Ch. 6. ACIDS & BASES 6-1. Definitions 6-1. Definitions Alchemist’s Alchemist’s Acids: sour, release gases by reacting with metals, turn litmus paper red Acids: sour, release gases by reacting with metals, turn litmus paper red Bases: bitter, slippery, turn litmus paper blue, neutralize acids Bases: bitter, slippery, turn litmus paper blue, neutralize acids Arrehnius, 1887 Arrehnius, 1887 Acids: produce H + by dissociation in an aqueous soln. Acids: produce H + by dissociation in an aqueous soln. Bases: produce OH - by dissociation in an aqueous soln. Bases: produce OH - by dissociation in an aqueous soln. Brǿnsted & Lowry, 1923 Brǿnsted & Lowry, 1923 Acids: donate H +. Acids: donate H +. Bases: accept H +. Bases: accept H +. Lewis, 1938 Lewis, 1938 Acids: aceept electron pairs Acids: aceept electron pairs Bases: donate electron pairs Bases: donate electron pairs

6-2. Examples & Amphiprotic (Ampholytes) 6-2. Examples & Amphiprotic (Ampholytes) Acids & conjugate bases (or vice versa) Acids & conjugate bases (or vice versa) HCO 3 - = H + + CO 3 2- HCO 3 - = H + + CO 3 2- Also Also HCO H + = H 2 CO 3 HCO H + = H 2 CO 3

6-3. Strong vs. Weak Acids 6-3. Strong vs. Weak Acids Called upon the extension of dissociation Called upon the extension of dissociation Strong acids: HCl, HNO 3, H 2 SO 4, H 3 PO 4 Strong acids: HCl, HNO 3, H 2 SO 4, H 3 PO 4 Weak acids: Acetic acids, HF, H 2 CO 3 Weak acids: Acetic acids, HF, H 2 CO 3

6-4. Humic/Fulvic Acids 6-4. Humic/Fulvic Acids Non-humic substances Non-humic substances Organic compounds having definite physical and chemical characteristics Organic compounds having definite physical and chemical characteristics Proteins, aldehydes, carbohydrates, amino acids Proteins, aldehydes, carbohydrates, amino acids (easily) Biodegradable (easily) Biodegradable Humic substances: biologically refractive Humic substances: biologically refractive Acidic, dark colored, aromatic, MW 100-more than a few 1,000 Acidic, dark colored, aromatic, MW 100-more than a few 1,000 Fulvic acids: soluble in both acids and bases, lowest MW masterial in humic substances Fulvic acids: soluble in both acids and bases, lowest MW masterial in humic substances Humic acids: soluble only in basic solutions Humic acids: soluble only in basic solutions Humin: insoluble in either acidic or basic solutions Humin: insoluble in either acidic or basic solutions

6-5. pH 6-5. pH Definition: pH = -log 10 a H+ Definition: pH = -log 10 a H+ Significance: Controls the following processes Significance: Controls the following processes Dissolution and precipitation of most minerals Dissolution and precipitation of most minerals Acid-base equilibria Acid-base equilibria Adsorption and desorption Adsorption and desorption Biologically mediated process Biologically mediated process Redox reactions Redox reactions Show a few example reactions Show a few example reactions See Fig. 5.1 on p.151 for pH probe See Fig. 5.1 on p.151 for pH probe

6-6. Carbonic Acids 6-6. Carbonic Acids Carbon dioxide equilibria Carbon dioxide equilibria Dissociation of carbonic acids Dissociation of carbonic acids See p , eqns (5.12) –(5.26) See p , eqns (5.12) –(5.26) Can you draw Fig.5.2 on p.156 ? Can you draw Fig.5.2 on p.156 ?

6-7. pH of Water in Equil. w/ Various P CO pH of Water in Equil. w/ Various P CO2 Refer eqn. (5.27) on p.158. Refer eqn. (5.27) on p.158. Controls on P CO2 Controls on P CO2 See Table 5.3 on p.157 See Table 5.3 on p.157 Respiration coefficient (RC) Respiration coefficient (RC) RC=(CO 2 produced/O 2 consumed) RC=(CO 2 produced/O 2 consumed)

6-8. Acidity 6-8. Acidity Definition: Capacity of water to produce (or donate) proton Definition: Capacity of water to produce (or donate) proton Causes: Causes: Acids: HSO 4 - = H + + SO 4 2- Acids: HSO 4 - = H + + SO 4 2- Salts of strong acids and weak bases: NH 4 Cl + H 2 O = NH 4 OH + H + + Cl - Salts of strong acids and weak bases: NH 4 Cl + H 2 O = NH 4 OH + H + + Cl - Hydrolysis of metals: Al 3+ + H 2 O = AlOH 2+ + H + Hydrolysis of metals: Al 3+ + H 2 O = AlOH 2+ + H + Oxidation & Hydrolysis: Fe H 2 O O 2 = Fe(OH) 3 + 2H + Oxidation & Hydrolysis: Fe H 2 O O 2 = Fe(OH) 3 + 2H +

