2 Water Electroneutrality Balanced: ∑ Cations = ∑ Anions∑ CC x eC = ∑ CA x eACC = moles / liter of cation c (one mole = 6.02x1023)eC = number of charges per molecule of cation CA subscript is for anionsPure water: [H+] = [OH-]Natural water: [H+] = [OH-] + [HCO3-] + 2[CO3-2]blog.ssis.edu.vn/
4 Why do we care about pH? Biological systems Corrosivity Extremes are disruptiveCorrosivityEquilibrium relationships involving H+
5 Good pH rangesFreshwater...Marine ...Drinking...Soda?
6 Alkalinity Capacity of water sample to neutralize an... Capacity of a water sample to take H+ without significant change in...Common ions that give water alkalinityHCO3-, CO32- & OH-
7 Acidity Capacity of water to take OH- without significant pH change Opposite of AlkalinityCapacity of water sample to neutralize a baseCommon ion: H+For Acid Mine DrainageAcidity =~f(Fe2+, Fe3+, Al, Mn, and H+)csmphotos.files.wordpress.com
8 pH, Alkalinity & Acidity Waters with same pH can have different amounts of Alkalinity or AcidityChemicals may be present that can react with or release H+ or OH-The lower the alkalinity (or acidity), the easier it is to change pH
9 Reversible Reactions aA + bB ↔ cC + dD A & B can react to form C and D C & D can react to form A and BAfter sufficient time has passed, equilibrium is reachedEquilibrium can be perturbed by adding more reactant or product
10 Reversible Reactions: Gas - Solution At equilibriumGaseous chemical dissolvesDissolved chemical volatilizesAt equal rateHenry’s LawExample…GasDissolved
11 Reversible Reactions In Solution At equilibriumReactants become productsProducts become reactantsAt equal rateExample…
12 Reversible Reactions: Solution - Precipitate At equilibriumPrecipitate is formedPrecipitate is dissolvedAt equal ratesExample…
13 K is constant at given temperature & relatively low concentration Equilibrium EquationaA + bB ↔ cC + dDK is constant at given temperature & relatively low concentration
14 Example - Pure Water What is water? ... Reversible equation?... Equilibrium equation?...At 25 C, Kw = ...Even if other sources of H+ (acids) or OH- (bases) are presentWhich substance predominates?...
15 pH of Pure Water @ 25 C? Follow these steps Write down species... Write electroneutrality eq...Write equilibrium equation..Solve equations...
16 Now, add… OH- ? (add a strong base) Systems finds new equilbrium, with less H+ and more OH-pH goes upH+ ? (add a strong acid, precipitate iron,…)Systems finds new equilbrium, with more H+ and less OH-pH goes down.Either way, [H+] [OH-] = Kw still holds
17 Precipitation of Limestone CaCO3-2 ↔ Ca+2 + CO3-2[Ca+2] [CO3-2] = KspKsp = Solubility productDo not include precipitate in equilibrium equation[Ca+2] [CO3-2] < Ksp?...[Ca+2] [CO3-2] > Ksp?...
19 Carbonate Equilibrium System (CES) Keeps natural waters in good pH rangeCommon ChemicalsCarbon Dioxide, CO2Carbonic Acid, H2CO3Bicarbonate, HCO3-Carbonate, CO3-2Solids containing Bicarbonate, Carbonate, Hydroxide, and (though rarely) certain Oxides (CaO),…
20 Main Sources CO2 from atmosphere Solids containing carbonate CO2 g CO2 aqH2CO3HCO3CO3Solid CO3,…AtmosphereSolutionPrecipitateCO2 from atmosphereByproduct ofbiological or chemical reactionscombustionSolids containing carbonatee.g., limestoneByproduct of sea life
21 CES Reactions Reversible Equilibrium (at 25C) CO2 g ↔ CO2 aq CO2 aq + H2O ↔ H2CO3H2CO3 ↔ H+ + HCO3-HCO3- ↔ H+ + CO3-2Ca+2 + CO3-2 ↔ CaCO3CO2 aq = CO2 g / 1637 atm[H2CO3] / [CO2 aq] = 1.58 x 10-3[H+] [HCO3-] / [H2CO3*] = 4.47x10-7 M[H+] [CO3-2] / [HCO3-] = 4.68x10-11 M[Ca+2] [CO3-2] = 3.80 x 10-9 M2Where CO2 g is in atm & CO2 aq is in mole fraction
