Although chelometric titrations are now being widely used for inorganic analysis, the majority of applications have been confined to the use of EDTA and NTA. Recently a few methods have been suggested using other complexans as reagents in order to improve the selectivity of the method 1-4.
The present paper deals with the use of hydroxyethylethylenediaminetriacetic acid (HEDTA) both as a reagent and as a masking agent. The pK (lO.7) for the manganese-HEDTA complex is sufficiently low to suggest that at a pH of approximately 5 very little of the complex would be formed. The apparent stability constant for Mn at pH 5 is less than 5-5 and as such one would not expect Mn to interfere in titrations at this pH. The apparent stability constants (at p~ 5) of nickel and copper are high enough (>8) so that a chelometric titration should be possible provided a suitable indicator were available.
In view of the color of the HEDTA complexes with nickel and copper, a fluorescent end-point appeared most appropriate. Preliminary titrations revealed that Calcein W and Calcein Blue gave rather sluggish end-points in the back-titration of an excess of HEDTA with a standard copper solution. It was apparent that the stability constants of copper with HEDTA and the indicators were not sufficiently far apart for a titration.
Two new indicators not previously described in the literature were available in which n-methylglycine had been substituted for iminodiacetic acid as the chelating substituent attached to the parent fluorescent indicator. The replacement of one of the carboxylic acid groups in Calcein W and Calcein Blue by the non-chelating methyl group ~vould be expected to lower the stability constant of the copper-indicator complex. These indicators which are a condensation product of n-methylglycine, formaldehyde and fluorescein (and 4-methylumbelliferone) are named Methyl Calcein and Methyl Calcein Blue respectively. They are commercially available from the G. Frederick Smith Chemical Co., Columbus, Ohio. Either indicator gives a good end-point in the back-titration of an excess of HEDTA with a standard copper solution.
Since the stability constant for the aluminum-HEDTA complex has not been reported, a preliminary investigation of the feasibility of using HEDTA as a reagent for aluminum was initiated. An aluminum solution was prepared and standardized using an excess of EDTA and a back-titration with a standard copper solution. The standardization of the aluminum solution was then repeated, but HEDTA was sub-