Page 89 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102
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COO- Br-, I-, SCN- with Ag+ 7)
Cl C Cl
Pink → reddish-violet
HO O OH (Best in acetic acid
+ solution useful down
to pH 1-2).
Tetrabromofluorescein (eosin)
Red → purple
COO- (Accurate if
Br C Br (NH4)2CO3 is added)
Orange red → blue-
HO O OH red (pH 4-7)
+
Br Br
Dichloro-tetraiodo-fluorescein (Rose I- in presence of Cl-
Bengal) with Ag+
Di-iodo-dimethyl-fluorescein I- with Ag+
Determination of mixtures
a) Chloride-iodide mixture:
By direct titration using Fajans method, fluorscein indicator fortotal halide then the iodide is determined
using eosin.
b) Bromide-iodide mixture:
This is similar to chloride iodide mixture, total halide with eosin or fluorescein then for I- use
diiododimethyl fluorescein.
c) Chloride-cyanide, bromide-cyanide or iodide-cyanide mixtures:
Such mixtures are analyzed using a combination of Liebig and Volhard methods. Liebig method
determines the cyanide ion while the total is determined by Volhard method.
2 CN- + Ag+ ↔ [Ag(CN)2]-
[Ag(CN)2]- + Ag+ ↔ Ag-Ag(CN)2
References
1- Principles of Instrumental Analysis, Douglas A. Skoog, F. James Holler, Stanley
R.Crouch, 7thedition,2007.
2- Quantitative Chemical Analysis, Daniel C.Harris, 9th edition, W.H. Free man and
Company, NY,USA,2015.
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