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Experiment 2

Experiment 2. Determination of Water Hardness. Water Hardness. Hard water is due to metal ions (minerals) that are dissolved in the ground water. These minerals include Ca 2+ , Mg 2+ , Fe 3+ , SO 4 2- , and HCO 3 -

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Experiment 2

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  1. Experiment 2 Determination of Water Hardness

  2. Water Hardness • Hard water is due to metal ions (minerals) that are dissolved in the ground water. These minerals include Ca2+, Mg2+, Fe3+, SO42-, and HCO3- • The concentration of the Ca2+ ions is greater than the concentration of any other metal ion in our water • Water hardness is usually expressed in ppm CaCO3

  3. Why Be Concerned About Hard Water? • Originally, water hardness was defined as the measure of the capacity of the water to precipitate soap • Hard water does cause soap scum, clogs pipes and clogs boilers as limescale

  4. Hard water can be softened by boiling • Mineral deposits are formed by ionic reactions resulting in the formation of an insoluble precipitate of calcium carbonate • Ca2+ + 2HCO3- CaCO3 + H2O + CO2

  5. Soaps are long chain fatty acids • Soap scum is formed when the Ca2+ ion binds with the soap. This causes an insoluble compound that precipitates to form the scum you see. Soap actually softens hard water by removing the Ca2+ ions from the water • When hard water is heated, CaCO3 precipitates out, which then clogs pipes and industrial boilers. This leads to malfunction or damage and is expensive to remove

  6. Temporary Hardness • Temporary Hardness is due to the bicarbonate ion, HCO3-, being present in the water. This type of hardness can be removed by boiling the water to expel the CO2, as indicated by the following equation • Ca2+ + 2HCO3- CaCO3 + H2O + CO2 • Bicarbonate hardness is classified as temporary hardness

  7. Permanent hardness • Permanent hardness is due to the presence of the ions Ca2+, Mg+2, Fe3+ and SO42-. This type of hardness cannot be eliminated by boiling • The water with this type of hardness is said to be permanently hard

  8. Objectives • To quantitatively determine Total, Permanent, and Calcium hardness in a sample of tap water • To gain some basic analytical knowledge through analysis of water samples • To become familiar with terminology such as ppm and to apply techniques learned from volumetric analysis to basic environmental analysis

  9. Complexometric Titration • Water hardness is usually determined by titrating with a standard solution of ethylenediamminetetraacetic acid, EDTA • EDTA is a complexing, or chelating agent used to capture the metal ions • This causes the water to become softened, but the metal ions are not removed from the water • EDTA simply binds the metal ions to it very tightly

  10. Ethylenediamine-tetraacetic acid - EDTA H2X + Ca2+ CaX + 2H+

  11. EDTA • EDTA is a versatile chelating agent • A chelating agent is a substance whose molecules can form several bonds to a single metal ion • Chelating agents are multi-dentate ligands. A ligand is a substance that binds with metal ions to form a complex ion • Multidentate ligands are many clawed, holding onto the metal ion to form a very stable complex • EDTA can form four or six bonds with a metal ion

  12. EDTA • It is frequently used in soaps and detergents because it forms complexes with calcium and magnesium ions • Certain enzymes are responsible for food spoilage. EDTA is used to remove metal ions from these enzymes • Used to promote colour retention, and to improve flavour retention in foods

  13. Titrations • Use one tablet of indicator to develop a good colour • Titrate water with EDTA until colour changes from red to blue

  14. Titrations EDTA solution in the burette Take approx. 100 mL of the EDTA solution from the container at the sink Dissolve the indicator tablet fully before starting the titration Water sample in the conical flask

  15. Titrations • Titrate for total hardness • Titrate a boiled sample for permanent and hence temporary hardness • Add murexide to a sample at pH 12 to precipitate any Mg2+ as Mg(OH)2. Then titrate to obtain Calcium and hence Magnesium hardness

  16. Treatment of Results • Water hardness is usually expressed as ppm CaCO3. Since the reaction between calcium or magnesium ions and EDTA has a 1:1 ratio, hardness is given by.... • ppm CaCO3 = 0.02 x [titration vol] x 105 • ppm = mg/L

  17. ppm CaCO3 = 0.02 x [titration vol] x 105 • M1V1 = M2V2[n1,n2=1] • M1 x 10-2 = 0.02 x [volume] • M1 = 0.02 x [volume] x 102 • Mol mass CaCO3 = 100 g mol-1 • gL-1 = 0.02 x [volume] x 102 • ppm = 0.02 x [volume] x 105

  18. Temporary hardness = total – permanent • Magnesium hardness = total – calcium

  19. Report • Experimental observations • Balanced chemical equations • All titration results • Calculations • Completed table

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