water science electroneutrality, ph, alkalinity, acidity reversible equations carbonate equilibrium...
TRANSCRIPT
Water Science• Electroneutrality, pH, Alkalinity, Acidity• Reversible Equations• Carbonate Equilibrium System• Examples
• Rain• Ocean pH• Acid Rain• Acid Mine Drainage
naturescrusaders.files.wordpress.com
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]
blog.ssis.edu.vn/
pH - reporting H+ concentration• pH = - log {H+}
• {} = activity, the effective concentration
• pH = - log [H+] for dilute solutions• [] = Molar
concentration (moles/Liter)
• [H+] = 10-pH
www.craterlakeoregon.org/
Why do we care about pH?• Biological systems
• Extremes are disruptive
• Corrosivity
• Equilibrium relationships involving H+
www.safetrack.se/
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-
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+)
csmphotos.files.wordpress.com
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
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
Reversible Reactions: Gas - Solution• At equilibrium
• Gaseous chemical dissolves • Dissolved chemical volatilizes• At equal rate
• Henry’s Law
• Example…
Gas
Dissolved
Reversible Reactions In Solution• At equilibrium
• Reactants become products • Products become reactants• At equal rate
• Example…
Reactants
Products
Reversible Reactions: Solution - Precipitate• At equilibrium
• Precipitate is formed • Precipitate is dissolved• At equal rates
• Example…
Reactants
Precipitate
www.struvite.info/
Equilibrium Equation
aA + bB ↔ cC + dD
[C]c [D]d
[A]a [B]b = k
K is constant at given temperature & relatively low concentration
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?...
pH of Pure Water @ 25 C?• Follow these steps
• Write down species...• Write electroneutrality eq...• Write equilibrium equation..
• Solve equations...
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
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?...
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),…
Main Sources• CO2 from atmosphere
• Byproduct of • biological or chemical reactions• combustion
• Solids containing carbonate• e.g., limestone
• Byproduct of sea life
CO2 g
CO2 aq
H2CO3
HCO3
CO3
Solid CO3,…
Atmosphere
Solution
Precipitate
CES Reactions
Reversible
• CO2 g ↔ CO2 aq
• CO2 aq + H2O ↔ H2CO3
• H2CO3 ↔ H+ + HCO3-
• HCO3- ↔ H+ + CO3
-2
• Ca+2 + CO3-2 ↔ CaCO3
Equilibrium (at 25C)
• 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
Carbonate System & pH• Typical Natural Water with 100 mg/L alkalinity as CaCO3
pH
100
mg
/L a
s C
aC
O3
6.5 118.5
50
CO2
CO3
HCO3
OH
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
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+
H+ Lowers pH
Atmosphere• Nitrogen - 0.781 atm• Oxygen - 0.209 atm• Argon - 0.0093 atm• CO2 - 0.00033 atm
• When this example was created. It’s now 0.00040 atm (398.78 ppm)
• Misc. - 0.0004 atm• TOTAL - 1.0 atm
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 = 55.56 mol/l
•
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]
•
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…
•
•
Solution: pH of Rain (4)• pH = - log [H+] = -log (2.13x10-6) =…
• Check assumptions? Use equilibrium equations:
• [OH-] = 10-14 M2 / [H+] = 10-14 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)
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]
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
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”, ocean.nationalgeographic.com.
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
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
Mine Side ViewWater Infiltration
AnaerobicConditions
Low pHHigh AcidityHigh metals
FeS2 + O2 + H2O → Fe2+ + SO4
2- + H+ Fe2+ + O2 + H+ → Fe3+ + H2O
Fe3+ + H2O → Fe(OH)3 ↓ + H+
Presence of limestone will buffer the mine water, resisting pH change
Even lower pH
Oxidation of organic matter can lead to high concentrations of CO2 in mine headspace
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]