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New guy, please be gentle. I might ask questions not according to conventions, I am not a chemist nor do I study chemistry. Feel free to adapt the question and the tagging.

I am preparing a solution for Fenton's reagents according to [1], mixing 0.750 g $\ce{FeSO4 * 7 H2O}$ (blueish green, cyan almost) to 50 ml deionized (milli-q) $\ce{H2O}$ ([1] uses a slightly higher concentration, but we adapted it according to another publication). This solution needs to be adjusted to $\mathrm{pH}~3$, which, according to [1], can be done with $\ce{H2SO4}$. Now, even if I adjust the solution to well under $\mathrm{pH}~3$, let's say $\mathrm{pH}~1.5$, I still get fallout (presumably a ferrous(III) compound) and a generally turbid solution.

I don't understand what might be falling out and why the solution is turbid in general. What might be happening here? Is the solution somehow reacting with the surrounding air? Could it be further reacting with the water?

turbid ferrous sulfate heptahydrate solution

2. Fenton’s Reagent

This reagent has two components: $30\%$ (v/v) $\ce{H2O2}$ with an iron catalyst. The catalyst solution was composed of 20 g of iron(II) sulfate heptahydrate in 1 l of filtered RO water. Tagg et al.(34) tested this reagent within the context of extracting microplastics from wastewater. The authors identified this as the optimum concentration. The catalyst solution was adjusted to $\mathrm{pH}~3.0$ using concentrated sulfuric acid.

References

  1. Hurley, R. R., Lusher, A. L., Olsen, M., & Nizzetto, L. Validation of a method for extracting microplastics from complex, organic-rich, environmental matrices. Environmental science & technology, 2018.
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    $\begingroup$ What water do you use? Is it distilled/deionized water, or just tap water? Also, what does the solid iron sulfate you are dissolving look like? $\endgroup$ – andselisk♦ Jan 28 '19 at 12:46
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    $\begingroup$ Deionized (milli-q). The iron sulfate is blueish green, cyan almost. $\endgroup$ – thymaro Jan 28 '19 at 12:52
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    $\begingroup$ Looks about right; I'd probably just filter the solution and proceed further. $\endgroup$ – andselisk♦ Jan 28 '19 at 12:56
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    $\begingroup$ @andselisk thx for the edit. This will be of assistance next time I post a question on chem.se. $\endgroup$ – thymaro Jan 28 '19 at 13:07
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    $\begingroup$ Yep, the concentration of $\ce{Fe(II)}$ might be a bit lower after filtration, but I presume you are not performing any quantitative analysis with Fenton's reagent anyway, so it shouldn't be a big deal. And you are very welcome (I included your comment regarding the precursors into the question); probably someone else here will provide you with better suggestion:) $\endgroup$ – andselisk♦ Jan 28 '19 at 13:11

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