How to identify ferrous compound from ferric?

Question

I got a sample (in powder form) which contains ferrous phosphate and may also contain ferric phosphate as impurity. How can I identify the presence of ferric phase(ie impurity phase in my case)?

Answer 1

Measure the total Fe concentration by atomic absorption spectrometry

Measure the ferrous Fe concentration by colorimetric method with
2,4,6-tri(2'-pyridyl)- 1,3,5-triazine

Calculate the ferric Fe concentration as the difference of the above values.

Answer 2

What do you want to use it for and how much do you think is % the impurity?

In aqueous solutions Fe+2 oxidizes over time to Fe+3 anyway.
FePO4 has very low solubility in water (Ksp= 9.91* 10^-16 at 25 C according to http://www.ktf-split.hr/periodni/en/abc/kpt.html )
so if you are planning to dissolve it in water, the impurity at least will not dissolve. The problem is that your true concentration will be less than what you want...

If you have KSCN you can add it to a solution of your compound. Fe+2 should have a green solution
Fe+3 forms bright red complexes with SCN- (like blood; it has been used for special effects because of this)
So your solution would have a brownish colour (combination of green and red) depending on how much Fe+3 you have.
However if you want to quantify it you would have to find another way.
Remember that In aqueous solutions Fe+2 oxidizes over time to Fe+3 so if you do this test don't wait for too long. If the colour change is not almost instant there shouldn't be much Fe+3.