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Buffers

Buffers. What Are They?. Solutions that resist changes in pH with addition of small amounts of acid or base Require two species: an acid to react with OH - and a base to react with H + It is necessary that the acidic and basic species not consume one another through a neutralization reaction

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Buffers

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  1. Buffers

  2. What Are They? • Solutions that resist changes in pH with addition of small amounts of acid or base • Require two species: an acid to react with OH- and a base to react with H+ • It is necessary that the acidic and basic species not consume one another through a neutralization reaction • Usually acid-base conjugate pair • Can be prepared by mixing a weak acid or base with a salt of that acid or base

  3. How Does It Work? • Buffer Solution: HC2H3O2 and NaC2H3O2 HC2H3O2  H+ + C2H3O2- • Small amount of acid added shifts left  limits H+ thus limits pH • If small amount of base added  H+ reacts with OH- shifts equilibrium right to make more H+ thus resists pH change

  4. General Form • HX  H+ + X- • Ka = [H+][X-] [HX] • [H+] = Ka[HX] [X-]

  5. pH is determined by two factors: value of Ka and ratio of concentrations of conjugate acid-base pair • If the amounts of HX and X- are large compared to the amount of acid or base added, the pH doesn’t change much • Most effective where [HX] is about equal to the [X-] ; pH is about equal to the pKa

  6. Equation • Henderson-Hasselbalch Equation pH = pKa + log [base] [acid]

  7. Example • Calculate the pH of a solution formed from .10M formic acid and .20M potassium formate.

  8. Example • What must be the concentration of NH4Cl in a .10M solution of NH3 if the pH is to be 9.00?

  9. Example • Calculate the concentration of sodium formate that must be present in a .10M solution of formic acid to produce a pH of 3.80.

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