I am investigating the effect of aspirin hydrolysis for a high school experiment.
My teacher told me to be careful with some of the buffer solutions I planned to make because they might form precipitates with Fe3+ ions. Apparently OH- ions and PO4 3-
I need to make 100ml of pH buffer solutions of around 1.2, 4.8, 7.2, 9.2 and 10.6
I did not know that pH is temperature dependent and am struggling to know how to make buffer solutions with the correct pH at 37oC and 60oC
Any suggestions would be greatly appreciated!
Hi Amy Prince ,
It's great to see you learning about geochemistry so early on and that your teacher is supportive.
I did my PhD work on how photosynthetic microbes interact with metals and one that is very well-studied is iron so I may be able to help.
As a starting point, regardless of the buffer you use, Fe3+ will likely precipitate if oxygen is presence in the absence of another molecule that will keep it dissolved in solution (we call these chelating molecules). So that is the first thing you need to address, is the Fe3+ added in such a way that it will stay in solution. This should not be an issue at acidic pH where iron will stay in solution (see the phenomenon of acid mine drainage if you want to see this at an extreme level!) but it will be an issue as you pass pH >4.
I am not sure what your ability to obtain buffers is but one that we typically use for controlling trace metal concentrations in that scenario is a molecule called MOPS:
https://www.sigmaaldrich.com/catalog/product/sigma/m1254?lang=en®ion=CA (protonated form for acidic pH)
https://www.sigmaaldrich.com/catalog/product/sigma/m9381?lang=en®ion=CA&gclid=CjwKCAiA2fjjBRAjEiwAuewS_Yvs4yfBACMgJsFCcus_h0VXdLP2SjulPI8e2XZcyPh8SWNOmsPsGBoCD6oQAvD_BwE (sodium salt for close to neutral)
This avoids issue with adding a lot of phosphate, which is typically not desirable and it is not really used as a nutrient by microbes so it stays active for a long time. It's also pretty forgiving when you are adjusting the pH. Typically we use a concentration of 25 mM (mmol per litre).
On a side note, if you're looking for a way to measure the Fe3+ vs Fe2+ in solution you can look to something called the ferrozine assay which is widely used even in university labs to work with iron.
I hope you found this helpful and good luck in your experiments!
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