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PKa concepts Ionization = the process in which ions are formed from neutral compounds; Dissociation = the separation of the ions of an electrovalent compound.

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Presentation on theme: "PKa concepts Ionization = the process in which ions are formed from neutral compounds; Dissociation = the separation of the ions of an electrovalent compound."— Presentation transcript:

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 pK a can be obtained simply by taking logarithms (to the base 10) of the previous equation: log 10 K a = log 10 [H + ] + log 10 [A - ] - log 10 [HA] Therefore -log 10 [H + ] = -log 10 K a + log 10 [A - ] - log 10 [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 pK a values, we are able to express the strength of an acid (i.e. its tendency to dissociate) with reference to the pH scale. If K a is large, then pK a will have a low numerical value. E.g., Hydrochloric acid, HCl has a pK a = -3 Acetic acid, CH 3 COOH has a pK a = 4.77 A strong acid is one which is largely, or completely, dissociated, and which therefore has a high K a value (and low pKa). A weak acid is one that is only slightly dissociated in solution, and has a low K a 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

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10 Acidity of various soil functional groups Some inorganic surface functional groups are more likely to deprotonate or dissociate than others pKa of Al(OH 2 ) + = ~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

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

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16 Colloid Negative charge Positive charge % constant% variable Humus20001090 Vermiculite1200955 Smectite1000955 Illite4008020 Kaolinite124595 Fe & Al Oxides 550100


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