How much contamination are you experiencing? i.e. how much iron is too much for the next experiment?

Where is the iron coming from?: experimental solutions or from the laboratory plumbing etc? If it is from your solutions and experiments, and you are doing trace element work, then you may need to get a new set of glassware and keep it for "clean work" only. Use the contaminated glassware for other jobs.

I have heard of chromic acid being used for cleaning glassware, but it is rather toxic and would probably render your glassware unsuitable for Cr analysis afterwards. You could try a 24h soak in pyroneg or another laboratory glassware detergent. Citranox or one of the Decon brand detergents might also help. Rinse well and try the 10% nitric acid for 24h after that.

Dear Dr. Wheal, thank you very much for your answer.

About your questions, I haven't measured the level of contamination in my glassware yet... but if I load 100 mL of distilled water in a three-necked round bottom flask, 10 ppm of iron contamination would be enough to interfere in my experiments.

Iron is coming from experimental solutions... we thought about separating the glassware for "clean work" only, but this would request more space and logistics. That's why I wanted to see if there's another solution, maybe changing the way we clean the glassware could help. I heard from a colleague that chromic acid could also be used, but is very dangerous as you also mentioned... I will try to do as you suggested for now, and I hope to obtain better results.

Again, thank you very much for your answer :)

Maybe you want to check:

https://www.researchgate.net/post/How-can-I-clean-a-beaker-stained-with-ferric-chloride-solution

https://www.researchgate.net/post/How-can-I-remove-iron-oxide-from-teflon-paddle-stirrer