【正文】 gm o l e s o f B Hx 7 . 6 0 = 8 . 0 7 5 + l o g0 . 0 6 3 5 x x = 0 .0 1 5 9 m o l?Example T his m a ny m ol e s of N a O H i s c onta i ne d in 9 m ol 31 6 m ol / LmL?Preparing a buffer in real life see p 194! Buffer capacity: is a measure of how well a solution resists changes in pH when strong acid or base is added badc dcdp H dp H? ??＝ Buffers (continued) Where ca and cb are the number of strong acid and strong base per liter needed to produce a unit change in pH. The greater the buffer capacity, the more resistant the solution is to pH change. 22: [ ] [ ] [ ] [ ][ ] [ ] ( 1 )[ ] [ ]1 ( 2 )[ ] [ ] ( [ ] )waM O HaM O H w aaCB E H O H B MK c Kc M HH H Kd c K c Kd H H H K? ? ? ?????? ? ?? ? ?? ? ? ? ??? ? ? ??? Buffer: cHB mol/L HB + CB mol/L NaB, c= cHB+ CB ? Adding cMOH strong base MOH Buffers (continued) 1l og l n ( 3 )11 ( 4)[ ] [] ( 5)[ ] [ ] [ ] [ ]HM O H M O H M O HpH H HdpHHdH Hdc dc dcdHHd pH d H d pH d HH??????? ??????? ? ? ?? ? ? ?? ? ? ???? ? ??????????? ? ? ? ?????2 ( 6)[ ] ( [ ] ) ( 7)awaHBH O Hc K HKH H K? ? ??????????? ?????? ? ? Buffers (continued) 3( [ ] ) 0[ ] ( [ ] )[ ] , or , is m a xi m umaaHBaaac K H Kdd H H KH K pH pK???????? ? ????HBHBHB2 . 3 0 3 c= 0 . 5 8 c4S o , th e h ig h e r c , th e b ig g e r ??? Buffers (continued) 在什么情況下，緩沖溶液的緩沖容量最大呢 So the bigger the conc. of the buffer solution, and the ratio of [HB]/[B] is more approach 1:1, the higher the buffer index th e r a tio o f H B a n d B is n o t 1 : 1 ,Su p p o s e [ H B ] : [ B ] 1 : 1 0If? Buffers (continued) 2[]2 . 3 0 3( [ ] )aHBac K HHK???? ?2 . 3 0 3 HB Bc ???[ HB ] [ B ] 03 c cc??1 1 02 . 3 0 31 1 1 10 . 1 9cc? ? ? ?? Buffers (continued) In choosing a buffer, seek one whose pKa is as close as possible to the desired pH. The useful pH range of a buffer is usually considered to be pKa177。 1 pH unit. Outside this range, there is not enough of either the weak acid or the weak base to react with added base or acid. Buffer capacity can be increased by increasing the concentration of the buffer. Indicator p239 For example: methyl orange (MO) 甲基橙 Indicator： A weak acid or weak base whose various protonated species have different colors. T r a n s i t i o n r a n g e :? ? ? ?? ?H I na A c i di c f or m HI n a nd b a si c f or m I n i n a n e qui l i br i um H I n H I n whe n 10 , pH=pK pH+l og H I naInInH I nH I nK?????????????? ? ? ?? ? ? ?????????? ?? ?H I nH I nba si c c ol ora c i di c =p 1 , 1 , pH =p 1 , 10 1 , pH =c ol ort he or e t i c a l t r a ns i t i on p oi np taInaInaKKK???????????????? ? ??? Indicator Transition range (變色范圍 ) p H = p 1aK ?Factors affecting the transition interval ? Concentration of the indicator ? Mixing indicator ? Ionic strength ? Temperature Indicator(continued) The concentration of twocolor indicator has no effect on the transition interval, but for monocolor indicator, its conc. does affect the transition interval. ? ?? ?00a a a11aaH I nT w o c o l o r : p H = p K l o g p K l g p K l o gH I nM o n o c o l o r : p H = p K l o g p K l gccIncaaIn??????? ? ? ? ??????? ? ?????c i s t h e t o t a l c o n c e n t r a t i o n o f i n d i c a t o r , a i s t h e m i n i m u m c o n c e n t r a t i o n w e c a n o b s e r v e t h e c o l o r c h a n g e . Indicator(continued) ? Two indicators which has similar Ka values (two color plemented) ? One indicator + one dye (use as a color background) aa a c id c o l o r b a sic c o l o rB r o m o c r e so l g r e e n y e l l o w b l u e( B CG , p K = 4 .9 )M e th y l r e d( M O , p K = 5 .2 ) r e d y e l l o wB CGa + M O( p K = 5 .1 ) o r a n g e g r e e n Indicator(continued) Mixing indicator Principle of acidbase titration Titrating a strong acid with a strong base Titration reaction Equilibrium constant 2H O H H O＋ －＋ ＝1 4 . 0 01 / 1 0wtKK??For each type of titration studied, our goal is to construct a graph showing how the pH changes as titrant is added. The first step in each case is to write the chemical reaction, then use that reaction to calculate position and pH after each addition of titrant. Because the equilibrium constant for this reaction is 1014, it is fair to say that ―it goes to pletion. Any amount of H+ added will consume a stoichiometric amount of OH There are four regions of the titration curve that represent different kinds of calculations ? Before titration, pH is determined by the initial concentration of HCl ? Before reaching the SP, the pH is determined by excess H+ in the solution ? At the SP, H+ is just sufficient to react with all the OH to make H2O. The pH is determined by the dissociation of water ? After the SP, pH is determined by the excess OH in the solution. Principle of acidbase titration Fraction F Titrated Strong acid Present Equation F=0 HX [H+]