Determination of Acid Concentration in Etching Baths

The application note describes the thermometric determination of a mixture of phosphoric acid (H₃PO₄) and sulfuric acid (H₂SO₄) by titration with Sodium Hydroxide (NaOH). No thermal indicator is required for this titration.

Background:

Etching baths are often composed of mixtures of acids. To ensure the same etching power over time, the concentrations of the different acids have to remain constant. It is therefore imperative to be able to know accurately the concentrations of all the acids of the mixture to correct the etching solution with the right amount of acid.

The thermometric titration allows a rapid and accurate analysis of the components of the etching bath that can be fully automated with an autosampler and the Thermometric sensor.

In thermometric titration, the change in reaction heat (i.e. enthalpy change) is monitored in the course of a titration by measuring, with a thermometric probe, the small difference in temperature induced by the chemical reaction.

Reaction

1st EQP:

H₂SO₄ + NaOH  ---------->  Na⁺ + HSO₄^- + H₂O

HSO₄^-  + NaOH  ---------->  Na⁺ + SO₄^2- + H₂O

H₃PO₄ + NaOH  ---------->  Na⁺  + H₂PO₄^1- + H₂O

2nd EQP:

H₂PO₄^- + NaOH ---------->  Na⁺ + HPO₄^2- + H₂O

(For the method to measure the blank value, download method M770)