ARGENTOMETRY Titration with AgNO3 standard solution SMK Negeri 13 Bandung

In the name of Allah, the precious, the merciful May God give us knowledge and bless us wishful Teknologi dan Rekayasa

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 Teknologi dan Rekayasa

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 Teknologi dan Rekayasa

Prerequisite Before beginning this Chapter, you are expected to comprehend: Ksp and solubility product Saturated, unsaturated, super saturated Substitution Reactions Teknologi dan Rekayasa

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) Teknologi dan Rekayasa

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. Teknologi dan Rekayasa

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) Teknologi dan Rekayasa

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 Teknologi dan Rekayasa

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 Teknologi dan Rekayasa

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 Teknologi dan Rekayasa

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) Teknologi dan Rekayasa

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 Teknologi dan Rekayasa

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) Teknologi dan Rekayasa

Fajans’s Method Adsorption indicator : Fluorescent Condition : a few alkaline pH 7 - 10 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 Teknologi dan Rekayasa

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 Teknologi dan Rekayasa

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 Teknologi dan Rekayasa

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) Teknologi dan Rekayasa

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) Teknologi dan Rekayasa

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+. Teknologi dan Rekayasa

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? Teknologi dan Rekayasa

1. Mask the AgCl precipitate AgCl will not contact with SCN- and they will not react Masking agent : organic compound e.g. : nitrobenzene Teknologi dan Rekayasa

2. Add KNO3 then boil for 3 minutes, titrate immediately K+ will desorp Ag+ Boiling will coagulate the precipitate Teknologi dan Rekayasa

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- Teknologi dan Rekayasa

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. Teknologi dan Rekayasa

Terima kasih Teknologi dan Rekayasa