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We have been examining an ion-pair between eosin (anion) and  
cetylpridinium chloride (cation) (a quaternary ammonium compound) by  
spectrophotometry (542 nm) and are seeking to determine the ion-pair  
association constant (K).

Investigations in our lab have shown that at pH 8.0 there are two  
complexes; 1:1 (eosin:cetylpyridinium chloride) and 1:2.  In these studies  
we have kept the total eosin concentration constant, varied the total  
cetylpyridinium chloride over the range we are investigating and recorded  
the associated absorbance readings which correspond to the concentration  
of the complexes being formed.

What we want to know is the ion-pair constants for both the 1:1 (K1) and  
1:2 (K2) complexes.  Does anyone have a mathematical model we could use to  
calculate these constants, and if so, can we use it?

If you can help, could you give us your fax number so we can communicate,  
or alternatively you could fax us (0064 3 479 7034) or write to us at:

Leo J. Schep,
School of Pharmacy,
University of Otago,
P.O Box 913,
Dunedin,
New Zealand.

Many thanks

     L. Schep and K. Setiawan.

e-mail: alastair@farli.otago.ac.nz