[AhmedEzatAlzawalaty]

What is the cause of violet color in the reaction of ferric with salycilic acid?

[ARGOS++]

Dear ahmedezat,
It’s quite easy, Phenols like free Salycilic Acid forms a simple 1:1 Complex
with Fe³⁺ (Of course, including some water?), which in your case is blue,
and detectable at ~540nm, if you do it in aqua.
It can be “hold” linear (Beer-Lambert!) over a wide concentration range!
There is a standard method written!

Good Luck!
                   ARGOS++

[AhmedEzatAlzawalaty]

thank u ARGOS++