AQUEOUS SOLUTION GEOCHEMISTRY

• Acid = substance containing hydrogen which gives free hydrogen (H⁺) when dissolved in water

• Base = substance containing the OH group that yields free (OH^-) when dissolved in water

• An acid solution is one containing an excess of free H⁺, and a base is one containing excess of free OH^-. A reaction between an acid and a base is usually called neutralization.
  For example:

• HCl (acid) + NaOH (base) ==> H₂O + NaCl
  which are dissociated into ions:
  H⁺ + Cl^- + Na⁺ + OH^- ==> H₂O + Na⁺ + Cl^-
• i.e. Na⁺ and Cl^- are unaffected.

• pH = inverse log of the concentration (activity) of free H⁺, or pH = -log [H⁺]

• Water dissociates into H⁺ and OH^-;
• the dissociation constant is: K_water = [H⁺] [OH^-] =10^-14

• So there has to be 10^-7 moles each of H⁺ and OH^- in a kilogram of neutral solution at standard temperature of 25°C. One mole is 6.023 x 10²³ atoms (or molecules) and H₂O has a molecular weight of 18 grams per mole. One kilogram of water has about 1000/18 = 55.6 moles of water or about 3.35 x 10²⁵ atoms of oxygen and about twice that number (6.7 x 10²⁵ atoms) of H⁺ (the amount of free H⁺ or free OH^- is relatively small compared to the amount of undissociated H₂O).

• pH ranges at 25°C from 0 to 14; pH < 7 = acidic solution; pH > 7 = basic solution. If HCl or another acid is added then pH decreases; if NaOH or another base is added then pH increases.

• pH increases as carbonic acid (a weak acid) dissociates: When carbon dioxide combines with water, such as what happens in the atmosphere when fossil fuels are burned, carbonic acid is formed: H₂O + CO₂ ==> H₂CO₃. Free H⁺ are made available during successive dissociations:

• H₂CO₃ ==> H⁺ + HCO^3- carbonic acid to bicarbonate, occurs at pH ~6.4
• HCO₃ ==> H⁺ + CO₃^2- bicarbonate to carbonate, occurs at pH ~10.3

Remember, free H⁺ is available only when acidic, or when pH < ~7. The dissociation of bicarbonate to carbonate occurs when there is too much OH^- in the system and H⁺ is "released" to balance out the base.

• Dissolved Cations and Anions in Water

  Cations = electron donors, positively charged: Na⁺, K⁺, Mg⁺⁺, Ca⁺⁺, Fe⁺⁺ or Fe⁺⁺⁺, Mn⁺⁺, Al⁺⁺⁺

Anions = electron acceptors, neg. charged: Cl^-, F^-, I^-, Br^-, SO₄^--, CO₃^--, HCO₃^-, NO₃^--, NO₂^-

Metals = act like cations mostly: Cu, Zn, Pb, Co, Ni, Cr, As, Se, Mo, etc.

• Water Analyses - Need to have cation-anion balance

millequivalent (MEQ) = mole equivalent charge or anion or cation, measure of total charge due to the ion in question dissolved in the solution. Start with concentration, divide by mole wt., multiply by charge: XX mg/L / MW x CHG = MEQ

Example: NaCl in solution, Na = 50 mg/L (50 ppm): 50/23 x 1 = 2.17 MEQ

Cl = 77 mg/L (77 ppm): 77/35.5 x -1 = -2.17 MEQ

So, if the total cation and anion MEQ’s are not balanced, some error exists in the analysis.

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