22 Carbonate System & pH HCO3 CO3 mg/L as CaCO3 CO2 OH pH Typical Natural Water with 100 mg/L alkalinity as CaCO3100mg/L as CaCO36.5118.550CO2CO3HCO3OHpH
23 Carbonate System Scenarios Open / ClosedOpen - in equilibrium with atmosphereopen container, streams & shallow lakes, upper oceanClosed - not in equilibriumClosed tank, quick reactions, deep regions of water bodies, soil gasesHomogeneous / HeterogeneousHomogeneous – in equilibrium with CO3 solidsHeterogeneous– not in equilibrium with CO3 solidsClosed & heterogeneousbottom of stratified lake with floor of limestonewater treatment unit with soda ash / acid or base / CO2 & some CaCO3 precipitates
24 Example - Natural pH of Rain CO2 in atmosphere naturally “shifts” pH of rain from 7 to ?Open systemNo solid source of CO3CO2 gCO2 aqH2CO3HCO3-CO3-H+Lowers pH
25 Atmosphere Nitrogen - 0.781 atm Oxygen - 0.209 atm Argon - 0.0093 atm CO atmWhen this example was created. It’s now atm ( ppm)Misc atmTOTAL atm
26 Solution: pH of Rain (1)Atmosphere to raindrop - Henry’s Law: CO2 aq = CO2 g / 1637 atmConvert to M: [CO2 aq] = CO2 aq x MwMw = molar density of water = mol/l
27 Solution: pH of Rain (2)Reaction with water forms carbonic acid - Equilibrium Equation: [H2CO3 aq] = 1.58x10-3 M • [CO2 aq][H2CO3* aq] = [CO2 aq] + [H2CO2 aq]
28 Solution: pH of Rain (3)Dissociation of carbonic acid - Equilibrium Equation: [H+] [HCO3-] = 4.47 x 10-7 M • [H2CO3*]Two unknowns? Use electroneutrality: [H+] = [OH-] + [HCO3-] + 2[CO3-2]Rain water is acidic, so [OH-] & [CO3-2] will be small giving [H+] ≈ [HCO3-], leading to…
29 Solution: pH of Rain (4) pH = - log [H+] = -log (2.13x10-6) =… Check assumptions? Use equilibrium equations:[OH-] = M2 / [H+] = M2 / 2.13x10-6 M = 4.68x10-9 M (small, OK))[CO3-2] = 4.68x10-11 M • [H+] / [HCO3-] = 4.68x10-11 M • 2.13x10-6 M / 2.13x10-6 M = 4.68x10-11 M (small, OK)
30 pH of Rain CO2 in atmosphere goes up?... Use this method for different conditions?Need to check assumptions about relative concentration of [OH-] & [CO3-2]
31 Atmospheric CO2 & Ocean pH Oceans can be CO2 sink or sourceCurrently: sink, taking ~ 30% of anthropocentric CO2Primary mechanismsCarbonate equilibrium system (Henry’s Law,…)Biological PumpLiving creatures take up carbon, some are trapped in sea bedToo much CO2?pH drop of oceans could effect sea organismsFrom 1751 to 1994 surface ocean pH estimated to have dropped from ~ 8.18 to 8.10
32 Ocean pH Change~half Anthropogenic CO2 has been absorbed by ocean so farCurrently absorbing ~1/322 M tons / dayCould drop to 7.6Drastic effect on shell-forming organismsNational Geographic (2014) “Ocean Acidification”, ocean.nationalgeographic.com.
33 Acid Rain Rain with pH below 5.7 Most prevalent cause: SOx produced from burning coalSOx reacts with water to form sulfuric acidSulfuric acid dissolves into atmospheric water and dissociates into H- and SO4-2
34 Precipitation pH downwind of Ohio Valley power plants
36 CES, Alkalinity, Acid Rain’s Effect Carbonate solidsCommon source of alkalinity in natural waters (as they dissolve)Lakes and streams with alkalinity can accept acid rain without big pH changesAlkalinity buffers acid rainAcid rain can damage water bodies that don’t have alkalinity
39 Mine Side View Water Infiltration pH – 4.4 (low) [drops to ~2 after seep]Acidity 434 ppm CaCO3 (high)Partial Pressure CO2 in mine headspace - ~6% [200x higher than atmosphere]FeS2 + O2 + H2O →Fe2+ + SO42- + H+Fe2+ + O2 + H+ → Fe3+ + H2OFe3+ + H2O → Fe(OH)3 ↓ + H+Even lower pHAnaerobicConditionsLow pHHigh AcidityHigh metalsOxidation of organic matter can lead to high concentrations of CO2 in mine headspacePresence of limestone will buffer the mine water, resisting pH change