Buffers A buffer is a mixture of chemicals that make a solution resist a change of pH pH remains relatively constant when adding an acid or base One kind of buffer system is a solution of a weak acid and one of its salts What is a weak acid? What qualifies as “one of its salts?”

Weak acids vs. Strong acids Weak acids DO NOT IONIZE completely HCl HBr HI HNO3 H2SO4 Strong acids have “large & very large” Ka values meaning they DO IONIZE completely It’s the free H+ (or OH-) that affects the pH

The classic example of a weak acid is acetic acid CH3COOH (HC2H3O2) What about its salt? The salt can be any of the following: NaCH3COO LiCH3COO KCH3COO Ca(CH3COO)2 The ca+ion is only a spectator All of these salts completely ionize in water so the buffer looks like this ….

CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH

Add in acid (H+) CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH

When you add in H+ it will bond to the acetate ion. H + + CH3COO-  CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH

Add in base (OH-) CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH

When you add in OH- it bonds with the CH3COOH HOH HOH HOH CH3COO- HOH CH3COO- OH- + CH3COOH  HOH + CH3COO- CH3COO- CH3COO- CH3COO- CH3COO- CH3COO- HOH CH3COO- HOH CH3COO- CH3COO- CH3COO- CH3COO- CH3COO- HOH CH3COO- CH3COO- HOH CH3COO-

Buffer Capacity There comes a point when the buffer is “used up” It can no longer take H+ (or OH-) ions out of solution and the pH drastically changes

Practice Write an equation to show the addition of an acid and a base to the following buffer systems NH4+ / NH3 CH3COOH / CH3COO- H2PO4- / HPO4-2

Practice Answers H+ + CH3COO-  CH3COOH OH- + CH3COOH  CH3COO- NH4+ / NH3 H+ + NH3  NH4+ OH- + NH4+  NH3 CH3COOH / CH3COO- H+ + CH3COO-  CH3COOH OH- + CH3COOH  CH3COO- H2PO4- / HPO4-2 H+ + HPO4-2  H2PO4- OH- + H2PO4-  HPO4-2

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