1. ENVE 201 Environmental Engineering Chemistry 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

4. 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)

5. 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.

6. Hydrogen Electrode Used to measure hydrogen ion activity Pure water dissociates to produce hydrogen and hydroxyl ions H 2 O  H + + OH - H ions  10 -7 mol/L OH ions  10 -7 mol/L Equilibrium equation?

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

8. 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

9. Sorenson, 1909 pH = - log {H + } pH range 0 -14 pH 7 @25 o C represent absolute neutrality Neutrality @ 0 o C  pH 7.5 Neutrality @ 60 o C  pH 6.5 K w changes with change in temperature  pH of neutrality changes with temperature

10. 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

11. IndicatorpH RangeQuantity per 10 mlAcidBase Thymol Blue1.2-2.81-2 drops 0.1% soln. in aq.redyellow Pentamethoxy red1.2-2.31 drop 0.1% soln. in 70% alc.red-violetcolorless Tropeolin OO1.3-3.21 drop 1% aq. soln.redyellow 2,4-Dinitrophenol2.4-4.01-2 drops 0.1% soln. in 50% alc.colorlessyellow Methyl yellow2.9-4.01 drop 0.1% soln. in 90% alc.redyellow Methyl orange3.1-4.41 drop 0.1% aq. soln.redorange Bromphenol blue3.0-4.61 drop 0.1% aq. soln.yellowblue-violet Tetrabromphenol blue3.0-4.61 drop 0.1% aq. soln.yellowblue Alizarin sodium sulfonate3.7-5.21 drop 0.1% aq. soln.yellowviolet α-Naphthyl red3.7-5.01 drop 0.1% soln. in 70% alc.redyellow p-Ethoxychrysoidine3.5-5.51 drop 0.1% aq. soln.redyellow Bromcresol green4.0-5.61 drop 0.1% aq. soln.yellowblue Methyl red4.4-6.21 drop 0.1% aq. soln.redyellow Bromcresol purple5.2-6.81 drop 0.1% aq. soln.yellowpurple Chlorphenol red5.4-6.81 drop 0.1% aq. soln.yellowred Bromphenol blue6.2-7.61 drop 0.1% aq. soln.yellowblue p-Nitrophenol5.0-7.01-5 drops 0.1% aq. soln.colorlessyellow Azolitmin5.0-8.05 drops 0.5% aq. soln.redblue Phenol red6.4-8.01 drop 0.1% aq. soln.yellowred Neutral red6.8-8.01 drop 0.1% soln. in 70% alc.redyellow Rosolic acid6.8-8.01 drop 0.1% soln. in 90% alc.yellowred Cresol red7.2-8.81 drop 0.1% aq. soln.yellowred α-Naphtholphthalein7.3-8.71-5 drops 0.1% soln. in 70% alc.rosegreen Tropeolin OOO7.6-8.91 drop 0.1% aq. soln.yellowrose-red Thymol blue8.0-9.61-5 drops 0.1% aq. soln.yellowblue Phenolphthalein8.0-10.01-5 drops 0.1% soln. in 70% alc.colorlessred α-Naphtholbenzein9.0-11.01-5 drops 0.1% soln. in 90% alc.yellowblue Thymolphthalein9.4-10.61 drop 0.1% soln. in 90% alc.colorlessblue Nile blue10.1-11.11 drop 0.1% aq. soln.bluered Alizarin yellow10.0-12.01 drop 0.1% aq. soln.yellowlilac Salicyl yellow10.0-12.01-5 drops 0.1% soln. in 90% alc.yelloworange-brown Diazo violet10.1-12.01 drop 0.1% aq. soln.yellowviolet Tropeolin O11.0-13.01 drop 0.1% aq. soln.yelloworange-brown Nitramine11.0-13.01-2 drops 0.1% soln in 70% alc.colorlessorange-brown Poirrier's blue11.0-13.01 drop 0.1% aq. soln.blueviolet-pink Trinitrobenzoic acid12.0-13.41 drop 0.1% aq. soln.colorlessorange-red pH Indicator Projects

12. 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

13. 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

14. pOH or hydroxy ion activity is important in precipitation reactions involving formation of hydroxides e.g. Precipitation of Mg 2+ in softening of water with lime