Water is one of the most important substances on the Earth from more than millions of years ago, taking up three quarters of the planet surface. Almost 97% of the total water is salted (seas and oceans), only 3% is sweet. Only a small percentage, about 2%, is on the ice- cup and the rest is on the continental waters.

This parameter is a way to describe the minerals contents dissolved in it (Ca 2+ and Mg 2+ ). These salts create incrustations in the pipes of hot water, and interfere with the sparkling action of the soaps.

TOTAL HARDNESS It is the total addition of concentration of Ca 2+ and Mg 2+ salts It is expressed like mg CaCO3 /l or French degrees (1ºF = 10mg/l) TEMPORARY HARDNESS It measures the concentration of Ca(HCO 3 ) 2 and Mg(HCO 3 ) 2 It disappears by boiling and precipitate carbonates. The reaction is: PERMANENT HARDNESS It exists after boiling water We can calculate it subtracting the two previous kind of hardness:

Materials Burette 10ml 3 Erlenmeyer flasks 100 ml 1 Volumetric flasks 50 ml 1 Glass beakers 50 ml 1 Graduated cylinder 25ml Pipette beakers 10ml Weighing scales

Chemical indicator NET alcoholic solution 0,6% Buffer solution NH 4 + /NH 3 pH=9,5-10 EDTA solution 0.1M Reactive agents

The most frequent components in natural water are Calcium and Magnesium salts, mainly bicarbonates, chlorides and sulfates combined to the Earth and the rocks. There are several kind of hardness depending on the effects produced by the hot.

Water hardness is due to the addition of metallic cations except the alkaline one. Although sometimes iron, aluminium, manganese and strontium contribution can be important, but hardness is mainly due to Ca 2+ and Mg 2+ ions. With an EDTA solution, the concentration of both cations can be worked out. It is necessary a buffer solution NH 4 + /NH 3 pH=9,5-10 and NET as an indicator. Then Ca 2+ and Mg 2+ concentration can be determinated at the same time.

1. Prepare a 50 ml aliquot of the problem substance and put it in the Erlenmeyer flask.

Add 2ml of buffer solution NH 4 + /NH 3 pH=9,5-10 and…

….6 drops of alcoholic solution of NET 0,6%.

…and write down the volume we have spent.

Calculate the concentration of Ca 2+ and Mg 2+ keeping in mind that EDTA can make complexes with both ions

We started with 306 ml de EDTA We have spent 12 ml

The result of the analysis is 240 mg/l = 24ºHF If hardness has a value between 201 and 300 mg CaCO 3 /liter water is considered as a HARD ONE

Students: Marta Granados Javier Martínez Sergio Toro David Fernández Víctor Granados Daniel Ruiz