Can anyone tell me what is the meaning of total acid and free acid? What is the difference between the two? D.K.
Total and free acid values are often measured and used to control phosphate systems, which are acidic by design (to initiate the phosphate reaction). The first titration would be for free acid. This would involve taking a bath sample and titrating it with a known standard such as sodium hydroxide (probably 0.1N or 1.0 N). An indicator is added to the solution that changes color when the titration reaches a certain pH. A common indicator for this would be bromphenol blue which changes from yellow to blue when passing through the pH range around 3.5-4.0.
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.
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. The corrective action would be to add phosphoric acid to bring it in line with the supplier’s recommendations. Too low a value could indicate too much phosphoric acid in the system which will not “build” a coating since it would tend to strip it as quickly as it forms. The supplier may have a chemical to adjust for this (possibly mono- or disodium phosphate), or may just tell you to run some scrap parts or steel wool through the system to use some of the available acid. This value will tend to rise and fall around some centerpoint that you will control around (i.e., five plus/minus 1 point).
The total acid value is meant to indicate the total amount of acid that has been put into the system and is a combination of both free acid and that which has been “neutralized” through reaction and combination with iron. In general, this number will tend to climb over time as the bath ages and, when combined with a known production quantity, may be able to give you an indication as to when to dump the bath.
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 (which basically is another way to determine free acid)
Free and Total Acid Values Revisited
An astute reader pointed out that I switched problems/corrective actions in the question from the October column regarding Free and Total Acid Values.
It should have stated that too low a value of free acid would not initiate the phosphate reaction and would require adding phosphoric acid to bring it in line with the supplier’s recommendation. Too high a value of free acid would indicate too much phosphoric acid in the system and may not allow the phosphate coating to “build” since it will strip it off as it is forming. The corrective action for this would be to make a chemical adjustment with mono- or disodium phosphate or run some scrap steel through to use up some of the acid.