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Water Science Electroneutrality, pH, Alkalinity, Acidity

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Presentation on theme: "Water Science Electroneutrality, pH, Alkalinity, Acidity"— Presentation transcript:

1 Water Science Electroneutrality, pH, Alkalinity, Acidity
Reversible Equations Carbonate Equilibrium System Examples Rain Ocean pH Acid Rain Acid Mine Drainage

2 Water Electroneutrality
Balanced: ∑ Cations = ∑ Anions ∑ CC x eC = ∑ CA x eA CC = moles / liter of cation c (one mole = 6.02x1023) eC = number of charges per molecule of cation C A subscript is for anions Pure water: [H+] = [OH-] Natural water: [H+] = [OH-] + [HCO3-] + 2[CO3-2]

3 pH - reporting H+ concentration
pH = - log {H+} {} = activity, the effective concentration pH = - log [H+] for dilute solutions [] = Molar concentration (moles/Liter) [H+] = 10-pH

4 Why do we care about pH? Biological systems Corrosivity
Extremes are disruptive Corrosivity Equilibrium relationships involving H+

5 Good pH ranges Freshwater... 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 alkalinity HCO3-, CO32- & OH-

7 Acidity Capacity of water to take OH- without significant pH change
Opposite of Alkalinity Capacity of water sample to neutralize a base Common ion: H+ For Acid Mine Drainage Acidity =~f(Fe2+, Fe3+, Al, Mn, and H+)

8 pH, Alkalinity & Acidity
Waters with same pH can have different amounts of Alkalinity or Acidity Chemicals 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 B After sufficient time has passed, equilibrium is reached Equilibrium can be perturbed by adding more reactant or product

10 Reversible Reactions: Gas - Solution
At equilibrium Gaseous chemical dissolves Dissolved chemical volatilizes At equal rate Henry’s Law Example… Gas Dissolved

11 Reversible Reactions In Solution
At equilibrium Reactants become products Products become reactants At equal rate Example…

12 Reversible Reactions: Solution - Precipitate
At equilibrium Precipitate is formed Precipitate is dissolved At equal rates Example…

13 K is constant at given temperature & relatively low concentration
Equilibrium Equation aA + bB ↔ cC + dD K 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 present Which 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 up H+ ? (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] = Ksp Ksp = Solubility product Do 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 range Common Chemicals Carbon Dioxide, CO2 Carbonic Acid, H2CO3 Bicarbonate, HCO3- Carbonate, CO3-2 Solids containing Bicarbonate, Carbonate, Hydroxide, and (though rarely) certain Oxides (CaO),…

20 Main Sources CO2 from atmosphere Solids containing carbonate CO2 g
CO2 aq H2CO3 HCO3 CO3 Solid CO3,… Atmosphere Solution Precipitate CO2 from atmosphere Byproduct of biological or chemical reactions combustion Solids containing carbonate e.g., limestone Byproduct of sea life

21 CES Reactions Reversible Equilibrium (at 25C) CO2 g ↔ CO2 aq
CO2 aq + H2O ↔ H2CO3 H2CO3 ↔ H+ + HCO3- HCO3- ↔ H+ + CO3-2 Ca+2 + CO3-2 ↔ CaCO3 CO2 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 M2 Where 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 CaCO3 100 mg/L as CaCO3 6.5 11 8.5 50 CO2 CO3 HCO3 OH pH

23 Carbonate System Scenarios
Open / Closed Open - in equilibrium with atmosphere open container, streams & shallow lakes, upper ocean Closed - not in equilibrium Closed tank, quick reactions, deep regions of water bodies, soil gases Homogeneous / Heterogeneous Homogeneous – in equilibrium with CO3 solids Heterogeneous– not in equilibrium with CO3 solids Closed & heterogeneous bottom of stratified lake with floor of limestone water 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 system No solid source of CO3 CO2 g CO2 aq H2CO3 HCO3- CO3- H+ Lowers pH

25 Atmosphere Nitrogen - 0.781 atm Oxygen - 0.209 atm Argon - 0.0093 atm
CO atm When this example was created. It’s now atm ( ppm) Misc atm TOTAL atm

26 Solution: pH of Rain (1) Atmosphere to raindrop - Henry’s Law: CO2 aq = CO2 g / 1637 atm Convert to M: [CO2 aq] = CO2 aq x Mw Mw = 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 source Currently: sink, taking ~ 30% of anthropocentric CO2 Primary mechanisms Carbonate equilibrium system (Henry’s Law,…) Biological Pump Living creatures take up carbon, some are trapped in sea bed Too much CO2? pH drop of oceans could effect sea organisms From 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 far Currently absorbing ~1/3 22 M tons / day Could drop to 7.6 Drastic effect on shell-forming organisms National Geographic (2014) “Ocean Acidification”,

33 Acid Rain Rain with pH below 5.7 Most prevalent cause:
SOx produced from burning coal SOx reacts with water to form sulfuric acid Sulfuric acid dissolves into atmospheric water and dissociates into H- and SO4-2

34 Precipitation pH downwind of Ohio Valley power plants

35 Sulfur Cap & Trade Program

36 CES, Alkalinity, Acid Rain’s Effect
Carbonate solids Common source of alkalinity in natural waters (as they dissolve) Lakes and streams with alkalinity can accept acid rain without big pH changes Alkalinity buffers acid rain Acid rain can damage water bodies that don’t have alkalinity

37 Acid Mine Drainage

38 Red Oak Mine Site

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+ + H2O Fe3+ + H2O → Fe(OH)3 ↓ + H+ Even lower pH Anaerobic Conditions Low pH High Acidity High metals Oxidation of organic matter can lead to high concentrations of CO2 in mine headspace Presence of limestone will buffer the mine water, resisting pH change

40 Red Oak Seep seep discharge

41 Plan View Not to scale

42 Red Oak Remediation Post injection, seep characteristics were pH - 6.3, Alkalinity - 150 mg/L as CaCO3, Fe- 120 mg/l; however, this level of treatment lasted only 15 months.

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