EXPERIMENT 5

Experiment to Determine the Total Hardness in Water

Apparatus

Burette
Pipette
Filter
Conical Flask
Beakers
.02 m EDTA
Buffer solution
Erichrome
Black T

Method

Fill a burette with standard EOTA solution. Measure out 100 cm3 of the water sample into a conical flask.

Add about 2 cm3 of the buffer solution and a pinch of erichrome black T. In the presence of the metal ions in the water, the indicator is red.

Filtrate with the EDTA solution until the end point is reached. This is the point at which all the metal ions have become complexed by the EDTA, and when this happens the colour of the free indicator (blue) appears.

As the end point is approached, the EDTA must be added slowly and the flask swirled continuously because the reaction between the EDTA and metal ions is fairly slow.

Carry out several filtrations and obtain an average. Calculate the amount of hardness in water in mg/litre of CaCo3.

CaCo3 = Calcium Carbonate.

Results

Volume of water analysed = 100 cm3
Volume of 0.02 m EDTA needed = 6.23 cm3

V1M1/ N1 = V2M2 /N2

100 x X/7 = 6.23 x 0.02/1

X = Molarity of Ca2+ in water

X = 6.23 x 0.02/100

= 0.001246 mol/L
= 0.1246 g CaCo3 / L

= 124.6 mg/L of CaCo3

This result indicates that the water is moderately soft.