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)

Fractional composition diagram for EDTA

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

-

--

-

-

EBT is blue in a buffered solution at pH 10, it turns red when Ca2+ ions were added.

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

calcium carbonate HCl(aq)

EBT (Eriochrome Black T) MgCl2

buffer solution (pH 10)

Chemicals

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：

(Step B) Estimation of Hardness of Water I II

volume of EDTA added mLvolume after subtracting volume of blank titration mLaverage volume mLmmole of EDTA mmol[Ca2+] Mhardness of water ppm CaCO3