SOFT AND HARD WATER

M.Morelli I.T.I. Malignani - Udine

SOFT AND HARDSOFT AND HARD WATER WATER


• what’s the stuff you get from the tap ?

• serious problems can stem if unsuitable kind of water is used;


what you should know is that water what you should know is that water must undergo special treatments must undergo special treatments before it’s supplied to towns :before it’s supplied to towns :

• first water flows through a screen that stops gross rubbish


• then it is pumped on a bed of coarse sand which eliminates the most parts of solid material;

• hence, in a sedimentation tank, special chemicals make small solids stick together, so that they settle on the bottom;


• from the top of the tank water flows through a sand filter where also the smallest solid particles are trapped;

• finally chlorine gas is added to kill bacteria and give clean sterilized water, fit to drink, that can be stored.

but what kind of water is this ?but what kind of water is this ?


• depending on its source, water contains various dissolved compounds, that have not been removed by previous treatments, and especially :

calcium sulphate, calcium hydrogen carbonate, magnesium sulphate and magnesium hydrogen carbonate.


these compounds may cause serious these compounds may cause serious drawbacks, actually :drawbacks, actually :

• dissolved solids require more energy to reach the boiling point ( especially expensive in power stations);

• scale deposition in boilers and pipes;

• stones precipitation in bladder and kidney ( not completely known );


• all these seem to be good reasons to be into water hardness;

• water hardnesswater hardness results when water is particularly rich in compounds containing:

CaCa and MgMg


• hard water is mainly caused by carbonate, which is produced when rain falls on limestone and chalk rocks;

• these are not soluble in pure water, but dissolve in rain water that contains CO2

as in the following reaction :


HH22O (l) + COO (l) + CO22(g) + CaCO(g) + CaCO33 (s) Ca(HCO (s) Ca(HCO33))22 (aq) (aq)

water + carbon dioxide + calcium carbonate calcium hydrogen carbonate

• the same process concerns also other kinds of rocks, as:

-- dolomitedolomite CaCOCaCO33 , MgCO , MgCO33

- gypsum CaSO- gypsum CaSO44·2H·2H22OO


• is everything clear ?

• is everyone aware of the importance

of the topic?

• for these special people our chemistry department offers a “freefree“ hardness test

and now, to workand now, to work : :


Determination of hardness of Determination of hardness of waterwater

• by soap solution -by soap solution - the hardness can be gauged by its abilitity to form a thick lather with soap

• by EDTA titration -by EDTA titration - is an analytical measurement using a polydentate ligand or a chelating ( claw like ) ligand EDTA


1. Fill the test tube to about 1/3 the capacity with sea, tap, rain, and hard

water samples.2. place soap solution in a buret3. add 2mL of soap solution from the buret

to each sample4. cap the test tube and shake vigorously5. put each test tube in the rack and wait 20

seconds if the lather remains, we can say 2mL of soap solution was required

6. if a lather does not remain, add another 2 mL and shake again and wait 20 seconds

7. go on in this way and record the volume of soap solution required to produce a permanent lather in each sample of water; can you now arrange the samples in order of increasing hardness ?

conclusionconclusion: water that lathers easily is called soft, whereas water that lathers easily is called soft, whereas water that produces very little lather and a water that produces very little lather and a

scum, is termed hard. scum, is termed hard.

First method:First method:


Second method : EDTA titrationSecond method : EDTA titration

• Ethylenediamine tetraacetic acid and its sodium salts (abbreviated EDTA) form a chelated soluble complex when added to a solution of certain metal cations.

• If a small amount of a dye such as Eriochrome Black T is added to an aqueous solution containing calcium and magnesium ions at a pH of 10.0 ± 0.1, the solution will become wine red.

• If EDTA is then added as titrant, the calcium and magnesium will be complexed. After sufficient EDTA has been added to complex all the magnesium and calcium the solution will turn from wine red to blue.

