# Irving Rossotti Method Lab Report

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2. Irving Rossotti Method
Calvin-Bjerrum pH titration technique as adopted by Irving & Rossotti is generally used for determining the proton-ligand and metal-ligand formation constants. The procedure consists of:
(A) Determination of the formation curve of the system. This is expressed as a plot of
n
(formation function) against pL for metal ligand system and a plot of n
A against pH for a proton-ligand system (Definitions of the terms
n
,
n
A and pL are given below).
(B) The calculation of the values of formation constants by solution of the formation function of the system or otherwise.
(C) The conversion or the stoichiometric constants into thermodynamic constants. n term, was introduced by Bjerrum who called it the 'formation functions'
According to Irving and Ressotti,
n
A and
n
can be expressed as-
n
A = o n
TL
V V
Y TLo V V N E
( )
( )( )
1
1

 
 

....................(8.8)
n =
(V V ) n .TCMο
(V V ) (N E ) TL
1
ο o iii1 n ο

  
....................(8.9)
Where Vo is the initial volume of the solution, Eo, TLo are the initial concentrations of the mineral acid and the reagent respectively and V', V'' and V''' are the volume of alkali of a given normality, N, required during the acid, the ligand and the metal titration respectively at a given pH (B). While the term Y gives the number of titrable hydrogen ions arising from the chelating agent and TMo gives the initial concentrations of the metal.
From the observed values of [L] for each n value, values of pL- are calculated utilising the equation given by Irving and Rossotti: pL- log10 

0
0 .
)
log
( 1
TCL n TCM anti n j n o
H n n 

. o iii V
V 0 V ....................(8.10)
Values of proton-ligand formation constants, KH
1 , KH2 etc. obtained from the proton-ligand formation curves plotted between values of
n
A and pH [Fig. 4(b)].
The pH value at