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Buffers. Chem 12A Mrs. Kay. Buffers help maintain a constant pH. They are able to accept small quantities of acids and bases without drastically changing their pH A buffer is composed of a weak acid molecule in equilibrium with its conjugate base and hydrogen ion (H+). 

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buffers

Buffers

Chem 12A

Mrs. Kay

slide2
Buffers help maintain a constant pH.
  • They are able to accept small quantities of acids and bases without drastically changing their pH
  • A buffer is composed of a weak acid molecule in equilibrium with its conjugate base and hydrogen ion (H+). 
  • CH3COOH    < --- >    CH3COO-    +   H+
acidic buffer solution
Acidic buffer solution:
  • An acidic buffer solution is simply one which has a pH less than 7.
  • Acidic buffer solutions are commonly made from a weak acid and one of its salts - often a sodium salt.
  • Example: acetic acid and sodium acetate

CH3COOH and CH3COONa

alkaline buffer solutions
Alkaline buffer solutions
  • An alkaline buffer solution has a pH greater than 7.
  • Alkaline buffer solutions are commonly made from a weak base and one of its salts.
  • A frequently used example is a mixture of ammonia solution and ammonium chloride solution. NH3 and NH4Cl
how do buffers work
How do buffers work?
  • It has to contain things which will remove any hydrogen ions or hydroxide ions that you might add to it - otherwise the pH will change.
slide6
Below is a weak acid, if we added CH3COONa, its like adding CH3COO-

CH3COOH < -- > CH3COO- + H+

There will be a shift to the left

  • The solution will therefore contain these important things:
  • lots of un-ionised acetic acid;
  • lots of acetate ions from the sodium acetate;
  • enough hydrogen ions to make the solution acidic.

Other things (like water and sodium ions) which are present aren't important to the argument.

slide7
Look to below website for a flash on how buffers work:
  • http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/buffer12.swf
buffer capacity and ph
Buffer capacity and pH
  • Buffer capacity is the amount of acid/base the buffer can neutralize before the pH begins to drastically change.
  • The pH of the buffer depends on the Ka for the acid and on the relative concentrations of the acid/base of the buffer
calculations
Calculations

What is the pH of a buffer that is 0.12 M in lactic acid (HC3H5O3) and 0.10 M of sodium lactate? For lactic acid, Ka= 1.4 x 10-4

  • Determine the important species involved
  • Set up ice table
  • Set up equilibrium expression
  • Plug in concentrations at equilibrium
slide11
Ka = 1.4 x 10-4 =[C3H5O3 –][H+]

[HC3H5O3]

  • Because of the small Ka, we expect x to be small relative to 0.12 and 0.10 M, so we can simplify our equation to…
  • Ka = 1.4 x 10-4 =[0.10][x]

[0.12]

Solve for x, x =1.7 x 10-4

slide12
Finally, solve for the pH of the buffer

pH = - log [1.7 x 10-4] = 3.77

Notice, when you put the value of x into the original equation it makes no difference for your final answer.

addition of strong acids bases to buffers
Addition of strong acids/bases to buffers:
  • We assume that the strong acid or base added is consumed completely by the reaction with the buffer
  • HX < -- > H+ + X-
  • By adding a strong acid, the shift is to the left, therefore increasing [HX] and decreasing [X-]
  • By adding a strong base, the OH- is used up by HX to produce X-, so [X-] increases and [HX] decreases.
calculation
Calculation
  • A buffer is made by adding 0.300 mol of HC2H3O2 and 0.300 mol of NaC2H3O2 to 1.0 L of water. The pH of the buffer is 4.74. Calculate the pH of this solution if 0.020 mol of NaOH is added (ignore volume changes)
slide16
Ka = (x)(0.320) / (0.280) = 1.8 x 10-5 so x = [H3O+]= (0.280)(1.8 x 10-5 )/0.320 = 1.6 x 10-5
  • pH = - log (1.6 x 10-5)= 4.80