i read somewhere that when (dilute?) acid is added to chlorophyll, the magnesium ion is removed. but it doe snot go into detail and i cannot find it anywhere else.

can someone tell me which acids this is and explain the mechanism and/or provide and equation please?

There are a couple possible avenues one can take to remove the Mg(2+) from chlorophyll to form pheophytin.

 

One is simply heating green leaves, denaturing the cell membranes that release acids, which displace the Mg ions with hydrogens. Another way uses oxalic acid to displace the Mg ion...

 

Here's an ACS paper outlining an experiment using another acid and a series of concentrations.