The total hardness of water (i.e.
the concentration of dissolved cations such as Ca2+,
Mg2+, Fe3+, etc.) is commonly determined by
the titration of a water sample with a standardized solution of
the disodium salt of ethylenediaminetetraacetic acid (EDTA) using
eriochrome black T or calmagite as an indicator. Calmagite has
the advantage of forming indefinitely stable solutions and will
be substituted without any other modification to the procedure.
The reaction that takes place can be
Y4- + Ca2+
at pH 10,
Y4- is the EDTA anion and
CaY2- is the complex ion. The CaY2- complex
is extremely stable a pH 10 and the titration proceeds
essentially to completion.
Endpoint detection is achieved by use
of the visual indicator, calmagite. Calmagite is a complexing
agent as is EDTA, but EDTA forms more stable complexes than
calmagite. The endpoint detection is as follows:
- At pH 10, HIn2- and Mg2+
form a red complex. (To ensure the presence of Mg2+
a small amount of MgCl2 is added to the
Mg2+ + Hin2-
MgIn- (red) + H+
- EDTA forms a weaker complex with Mg2+
than Ca2+. Ca2+ reacts with Y4-
first, leaving the red MgIn- solution
Ca2+ + MgIn-
(red) + Y-4 ó CaY-2 + MgIn- (red)
- When all the Ca2+ is
titrated by Y4-, MgIn- reacts with
MgIn- (red) + Y4-
+ In3- (colorless)
- In-3 hydrolyzes.
+ H2O ó Hin2- (blue) + OH-
If one knows the concentration of the
titrant (often expressed in terms of the titer value, mg CaCO3/mL
titrant) the hardness of the water sample can be found. Water
hardness is usually reported as parts per million (ppm) of CaCO3
even though CaCO3 is not the species in solution.
Remember that ppm is equivalent to mg/Kg or mg/liter for dilute
- Determine the amount of the disodium
salt of EDTA to weigh out to make one liter of 0.01 F
- Determine the amount of CaCO3
to weigh out to make 500.0 mL of a solution that contains
the equivalent of 0.800 mg CaCO3/mL.
- Determine how many mL of the CaCO3
solution in problem 2 should be titrated with the Na2EDTA
solution to require 40.0 mL of titrant.
- Find the titer value of an EDTA
solution in mg CaCO3/mL if 50.00 mL of the
CaCO3 standard with 0.800 mg CaCO3/mL
requires 45.00 mL of the EDTA titrant to reach a
- If 100.00 mL of a water sample was
found to require 14.90 mL of the EDTA titrant of problem
4 to reach the calmagite endpoint, find the hardness of
the water in ppm CaCO3.
Preparation and Standardization of the
Save the EDTA solution for the Ion Exchange experiment
- Weigh out enough Na2EDTA
into a clean 400 mL beaker to prepare 1.0 L of 0.01 F
EDTA (see preliminary calculation 1). Add about 0.1 g
the solids in distilled water and transfer to 1 liter
bottle. If some of the solids will not dissolve, add
small amounts of 0.1 N NaOH until all solids
dissolve. Dilute to 1 L and mix thoroughly.
- Use the standard calcium solution to
make 250 mL of a standard that will be ~0.8 mg/mL. Obtain
not more than ~25 mL of this standard from the dispenser
and record in your notebook the concentration of this
standard. Dilute volumetrically and record in your
notebook the concentration of your standard solution.
- Pipet a 25.00 mL portion of the
standard into a 250 mL Erlenmeyer flask and add 25 mL of
distilled water and 5 mL of an ammonia-ammonium chloride
buffer (reagent shelf). Then add 5 drops of calmagite
solution to the solution to be titrated.
- Titrate carefully with the EDTA
solution to the point where the color changes from wine
red to pure blue (no trace of red left). Repeat the
titration with two other portions. Calculate the calcium
carbonate titer value of the EDTA solution.
Determination of Total Hardness of Water
- Turn in to the laboratory instructor a
clean, 250 mL, volumetric flask clearly marked with your
name and section number.
- Dilute the sample given to you to
- Transfer 50.00 mL of the sample of a
250 mL Erlenmeyer and titrate in the same manner as in
the standardization. Remember to add 5 mL of buffer!
Repeat the titration two more times. Calculate the total
hardness of the water as ppm CaCO3. Remember
that the volumes of standard and unknown are different.
- What effect on the total hardness
would be observed if the water sample contained: (a) 50
ppm Na? b. 100 ppm NH3?
- What effect on the total hardness
would be observed if the soluble cationic species were
primarily Mg2+ instead of Ca2+?
- Propose a titration scheme by which
one could differentiate between Mg2+ hardness
and Ca2+ hardness.
- Express the hardness of a water sample
in terms of grains CaCO3 per gallon if it has
a hardness of 100 ppm CaCO3. (There are 700
grains per pound).
Turn in unknown hardness as ppm CaCO3.
This number is 100% of your score.
D. C. Harris Quantitative Chemical
Analysis 4th Ed., W. H. Freeman and Company, New York 1995
Chapters 5 and 1
Tuesday, August 03, 2004