Significance Significance Attacking geological material Attacking geological material Increase solubilities of (hazardous) metals Increase solubilities of (hazardous) metals Limit water resources usage Limit water resources usage Measurement Measurement Titration by 0.02N NaOH (EPA) or N NaOH (USGS) Titration by 0.02N NaOH (EPA) or N NaOH (USGS) End points: pH = 8.3 End points: pH = 8.3 Reports as Reports as mg/L H + mg/L H + meq/L H + meq/L H + mg/L CaCO3 mg/L CaCO3 mg/L H2SO4 mg/L H2SO4

6-9. Alkalinity 6-9. Alkalinity Definition: Capacity of water to consume (or accept) proton Definition: Capacity of water to consume (or accept) proton Causes: Causes: Cartbonate alkalinity = mHCO mCO 3 2- Cartbonate alkalinity = mHCO mCO 3 2- Caustic alkalinity = mOH - Caustic alkalinity = mOH - Other alkalinities: NH 3, silicate, borate, etc. Other alkalinities: NH 3, silicate, borate, etc. Total alkalinity=sum of all threes above Total alkalinity=sum of all threes above

Significance Significance Indicate the tolerance (buffer capacity) of s system to the acid impact Indicate the tolerance (buffer capacity) of s system to the acid impact Measurement Measurement Titration by 0.02N HCl or H 2 SO 4 Titration by 0.02N HCl or H 2 SO 4 End points: pH = 4.5 (actually it depends on C T ) End points: pH = 4.5 (actually it depends on C T ) Reports as Reports as mg/L CaCO3 mg/L CaCO3

A weak monoprotic acid A weak monoprotic acid HA = H + + A - HA = H + + A - K = [H + ] [A - ]/[HA] K = [H + ] [A - ]/[HA] C = [HA] + [A - ] C = [HA] + [A - ]  o = [HA]/C = [H + ]/(K+ [H + ])  o = [HA]/C = [H + ]/(K+ [H + ])  1 = [A - ] /C = K/(K+ [H + ])  1 = [A - ] /C = K/(K+ [H + ]) Titration with NaOH Titration with NaOH Charge balance: [H + ] + [Na + ] = [A - ] + [OH - ] Charge balance: [H + ] + [Na + ] = [A - ] + [OH - ] C B = [A - ] + [OH - ] - [H + ] C B = [A - ] + [OH - ] - [H + ]  = dC B /dpH = d[A - ]/dpH + d[OH - ]/dpH - d[H + ]/dpH  = dC B /dpH = d[A - ]/dpH + d[OH - ]/dpH - d[H + ]/dpH = Cd  1 /dpH + d[OH - ]/dpH - d[H + ]/dpH = Cd  1 /dpH + d[OH - ]/dpH - d[H + ]/dpH where where Cd  1 /dpH = 2.3C K[H + ]/(K+ [H + ]) 2 = 2.3  o  1 C Cd  1 /dpH = 2.3C K[H + ]/(K+ [H + ]) 2 = 2.3  o  1 C d[OH - ]/dpH = 2.3[OH - ] d[OH - ]/dpH = 2.3[OH - ] -d[H + ]/dpH=-2.3[H + ] -d[H + ]/dpH=-2.3[H + ]  = 2.3([H + ] + [OH - ] +  o  1 C)  = 2.3([H + ] + [OH - ] +  o  1 C)

For a number of monoprotic acids For a number of monoprotic acids  = 2.3([H + ] + [OH - ] +  1o  11 C 1 +  2o  21 C 2 +  3o  31 C 3 +  4o  41 C )  = 2.3([H + ] + [OH - ] +  1o  11 C 1 +  2o  21 C 2 +  3o  31 C 3 +  4o  41 C ) =  water  HA1  HA2  HA3  HA4  =  water  HA1  HA2  HA3  HA4  For a polyprotic acid For a polyprotic acid  =  water  HnA  Hn-1A  Hn-1A   =  water  HnA  Hn-1A  Hn-1A  For a mineral For a mineral 2KAl 3 Si 3 O 10 (OH) 2 + 2H + = 3Al 2 Si 2 O 5 (OH) 4 + 2K + 2KAl 3 Si 3 O 10 (OH) 2 + 2H + = 3Al 2 Si 2 O 5 (OH) 4 + 2K +

For a mineral For a mineral Muscovite-kaolinite Muscovite-kaolinite 2KAl 3 Si 3 O 10 (OH) 2 + 2H + = 3Al 2 Si 2 O 5 (OH) 4 + 2K + 2KAl 3 Si 3 O 10 (OH) 2 + 2H + = 3Al 2 Si 2 O 5 (OH) 4 + 2K + K= ([K + ] / [H + ] ) 2 K= ([K + ] / [H + ] ) 2 Titrate with HCl Titrate with HCl Charge balance: [H + ] + [K + ] = [Cl - ] + [OH - ] Charge balance: [H + ] + [K + ] = [Cl - ] + [OH - ] C A = [Cl - ] = [H + ] + [K + ] - [OH - ] C A = [Cl - ] = [H + ] + [K + ] - [OH - ] = [H + ] + K/[H + ] 1/2 - [OH - ] = [H + ] + K/[H + ] 1/2 - [OH - ] Differentiate the above equation and change the sign  buffer capacity Differentiate the above equation and change the sign  buffer capacity See Fig on p. 186 See Fig on p. 186

Assignment Assignment P. 190: Problem 2, 3, 4, 8 P. 190: Problem 2, 3, 4, 8