What is the Acid Source? Where is Acid Rain Falling? Chemistry of Acid Rain Formation. What Determines Lake Susceptibility to Acidification?

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1 Acid Precipitation and Remediation of Acid Lakes What is the Acid Source? Acid Rain Peristaltic Pump ph Probe Lake Data source: Chemistry of Acid Rain Formation Where is Acid Rain Falling? Combustion product precursors to acid rain SO2 Reactions NO Strong acids H 2SO 4 Sulfuric acid HNO Nitric acid What Determines Lake Susceptibility to Acidification? Acidification = f(acid inputs, ANC) Acid inputs = f(power plants, cars, wind currents, mine tailings) Acid Neutralizing Capacity = f(?) Carbonates obtained from dissolution of minerals such as CaCO (calcite or aragonite) MgCO (magnesite) CaMgCO (dolomite)... Fate of Strong Acids in the Environment Strong acids completely dissociate in water pk= - pk = -, pk 2 =.9 HNO H 2SO4 2H H NO SO 2 4 If 0. mol of nitric acid is added to liter of pure water, what is the concentration of H +? 0. M What is the ph? [ph = -log[h + ]] What else can happen to the hydrogen ions if it isn t pure water? reactions

2 Fate of Strong Acids: Reactions Weak acids/bases can react with the added H + and reduce the final concentration of H + Examples of weak acids and bases in the environment: carbonates carbonate, bicarbonate, carbonic acid organic acids (A ) acetic acid (pk = 4.7) When K A = [H + ] then [A - ] = [HA] species definition 2 * K H CO H HCO K 0 6. HCO Carbonate System CO >> 2 H 2( aq) H CO 2 CO 2 2 K H CO K pk pk HCO Equilibrium Constant Alpha Notation Alpha Notation C T Total carbonate species define Alpha Notation ph Diagram All carbonate species concentrations are related to the hydrogen ion concentration = _ pk ph pk 2 alpha0 alpha alpha2 Add acid to a carbonate solution at ph 9. What happens?

3 Acid Neutralizing Capacity (ANC) The ability to neutralize (react with) acid ANC has units of moles of protons/l or eq/l Possible reactants in environment: ANC [HCO ]+2[CO2 2 ]+[OH - ]-[H + ] Hydrogen Ion Concentration: The Master Variable 2 2 (ph, C T ) 0 ANC properties ANC Example ANC = capacity to react with H + minus the concentration of H + ANC can be positive or negative ANC is conservative with respect to mixing Example: 0 liters of a solution with an ANC of 0. meq/l is mixed with 5 liters of a solution with an ANC of meq/l. What is the final ANC? Suppose we add mm Ca(OH) 2 to distilled water. What is the ANC? What is the resulting ph if the system is closed to the atmosphere? p(oh)= ph= no carbonates / [H + ] is small More Complications: Open to the Atmosphere Natural waters exchange carbon dioxide with the atmosphere The total concentration of carbonate species is affected by this exchange Is ANC affected? NO! 2 2 Henry s constant ANC Example (continued) Suppose we aerate the mm Ca(OH) 2 solution. What happens to the ph? 2 All the alphas are functions of ph and it is not possible to solve explicitly for [H + ]. Solution techniques numerical methods (ph=9, C T =0.0057M) Beware of precision! 0 Solve for ph rather than for [H+] to make it possible for SOLVER to find a solution!

4 Open vs. Closed to the Atmosphere What is conserved in an open (volatile) system? ANC What is conserved in a closed (nonvolatile) system? ANC C T For conservative species we can use the Completely Mixed Flow Reactor equation to model a well mixed lake Completely Mixed Flow Reactor Q, C in Q, C V, C Mass in Mass out = Increase of Mass in reactor Mass balance Hydraulic Residence Time Completely Mixed Flow Reactor / / What is C when t is large? C in Set up integration Equation applies to any conservative species. C 0 = time zero concentration in reactor C in = influent concentrationand effluent! C = concentration in the reactor as a function of time Three Equations for C T in CMFR CMFR for conservative species. (True if nonvolatile!) C e -t/θ T C T 0 Assuming no carbonates in influent If in equilibrium with atmospheric CO 2... Can we measure C T? What is the measured concentration of carbonates? Measured C T? 2 2 What is ANC? Measuring ANC: Gran Titration The sample is titrated with a strong acid to "cancel" the sample ANC At the equivalence point the sample ANC is zero Further titration will result in an increase in the number of moles of H + equal to the number of moles of H + added. Use the fact that ANC is conservative... / /

5 Conservation of ANC Need to find ANC T and V e T = titrant S = sample 0 V e = equivalent volume = volume of titrant added so that ANC = 0 0 ANC of Titrant 2 Why? N T = [H + ] Could solve for V e, but what is ANC T+S? When can we measure ANC with a ph probe? When ph is so low that no reactions are occurring. ANC of Titrated Sample When is this true? For ph << pk Finally! An equation for equivalent volume! Gran Function A better measure of the equivalent volume can be obtained by rearranging the equation so that linear regression on multiple titrant volume ph data pairs can be used. Define F as: Gran Plot Gran Plot using ph First Gran Function Volume of Titrant (ml) N slope = t V abscissa intercept of V e s Minimum value of F before it is worth attempting analysis? ph<4. points with F >0.000 Algorithm for choosing points to include? V e F plotted as a function of V t. The abscissa has units of ml of titrant and the ordinate is a Gran function with units of [H + ].

6 Calculating ANC The ANC is obtained from the equivalent volume. TheANCoftheacidraincanbeestimated fromitsph.atlowph(<pk )mostofthe carbonates will be carbonic acid and thus for ph below about 4. the ANC equation simplifies to 2 Titration Technique Titrate with digital pipette Accuracy is important Measure ph before first addition of titrant Measure ph after each addition of titrant After ANC is consumed Gran function will be linear What should the incremental titrant volume be? Techniques to speed up titration Titrant Incremental Volume Minimum? None except your patience Maximum? Constraints? Number of data points or more Before reaching ph How do we determine titrant volume? H + is conservative! Freshwater ph: water quality issues/challenges Source of Acid Rain History of Acid Rain Research Fate of strong acids in the environment Reactions Carbonate System Dissociation constants Alpha notation p notation (pk, etc) Volatile (open) vs non volatile (closed) Acid Neutralizing Capacity (ANC) Defined Measured Gran Plot Titration The Global Problem of Acid Rain Data source: Copyright by Holt, Rinehart and Winston. All rights reserved.

7 History of Acid Rain Research Lake and/or Watershed Remediation British writer noted the corrosive effect of polluted, acidic city air on the deterioration of marble sculptures 600s s 960s 970s 980 Robert Angus Smith associated acidic rain with industrial pollution in Manchester, England Eville Gorham hypothesized SO 2 emissions could lead to acid rain Harold Harvey convinced the world of the causes and effects of acid rain New York Times made the issue popular with a series of reports on damage to forest ecosystems wrought by acid rain US Congress passes theacid Deposition Act Add a soluble mineral such as lime (CaCO ) or sodium bicarbonate (NaHCO ) Application options spread on watershed meter into stream apply directly to lake At what ph is ANC 0? 2 concentration (moles/l) ph H2CO HCO CO H+ OH-