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Hardness

Hardness. Objective to understand the chemical basis of water hardness, how it originates, and ways it can affect water distribution systems. to know the methods for measuring and expressing hardness, and its relationship with other water quality parameters.

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Hardness

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  1. Hardness Objective • to understand the chemical basis of water hardness, how it originates, and ways it can affect water distribution systems. • to know the methods for measuring and expressing hardness, and its relationship with other water quality parameters. • to know the significance of water hardness in Environmental Engineering

  2. Hardness • Poor lathering of soap in water. • Scale deposition in Boilers and Kettles • Importance • Economic • Soap Costs • Maintenance of Boilers • Health • Putative link between Magnesium and heart disease (negative correlation) • Soft Waters and Lead pipes • Strength of Bones

  3. Causes of Hardness • Soap Effects • Divalent Cations • Especially • Calcium Ca2+ • Magnesium Mg2+ • To a lesser extent • Strontium Sr2+ • Iron Fe2+ • Manganese Mn2+ • What’s Happening C18 Stearate molecules linked by divalent Metals - cause Scum O O- M2+ O- O

  4. Causes of Hardness • Scale Formation • Ions start in solution • Temperature alters Carbon dioxide solubility, CO2 volatilises. • Equilibrium of bicarbonate shifts Ca2+ + 2HCO3- CaCO3 (s) + CO2(g) + H2O Scale

  5. Origins of Hardness • Hardwater • Rainwater dissolves CO2from air to form carbonic acid. H2O + CO2 H2CO3 • Groundwater formed where soil is thick over limestone, CO2 from soil respiration (microorganisms) • Limestone is dissolved MgCO3 (s) + H2CO3 Mg2+ + 2HCO3- CaCO3 (s) + H2CO3 Ca2+ + 2HCO3- • Softwater • Thin soil , little limestone

  6. Solubility of Minerals rain AIR CO2 Dissolves Bacteria Release CO2 (Respiration) SOIL CO2 + H2 O H2 CO3 CaCO3 + H2 CO3 Ca(HCO3)2 (aq) LIMESTONE MgCO3 + H2 CO3 Mg(HCO3)2 (s) CaCO3 + H2 CO3 Ca(HCO3)2 (s)

  7. Measurement of Hardness • Three Approaches • Soap Method (Obsolete) • Calculation (Sum of all Divalent Metal ions , M2+ ). AAS ICP • EDTA Method Express result as: mg CaCO3/l • Calculation Method 1. Obtain Analysis for each of the divalent metal ions 2. Express each metal ion as mg CaCO3/l • Hardness mg CaCO3/l = M2+(mg/l) x 50/EW(M2+) • Sum all Individual values from 2.

  8. Calculation Method • Bottled Mineral Water (eg. Perrier, ASDA) • Gives amounts of several Cations and Anions (mg/l) • Only divalent cations of interest Ca2+ 48.0Na+ 1.2 Mg2+ 8.5K+ 13.3 • Calculation • CATION EW Hardness Ca2+ 20 48 x 50 x 1/20 = 120 mg CaCO3/l Mg2+ 12.2 8.5 x 50 x 1/12.2 = 34.8 mg CaCO3/l Total Hardness 154.8 mg CaCO3/l • Disadvantage • Individual analyses are required • Expensive non-portable equipment

  9. EDTA Method • Simple Accurate • EDTA (Ethylenediamine tetraacetic acid) • Chelating Agent - binds M2+ very strongly • Indicator Eriochrome • binds M2+ more weakly than EDTA • Is Coloured • Titrate with EDTA (competes for the M2+ bound to Eriochrome). • When all M2+ removed from Eriochrome colour changes Eriochrome-M2+ (Wine Red) Eriochrome (Blue-tint) • Extended Method • Calcium ions Ca2+ with Eriochrome Blue at pH 12 - pH 13 • Magnesium ions Mg2+ with Eriochrome Black at pH 4

  10. Types of Hardness • Specific Cations • Calcium or Magnesium Hardness • Total Hardness - Calcium Hardness = Magnesium Hardness • Specific Anions • Carbonate Hardness (formerly called Temporary Hardness) • Divalent cations associated with carbonate or bicarbonate anions • Non-Carbonate Hardness (NCH) (formerly called Permanent Hardness) • NCH = Total Hardness - Carbonate Hardness • Divalent cations associated with sulphate, chloride, or nitrate anions. • Pseudo- Hardness • Salt Water - Common Ion Effect (Na+ with soap)

  11. Specific Anions • the general condition is as follows : Carbonate Hardness (CH) is equal to either the Total Hardness (TH) or the Alkalinity, whichever is the lowest value. When TH > Alkalinity • there is more calcium and magnesium than carbonate and bicarbonate • some Non-Carbonate Hardness must be present Then CH =Alkalinity (bicarbonate and carbonate) When TH < Alkalinity • there is less calcium and magnesium than carbonate and bicarbonate Then CH = TH

  12. Interpretation • Range of Hardness RESULT RELATIVE (mg CaCO3 /l) HARDNESS 0 - 75 Soft 75 - 150 Moderately Hard 150 - 300 Hard > 300 Very Hard • Bottled Mineral Water • Total Hardness 154.8 mg CaCO3/l • Total Alkalinity 110 mg CaCO3/l • Carbonate Hardness 110 mg CaCO3/l • Non-Carbonate Hardness (TH - CH) 44.8 mg CaCO3/l

  13. Langelier Saturation Index (LSI) • Predicts whether Scale will deposit from water. • Based on the pH at which water is saturated with CaCO3 ( pHs ) • LSI defined as: LSI = pH - pHs LSI > 0 Water is supersaturated, CaCO3 Scale tends to deposit. LSI = 0 Water saturated (equilibrium). LSI < 0 Water undersaturated, tends to dissolve CaCO3 Scale .

  14. Exercise (Hardness) • A groundwater sample has the following analysis of the individual species. • Find the Carbonate Hardness, Total Hardness, and Non-carbonate Hardness. Also classify the Relative Hardness of the water. • If EW of Calcium is 20 g/equivalent, what would the Atomic Absorption analysis result (mg/l) have been for this ion before its conversion to units of “mg CaCO3”. (EW CaCO3 is 50 g/equivalent) ION Concentration (mg CaCO3) Ca 2+ 187 Mg 2+ 164 Na + 22 HCO3- 246 Cl - 14 SO42- 113 Answers (CH 246, TH187+164=351, NCH 351-246=105 ) mg CaCO3/l., very hard 75 mgCa/l

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