pH, acid neutralizing capacity & acid rain Announcements Canoe trip canceled due to high water (discharge more than doubled between last Thursday and Sunday!
pH, acid neutralizing capacity & acid rain
AnnouncementsCanoe trip canceled due to high water (discharge more
than doubled between last Thursday and Sunday!
Announcements, con't
Independent projects: Meeting time Monday/Tuesday/Wednesday this week?
Exams: excellent!mean: 76.8 +- 7.6 sd (out of 90 total points)
Guest Lecture on Wednesday: rm 66 Mudd
pH
" puissance d'hydrogène" ~ Strength of hydrogenDisassociation of water: H2O <-> H+ + OH-
– Dissociation constant of water Kw = [H+][OH-] = 10 -14
[H2O]
[H2O]= 1= activity of water
So Kw = [H+][OH-] = 10 -14 [ ]=conc. in moles/Liter
pH = -log[H+]
Adding acid increases
Which decreases the base
The pH scale
pH ranges from 0 (acidic) to 14 (basic).
pH is a unit-less measure on a log10 scale, so there is a tenfold difference in [H+] between increments (e.g., pH = 2 and pH = 3).
At pH = 7, [H+] = [OH-]
pH examples:
1. [OH-] = 10-8; what is the pH?
2. If the pH = 6.4 , what is the hydrogen ion concentration?
[H+]= Kw / [OH-] = 10 -14/ 10 -8 = 10 - 6
-log[10 - 6] = 6 = pH
[H+] = 10 -pH = 10 -6.4 = 3.98x10 -7
The pH of natural waters
• Commonly between 4 and 9• Acid rain is a big concern• But some lakes are naturally acidic…
Sphagnum bogs
• Sphagnum moss exchanges H+ ions in order to uptake Ca2+ ions
Volcanic action or sulfur springs
• H2S (hydrogen sulfide) oxidized by bacteria to form H2SO4 (sulfuric acid)
DIC - dissolved inorganic carbonThe carbonate species
CO2 (gas); dissolves as mixture of CO2 & H2CO3 (carbonic acid)
HCO3- (bicarbonate)
CO32- (carbonate) completely disassociated
• Which species is present depends on pH
Carbonate species disassociation constants
Kh = [H2CO3]/pCO2 dictates the amount of CO2 dissolved in the water Kh= 200x that of O2
K1 = [H+][HCO3-]/[H2CO3] = 10 -6.3 (at 15oC)
therefore, at pH = 6.3, [HCO3-] = [H2CO3]
K2 = [H+][CO32- ]/[HCO3
-] = 10 -10.3 (at 15oC)
therefore, at pH = 10.3, [CO32- ] = [HCO3
-]
The carbonate buffering system- maintains pH
CO2 (gas) <> CO2 + H2O <> H2CO3 <> HCO3- + H+ <> CO3
2- + H+ (carbonic acid) (carbonate)(bicarbonate)
How does a buffering system work?
Distribution of carbonate species according to pH
CO2 (gas) <> CO2 + H2O <> H2CO3 <> HCO3- + H+ <> CO3
2- + H+
K1 K2
When a system is at high pH (little to no CO2 present)…
Calcium bicarbonate forms when CO2 is taken up during photosynthesis and system becomes more basic (high pH)
Calcium bicarbonate is not soluble at high pH
Phytoplankton and macrophyte leaves act as nuclei for precipitation
CO32- + Ca2
+ <> CaCO3 (solid)
Processes affecting inorganic carbon
• Atmospheric exchangeFluxCO2 = k ([CO2 air] - [CO2water])– k depends on wind
• Groundwater inputsCaCO3 + CO2 + H2O --> Ca2+ + 2HCO3
-
• Lake metabolism
• CaCO3 precipitation
Acid neutralizing capacity (alkalinity)
ANC = a measure of the lake water's capacity to buffer pH change - ANC determined by titrating a water sample with a strong acid and monitoring change in pH- added H+ is taken up first by carbonate, then by bicarbonateat approx pH=4.5, all carbonate and bicarbonate is converted to carbonic acid and the buffering capacity is exhausted- At this point, change in pH is directly proportional to the amount of acid added.- The amount of acid added up to pH= 4.5 is used to calculate ANC (in µeq/L).