1. EDTA Titration of Ca2+ and Mg2+ in Natural Waters
Lab 11:
EDTA Titration of Ca2+ and Mg2+ in Natural Waters
By: Samantha Cabrera

2. Background
Natural water contains some dissolved minerals; the two most common ones are calcium and magnesium ions.
These ions can interfere with the ability of soaps and detergents to clean, reduce the quality of water that we drink, create soap scum, etc.
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3. Purpose
To determine the concentration of calcium and magnesium ions (also known as “hardness”) by complexometric titration with EDTA.

4. metal ion + ligand = complex ion
Complexometric titration:
• based on complex ion formation between
titrant and analyte
• stoichiometry is 1:1 (just like an acid-
base titration)
metal ion ligand = complex ion
(analyte = Ca2+, Mg2+) (titrant)

5. ethylenediametetraacetic acid
EDTA:
ethylenediametetraacetic acid
• A Lewis base that donates electrons to metal
ions (Lewis acids)
• A chelating ligand: binds to more than 1 metal
ion; it has 6 binding sites
• Forms stable complexes with metal ions in a
1:1 ratio

6. Reactions
EDTA will coordinate around the metal ion in the solution, forming a complex.
Ca Y4- ↔ [CaY]2-
Mg Y4- ↔ [MgY]2-
EDTA will react with calcium first because the CaY2- complex (log Kf = 10.65) is more stable than the MgY2- complex (log Kf = 8.79).

7. Finding the Endpoint: Metal Ion Indicators
Eriochrome black T indicator &
solid hydroxynaphthol blue
Finding the Endpoint: Metal Ion Indicators
Such indicators are compounds whose color changes when they bind to a metal ion.
MgIN EDTA → MgEDTA IN
1. Indicator is added, binding to Mg2+ ion = RED
2. EDTA reacts with all free Mg2+ ions
3. EDTA takes the Mg2+ ion attached to the
indicator
4. Free indicator = BLUE (the endpoint)
Finding the Endpoint: Metal Ion Indicators
Such indicators are compounds whose color changes when they bind to a metal ion.
MgIN EDTA → MgEDTA IN
1. Indicator is added, binding to Mg2+ ion = RED
2. EDTA reacts with all free Mg2+ ions
3. EDTA takes the Mg2+ ion attached to the
indicator
4. Free indicator = BLUE (the endpoint)
Red Colorless Colorless Blue

8. Moles of EDTA = Total moles of Ca2+ and Mg2+
Overview of Steps
Titration #1
Mixture of Ca2+ and Mg2+
titrate with EDTA
indicator: Eriochrome Black T
Moles of EDTA = Total moles of Ca2+ and Mg2+

9. precipitate Mg(OH)2 with Ca2+ remains Titrate with EDTA
Titration #2
Mixture of Ca2+ and Mg2+
precipitate Mg(OH)2 with
50 wt% NaOH
Ca2+ remains
Titrate with EDTA
indicator: solid hydroxynaphthol
blue
Moles of EDTA = moles of Ca2+
Moles of Mg2+ = Total moles – moles of Ca2+

10. What type of EDTA titration is performed in this experiment?
Question:
What type of EDTA titration is performed in this experiment?
a) Direct titration
b) Back titration
c) Displacement titration
d) Indirect titration

11. The analyte (Ca2+ and Mg2+) is directly titrated with the EDTA.
Answer:
a) Direct titration
The analyte (Ca2+ and Mg2+) is directly titrated with the EDTA.

12. EDTA is directly added to analyte Back titration:
Direct titration:
EDTA is directly added to analyte
Back titration:
an excess of EDTA is added if the metal blocks the
indicator, complex forms too slowly, or metal
precipitates without EDTA; the excess is titrated with
a metal ion of known concentration
Displacement titration:
analyte is treated with another metal-EDTA complex; the analyte displaces the second metal, which is then titrated with EDTA
Indirect titration:
for finding the concentrations of anions; metal is
added to precipitate the anion, and the excess metal is titrated with EDTA
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