Chapter 7-Precipitation and Complexation Methods of Analysis Flashcards
•require the formation of relatively insoluble substances or precipitates to cause the reactions to go to completion. •solution must be titrated
Volumetric Precipitimetry
•cessation of precipitation or the appearance of turbidity
•use of internal indicators
•instrumental methods (i.e. potentiometric or amperometric)
Determination of End Point
Instrumental methods used in determining end point?
potentiometric or amperometric
3 indicators of volumetric precipitimetry:
- Ferric Ammonium Sulfate (FAS or Ferric Alum)
- Potassium Chromate Test Sol’n
- Adsorption Indicators
• used in both direct and residual titration
a. employs standard ammonium thiocyanate solution
b. thiocyanate reacts with Hg or Ag ions present to form white precipitate of silver or mercuric thiocyanate
Ferric Ammounium Sulfate (FAS or Ferric Alum)
thiocyanate reacts with __ or __ ions present to form white precipitate of silver or mercuric thiocyanate.
Ag or Hg
• forms a red precipitate of silver chromate which is seen against the background of white silver chloride
Potassoum Chromate TS
used in the analyses of halides by direct titration with silver nitrate solution
Adsorption Indicators
3 Adsorption Indicators
- Dichlorofluorescien(DF) TS
- Eosin Y TS
- Tetrabromophenolphthalein Ethyl Ester (TEE)
TS
2 Standard Sol’n
a) 0.1 N Silver Nitrate
b) 0.1 N Ammonium Thiocyanate
3 Types of Volumetric
Precipitation Methods
- Mohr Method
- Volhard Method
- Fajans Method
• named after Karl Friedrich Mohr
• for determining chloride content (drinking water)
Mohr Method
•indirect or back titration procedure
• for determination of anions that precipitate silver like Cl-, Br-, I-, SCN-
• uses ferric ammonium sulfate/ NH4 Fe(SO4)2 as indicator
• named after Jacob Volhard
Volhard Method
Is added to form a film over the precipitated AgCl
Nitrobenzene
• makes use of adsorption
indicators
• typically DCF for Cl-
• the endpoint is marked by green suspension turning to
pink
• Eosin is suitable for titrating
against Br-, I-, SCN-
• named after Kazimierz Fajans
• the most recent silver halide
method
Fajans Method
• is a type of titration involving
silver ions
• used to determine Cl- in a
sample
• sample is titrated against a
solution of AgNO3
Argentometry
• precipitate organic nitrogen
compounds such as alkaloids,
amines
• also used for ammonium, K and Ag ions
• uses chloroform as extraction indicator
Sodium Tetraphenylboron Titrations
Application of Precipitation Method
• salt content in crisps, ketchup and food
• iodide in iodized salt
• silver content in coins
• sulfate content in mineral water
For metal determination
using EDTA
Complexation Methods
EDTA
Ethylenediamine Tetraacteate
When a metal ion combines
with a molecule which can
donate electrons.
Complex
Combining molecule contains
two or more groups that donate electrons.
Chelate
• will react with metal ions to form a water soluble, stable complex or chelate
compound
• the 4 oxygen and 2 nitrogen •atoms of it make hexadentate molecule
• (disodium salt) - more preferred over the free acid because it is more water
soluble, nonhygroscopic and very stable
EDTA
EDTA make _____ molecule
Hexadentate
• determination of metal in the
presence of another metal
• Zn will not interfere with Ca
titrations at a pH of 13
Masking
5 Masking Agent
- Potassium Cyanide
- Ammonium Fluoride
- Triethanolamine
- Thioglycol
- Ascorbic acid
• Masking Agent
• Co, Ni, Cu, Zn
Potassium Cyanide
• Masking Agent
• Ca, Mg, Al to allow titration of Zn
Ammonium Fluoride
• Masking Agent
• suppress Al-EDTA complex
Triethanolamine
• Masking Agent
• inactivate Hg, Cu for titrati-
on of Zn
Thioglycol
• for Ca, Mg and Zn
• Titrant: Mercuric nitra.
• Indicators: Hydroxynaphthol blue ,eriochrome black, xylenol orange
Direct Titration
• for Al and Bi compounds, Mn
• Titrant: Zinc sulfate
• Indicators: dithizone, eriochrome black
Residual Titration
