7/23/2019 Analytical Chemistry Notes Chapter 13

http://slidepdf.com/reader/full/analytical-chemistry-notes-chapter-13 1/2

Henry Su December 12, 2001

Chapter Notes Chemistry 221

Chapter 13 Dr. Rahni

Titrating Polyfunctional Acids and Bases

13A—Polyfunctional Acids-Phosphoric Aci is a typica! po!y"unctiona! aci

H3P#$ % H2#⇔ H2P#$- % H3#

%  & a1 ' (H3#%  ) (H2P#$

- ) ' *.11e-3

  (H3P#$ )

H2P#$- % H2#⇔ HP#$

-2 % H3#%  & a2 ' (H3#

%  ) (H2P#$-2  ) ' +.32e-

  (H2P#$- )

HP#$-2 % H2#⇔ P#$

-3 % H3#%  & a3 ' (H3#

% ) (P#$-3  ) ' $.e-13

  (HP#$-2)

& a1 & a2 & a3

13B—Describing Polyfunctional Bases

C#3-2 % H2#⇔ HC#3

- % #H-  &  b1 ' (HC#3- ) (#H- ) ' & /  ' 2.13e-$

  (C#3-2) & a2

HC#3- % H2#⇔ H2C#3 % #H-  &  b2 ' (H2C#3) (#H- ) ' & /  ' +.*e-$

  (HC#3-) & a2

13C—Finding the pH of Solutions of Aphiprotic Salts

-Amphiprotic sa!ts are "orme urin neutra!iation titration o" po!y"unctiona!

acis an bases

-An amphiprotic sa!t is a species that can act as an aci an as a base /hen

isso!e in a suitab!e so!ent

HA- % H2#⇔ A-2 % H3#% aciic4

HA- % H2#⇔ H2A % #H- basic4

(H3#%) ' & a1 & a24 appro5.4 e6 13-$4

75amp!e8 Ca!cu!ate the hyronium ion concentration o" a 0.100 9 NaHC#3 so!ution.:e "irst e5amine the assumptions !eain to 76uation 13-$. ;he issociation constants "or H2C#3 

are & a1 ' 1.e-$ an & a2 ' $.+<e-11.

(H3#%) ' 1.e-$ 5 $.+<e-114 ' .$e- 9

7/23/2019 Analytical Chemistry Notes Chapter 13

http://slidepdf.com/reader/full/analytical-chemistry-notes-chapter-13 2/2

13D—Constructing Titration Cur!es for Polyfunctional Acids

-Compouns /ith t/o or more aci "unctiona! roups yie! mu!tip!e enpoints in a

titration, proie the "unctiona! roups i""er su""icient!y in strenths as acis.

-=nc!ue in a titration cure "or a /ea> iprotic aci an stron base are the "o!!o/in81-=nitia! pH

-=nitia!!y, on!y the "irst issociation ma>es an appreciab!e contribution to

(H3#%)

2-?irst @u""er Reion

-;he aition o" minima! base resu!ts in the "ormation o" a bu""er

consistin o" the /ea> aci H29 an its conuate base H9-. ;he so!ution

can be treate as a simp!e bu""er system.3-?irst 76uia!ence Point

$-Secon @u""er Reion

-?urther aitions o" base to the so!ution create a ne/ bu""er system

consistin o" H9

-

 an 9

-2

.-Secon 76uia!ence Point

+-pH @eyon the Secon 76uia!ence Point-pH is etermine by the concentration o" the e5cess #H -

13"—Dra#ing Titration Cur!es for Polyfunctional Bases

-;he construction o" a titration cure "or a po!y"unctiona! base is pretty much the same

issue as /ith a po!y"unctiona! aci, on!y /ith a neatie s!ope, !i>e a 10B horionta!!y

"!ippe po!y"unctiona! aci titration cure.

13F—The Coposition of Polyprotic Acid Solutions as a Function of pH

-A!pha a!ues can be ca!cu!ate "or po!y"unctiona! acis an bases in the "o!!o/in

e6uation80' (H29)

  ct ct ' (H29) % (H9-) % (9-2)

1 ' (H9- )

  ct

2 ' (9 -2  )   0 % 1 % 2 ' 1

  ct  sum o" a!pha a!ues must e6ua! unity4

-Note that the enominator is the same "or each e5pression. Note a!so that the "ractiona!amount o" each species is "i5e at any pH an is independent  o" the tota! concentration c t.