Experiment 21
Determination of the hardness of water
Purposes
1. To know the basic principle of
complexometric titration.
2. Grasp the condition and the method of
operation to determine the hardness of water.
Principles
Hardness of water is to mean the total
concentration of calcium and magnesium
ions, which expressed as c(Ca2+)/mmol·L-1.
The determination of hardness is a useful
analytical process for measuring the quality of
water for household and industrial uses. The
importance for the latter is due to the fact that
hard water, upon heating, precipitate calcium
carbonate, which then clogs boilers and pipes.
How to determine the csum of
the water sample solution?
Water hardness is ordinarily determined
by the EDTA titration after the sample has
been buffered to a pH of 10, and Eriochrome
black T serves as the indicator.
Firstly, the pH of water sample solution
was adjust to pH 10 and kept stable with the
addition of NH3-NH4Cl buffer solution in
order to guarantee the titration was carried
out in a proper pH environment. Then the
solution is wine red with the addition of
chrome black T indicator (H2In-), because
the chrome black T combine with a part of
Mg2+ ions to form a wine red complex.
Why does this happen?
Because there is a sequence of the stability
of the complex:
CaY2- ＞ MgY2- ＞ MgIn- ＞ CaInso the adding chrome black T give priority
to combine with a part of Mg2+ ions to form a
wine red complex. After that, use EDTA
standard solution to titrate the csum (Ca2+ and
Mg2+ ) of the water sample solution.
EDTA will usurp the Mg2+ ions from the complex
derived from Mg2+ ions and the chrome black T
after it combined with all free Ca2+ ions and free
Mg2+ ions in the solution because it can form a
more stable colorless complex with Mg2+ ions
than the chrome black T. Finally, the color of the
solution turn blue with all free chrome black T
released from the red complex derived from Mg2+
ions and the chrome black T at the end point of
the titration.
• CaY2- > MgY2- > MgIn- > CaIn1) Before EDTA was added (pH ≈10)
Mg2+ + HIn2- → MgIn- + H+
(blue)
(red)
2) Among titration (pH =8 ~10)
Ca2+ +
HY3- → CaY2- + H+
Mg2+ + HY3- → MgY2- + H+
3) End point (pH≈8)
MgIn- + HY3- → MgY2- + HIn2(red) (colorless) (colorless) (blue)
According to the consumption of EDTA
standard solution, we can calculate the csum of the
water sample solution.
c(EDTA )V (EDTA )
Hardness of water (mmol/L) 
1000
Vwater sample
Procedure
1.Preparation of 0.01mol·L-1 EDTA
standard solution
Weigh about 1.5 g of EDTA with the
platform balance, put it into a clean small
beaker, dissolve the solid with distilled
water, transfer the solution to a 500mL beaker,
dilute to about 400 mL, mix the solution.
2．Standardization of EDTA
standard solution
 Prepare a standard MgCO3 solution as follows:
Weigh accurately 0.20~0.22g of MgCO3 which
has been previously dried at 110℃ into a clean
beaker, add 5 drops of distilled water to moisten
the solid, add about 3mL 3mol·L-1 HCl solution
dropwise. After the solid has dissolved, add 50mL
of distilled water (or deionized water). Transfer it
quantitatively to a 250mL volumetric flask and
dilute to the mark, mix the solution throughout.
 Pipet 20.00 mL MgCO3 standard solution
into a 250 mL conical flask, add 4 drops
of 9 mol/L NH3·H2O, and 8 mL NH3-NH4Cl
buffer solution of pH 10 and small amount
of EBT indicator. Titrate with EDTA
standard solution to the point where the
color changes from wine-red to pure blue.
Repeat the titration twice and calculate
the concentration of EDTA solution
20.00
m(MgCO 3 ) 
 1000
250.00
c(EDTA)=
M (MgCO 3 )  V (EDTA)
3．Determination of water hardness
Pipet 50.00 mL water sample into a conical
flask, add add 4 drops of 9 mol/L NH3·H2O, and
5 mL NH3-NH4Cl buffer solution of pH 10 and
small amount of EBT indicator. Titrate with
EDTA solution until the color changes from
wine-red to pure blue. Repeat the titration twice
and according to the consumed volumn of EDTA
solution to calculate the total hardness of water.
Recording and treating data
Table 1 Standardization of EDTA solution
Experiment No.
m (Mass of MgCO3)/g
Indicator
The change of color at the end
point of titration
V(MgCO3)/mL
V Initial(EDTA)/mL
V Final (EDTA)/mL
Vconsume(EDTA)/mL
c(EDTA)/mol·L-1
cAverage (EDTA) /mol·L-1
1
2
3
Table 2 Determination of hardness of water
Experiment No.
Indicator
The change of color at the
end point of titration
V(water sample)/mL
V Initial(EDTA)/mL
V Final (EDTA)/mL
Vconsume(EDTA)/mL
c (Total hardness )/mol·L-1
cAverage (Total hardness)/mol·L-1
1
2
3
50.00 50.00 50.00