If oh ions are added to the buffered solution they

This preview shows page 7 - 9 out of 11 pages.

IfOH-ions areaddedto the buffered solution, they reactwiththe acidcomponentof the buffer:[4]This reaction results in a slight decreaseinthe[HX]anda slight increaseinthe [X-J, aslongas theamountsof HXandX-inthe buffer are large com-paredto theamountof theaddedOH-.In that case, the ratio [HX]l[X-]doesn'tchange much,andthusthe change in thepHis smalLIfHtions areaddedto the buffered solution,theyreactwiththebasecomponentof the buffer:H'(ll'l),X-(Il'll--;.HX(aq)[5]This reaction causes a slight decrease in the [X-janda slight increase in the[HX]. Once again, as long as the change in the ratio[HXli[X~]is small, thechangeinthepH\\'illbesmalLBuffers resist changesinpHmost effectiwlywhenthe concentrations ofthe conjugate acid-base pair, HXandX,areaboutthe same.Wesee fromex-aminingEquation[3]thatunderthese conditions their ratio is close to one,andthusthe[H+]is approximately equal to. For this reason~vetryto se-lect a bufferwhoseacid formhasapKaclose to the desiredpH.Becauseweare interested inpH,let's take thenegath'elogarithmofbothsides of Equation[3]andobtain[HX]-logKa10gTx~fBecause-log[H']=pHand-log:Ka:=pKiUwehavepH=pKlandmakinguse of the properties of logarithms (seeAppendixB)wehave[6]andingeneral[conjugate base]pH=pK"-'-[7][weak acid]This relationshipisknownas theHClldersoN-Hllsselbalchequation.Biochemists,biologists, and others vvho frequentlywork'with buffers often use this equa-tionto calculate thepHof buffers.Whatmakes this equation particularly con-venient is thatwecan norntally neglect theamountsof the acidandbase ofthe buffer that ionize. Therefore,wecan use theinitial concentrationsof theacidandconjugate base components ofthebuffer directly in Equation[7].
264Experiment24Hydrolysisof SaltsandpHofBufferSolutionsEXA:\1PLE24.3vVhatisthepHof a buffer thatis0.120AIin lactic acid, HC3Hi03,and0.100AIin sodium lactate, '\JaC3H"O}Forlactic=LIx 10-4.SOLUTION:Because lactic acidisaweak acid, wewillassume that its initialconcentrationis0.120 Mandthat noneofit has dissociated.Wewillalso assumethat the lactate ion concentrationisthat of thesaltsodium lactate,0.100 M.Letxrepresenttilt'concentration in mol Lofthe lactic acid thilt dissociate. The initialand equilibrium concentrati,l11s i!woh'edinthis equilibrium areThe equilibrium concentrations arego\ernedbythe equilibrium expressionx(0.100~xlK"=1.40.120xBecauseK"issmall and the presence of aCOll"imOnion, wexto be smallrelative to0.12and0.10M.Thus, our equation can be simplified to giver(O100)1.-1x,10,~0.120Solvingforxa \'alue that justifiesourneglectingit:( O..20)"10~)1.7x[W]=\0.100(UpH=1.7x10~l3.77Alternati\'ely,wecould han:, used the Henderson-Hasselbalch equation to calcu-late thepHdirectl\':[00'(°·100\1M0.120!

Upload your study docs or become a

Course Hero member to access this document

Upload your study docs or become a

Course Hero member to access this document

End of preview. Want to read all 11 pages?

Upload your study docs or become a

Course Hero member to access this document

Term
Fall
Professor
N/A
Tags
pH, Buffer solutions

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture