Buffers

Buffers

A bufer solution contains a conjugate acid–base pair with both the acid and base in reasonable concentrations. The acidic component reacts with added strong bases. The basic component reacts with added strong acids. The operation o a bufer solution depends on the common ion efect, a special case o Le Chatelier’s Principle. When a solution o a wea electrol!te is altered b! adding one o its ions rom another source" the ioni#ation o the wea electrol!te is suppressed. This beha$ior is termed the common ion efect.

Weak Acids Plus Salts o Weak Acids

How do bufer solutions work? Acidic bufer solutions We%ll tae a mi&ture o ethanoic acid and sodium ethanoate as t!pical. 'thanoic acid is a wea acid" and the position o this e(uilibrium will be well to the let) *C+*,-+

a(/

 0 *+-

l/

C+*,-+1

a(/

0 *,-0

a(/

Adding sodium ethanoate to this adds lots o e&tra ethanoate ions. According to Le Chatelier%s Principle" that will tip the position o the e(uilibrium e$en urther to the let.  The solution will thereore thereore contain contain these important important things) •

lots o un1ionised ethanoic acid2

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lots o ethanoate ions rom the sodium ethanoate2

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enough h!drogen ions to mae the solution acidic.

-ther things lie water and sodium ions/ which are present aren%t important to the argument. 3olutions that contain a wea acid plus a salt o the wea acid are alwa!s less acidic than solutions that contain the same concentration o the wea acid alone.

 Adding  Adding an acid to this bufer bufer solution *C+*,-+

a(/

 0 *+-

C+*,-+1

l/

a(/

0 *,-0

a(/

 The bufer solution must remo$e most o the new h!drogen ions otherwise the p* would drop maredl!.  The addition o the h!drogen ions will cause the s!stem to be stress" stress" and it will shit let to alle$iate the stress. *!drogen ions combine with the ethanoate ions to mae ethanoic acid. This causes the re$erse reaction to predominate" with the acetate ions combining with the added protons to orm acetic acid. 3ince most o the added h!drogen ions * 0/ are remo$ed through the re$erse reaction" reaction" the p* will not change $er! much 1 but because o the e(uilibria e(uilibria in$ol$ed" it will all a little bit.

 Adding  Adding an alkali alkali to this bufer bufer solution Alaline solutions contain h!dro&ide ions and the bufer solution remo$es most o these ions b! their combination with the h!dronium ions present in the bufered s!stem according to) *,-0

a(/

 0 -*1

a(/

 + *+-

→

l/

 This causes a stress on the bufered s!stems e(uilibrium" a decrease in h!dronium h!dronium ion concentra concentration tion.. The s!stem s!stem will thereor thereore e shit shit to increase increase the h!droniu h!dronium m ion concentra concentration tion through the increased ioni#ation o the wea acid" *C +*,-+. 4ue to the release o h!drogen ions" through the orward reaction" the p* will not change $er! much 1 but because o the e(uilibria in$ol$ed" it will rise a little bit.

Bufer Capacity 5ufered s!stems can resist the addition o both h!dronium ions or h!dro&ide ions" but onl! as long as these additions do not e&ceed the bufer capacit!. A bufe buferred s!st s!stem em resis esists ts a p* chan change ge with with the the addi additi tion on o an acid acid thr through ough the the combination o the conjugate base anion o a wea acid/ with the added h!drogen ions rom the acid/. This remo$al o the added * 0 through the combination with the conjugate base base will will conti continue nue 67T8L all o the conjugat conjugate e base base is consum consumes" es" at which which point point an! an! 0 additional *  that is added will cause the p* to drop A bufered s!stem resists a p* change with the addition o h!dro&ide b! the ioni#ation o  more o the wea acid to replace an! o the lost h!dronium ion that combined with the added h!dro&ide. This replacement will continue until all o the wea acid is ioni#ed" and then an! more h!dro&ide h!dro&ide added will then be able to cuase the p* to rise. •

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