Determination of iron, zinc and manganese

Have you determined iron, copper, zinc and manganese? Let's exchange and study. I have a few questions for your advice! 1 Iron: how to determine the end point? The national standard says purple, but when I titrate it, it only shows for a moment, and then shake it up a little bit, and then it's not purple, it seems to be deeper than green, and it's not like green, so I can't say any color. 2 Manganese: the details are difficult to control. Can not master the national standard of 220-240 degrees, each time the result is low. 3 Zinc: what do you need to pay attention to? The result page of my work is low. I don't know why.