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(9)Volumetric Precipitimetry

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allelipraise's version from 2018-06-06 19:11

Section 1

Question Answer
Volumetric Precipitimetryrequires the formation of relatively insoluble substances or precipitates to cause the reactions to go to sufficient completion to be quantitative in nature
solubility product constantssparingly soluble salts, Ksp
Halogenoids (Pseudohalides):CN-, SCN-, OCN-, N3-, Fe(CN)63-
HalidesF-, Cl-, Br-, I-
Endpointsreaction in analysis by precipitation methods may be determined in the following ways
Gay Lussac’s MethodCessation of precipitation
Liebig Methodappearance of a turbidity
Mohr Method, Volhard Method, Fajan’s MethodUse of internal indicators,
potentiometic or amperometric Instrumental methods
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Section 2

Question Answer
Mohr Methoddirect titration method, generally for the assay of halides, against silver nitrate VS
Mohr MethodYellow to red (?) method
Mohr Method The required pH is neutral to slightly alkaline (pH 8.0). Titration should be carried out at room temperature because solubility of silver chromate is increased with rising temperature.
undertitration Excessive indicator (Mohr)
overtitrationInsufficient indicator(Mohr)
Volhard Methodindirect or residual method, generally for the assay of halides, wherein the excess silver nitrate VS is back-titrated with a standard thiocyanate solution
determination of iodideDetermination of what uses ferric indicator must be added only after an excess of the silver nitrate has been added.
Modified Volhard Methoddirect method, generally for the assay of mercury (II), against standard thiocyanate solution
Fajans’ Methodadsorption or desorption of colored indicators occurs near equivalence point resulting in color change of the precipitate
Fajans’ MethodFew precipitation reactions that produce colloidal precipitate rapidly
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Section 3

Question Answer
Dichlorofluorescein TSIridescent greenish-yellow color in solution; turns pink when adsorbed that requires pH=4
Dichlorofluorescein TSbeing more highly ionized than fluorescein, gives a higher concentration of anions, even in slightly acid solution.
Eosin Yellow (Eosin Y) TSEven more highly ionized than DCF allowing titration at lower pH (avoid precipitation of other silver compounds)
Eosin Yellow (Eosin Y) TSFluorescent yellowish-green in solution; turns pink when adsorbed pH=2
0.1 N Silver Nitrate VS- photosensitive - stains skin and clothes - primary standard: Sodium Chloride - standardization: Fajans’ method (eosin Y TS)
0.1 N Ammonium Thiocyanate VS- secondary standard: 0.1 N AgNO3 VS - standardization: Volhard method (ferric alum TS)
0.02 M Sodium Tetraphenylboron VSfor the analysis of organic nitrogen compounds as alkaloids, amines, and quaternary salts, as well as ammonium, potassium, and silver ions
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