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Hardness Objective n to understand the chemical basis of water hardness, how it originates, and ways it can affect water distribution systems. n to know the methods for measuring and expressing hardness, and its relationship with other water quality parameters. n to know the significance of water hardness in Environmental Engineering
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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
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Causes of Hardness Soap Effects Divalent Cations Especially CalciumCa 2+ MagnesiumMg 2+ To a lesser extent StrontiumSr 2+ Iron Fe 2+ ManganeseMn 2+ What’s Happening C18 Stearate molecules linked by divalent Metals - cause Scum O O-O- O O-O- M 2+
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Causes of Hardness Scale Formation Ions start in solution Temperature alters Carbon dioxide solubility, CO 2 volatilises. Equilibrium of bicarbonate shifts Ca 2+ + 2HCO 3 - CaCO 3 (s) + CO 2(g) + H 2 O Scale
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Origins of Hardness Hardwater Rainwater dissolves CO 2 from air to form carbonic acid. H 2 O + CO 2 H 2 CO 3 Groundwater formed where soil is thick over limestone, CO 2 from soil respiration (microorganisms) Limestone is dissolved MgCO 3 (s) + H 2 CO 3 Mg 2+ + 2HCO 3 - CaCO 3 (s) + H 2 CO 3 Ca 2+ + 2HCO 3 - Softwater Thin soil, little limestone
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Solubility of Minerals SOIL AIR LIMESTONE rain Bacteria Release CO 2 (Respiration) CaCO 3 + H 2 CO 3 Ca(HCO 3 ) 2 (s) MgCO 3 + H 2 CO 3 Mg(HCO 3 ) 2 (s) CO 2 + H 2 O H 2 CO 3 CaCO 3 + H 2 CO 3 Ca(HCO 3 ) 2 (aq) CO 2 Dissolves
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Measurement of Hardness Three Approaches Soap Method (Obsolete) Calculation (Sum of all Divalent Metal ions, M 2+ ). AAS ICP EDTA Method Express result as:mg CaCO 3 /l Calculation Method 1.Obtain Analysis for each of the divalent metal ions 2.Express each metal ion as mg CaCO 3 /l Hardness mg CaCO 3 /l = M 2+ (mg/l) x 50/EW (M 2+ ) Sum all Individual values from 2.
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Calculation Method Bottled Mineral Water (eg. Perrier, ASDA) Gives amounts of several Cations and Anions (mg/l) Only divalent cations of interest Ca 2+ 48.0 Na + 1.2 Mg 2+ 8.5 K + 13.3 Calculation CATIONEWHardness Ca 2+ 2048 x 50 x 1/20 = 120 mg CaCO 3 /l Mg 2+ 12.28.5 x 50 x 1/12.2 = 34.8 mg CaCO 3 /l Total Hardness 154.8 mg CaCO 3 /l Disadvantage Individual analyses are required Expensive non-portable equipment
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EDTA Method Simple Accurate EDTA (Ethylenediamine tetraacetic acid) Chelating Agent - binds M 2+ very strongly Indicator Eriochrome binds M 2+ more weakly than EDTA Is Coloured Titrate with EDTA (competes for the M 2+ bound to Eriochrome). When all M 2+ removed from Eriochrome colour changes Eriochrome-M 2+ (Wine Red) Eriochrome (Blue-tint) Extended Method Calcium ions Ca 2+ with Eriochrome Blue at pH 12 - pH 13 Magnesium ions Mg 2+ with Eriochrome Black at pH 4
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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)
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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
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Interpretation Range of Hardness RESULT RELATIVE (mg CaCO 3 /l)HARDNESS 0 - 75Soft 75 - 150Moderately Hard 150 - 300Hard > 300Very Hard Bottled Mineral Water –Total Hardness154.8 mg CaCO 3 /l –Total Alkalinity110 mg CaCO 3 /l –Carbonate Hardness110 mg CaCO 3 /l –Non-Carbonate Hardness (TH - CH)44.8 mg CaCO 3 /l
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Langelier Saturation Index (LSI) Predicts whether Scale will deposit from water. Based on the pH at which water is saturated with CaCO 3 ( pH s ) LSI defined as: LSI = pH - pH s LSI > 0Water is supersaturated, CaCO 3 Scale tends to deposit. LSI = 0Water saturated (equilibrium). LSI < 0Water undersaturated, tends to dissolve CaCO 3 Scale.
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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 CaCO 3 ”. (EW CaCO 3 is 50 g/equivalent) ION Concentration (mg CaCO 3 ) Ca 2+ 187 Mg 2+ 164 Na + 22 HCO 3 - 246 Cl - 14 SO 4 2- 113 Answers(CH 246, TH187+164=351, NCH 351-246=105 ) mg CaCO 3 /l., very hard 75 mgCa/l
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