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

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1 M.Morelli I.T.I. Malignani - Udine
SOFT AND HARD WATER M.Morelli I.T.I. Malignani - Udine

2 M.Morelli I.T.I. Malignani - Udine
what’s the stuff you get from the tap ? serious problems can stem if unsuitable kind of water is used; M.Morelli I.T.I. Malignani - Udine

3 M.Morelli I.T.I. Malignani - Udine
what you should know is that water must undergo special treatments before it’s supplied to towns : first water flows through a screen that stops gross rubbish M.Morelli I.T.I. Malignani - Udine

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

5 but what kind of water is this ? M.Morelli I.T.I. Malignani - Udine
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 ? M.Morelli I.T.I. Malignani - Udine

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

7 M.Morelli I.T.I. Malignani - Udine
these compounds may cause serious 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 ); M.Morelli I.T.I. Malignani - Udine

8 M.Morelli I.T.I. Malignani - Udine
all these seem to be good reasons to be into water hardness; water hardness results when water is particularly rich in compounds containing: Ca and Mg M.Morelli I.T.I. Malignani - Udine

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

10 M.Morelli I.T.I. Malignani - Udine
H2O (l) + CO2(g) + CaCO3 (s) Ca(HCO3)2 (aq) water + carbon dioxide + calcium carbonate calcium hydrogen carbonate the same process concerns also other kinds of rocks, as: - dolomite CaCO3 , MgCO3 - gypsum CaSO4·2H2O M.Morelli I.T.I. Malignani - Udine

11 M.Morelli I.T.I. Malignani - Udine
is everything clear ? is everyone aware of the importance of the topic? for these special people our chemistry department offers a “free“ hardness test and now, to work : M.Morelli I.T.I. Malignani - Udine

12 Determination of hardness of water
by soap solution - the hardness can be gauged by its abilitity to form a thick lather with soap by EDTA titration - is an analytical measurement using a polydentate ligand or a chelating ( claw like ) ligand EDTA M.Morelli I.T.I. Malignani - Udine

13 M.Morelli I.T.I. Malignani - Udine
First method: 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 buret 3. add 2mL of soap solution from the buret to each sample 4. cap the test tube and shake vigorously 5. 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 ? conclusion: water that lathers easily is called soft, whereas water that produces very little lather and a scum, is termed hard. M.Morelli I.T.I. Malignani - Udine

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

15 M.Morelli I.T.I. Malignani - Udine
laboratory equipment materials apparatus buret beaker Erlenmayer flask pipette filler bulb funnel graduated cylinder volumetryc flask reagents EDTA EBT ammonia buffer M.Morelli I.T.I. Malignani - Udine

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

17 M.Morelli I.T.I. Malignani - Udine
Chemists 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 = ppm (as CaCO3) 1 American degree = ppm 1 English degree = ppm 1 French degree = ppm M.Morelli I.T.I. Malignani - Udine

18 Milligrams Per Litre (mg/l) M.Morelli I.T.I. Malignani - Udine
Grains Per Gallon (gpg) Milligrams Per Litre (mg/l) °F Rating less than 1.0 less than 17.1 less than 7 Soft 1.0 – 3.5 17.1 – 60 7-15 Slightly Hard 3.5 – 7.0 60 – 120 15-22 Moderately Hard 7.0 – 10.5 120 – 180 22-35 Hard over 10.5 over 180 Over 35 Very Hard M.Morelli I.T.I. Malignani - Udine

19 M.Morelli I.T.I. Malignani - Udine
Water 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 RNa2 + Ca RCa + 2 Na+ M.Morelli I.T.I. Malignani - Udine

20 M.Morelli I.T.I. Malignani - Udine
Questions 1- Do all kinds of water behave the same way when soap is added ? 2- What about hard water in plumbings and boilers? 3- Chalk is insoluble in water. Why does it dissolve in rainwater? Write an equation for the reaction. 4- Why are EDTA solutions used to clean conctact lenses ? 5- Is rain water hard or soft ? M.Morelli I.T.I. Malignani - Udine

21 M.Morelli I.T.I. Malignani - Udine
Glossary dump verb [transitive always + adv/prep] = to put something such as a load, bag etc somewhere in a careless, untidy way lather noun [singular, uncountable] = a white mass of bubbles produced by mixing soap in water scale water pipes [uncountable] = a white substance that forms around the inside of hot water pipes or containers in which water is boiled scum noun [singular, uncountable] = an unpleasant substance that forms on the surface of a liquid stem = to stop something from spreading or growing; stem from :to develop as a result of something stuff noun [uncountable] = a kind of substance or material suds 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 M.Morelli I.T.I. Malignani - Udine

22 M.Morelli I.T.I. Malignani - Udine
Finally let’s have a little fun: insert the correct names in the figure below choosing among the following : groundwater treatment plant water tower rain ridge distribution home and factories spring well reservoir diverted for irrigation use M.Morelli I.T.I. Malignani - Udine

23 M.Morelli I.T.I. Malignani - Udine
SOFT AND HARD WATER M.Morelli I.T.I. Malignani - Udine

24 Reaction before the titration Ca+2 + EDTA-4 ------> CaEDTA-2
C10H12O8N2Ca carbons are black hydrogens are white oxygens are red nitrogens are blue calcium ion is green M.Morelli I.T.I. Malignani - Udine

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

26 end point of the titration Mg In + EDTA-4 ------> MgEDTA-2 + In-2
M.Morelli I.T.I. Malignani - Udine


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