• This is the end point of the titration.


laboratory equipmentlaboratory equipment

apparatus•buret •beaker•Erlenmayer flask•pipette filler bulb

• funnel• graduated cylinder• volumetryc flask

reagents•EDTA•EBT•ammonia buffer

materialsmaterials


experimental procedureexperimental procedure

• 100 cm³ of sample (V sample) are transferred to a titration vessel

• 5 cm³ of the ammonia buffer solution and a bit of indicator EBT are added.

• The obtained violet solution is titrated with the EDTA standard solution until the colour

of the solution is turned to a stable blue. • titrations is carried out for the determination

of water hardness.


Chemists express the amount of hardness inChemists express the amount of hardness in : :

– ppm ( parts per million ): mg of CaCO3 in 1 litre of water

– ° F = 1g of CaCO3 in 100 litre of water

different scales of "degrees" of water hardness exist :

1 German degree = 17.9 ppm (as CaCO3)1 American degree = 17.1 ppm 1 English degree = 14.3 ppm1 French degree = 10.0 ppm


Grains PerGallon

(gpg)

MilligramsPer Litre (mg/l)

°F RatingRating

less than 1.0 less than 17.1 less than 7 Soft Soft

1.0 – 3.517.1 – 60

7-15Slightly Hard Slightly Hard

3.5 – 7.060 – 120

15-22Moderately Moderately

Hard Hard

7.0 – 10.5120 – 180

22-35 HardHard

over 10.5over 180

Over 35 Very HardVery Hard


Water SofteningWater Softening• These mineral ions are removed using ion exchange

technology.

• This technology is based on removing certain ions from treated water, using resin that have the property of attracting specific types of loose ions in the water, while releasing other ions that were originally attached to it.

• When hard water passes through a column filled with small granules of resin, the Ca and Mg ions are exchanged for sodium ions

RNaRNa22 + Ca + Ca2+2+ RCa + 2 Na RCa + 2 Na++


Questions Questions

1- 1- Do all kinds of water behave the same way when soap is added ?

2- 2- What about hard water in plumbings and boilers?

3- 3- Chalk is insoluble in water. Why does it dissolve in rainwater? Write an equation for the reaction.

4- 4- Why are EDTA solutions used to clean conctact lenses ?

5-5- Is rain water hard or soft ?


GlossaryGlossarydump verb [transitive always + adv/prep] = to put something such as a load, bag etc somewhere in a careless, untidy waylather noun [singular, uncountable] = a white mass of bubbles produced by mixing soap in waterscale water pipes [uncountable] = a white substance that forms around the inside of hot water pipes or containers in which water is boiledscum noun [singular, uncountable] = an unpleasant substance that forms on the surface of a liquidstem = to stop something from spreading or growing; stem from :to develop as a result of somethingstuff noun [uncountable] = a kind of substance or materialsuds noun [plural] = the mass of bubbles formed on the top of water with soap in it;tap noun [countable] = a piece of equipment for controlling the flow of water, gas etc from a pipe or container;waterworks noun [plural] = the system of pipes and artificial lakes used to clean and store water before it is supplied to a town


Finally let’s have a little fun: insert the correct Finally let’s have a little fun: insert the correct names in the figure below choosing among the names in the figure below choosing among the following :following :

• groundwater

• treatment plant

• water tower

• rain

• ridge

• distribution home and factories • spring• well• reservoir

• diverted for

irrigation use


SOFT AND HARDSOFT AND HARD WATER WATER


Reaction before the titrationReaction before the titrationCa+2 + EDTA-4 ------> CaEDTA-2

C10H12O8N2Ca

• carbons are black • hydrogens are whitewhite • oxygens are red • nitrogens are blue

• calcium ion is greengreen


4


end point of the titrationend point of the titration

Mg In + EDTA-4 ------> MgEDTA-2 + In-2

SOFT AND HARD WATER

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