Please look at the following below links which may help you in your analysis:

https://carleton.ca/chemistry/wp-content/uploads/speclab.pdf

https://digitalcommons.lsu.edu/cgi/viewcontent.cgi?article=1196&context=gradschool_disstheses

Thanks

Dear Dr. Isam Eldin Hussein Elgailani ,

Thank you very much sir, I will check and study the reference.

جَزَاكَ اللَّهُ خَيْرًا

Best regards,

Saefudin

Follow the suggestion form Isam Eldin Hussein Elgailani .

You must be aware that iron 2+ doesn't occur very often of very long in water, and so your best aproach in the end (because of LoQs of the phenantroline method) might be calculation after determining the total iron content.

Cheers

Dear Mr. Mário Silva ,

Thank you for your suggestion.

As I know from Dr. Isam Eldin Hussein Elgailani reference, we can add Hydroxylamine Hydrochloride (2 NH2OH·HCl) to reduce Fe3+ to Fe2+.

Anyway, I really appreciate your suggestion and will take it into account.

Best regards,

Saefudin