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Argentometric Titration (Precipitation Titration)

Argentometric Titration (Precipitation Titration). Meeting 7. Analite + Titrant  precipitation  AgNO 3  ARGENTOMETRy. Principles: sedimentation of Ag(I) reaction. Diffuculties of titration: Hard to find the suitable indicators

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Argentometric Titration (Precipitation Titration)

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  1. Argentometric Titration (Precipitation Titration) Meeting 7

  2. Analite + Titrantprecipitation  AgNO3 ARGENTOMETRy Principles: sedimentation of Ag(I) reaction • 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

  3. Benefit : Determine Cl-, F-, Br- • Reaction: NaCl (aq) + AgNO3 (aq)  AgCl (s) + NaNO3 (aq) white Why is AgCl being precipitated?

  4. Solubility constant (KSp) AgCl (s) Ag+ (aq) + Cl- (aq) Ksp = [Ag+ ] [Cl-] ----- equillibrium Ksp > [Ag+ ] [Cl-] ----- become sediment

  5. 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

  6. Mohr Methods: Indicator  chromates • Ag+ + Cl- ==== AgCl (white crystal) • Ag + + CrO4= === AgCrO4 (red crystal surrounding AgCl surface)

  7. Volhard methods: Indicator: Fe (III)Ag+ + Cl- ==== AgCl (white crystal) Ag+ + SCN- AgSCN(s) Fe3+ + SCN- FeSCN2+(aq) End Point : white crystal in reddish solution

  8. Fajans Methods: Indicator fluororescense (Fl) Ag+ + Cl- AgCl(s) (AgCl).Ag-+ X- Primary layer secondair layer Cl- exceed E.P : HFl H+ + Fl- (AgCl).Ag+ Fl- (Pink precipitate)

  9. Penyelesaian : Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-  AgCl(s) + Na+(aq) + NO3-(aq) pCl- =-log [Cl-] = -log 10-1 = 1,00 [Ag+] = 0 a. Pada keadaan awal :

  10. 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

  11. 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

  12. 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

  13. e. Penambahan 60,0 mL (60,0 x 0,10) – (50 x 0,1) [ Ag+] = ——————————————— = 9,1 x 10-3 M 60,0 + 50,0 pAg = 2,04

  14. KURVA TITRASI

  15. Metode Mohr : Ag+(aq) + Cl- (aq) AgCl(s) Ksp = 1 x 10-10 2Ag+(aq) + CrO42-(aq) Ag2CrO4(s) Ksp = 2 x10-12 Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-  AgCl(s) + Na+(aq) + NO3-(aq) Berapa konsentrasi K2CrO4 yang diperlukan pada titik eakhir titrasi?

  16. 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

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