Rapid and Simple Spectrophotometric Method for Determination of Micro-Amounts of Cyclodextrins

Summary

Cyclodeitrins are known to form inclusion complexes with many kinds of organic dye molecules.

Phenolphtalein, the acid-base indicator forms stable colourless inclusion complexes with α-, β- and γ-cyclo-dextrins in aqueous solutions at pH 10,5. Complex stability constants /K_C/ of the different CD-complexes cure:

$${{\text{K}}_\beta }_{{\text{CD}}} = {\text{2}},{\text{16}}.{\text{1}}{0^{\text{4}}};{\text{ }}{{\text{K}}_\gamma }_{{\text{CD}}} = {\text{1}},{\text{92}}.{\text{1}}{0^{\text{3}}};{\text{ }}{{\text{K}}_\alpha }_{{\text{CD}}} = {\text{1}},{\text{5}}0.{\text{1}}{0^{\text{2}}}$$

. The molar ratio of phenolphtalein to any of the CDs is 1:1. The concentration range where the decrease of color intensity of phenolphtalein is proportional to CD concentrations was determined:

$$\eqalign{ & 0 - \,\,\,\,\,50\,\mu g/ml\,for\,\beta \,CD \cr & 0 - \,\,\,300\,\mu g/ml\,for\,\gamma \,CD \cr & 0 - \,3000\,\mu g/ml\,for\,\alpha \,CD \cr} $$

This method is also easily applicable to assort different batches of CD comparing the samples with standards.

Cyclodextrins are known to form inclusion complexes with many kinds or organic dye molecules (l,2,3).

One of the most frequently used acid-base indicator, phenolphtalein forms a very stable colourless inclusion complex with β-cyclodextrin in 4. 10^-3M Na₂CO₃ solution at pH 10,5. Stability constant /K_C/ was found to be relatively high, K_βCD = 2,16. 10⁴ M^-1 and can be easily measured spectrophoto-metrically (4). γ and α-cyclodextrin also form inclusion complexes with phenolphtalein under similar conditions but complex stability constants are very different from the one of β-cyclodextrin complex:

$$\eqalign{ & {{\text{K}}_\gamma }_{{\text{CD}}} = {\text{1}},{\text{92}}.{\text{1}}{0^{\text{3}}}{{\text{M}}^{ - {\text{1}}}} \cr & {{\text{K}}_\alpha }_{{\text{CD}}} = {\text{1}},{\text{5}}0.{\text{1}}{0^{\text{2}}}{{\text{M}}^{ - {\text{1}}}} \cr} $$

It must be mentioned that these stability constants were calculated using the commonly accepted and simple Benesi--Hildebrand method, because the concentrations of the components were not comparable, γ-cyclodextrin and α-cyclodextrin were taken in high excess.

Buvári and Barcza’s (4) calculation was based on the generally known relations of coordination chemistry because the concentrations of phenolphtalein and β-cyclodextrin were nearly identical. Their method delivered an indirect proof for the 1:1 ratio of components in the complex. The same 1:1 complex formation was assumed with γ-and α-cyclodextrins too.

In our measurements the concentration of phenolphtalein was 3. 10^-5 M, concentrations of cyclodextrins were varied between 0-4. 10^-5M for β-cyclodextrin, 0-3. 10^-4M for γ-cyclodextrin and 0-4. 10^-3M for α-cyclodextrin. The pH was supported by 4. 10^-3M Na₂CO₃.