Irving Rossotti Method Lab Report

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

Osterberg showed that the differential quotient of two external coordinates of a titration could be evaluated experimentally at selected values of pH, and that this function could be used to determine the difference in pL between two points differing in pH.

If pH 0 is selected such that at it pL0 is known, eg., a value low enough that no metal complex formation occurs, then (eq.) can be used to find pL at any other chosen pH.

Sarkar and Kruck showed the analogues function (δCH/δCM) could be determined experimentally and used to determine pM at a selected pH provided that at some reference pH0 the value of pM 0 was known –

Where pM = – log [Free metal M], pL= – log [Free ligand L], pH = – log [H+] and H

= moles of OH− consumed in the titration of the hydrogen ion liberated from the complexation reactions. This is a method used with computer program to calculate of the stability constant and species distribution of the complexes