QS Study

Free acidity: Existing acidity in the solution at a particular instant. The free acid value is used to tell you how much acid is available to initiate the phosphate reaction and exists in its original “active” state. Too high a value may indicate that the bath would have difficulty initiating the phosphate reaction.

Total acidity: It means the total amount of H+ contained by a particular amount of acid (Both weak and strong). To determine total acid content, you would repeat most of the same steps above, instead this time using a different indicator, possibly phenolphthalein. This will change from clear to pink in the pH range of 8.5-9. It is the total amount of all the available Hydrogen ions in a solution, i.e. ions that are both free in solution (as H+) and those that are bound to undissociated acids and anions


10 M HCl = 10 MH+ (Free acidity = total acidity)

10 M H2CO3 = 1 M H+ (total acidity > Free acidity)

These measurements are most often performed with a zinc phosphate line. An iron phosphate line will typically have a titration for total acid and a pH measurement.