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How buffers work

Dr. M. Sasvári: Chemistry Lectures. How buffers work. Further reading : Ebbing Gen. Chem. Solution of a Weak Acid or Base with another Solute Buffers Acid-Base Titration Curves Solution of a Weak Acid or Base Polyprotic Acids Base-Ionization Equilibria. Starting pH is not so low

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How buffers work

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  1. Dr. M. Sasvári: Chemistry Lectures How buffers work • Further reading: Ebbing Gen. Chem. • Solution of a Weak Acid or Base with another Solute • Buffers • Acid-Base Titration Curves • Solution of a Weak Acid or Base • Polyprotic Acids • Base-Ionization Equilibria Chem Eq 6

  2. Starting pH is not so low • “Buffer effect” around half neutralization • equivalence point is basic Difference to strong acid: Titration curves of weak acids Chem Eq 6

  3. Titration curves of acetic acid pH 10 Equivalence point 7 5 0,5 1 Buffer effect meqv NaOH • Titration of a 0.1 M acetic acid (10 ml) with 0.1 N NaOH: NaOH excess Start: around pH 3 Buffer effect at partial neutralization Equivalence point in BASIC range Start Chem Eq 6

  4. Na-acetate A- + Na+ Salt HA A- + H+ Weak acid Buffers are made of 2 components: Buffers • a weak acid • and its conjugated base (salt) Common ion: [A-] Mainly from the salt [HA] will be the dominant form of the weak acid [H+] is low Ionization is inhibited Chem Eq 6

  5. [H+][A-] [HA] [HA] [HA] [A-] [A-] [H+]= Ka pH=pKa - log Ka = [acid] pH=pKa - log [HA]=[A-] Common ion: [salt] [salt] pH=pKa + log [acid] Buffers: Calculations Henderson-Hasselbalch equation: Chem Eq 6

  6. 1 pH = pKa- 1 = 3.7 = 10 [Salt] [Salt] [Salt] 1 4.7 pH = pKa= 4.7 = [Acid] [Acid] [Acid] 1 10 pH = pKa+ 1 = 5.7 = 1 1:1 10:1 1:10 more salt more acid How Buffers work? Range of pH stabilization (acetic acid/Na-acetate buffer): A buffer stabilizes pH around its pKa value +1 Chem Eq 6

  7. Na-acetate A- + Na+ Salt HA A- + H+ Weak acid [salt - x] pH=pKa + log [acid + x] Principle of maintaining constant pH Acid is added to the buffer: H+ x meqv [Acid]+x [Salt] - x Calculation: pH will slightly decrease Chem Eq 6

  8. How Buffers work? A- HA H+ HA A- HA pH<pKa pH=pKa pH=pKa A- The buffer effect is based on the equilibrium of the weak electrolyte EQ Increasing the [H+] destroys the EQ INI Compensation: The system reacts with the H+ New EQ Chem Eq 6

  9. Na-acetate A- + Na+ Salt HA A- + H+ Weak acid H2O [salt + x] pH=pKa + log [acid - x] Principle of maintaining constant pH Base is added to the buffer: OH- x meqv [Acid]-x [Salt] + x Calculation: pH will slightly increase Chem Eq 6

  10. pH - 1 =pKa + log [salt + x] pH + 1=pKa + log [acid - x] [salt - x] [acid + x] Buffer Capacity Acid capacity: moles of H+ added to 1 L buffer,causing pH = -1 where x = the acid capacity (moles) Base capacity: moles of OH- added to 1 L buffer, causing pH = +1 where x = the base capacity (moles) Chem Eq 6

  11. more acid more salt 1:1 acid capacity B.C. base capacity a.c. the concentration and the ratio of the components Buffer capacity depends on: if more salt is present (pH is above pKa), acid capacity is bigger (the salt will react with the H+) at 1:1 ratio acid and base capacities are equal if more acid is present, (pH is below pKa), base capacity is bigger (the acid will react with the OH-) Chem Eq 6

  12. Starting pH is • “Buffer effect” around neutralization • equivalence point is Difference to strong base: Titration curves of weak bases not so high half acidic Chem Eq 6

  13. 7 Equivalence point Buffer effect Titration curves of ammonia • Titration of a 0.1 M NH4OH (50 ml) with 0.1 N HCl: Start: around pH 11 Buffer effect at partial neutralization Equivalence point in ACIDIC range Start HCl excess Chem Eq 6

  14. NH4+ NH4Cl + Cl- Salt NH3 NH4+ H+ + Weak base Calculation: [salt + x] pOH=pKb + log [base - x] Principle of maintaining constant pH Acid is added to the buffer: H+ x meqv [Base]-x [Salt] + x pH will slightly decrease Chem Eq 6

  15. NH4+ NH4Cl + Cl- Salt NH3 NH4+ H+ + Weak base OH- x meqv H2O Calculation: [salt -x] pOH=pKb + log [base + x] Principle of maintaining constant pH Base is added to the buffer: [Base]+x [Salt] - x pH will slightly increase Chem Eq 6

  16. Titration of polyprotic acid Example: Phosphoric acid (a triprotic acid) • Three steps of neutralization • Tree titration curves, superposed on each other • Three ranges of “Buffer effect” • Three equivalence point (third is difficult to reach) Difference to monoprotic acids: Chem Eq 6

  17. Titration curve of phosphoric acid Tree buffers Three neutralization steps pK1 pK2 pK3 Chem Eq 6

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