1. ARGENTOMETRY Titration with AgNO3 standard solution
SMK Negeri 13 Bandung

2. In the name of Allah, the precious, the merciful
May God give us knowledge and bless us wishful
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3. Objective After this chapter you are expected to be able to:
explain the basic principles of argentometric titration
explain how the indicators work in argentometric titration
explain conditioning and the direction
conduct the argentometric titration
arrange the report of argentometric titration
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4. Titrimetric Precipitation Method
The titration method which its reaction produces a difficult soluble compound
REMEMBER:
Ksp, Hkk, saturate, not saturate, oversaturated
substitution
“Weak Compound”, acid, base, salt which difficult to ionizations
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5. Prerequisite
Before beginning this Chapter, you are expected to comprehend:
Ksp and solubility product
Saturated, unsaturated, super saturated
Substitution Reactions
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6. Substitution Reactions
The reaction between ions from acid, base and salt, produce acid, base or another less soluble salt (precipitated easier) than reagents
Na2SO4 + BaCl2  BaSO4 + 2 NaCl
(aq) (aq) (s) (aq)
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7. Determination Argentometric usually involved:
Halides ion ( X: Cl-, Br-, I-)
Cyanide CN-
Thiocyanate SCN-
Because of the silver ion with the ions mentioned above, form a difficult soluble compound.
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8. Determination of End Point
Conventional Method
1. Formation of a coloured precipitate
2. Formation of a soluble coloured compound
3. Changed of colour because precipitate absorbs indicator
(Use of absorption indicators)
Instrumental Method
4. Determination of Turbidity/turbidity
(Turbidity Method)
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9. Substitution Indicators
Salt reacting with AgNO3 to form precipitate
Precipitate colour formed differs from main precipitate colour
Solubility of Ag precipitate - indicator > solubility of Ag-X ( main precipitate)
Ksp precipitate of sufficiently small so that precipitate suddenly formed at excess a few AgNO3
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10. The Mohr’s method
Indicator : CrO4= ion (chromate) as K2CrO4 saturated solution or 5 %
Condition : pH neutral
Conditioner : powder MgO ( if sample solution is acid)
Reaction : Cl- + Ag+  AgCl (titration)
(aq) (aq) (s) white
CrO4= + 2Ag+  Ag2CrO4 (TA)
(aq) (aq) (s) sorrel
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11. If the former precipitate ion concentration is very small
Hence “ precipitate” formed only from dissolved compound that is having colour ( Ksp has not been gone beyond)
Seen by our eye as discoloration of main precipitate suspension
The factualend point = discoloration of precipitate suspension from white become yellowiest
The theoretical end point = the forming of sorrel precipitate
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12. Adsorption Indicators
Organic acid or base that is adsorbed by the precipitate at the equivalence point
During the adsorption process, a change in indicator leads to color change of the adsorbed indicator. (we see it as the precipitate’s color change)
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13. Condition of Adsorption indicator
The precipitate should be at colloidal condition, the solution should not be too dilute
The indicator ion must be opposite charge to the ion of the precipitating agent
The indicator ion should not be adsorbed before the particular compound has been completely precipitated, but immediately after the end point
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14. Reactions Secondary adsorption
Cl- + Ag+  AgCl (Beginning of titration – The (s) white equivalence point)
Primary adsorption
AgCl + Ag+  (AgCl.Ag)+ (excess a few Ag+)
(s) white
Secondary adsorption
AgCl.Ag+ + Fl-  (AgCl.Ag.Fl) (s) pink(end point)
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15. Fajans’s Method Adsorption indicator : Fluorescent
Condition : a few alkaline pH
If the condition is too acid HFl difficult to be ionized, causing cannot be formed Fl-, as a result will not happened adsorption of indicator
If condition is too alkaline, Ag+ will precipitate as AgOH
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16. Some Adsorption indicators
Analit
Waran
Conditions
Solution (free indicator)
End at
end point
Fluorescent
Cl-, Br-, I-, SCN-
Green-yellow
pink
Neutral, a few alkaline
Dicloro®fluorescent
red
pH 4,4 – 7
Eosin
Br-, I-, SCN-
Pink
purple-red
pH 1 – 2, acetic acid
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17. Volhard’s Method
Indicator : Ion Fe3+ as Fe(NO3)3 atau Fe2(SO4)3.(NH4)2SO4 saturated
Condition : Acid
If the condition is neutral or alkaline, hence’ happened reaction:
Ag+ + OH-  AgOH
(s) white
Fe3+ + 3OH-  Fe(OH)3
(s) brown
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18. Reactions Standard applied : KSCN or NH4SCN
At determination [AgNO3] or [ KSCN ]
Titration
Ag+ + SCNˉ  AgSCN
(S) white
At the end point
Fe3+ + SCNˉ  FeSCN until Fe(SCN)63-
(orange to red blood)
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19. Determination of Cl- (orange to red blood) Cl- + Ag+  AgCl excess
Ag+ + SCN-  AgSCN
Fe3+ + SCN-  FeSCN until Fe(SCN)63-
(orange to red blood)
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20. Note:
The reaction between Fe3+ ion and SCN- occurs if only all of Ag+ ion has been reacted.
AgCl should be easier to precipitated than AgSCN (Ksp AgCl < Ksp AgSCN) so Cl- will not be replaced/substituted by SCN-.
Substitution of Clˉ by SCNˉ occurs if Ksp AgCl > Ksp AgSCN, the SCN- demands is bigger than Ag+.
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21. FACT WHAT DO WE DO? Ksp AgCl = 1.7 X 10-10 Ksp AgSCN = 1.0 X 10-12
The condition are not fulfilled.
WHAT DO WE DO?
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22. 1. Mask the AgCl precipitate
AgCl will not contact with SCN-
and they will not react
Masking agent : organic compound
e.g. : nitrobenzene
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23. 2. Add KNO3 then boil for 3 minutes, titrate immediately
K+ will desorp Ag+
Boiling will coagulate the precipitate
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24. The precipitate is removed and will not consume SCN-
3. Coagulate the precipitate by shaking, filter and then titrate the filtrate
The precipitate is removed and will not consume SCN-
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25. Remember Ag+ is very sensitive to light Keep it in amber bottle
Use a dark burette
Cover every clear glassware with black /carbon paper.
Under the light Ag+ decomposed to Ag2O, dark brown substance, hard removal dirt on skin or clothes.
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26. Terima kasih
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