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).