SCH4U
Determination of a Ksp by EDTA Titration
Purpose
To experimentally determine the solubility product constant (Ksp) of calcium hydroxide.
Background
Calcium hydroxide is a white, insoluble solid. However some of this solid dissolves in the water and
comes to equilibrium as shown in Reaction 1:
Reaction 1:
Ca(OH)2(s)
⇌
Ca2+(aq) +
2OH¯(aq)
The concentration of calcium and hydroxide are determined by the Ksp, the solubility product constant.
Therefore if one knows these concentrations, the Ksp value can be calculated. To determine the
concentration of these ions in a saturated solution, we will use a metal chelating agent to titrate the
calcium ions present in a saturated solution of Ca(OH)2 using Reaction 2:
Reaction 2:
Ca2+ (aq) +
EDTA4-(aq)
`
→
CaEDTA2- (aq)
(chelate)
A chelate is a complex that forms between metal ions and electron pairs on another molecule. In this
lab, the molecule EDTA (ethylenediaminetetracetic acid) (Figure 1) will be used to chelate the calcium
ions using the technique of titration. The endpoint of the titration will be detected using the indicator
Eriochrome Black T which forms a red-coloured complex with calcium ions and undergoes a colour
change to blue when the calcium ions are removed (or chelated) from the solution by EDTA.
Figure 1: Structure of EDTA
Materials [T 2]
- saturated Ca(OH)2 solution (limewater)
- 75 mL Na2H2EDTA solution (Approx: 3.8 g/1000 mL)
- ammonium chloride / ammonium hydroxide buffer (pH 10)
- Eriochrome Black T indicator (1:10 with NaCl (s))
Procedure: [T 4]
Preparation of Glassware
It is important that all glassware be prepared properly before a titration. Clouded glassware indicates the
presence of scale (CaCO3). This glassware should be first cleaned with 1 mol/L HCl and rinsed with tap
water. Clean all flasks and beakers by scrubbing with soapy water and triple rinsing with tap water.
(Triple rinsing does not mean that you fill the vessel to the brim three times. To triple-rinse, place a small
amount of solvent (5 mL) in the vessel and swirl to coat the entire inside surface. Discard and repeat as
needed). Lastly, rinse the glassware with a small amount of deionized water. To prepare the pipette,
rinse with tap water and deionized water. Rinse once with the sample solution before use by pipetting
and discarding the solution.
Preparation of the Buret
The buret should not be washed with soap to avoid contamination. Add deionized water to the buret
with the stopcock closed in rinse the inside walls by rotating the horizontal buret in your hands. If an
insoluble residue is visible in the buret, scrub the buret using the buret brush. Discard the water and
triple rinse the buret with deionized water using a wash bottle. Open the stopcock and allow some water
to pass through to ensure it is not blocked. Slowly pour about 10 mL of EDTA solution down the insides
of the buret to rinse the buret and stopcock. Discard this solution and then fill the buret with fresh EDTA
solution. Open the stopcock to clear any bubbles from the tip. Record the actual EDTA concentration
and the initial volume in the observations.
Sample Preparation:
Limewater is a saturated solution of calcium hydroxide. However the solid Ca (OH) 2 must be removed
by filtration prior to titration. Using a glass funnel and Whatman #1 filter paper, filter about 75 mL of the
saturated solution into a clean Erlenmeyer flask and retain the filtrate (the clear solution).
Discard the
undissolved calcium hydroxide and filter paper. Using a prepared volumetric pipette, aliquot 10.00 mL of
the filtrate into a 125 mL Erlenmeyer flask. Add 10.0 mL of deionized water with a graduated cylinder.
In the fumehood, carefully add 2.00 mL of ammonium hydroxide buffer using a 10 mL graduated pipette.
Lastly, add a few crystals of the indicator Eriochrome Black T to the flask and swirl to dissolve. The
indicator should produce red colour in the solution.
Titration
Titrate the sample with the EDTA solution while swirling the flask. Stop as soon as a single drop
produces a blue colour change that remains for at least 10 seconds. Repeat the titration at least two
more times or until a reproducible result is obtained. Discard the waste in the caustic waste bin. Empty
the buret and rinse with DI water. Store the buret in an open, inverted position on the clamp. Rinse
glassware thoroughly with tap water and return the equipment to the appropriate cupboard or drawer.
Analysis
1.
Using the the average volume of titrant used in the trials, calculate the moles of EDTA
required to chelate the calcium in Reaction 2. [T1]
2.
Using the stoichiometry of Reaction 2 and the pipette volume, calculate the calcium concentration
of the saturated Ca(OH)2 solution. [T1]
3.
Using the stoichiometry of Reaction 1, determine the concentration of hydroxide ions present in
the saturated Ca(OH)2 solution. [T1]
4.
Write the solubility product (Ksp) expression for Ca(OH)2. [T1]
5.
The concentrations determined in questions 2 and 3 are the equilibrium concentrations of Ca 2+
and OH¯ in the saturated solution. Using these values, calculate the Ksp value for Ca(OH)2. [T1]
6.
One published value for the Ksp of Ca(OH)2 is 5.02 x 10-6 at 25oC (1). Using this value,
calculate the theoretical calcium concentration present in a saturated solution. [T 2]
7.
Using the theoretical calcium concentration determined in question 6 and the experimental
calcium concentration from question 2, calculate the percentage error. [T 1]
Conclusion
Write a conclusion to summarize your results.
Questions:
1.
a) During filtration, tiny particles of solid Ca(OH)2 may pass through the pores in some filter
papers. If solid particles were present, what would happen to the concentration of dissociated
ions in your sample before the titration? Explain your response.
b) What would happen to these solid particles as the free calcium ions were removed (chelated)
during the titration? Expplain your response.
c) How would this problem affect the experimental value of Ksp? [T 3]
2.
Suppose that some volume of the filtered Ca(OH)2 solution that you prepared was mixed with an
equal volume of a 1.0 x 10-6 mol/L solution of iron FeCl3 (aq) . Would a precipitate form?
Support your answer with calculations. Include any relevant dissociation equations, solubility
equilibrium equations, Ksp equations Trial Ion Product in your answer.
Ksp(Fe(OH)3) = 2.79 x 10-39 (1). [T 3]
References:
1. Faculty of Chemistry and Technology, Split, Croatia (http://www.ktf-split.hr/en/)
Nov.21, 2010).
(accessed