Measuring corrosion of iron—Yr 12 EEI
Hi,
I'd like to get ideas/techniques on how students can measure the amount of rust produced when a nail (iron) corrodes.
In the past years, students have weighed the nails before and after the corrosion, and the difference was assumed to be the amount of rust formed. Would it be better to scrape off the rust as well and weigh the nail without the rust to get a more correct measurement of the mass of rust formed? The main issue I have with this method, is that it will not take into account the dissolved iron ions, which are also products of the corrosion. This leads me to the next question: Can I tell my students to disregard the dissolved irons ions if they wish to do this technique?
To measure the dissolved iron ions, more specifically Fe2+ ions, is it correct to use redox titration with permanganate ion? My students did this last year and found that there wasn't enough Fe2+ ions in the solution to get a decent amount of permanganate titre (endpoint was reached after 3–5 drops!), and so we've had to dilute the permanganate solution, which then makes the colour fainter and the endpoint was harder to detect. I thought this technique was promising because students can take samples of their solutions at different days and check for any trends.
I've also tried using a conductivity meter to measure the change in the amount of dissolved ions after corrosion, but it wasn't very successful, mainly because the probe we were using didn't pick up the range of microseimens in the solutions.
Can you please comment on the techniques I've described—whether or not we are on the right track. If yes, how can we modify it to get better results? Are there any other techniques that you can suggest that students can use?
I saw an interesting article in the RSC website: http://www.rsc.org/learn-chemistry/resource/res00000453/how-much-air-is-used-up-during-rusting?cmpid=CMP00005146—it measures the change in volume of air (oxygen) after rusting. Do you think this is a suitable technique to measure the extent to which the oxidation of Fe occurred? (Taken from the point of view of how much oxygen was used if iron is in excess instead of measuring the product produced.) In the method, the iron (steel wool) was not placed in the water to rust. Will this method still work should we decide to drop the steel wool in the water? I'm guessing not, because then the oxygen will come from the dissolved oxgen in water...
Thanks in advance. Sorry it was such a long one. :)