the free metal (in this experiment calcium) ions have reacted and thus removed as the complex ion . This includes the ones from the [Ca-EBT] complex that formed when EBT was added to the solution.
A run 4- keep 3 titration was performed in order to achieve more accurate results. Carelessness led to 5 runs of the first titration until 3 endpoints close
