Transcript Ch. 3. KINETIC VS. EQUILIBRIUM MODELING

Ch. 6. ACIDS & BASES
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6-1. Definitions
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Alchemist’s
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Arrehnius, 1887
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Acids: produce H+ by dissociation in an aqueous soln.
Bases: produce OH- by dissociation in an aqueous soln.
Brǿnsted & Lowry, 1923
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Acids: sour, release gases by reacting with metals, turn litmus paper
red
Bases: bitter, slippery, turn litmus paper blue, neutralize acids
Acids: donate H+.
Bases: accept H+.
Lewis, 1938
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Acids: aceept electron pairs
Bases: donate electron pairs
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6-2. Examples & Amphiprotic (Ampholytes)
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Acids & conjugate bases (or vice versa)
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HCO3- = H+ + CO32-
Also
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HCO3- + H+ = H2CO3
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6-3. Strong vs. Weak Acids
Called upon the extension of dissociation
 Strong acids: HCl, HNO3, H2SO4, H3PO4
 Weak acids: Acetic acids, HF, H2CO3
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6-4. Humic/Fulvic Acids
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Non-humic substances
Organic compounds having definite physical and chemical
characteristics
 Proteins, aldehydes, carbohydrates, amino acids
 (easily) Biodegradable
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Humic substances: biologically refractive
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
 Humic acids: soluble only in basic solutions
 Humin: insoluble in either acidic or basic solutions
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6-5. pH
Definition: pH = -log10aH+
 Significance: Controls the following processes
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Dissolution and precipitation of most minerals
 Acid-base equilibria
 Adsorption and desorption
 Biologically mediated process
 Redox reactions
 Show a few example reactions
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See Fig. 5.1 on p.151 for pH probe
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6-6. Carbonic Acids
Carbon dioxide equilibria
 Dissociation of carbonic acids
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See p.153-155, eqns (5.12) –(5.26)
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Can you draw Fig.5.2 on p.156 ?
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6-7. pH of Water in Equil. w/ Various PCO2
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Refer eqn. (5.27) on p.158.
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Controls on PCO2
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See Table 5.3 on p.157
Respiration coefficient (RC)
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RC=(CO2 produced/O2 consumed)
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6-8. Acidity
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Definition: Capacity of water to produce (or
donate) proton
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Causes:
Acids: HSO4- = H+ + SO42 Salts of strong acids and weak bases: NH4Cl + H2O =
NH4OH + H+ + Cl Hydrolysis of metals: Al3+ + H2O = AlOH2+ + H+
 Oxidation & Hydrolysis: Fe2+ +2.5H2O + 0.25O2 =
Fe(OH)3 + 2H+
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Significance
Attacking geological material
 Increase solubilities of (hazardous) metals
 Limit water resources usage
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Measurement
Titration by 0.02N NaOH (EPA) or 0.0248N NaOH (USGS)
 End points: pH = 8.3
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Reports as
mg/L H+
 meq/L H+
 mg/L CaCO3
 mg/L H2SO4
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6-9. Alkalinity
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Definition: Capacity of water to consume (or
accept) proton
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Causes:
Cartbonate alkalinity = mHCO3- + 2mCO32 Caustic alkalinity = mOH Other alkalinities: NH3, silicate, borate, etc.
 Total alkalinity=sum of all threes above
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Significance
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Indicate the tolerance (buffer capacity) of s system to
the acid impact
Measurement
Titration by 0.02N HCl or H2SO4
 End points: pH = 4.5 (actually it depends on CT)
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Reports as
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mg/L CaCO3
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6-10. Buffer Capacity
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Definition: Amount of base to change unit pH
𝑑𝐶𝐵
𝑑𝑝𝐻
=
𝑑𝐶𝐴
−
𝑑𝑝𝐻
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𝛽=
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Significance –Somewhat similar to alkalinity, and
additionally
Help understand the reactions controlling the pH
 Can be used to understand the evolution of pH and CO2
 mineral alteration esp. during diagenesis?
 Applied to our thinking of the buffering of the
environmental system w/r the conc. of other substances
 Buffer solutions
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Buffer capacity of water
Think of titration of pure water with NaOH
 Charge balance: [H+] + [Na+] = [OH-]
 CB = [Na+]
 Thus,
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CB = [OH-] - [H+] = Kw/ [H+] - [H+]
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𝑑𝐶𝐵 = −
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By the way, pH = -log10 [H+] = -ln [H+]/2.303
dpH = -1/(2.303 [H+]) d[H+]
d CB/dpH = b = 2.3(Kw/ [H+] + [H+]) = 2.3([OH-]+[H+])
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𝐾𝑤
[𝐻 + ]2
− 1 𝑑[𝐻 + ]
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A weak monoprotic acid
HA = H+ + A K = [H+] [A-]/[HA]
 C = [HA] + [A-]
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ao = [HA]/C = [H+]/(K+ [H+])
a1 = [A-] /C = K/(K+ [H+])
Titration with NaOH
 Charge balance: [H+] + [Na+] = [A-] + [OH-]
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CB = [A-] + [OH-] - [H+]
b = dCB/dpH = d[A-]/dpH + d[OH-]/dpH - d[H+]/dpH
= Cda1/dpH + d[OH-]/dpH - d[H+]/dpH
where
 Cda1/dpH = 2.3C K[H+]/(K+ [H+])2 = 2.3 aoa1C
 d[OH-]/dpH = 2.3[OH-]
 -d[H+]/dpH=-2.3[H+]
b = 2.3([H+] + [OH-] + aoa1C)
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For a number of monoprotic acids
b = 2.3([H+] + [OH-] + a1oa11C1 + a2oa21C2 + a3oa31C3
+ a4oa41C4 + . . . . . .)
 = bwater + bHA1 + bHA2 + bHA3 + bHA4 + . . . . . .
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For a polyprotic acid
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b = bwater + bHnA + bHn-1A + bHn-1A + . . . . . .
For a mineral
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2KAl3Si3O10(OH) 2 + 2H+ = 3Al2Si2O5(OH) 4 + 2K +
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For a mineral
Muscovite-kaolinite
 2KAl3Si3O10(OH) 2 + 2H+ = 3Al2Si2O5(OH) 4 + 2K +
 K= ([K+] / [H+] )2
 Titrate with HCl
 Charge balance: [H+] + [K+] = [Cl-] + [OH-]
 CA = [Cl-] = [H+] + [K+] - [OH-]
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= [H+] + K/[H+]1/2 - [OH-]
 Differentiate the above equation and change the sign 
buffer capacity
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See Fig. 5-11 on p. 186
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Assignment
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P. 190: Problem 2, 3, 4, 8