Comenius project«Come closer: my world is

your world »«For a drop of clear water»

Act. 27

Water is one of the most important substances on the Earth from more than 3.800 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 (Ca2+ and Mg2+ ). These salts create incrustations in the pipes of hot water, and interfere with the sparkling action of the soaps.

Determination of hardness

TOTAL HARDNESS It is the total addition of concentration of Ca2+ and Mg2+ salts It is expressed like mgCaCO3 /l or French degrees (1ºF =

10mg/l)

TEMPORARY HARDNESS It measures the concentration of Ca(HCO3)2 and Mg(HCO3)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:

TYPES 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 NH4+/NH3 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 Ca2+ and Mg2+ ions.

With an EDTA solution, the concentration of both cations can be worked out. It is necessary a buffer solution NH4

+/NH3 pH=9,5-10 and NET as an indicator. Then Ca2+ and Mg2+ concentration can be determinated at the same time.

PROCEDURE

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

STEPS

Add 2ml of buffer solution NH4

+/NH3

pH=9,5-10 and…

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

Pour the EDTA solution 0,1 M drop by drop with a burette….

…until the indicator changes from aubergine color into blue….

…and write down the volume we have spent.

Calculate the concentration of Ca2+ and Mg2+ keeping in mind that EDTA can make complexes with both ions

We started with 30’6 ml de EDTA We have spent 1’2 ml

CALCULATIONS

The result of the analysis is 240 mg/l = 24ºHFIf hardness has a value between 201 and 300 mg CaCO3/liter water is considered as a HARD ONE

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