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pKa concepts

pKa concepts. Ionization = the process in which ions are formed from neutral compounds; Dissociation = the separation of the ions of an electrovalent compound as a result of the action of a solvent (usually water). For a weak acid, which dissociates as follows: HA ↔   H + + A -.

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pKa concepts

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  1. pKa concepts Ionization = the process in which ions are formed from neutral compounds; Dissociation = the separation of the ions of an electrovalent compound as a result of the action of a solvent (usually water)

  2. For a weak acid, which dissociates as follows: HA ↔   H+ + A-

  3. An interesting and extremely useful relationship between pH and pKa can be obtained simply by taking logarithms (to the base 10) of the previous equation: log10Ka = log10[H+] + log10[A- ] - log10[HA] Therefore              -log10[H+] = -log10Ka + log10[A-] - log10[HA] Note: log a – log b = log (a/b) giving the Henderson-Hasselbalch equation:     

  4. The most convenient form of this Henderson-Hasselbalch equation, is

  5. Using pKa and pH relationship • By using pKa values, we are able to express the strength of an acid (i.e. its tendency to dissociate) with reference to the pH scale. • If Ka is large, then pKa will have a low numerical value. E.g., Hydrochloric acid, HCl has a pKa = -3 Acetic acid, CH3COOH has a pKa = 4.77 • A strong acid is one which is largely, or completely, dissociated, and which therefore has a high Ka value (and low pKa). • A weak acid is one that is only slightly dissociated in solution, and has a low Ka value.

  6. if we consider the situation where the acid is one-half (50%) dissociated, or where [A-] = [HA] (that is 50% negatively charged and 50% uncharged) then, substituting in the Henderson-Hasselbalch Equation pH = pKa + log(A-/HA) pH = pKa + log(1) Therefore pH = pKa + 0 and                   pH = pKa

  7. How to use H-H equation • When pH = pKa, the charged and uncharged species have ~equal concentrations. • When pH > pKa, the ionized (charged) form is dominant, so there will be more negative sites, therefore, as pH increases, CEC increases • When pH < pKa, the un-ionized, uncharged form is dominant, so there will be fewer negative sites, thus, as pH decreases, CEC decreases (and AEC increases)

  8. Why care about pKa in soils? • CEC increases as pH increases Early studies showed soil CEC was constant from pH 2.5 – 5 At pH > 5 the CEC of soil increased, especially in soils containing organic matter or non 2:1 clays • Organic and inorganic components of soil have functional groups that dissociate at various pH’s, leaving them with a negative charge that can attract cations

  9. Acidity of various soil functional groups • Some inorganic surface functional groups are more likely to deprotonate or dissociate than others pKa of Al(OH2)+ = ~5, (Al-OH-Si)+0.5 = ~7, SiOH = ~9.5 2:1 silicate minerals have more Si-OH groups and contribute less to pH-dependent charge than 1:1 minerals and metal oxides • SOM contributes the most negative charge 85-90% of charge due to deprotonation of COOH and phenolic OH groups which have pKa’s of 4 – 6 and 9 - 11

  10. http://www.biologie.uni-hamburg.de/b-online/ge16/10.gif

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