1. Dissociation and pH Dissociation of weak acids/bases controlled by pH Knowing the total amount of S and pH, we can calculate activities of all species and generate curves Example: H 2 S 1

2. Hydrogen Sulfide Activity Diagram 2

3. 3

4. Solubility of Quartz The oxides of many metals react with H 2 O to form bases SiO 2(s) + 2H 2 O  H 4 SiO 4 ° 4

5. Quartz Activity Diagram When including a solid, the activity diagram looks a little different – Showing fields of stability for each species Note: we don’t need to define initial log[SiO 2 ] concentration – Activity of solid = 1 5

6. 6 Quartz Activity Diagram

7. 7 H 4 SiO 4

8. Buffering of pH Weak acids and bases can buffer pH of a solution – pH changes very little as acid (or base) is added – Need both a protonated and unprotonated species present in significant concentrations e.g., H 2 CO 3(aq) and HCO 3 - Carbonic acid-bicarbonate is the major buffer in most natural waters Organic acids and sometimes silicic acid can be important buffers 8

9. pH Buffering capacity of an aquifer: Minerals as well as aqueous species Reactions with minerals: carbonate most important, fastest – CaCO 3 + H + ↔ Ca 2+ + HCO 3 - Silicates, slower, less important – 2KAlSi 3 O 8 + 2H 2 CO 3 + 9H 2 O  Al 2 Si 2 O 5 (OH) 4 + 2K + + 4H 4 SiO 4 + 2HCO 3 - H 2 CO 3 consumes acid, HCO 3 - creates alkalinity Ion exchange of charge surfaces – Negatively charged S - + H + ↔ SH 9

10. Dissolved Inorganic Carbon (DIC) Initially, DIC in groundwater comes from CO 2 – CO 2 (g) + H 2 O ↔ H 2 CO 3 ° Equilibrium expression with a gas is known as Henry’s Law – P CO2 : partial pressure (in atm or bar); pressure in atmosphere exerted by CO 2 – Assuming atmospheric pressure of 1 atm, P CO2 = 10 -3.5 ; concentration of CO 2 = 350 ppm At atm = 1, N 2 is 78%, P N2 = 0.78, O 2 21%, P O2 = 0.21 10

11. Dissolved Inorganic Carbon (DIC) P CO2 of soil gas can be 10-100 times the P CO2 of atmosphere P CO2 for surface water controlled by atmosphere and biological processes – Photosynthesis (day): drives P CO2 down, less H 2 CO 3, pH increases 6CO 2 + 6H 2 O + Energy ↔ C 6 H 12 O 6 + 6O 2 – Respiration: increases P CO2, more H 2 CO 3, pH drops 11

12. Dissolved Inorganic Carbon (DIC) In groundwater, no photosynthesis, no diurnal variations – CO 2 usually increases along a flow path due to biodegradation in a closed system – CH 2 O + O 2  CO 2 + H 2 O CH 2 O = generic organic matter 12

13. DIC and pH in Open System CO 2 can be dissolved into or volatilize out of water freely – Surface waters P CO2 is constant = 10 -3.5 atm at Earth’s surface 13

14. DIC and pH in Open System What is the pH of natural rainwater? – Controlled by DIC equilibrium – At 25°C, K CO2 = 10 -1.47 14

15. DIC and pH in Closed System In a closed system (no CO 2 exchange), for a given amount of TIC, speciation is a function of pH CO 2 + H 2 O ↔ H 2 CO 3 ↔ HCO 3 - + H + ↔ CO 3 2- + H + – At pH = 6.35, [H 2 CO 3 ] = [HCO 3 - ] – At pH = 10.33, [HCO 3 - ] = [CO 3 2- ] We can do same calculations we did for H 2 S 15

16. 16 Total DIC = 10 -1 M pH = 6.35 pH = 10.33 Common pH range in natural waters

17. Rainwater pH and P CO2 What if we double P CO2 (10 -1.75 atm) – [H 2 CO 3 ] = [10 -1.47 ] [10 -1.75 ] = 10 -3.22 – Doubling the P CO2 does not have a large effect on pH Acid rain can have pH < 4 – Due to other acids (nitric and sulfuric) that are injected into the atmosphere by vehicles and smokestacks 17

18. Special points about DIC, pH, and other weak acids At pH 6.35, K a1 = [H + ], therefore [H 2 CO 3 ] = [HCO 3 - ] – – Likewise, at pH 10.33, K a2 = [H + ], therefore [HCO 3 - ] = [CO 3 2- ] 18

19. Special points about DIC, pH, and other weak acids When pH = pK a, concentration of protonated in reactant = deprotonated in product – pK a = -log K a – for H 2 CO 3 ↔ HCO 3 - + H +, K a = 10 -6.35, pK a = 6.35 – so for H 4 SiO 4 ↔ H 3 SiO 4 - + H +, pK a = 9.71 – And for H 3 SiO 4 -  H + + H 2 SiO 4 2-, pK a = 13.28 19

20. Alkalinity Alkalinity = acid neutralizing capability (ANC) of water – Total effect of all bases in solution – Typically assumed to be directly correlated to HCO 3 - concentration in groundwater 20

21. Alkalinity Total alkalinity = [HCO 3 - ] + 2[CO 3 2- ] + [B(OH) 4 - ] + [H 3 SiO 4 - ] + [HS - ] + [OH - ] – [H + ] – Typically in groundwater, [HCO 3 - ] >> [CO 3 2- ], [B(OH) 4 - ], [H 3 SiO 4 - ], [HS - ], [OH - ], [H + ] – Whenever there are significant amounts of any of these other species, they must be considered Carbonate alkalinity = [HCO 3 - ] + 2[CO 3 2- ] + [OH - ] – [H + ] – Directly convertible to [HCO 3 - ] when it is >> than others Measured by titration of solution with strong acid 21

22. 22 Total DIC = 10 -1 M

23. Alkalinity Titration Determine end-point pH: – The pH at which the rate of change of pH per added volume of acid is at a maximum – Typically in the range 4.3-4.9 – Function of ionic strength – Reported as mg/L CaCO 3 – – HCO 3 - = alkalinity 0.82 23

24. Determining Alkalinity by Titration 24 Initial pH = 8.26 Rapid pH change Rapid pH change Slow pH change: Buffered Determine maximum pH change by: ΔpH ÷ mL acid added