Partitioning in a Cuvette

The partitioning of a chemical between
two immiscible
solvents
can be performed in a stoppered cuvette. Since the sample remains
intact during the spectroscopic analysis, it can be subjected to additional
extraction solvent, so further data can be obtained. This technique
has the potential to save time and money and can dramatically minimize
chemical waste. The mathematics of successive partitioning are developed,
so that the spectral absorption can be written as a function of the
volume of the extracting solvent, and the distribution equilibrium
constant KD can be determined by least-squares.
The solvatochromic dye Brooker’s merocyanine and methylene
blue were partitioned between octanol and water in a cuvette; the
data yielded reasonable fits; the values of KD were 0.026 and 0.054, respectively. The technique was applied
to the acid–base indicator methyl orange; while the data is
reproducible, the fit of the data indicates some systematic bias,
presumably because additional acid–base equilibria come into
play.