1. pH, acid neutralizing capacity & acid rain Announcements Canoe trip canceled due to high water (discharge more than doubled between last Thursday and Sunday!

2. 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

3. pH " puissance d'hydrogène" ~ Strength of hydrogen Disassociation of water: H 2 O H + + OH - –Dissociation constant of water Kw = [H + ][OH - ] = 10 -14 [H 2 O] [H 2 O]= 1= activity of water So Kw = [H + ][OH - ] = 10 -14 [ ]=conc. in moles/Liter pH = -log[H + ] Adding acid increases Which decreases the base

4. The pH scale pH ranges from 0 (acidic) to 14 (basic). pH is a unit-less measure on a log 10 scale, so there is a tenfold difference in [H + ] between increments (e.g., pH = 2 and pH = 3). At pH = 7, [H + ] = [OH - ]

5. 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

6. The pH of natural waters Commonly between 4 and 9 Acid rain is a big concern But some lakes are naturally acidic…

7. Sphagnum bogs Sphagnum moss exchanges H + ions in order to uptake Ca 2+ ions

8. Volcanic action or sulfur springs H 2 S (hydrogen sulfide) oxidized by bacteria to form H 2 SO 4 (sulfuric acid)

9. DIC - dissolved inorganic carbon The carbonate species CO 2 (gas); dissolves as mixture of CO 2 & H 2 CO 3 (carbonic acid) HCO 3 - (bicarbonate) CO 3 2- (carbonate) completely disassociated Which species is present depends on pH

10. Carbonate species disassociation constants K h = [H 2 CO 3 ]/pCO 2 dictates the amount of CO 2 dissolved in the water K h = 200x that of O 2 K 1 = [H + ][HCO 3 - ]/[H 2 CO 3 ] = 10 -6.3 (at 15 o C) therefore, at pH = 6.3, [HCO 3 - ] = [H 2 CO 3 ] K 2 = [H + ][CO 3 2 - ]/[HCO 3 - ] = 10 -10.3 (at 15 o C) therefore, at pH = 10.3, [CO 3 2 - ] = [HCO 3 - ]

11. The carbonate buffering system - maintains pH CO 2 (gas) <> CO 2 + H 2 O <> H 2 CO 3 <> HCO 3 - + H + <> CO 3 2 - + H + (carbonic acid)(carbonate)(bicarbonate) How does a buffering system work?

12. Distribution of carbonate species according to pH CO 2 (gas) <> CO 2 + H 2 O <> H 2 CO 3 <> HCO 3 - + H + <> CO 3 2 - + H + K1K1 K2K2

13. When a system is at high pH (little to no CO 2 present)… Calcium bicarbonate forms when CO 2 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 CO 3 2- + Ca 2 + <> CaCO 3 (solid)

14. Processes affecting inorganic carbon Atmospheric exchange FluxCO 2 = k ([CO 2 air] - [CO 2 water]) –k depends on wind Groundwater inputs CaCO 3 + CO 2 + H 2 O --> Ca 2 + + 2HCO 3 - Lake metabolism CaCO 3 precipitation

15. 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 bicarbonate at 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).