Download - The Amount of Calcium in Hard Water Sample
Water's hardness is determined by the concentration of
multivalent cations in the water. Multivalent cations are
cations (positively charged metal complexes) with a charge
greater than 1+. Usually, the cations have the charge of 2+.
Common cations found in hard water include Ca2+ and Mg2+ .
Hard Water
Temporary hard water: Temporary hardness of water can be reduced either by boiling the water, or by the addition of lime (calcium hydroxide) through the softening process of lime softening. e.g. water containing Ca(HCO3)2, Mg(HCO3)2
Permanent hard water: Permanent hardness of water is hardness (mineral content) that cannot be removed by boiling. e.g. water containing CaSO4, MgSO4, CaCl2, MgCl2
Hardness of water can be expressed by total amount of CaCO3
(unit: ppm)Hard water may cause industrial safety problem. e.g. As the water temperature increases, the more mineral deposits will appear in the water. It will cover the inner layer of the boiler to interrupt the heat transfer.
EDTA (Ethylene Diamine Tetraacetic Acid) is a hexadentate (“six-toothed”) ligand and chelating agent, i.e. its ability to “sequester” metal ions such as Ca2+ and Fe3+. After being bound by EDTA, metal ions remain in solution but exhibit diminished reactivity.
O
Ca
O
O
N
NO
O
CH2
CH2CH2
O
CH2
CH2
CH2O
O 2-
N CH2
H2C
H2C
HOOC
HOOC
CH2 N
CH2
CH2
COOH
COOHEDTA
Metal EDTA chelate(metal:EDTA=1:1)
OH
NN
OHO3S
O2N
The indicator EBT (Eriochrome Black T) (HIn2) reacts with Mg2+ to form the complex MgIn with wine red color. After chelating Mg2+ by EDTA, the color of indicator (HIn2) changes to sky blue color.
H2Y2(aq)+Ca2+(aq) → CaY2(aq)+ 2H+(aq)
H2Y2(aq)+MgIn(aq) → (wine red)
MgY2(aq)+HIn2(aq)+H+(aq) (sky blue)
EBT (Eriochrome Black T)
The purpose of adding buffer solution (pH 10):
EBT indicator requires an environment at pH above 7.3 for the desired color change: H2In- HIn2-+ H+ K = 5107
(wine red) (sky blue)
7.3 pH 10 ]In[H][HIn 105
]In[H]][H[HIn
2
27
2
2
-
--
-
-
Suitable pH values for titration of Ca2+, Mg2+ with EDTA are above 7.6 and 10.0, respectively:
KMY2− : formation constant of EDTA-metal complex
KCaY2− : 1.75×1010
KMgY2− : 1.72×108
•electronic balance
•beaker (250 mL)
•volumetric flask (100 mL)
•pipette
•pipet filler
•burette
•flask (125 mL)
Apparatus & Materials
The Amount of Calcium in Hard Water Sample
250 mL ddH2O
25 mL ddH2O
10 mL buffer (pH 10)10 drop of EBT15 drop of 0.03 M MgCl2(aq)
10 mL ddH2O
12 drop of HCl(aq)
(prepare it in the hood)
(add additional water to the mark of the flask)
0.016 M Ca2+ solution
(blank titration)
10 mL buffer (pH 10)10 drop of EBT15 drop of 0.03 M MgCl2(aq)
0.016 M Ca2+ solution
Ca2+ solution (unknown concentration)
10 mL buffer (pH 10)10 drop of EBT15 drop of 0.03 M MgCl2(aq)
(repeat titration twice)
(repeat titration again)
2.0 g EDTA .2Na(s)
(Step A) Calibration of EDTA Solution
(Step B) Estimation of Hardness of Water
(Step A) Calibration of EDTA·2Na Solutionweight of CaCO3 g[Ca2+] MCa2+ m molexhausted volume of EDTA solution for blank titration mLvolume of EDTA added mLvolume after subtracting volume of blank titration mL average volume mL[EDTA] M
Results: