why is it necessary to add phosphoric acid in addition to sulphuric acid in the dissolving of a steel sample?
To understand why both phosphoric acid and sulfuric acid are commonly used in the process of dissolving a steel sample, we need to consider the chemistry involved.
When it comes to dissolving steel, sulfuric acid alone has limited effectiveness. This is because steel contains chromium, which forms a protective oxide layer on the surface. This oxide layer prevents the acid from directly attacking the steel, making it difficult to dissolve.
However, the addition of phosphoric acid helps overcome this limitation. Phosphoric acid acts as a reducing agent, meaning it can react with the chromium oxide layer and reduce it, allowing the sulfuric acid to access the steel surface and dissolve it more effectively. This reduction reaction converts the chromium oxide into soluble chromium ions, which can then be dissolved by the sulfuric acid.
In summary, the combination of phosphoric acid and sulfuric acid is necessary in the dissolving of a steel sample because phosphoric acid helps to remove the protective chromium oxide layer, enabling the sulfuric acid to access and dissolve the steel effectively.