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ENVE 201 Environmental Engineering Chemistry 1

ENVE 201 Environmental Engineering Chemistry 1. pH Dr. Aslıhan Kerç. Definition of pH. Used to express the intensity of the acid or alkaline condition of a solution Express hydrogen ion concentration Express hydrogen ion activity. Why pH is important in environmental engineering?.

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ENVE 201 Environmental Engineering Chemistry 1

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  1. ENVE 201EnvironmentalEngineeringChemistry 1 pH Dr. Aslıhan Kerç

  2. Definition of pH • Used to express the intensity of the acid or alkaline condition of a solution • Express hydrogen ion concentration • Express hydrogen ion activity

  3. Why pH is important in environmental engineering? In environmental engineering pH must be considered in: Chemical coagulation Disinfection Water softening Corrosion control Microorganisms in biological treatment Sludge dewatering Oxidation of certain substances (such as cyanide)

  4. Common characteristics of Acids and Bases • All acids contain element hydrogen • Neutralization reaction between acids- bases? produce water • Bases contain hydroxyl groups • Strong acid ? Weak acid? • Strong acids and bases are highly ionized • Weak acids and bases are poorly ionized in aqueous solutions.

  5. Hydrogen Electrode • Used to measure hydrogen ion activity • Pure water dissociates to produce hydrogen and hydroxyl ions H2O  H+ + OH- H ions  10-7 mol/L OH ions  10-7 mol/L Equilibrium equation?

  6. Equilibrium Equation • Since concentration of water is constant, activity =1 • {H+} {OH-}=10-7 x 10-7 = 10-14

  7. When acid is added to water, hydrogen activity increases • When base is added to water hydoxile ion activity increases • {H+} and {OH-} can never be zero

  8. Sorenson, 1909 • pH = - log {H+} • pH range 0 -14 • pH 7 @25oC represent absolute neutrality • Neutrality @ 0oC  pH 7.5 • Neutrality @ 60oC  pH 6.5 • Kw changes with change in temperature  pH of neutrality changes with temperature

  9. Measurement of pH • Hydrogen electrode  absolute standard for pH measurement. Not practical in use • Indicators  calibrated with hydrogen electrode. Different color characteristics at different pH levels

  10. pH Indicator Projects

  11. Measurement of pH • Glass electrode  standard method of pH measurement. A potential is developed related to the hydrogen ion activity. Field instruments Benchtop models Standardized with buffer solutions of known pH values

  12. Interpretation of pH Data • pH data should be interpreted in terms of hydrogen ion activity • Approximately we can assume [H+] = {H+} pH + pOH = ? = 14 [H+] and [OH-] can never be zero

  13. pOH or hydroxy ion activity is important in precipitation reactions involving formation of hydroxides e.g. Precipitation of Mg2+ in softening of water with lime

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