[znaes97]

Hello,
I have to do a certain manual titration a couple of times a week. Salicylic acid is used as the indicator and the procedure calls to titrate (with 0.1M EDTA) until a bright yellow end point. The solution starts purple, goes brown-ish and then yellow/brown, then finally will go bright yellow. Is the end point in the yellow/brown range, or should it go to the full bright yellow? All help is greatly appreciated.

[jeffmoonchop]

What are the products producing the brown colour, and what are they producing the yellow colour? Would the titration end point be a point in between the brown/yellow or would it be when there is no longer any brown contaminating the yellow product?

[jeffmoonchop]

Sorry, I meant what causes the purple, just sub purple with brown from my above comments.

[znaes97]

What are the products producing the brown colour, and what are they producing the yellow colour? Would the titration end point be a point in between the brown/yellow or would it be when there is no longer any brown contaminating the yellow product?

The salicylic acid produces the purple color. I am not sure if I should stop the titration when I first observe any yellow or if I should go until it is bright yellow.

[jeffmoonchop]

Normally the end of a reaction is when the reactants are fully used up. How can you tell in this case that there are still reactants and not fully product?

[Borek]

The salicylic acid produces the purple color

Salicylic acid is colorless, so no, it is not producing the color on its own. It reacts with something that is present in the solution.

[rolnor]

Whats more in the solution? Some metal salt?

[znaes97]

Whats more in the solution? Some metal salt?

The chemical being tested is an acidic aluminum deoxidizer with <10% iron content and the titration is done to determine the iron content. Apologies if this is a simple question, I went to school for chemical engineering, not chemistry.

[Borek]

the titration is done to determine the iron content

Iron reacts with the salicylic acid to produce color - and the compound is purple. When you add EDTA you replace the salicylate complex with the ETDA complex, which as far as I remember is yellow.

Not seeing the colors and not ever doing the titration I can only guess, but these brownish colors are probably mix of purple/yellow, and the end point is when the purple disappears at all. Then when you add more EDTA the color should not change (so you can prepare an overtitrated sample to see what you are getting to).