These two experiments are probably most suited to Advanced Higher – possibly for an investigation.

They both involve burning the tea and extracting the iron/manganese in the form of a coloured compound that can be determined colorimetrically.

Iron & manganese in tea -Pupil Guide

Iron & Manganese in Tea – Teacher’s/Technician’s Guide

Analysis – Iron in tea, and tablets – Risk Assessment

Tea Calibration graph

Image by André Karwath  used under a Creative Commons License CC BY-SA 2.5