Transcript Document

Meeting 7
Principles: sedimentation of Ag(I)
reaction
Analite + Titrant  precipitation

AgNO3  ARGENTOMETRy
Diffuculties of titration:



Hard to find the suitable indicators
In some cases (mainly in dilute solution ),
reaction rate usually slow
The composition of sediment sometimes can
not be determined
• Benefit : Determine Cl-, F-, Br• Reaction:
NaCl (aq) + AgNO3 (aq)  AgCl (s) +
NaNO3 (aq)
white
Solubility constant (KSp)
AgCl (s)
Ag+ (aq) + Cl- (aq)
Ksp = [Ag+ ] [Cl-] ----- equillibrium
Ksp > [Ag+ ] [Cl-] ----- become sediment
Indicators in Argentometric
titration
a. Formation of colorfull sediment: (Mohr
methods: pH 6-10)
b. Formation of colorfull complexs
(Volhard methods, acid pH)
c. Using adsoprtion indicators (Fajans
Methods
Mohr Methods: Indicator 
chromates
• Ag+ + Cl- ==== AgCl (white crystal)
• Ag + + CrO4= === AgCrO4 (red crystal
surrounding AgCl surface)
Volhard methods: Indicator: Fe (III)
Ag+ + Cl-
====
Ag+ + SCNFe3+ + SCN-
AgCl (white crystal)
AgSCN(s)
FeSCN2+(aq)
End Point : white crystal in reddish solution
Fajans Methods: Indicator fluororescense (Fl)
Ag+ + ClAgCl(s)
(AgCl).Ag-+
Primary
layer
X-
secondair
layer
HFl
(AgCl).Ag+
H+ +
Fl-
Cl- exceed
Fl-
E.P :
(Pink precipitate)
Penyelesaian :
Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-  AgCl(s) + Na+(aq) + NO3-(aq)
a. Pada keadaan awal :
pCl- = -log [Cl-]
= -log 10-1
= 1,00
[Ag+] = 0
b. Penambahan 10,0 mL AgNO3
jumlah mmol NaCl setelah penambahan AgNO3
[Cl-] =
————————————————————————
volume campuran
jumlah mmol NaCl awal – jumlah AgNO3 yg ditambahkan
[Cl-] =
—————————————————————————————
volume campuran
(50,0 x 0,10) – (10,0 x 0,1)
[Cl-] = ——————————————— = 0,067 M
50,0 + 10,0
pCl = 1,17
c. Penambahan 49,9 mL AgNO3
(50,0 x 0,10) – (49,9 x 0,1)
[Cl-] = ——————————————— = 1,0 x 10-4 M
50,0 + 49,9
pCl = 1,17
d. Pada penambahan 50,00 mL
AgCl(s)
Ag +(aq) + Cl-(aq)
[Ag+][Cl-] = Ksp , pAg + pCl = 10
[Ag+] = [Cl-]
[Cl-]2 = 1x 10-10
[Cl-] = 1 x 10-5
pCl- = 5,00
e. Penambahan 60,0 mL
(60,0 x 0,10) – (50 x 0,1)
[ Ag+] = ———————————————
60,0 + 50,0
pAg = 2,04
= 9,1 x 10-3 M
KURVA TITRASI
Metode Mohr :
Ag+(aq) + Cl- (aq)
Ksp = 1 x 10-10
2Ag (aq) + CrO4 (aq)
Ksp = 2 x10-12
AgCl(s)
Ag2CrO4(s)
Na+(aq) +
+ Cl-(aq) + Ag+(aq) 2+ NO3-  AgCl(s) + Na+(aq) + NO3-(aq)
Berapa konsentrasi K2CrO4 yang
diperlukan pada titik eakhir titrasi?
Konsentrasi K2CrO4 yang diperlukan pada
titik akhir titrasi:
Kelarutan Ag2CrO4 = 8,4 x 10-5 mol/L
Kelarutan AgCl = 1 x 10-5 mol/L
Pada titik ekivalen pAg = pCl = 5,0
[Ag2+]2 [CrO42-] = 2 x 10-12
2 x 10-12
[CrO42-] = ————— = 0,02M
(1 x 10-5 )2