Determination of Hardness of Water
 
Natural water always contains a variety of dissolved ions, including Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, and some other. If concentrations of Ca and Mg ions are relatively large, the water is called hard.
Hard water may cause many problem. Therefore, it is important to have a simple method to determine hardness. Such method is based on a titration of the water sample with a solution of acomplexing agent, named ethylenediaminetetraacetate (EDTA). The common form of the agent is disodium salt Na2H2EDTA. It is colorless. It can be weighed and dissolve in water to form a stable solution. At high pH (> 10) the remaining protons leave EDTA forming EDTA4- anion:
This ion readily reacts with Mg2+ (and Ca2+) forming colorless complex:
Mg2+ + EDTA4- ® MgEDTA2-

All reactants are colorless as well. Therefore, to detect the end point of this reaction we need an indicator. For this purpose we will use an organic dye named Eriochrome Black T . This compound forms pink complexes with Mg2+ ions and with Ca2+ ions. Aqueous solutions of the indicator at pH 10 are blue. Thus when pink complex MgInd reacts with EDTA, it results in the blue solution of indicator:
MgInd + EDTA4- ® MgEDTA2- + Ind (charges are not balanced!)
The reaction occurs because the EDTA forms much more stable complex than the indicator. A similar equation can be written for calcium.

Reagents:
Standard EDTA solution.
Buffer (pH 10) : mixture of ammonia and ammonium chloride solutions in water.
Eriochrome Black T
Buret

Procedure
A 50 mL sample of tap water is usually reasonable. To the sample, add 5 mL of buffer, and 3-5 drops of indicator. Titrate with EDTA from a 50 mL buret and note when the color changes from red to blue. Repeat the titration two more times to accurately measure the total Mg2+ and Ca2+ concentration. 

Calculations:
At the equivalence point, number of moles of Mg before titrations is exactly equal to the number of moles of EDTA you have added.
NMg = NEDTA
Number of moles of any compound in solution NX can be written as
NX = CX VX, where CX is molarity, and VX is volume. Thus,
CMgVMg = CEDTA VEDTA,
and CMg = CEDTA VEDTA/ VMg

Experiment 4

Name __________________________ Date _________________

Concentration of EDTA

Volume of Mg sample 

Volume of EDTA

Titration No. 1:

Titration No. 2

Titration No. 3:

Average

Concentration of Mg (mMoles/L):