The arsenazo 2-(2-arsenophenyl)azo-7-(4-antipyril)azo-1,8-dihydroxy-3,6-naphthalenedisulphonic acid as spectrophotometric reagent and metallochromic indicator for calcium

Abstract

Ions Ca(II) react with the arsenazo [2-(2-arsenophenyl)azo-7-(4-antipyril)azo-1,8-dihydroxy-3,6-naphthalenedisulphonic acid] at pH 10.0 to produce a blue-violet complex with the maximum of absorbance at 578 nm. Its stoichiometry is 1 : 1 and its stability constant has the value of 8.56 . 104 at pH 10.0. Beer's law is obeyed in the range from 0.04 to 0.4 μg ml-1 of Ca(II), with a molar absorbance of 1.92 . 104 l mol-1 cm-1. The reagent has also been studied as the metallochromic indicator in the complexometric titration of Ca, and the colour change was specified by means of CIE chromaticity diagrams. According to Ringbom's concepts a transition pM of 4.93 is obtained when a 5.0 . 10-4 mol l-1 solution of Ca(II) is titrated.