1. 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

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  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+

4. 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

5. 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

6. 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

7. 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.

8. 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

9. 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

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 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

13. 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.

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 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