The analytical utility of heterocyclic azo compounds in colour-forming reaction is very familiar. Among them, 2-pyridylazo compounds, which have a hydroxy group in the o-position to the azo group, have proved very useful as metal indicators and also as spectrophotometric reagents. As a part of a program to investigate the correlation of the spectrophotometric sensitivity and the structure of the reagents, the chelating properties of the 1-(2-pyridylazo)-2-naphthol (PAN)analogue ofphenanthroline were studied 1. The sensitivity of the reagent for metals was found to be considerably higher than that of PAN. Extension of the n-system of the molecule should lead to even more sensitive reactions.
Replacement of the pyridine ring by quinoline should result in an increase in the molar absorptivities of metal chelates. This led to the synthesis of 2-(2-quinolylazo)-l-naphthol, hereafter referred to as ~-QAN.
The present paper describes a quantitative comparison of ~-QAN with PAN and a-PAN, 2-(2-pyridylazo)-l-naphthol, the establishment of the type of metal chelates formed, and determination of the equilibrium constants.
EXPERIMENTAL
Apparatus
Absorption spectra and quantitative spectrophotometric measurements were obtained with a Cary 14 spectrophotometer with matched silica cells. For pH measurements, a Denkikagaku digital pH meter Model HG-2 was used with a combination electrode. When the solution contained more perchlorate than 1 M, a double-junction calomel electrode was used with a filling solution of saturated ammonium nitrate. The pH meter was standardized against standard oxalate, phthalate, and phosphate buffer solutions. Acidity higher than pH 1.0 was calculated from the relation between motarity of perchloric acid and the activity coefficient which was interpolated from the data of Harned and Owen 2 by the method of Lagrange.
Calculations were performed with a NEAC 3100 computer.
Reagents
Stock solutions of the sodium salts of perchlorate, acetate, thiocyanate, sulfate, and bromide were made from analytical-grade reagents and purified by extraction with ~-QAN in carbon tetrachloride, followed by washing with several portions of carbon tetrachloride. The stock solutions of 1.10 -2 M metal perchlorates were 2-(2-QUINOLYLAZO)-I-NAPHTHOL AS SPECTROPHOTOMETRIC REAGENT