Development of a Novel Category of Azo Dyes Based on Coumarin and Metal Complexes for Potential Applications as an Acid–Base Titration Indicator and a Carbon Steel Corrosion Inhibitor in Acidic Conditions

This study aimed to design and synthesise a novel azo coumarin dye, 8-(3-phenyl-5-pyrazolylazo)-7-Hydroxy-4-Methyl Coumarin (PPHMC), and investigate its structural aspects. Additionally, the study sought to explore the capabilities of PPHMC in detecting metal ions and protecting carbon steel against corrosion. Structural characterisation of synthesised PPHMC was performed using elemental analysis and infrared spectroscopy. Spectrophotometric analysis determined optimal pH range and stoichiometric ratios of PPHMC complexes. Conductometry, pH-metry, spectrophotometry, and potentiometry techniques were employed to determine conditional acid–base equilibrium, ionisation constants, pH effects, and stability constants of metal ion complexes. Electrochemical impedance and polarisation procedures assessed PPHMC’s corrosion inhibition for carbon steel in a 1.0 M HCl solution. Computation and discussion of Langmuir adsorption isotherm and Gibbs free energy parameters were conducted. PPHMC formed metal ion complexes with stoichiometric ratios of 1:1 and 2:1. Complex stability constants followed the order CoII < NiII < CuII < ZnII. PPHMC demonstrated promise as an acid–base indicator and exhibited an average corrosion inhibition efficiency of > 94.7% for carbon steel. Adsorption on the steel surface followed the Langmuir isotherm, forming a protective layer against corrosion. The study successfully designed and characterised PPHMC, highlighting its applications in metal ion detection, acid–base indication, and corrosion prevention. These findings contribute to advancing knowledge in the field and emphasise the potential of PPHMC for diverse practical uses.