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I am trying to purify ferrous sulfate (for some electroplating experiments) and I came up with the following process:

  1. Dissolve 200g of iron fertilizer in 300ml of distilled water heated to 54 °C (because solubility is highest at that temperature).
  2. Add a splash of 15% sulfuric acid and some steel wool to prevent oxidation of iron(II) to iron(III).
  3. Filter the solution while it's hot, cool it to 0 °C and scrape out the ferrous sulfate crystals.

Now I am wondering about impurities, because I think it smelled like hydrogen sulfide when I add the steel wool. Manganese would create hydrogen sulfide while dissolving it in sulfuric acid and is a common alloying element in steel wool (besides carbon and silicon, which would both not dissolve, right?). Also I think my ferrous sulfate looks a bit blue-ish so that made me think that there might be some chromium and nickel in the solution.

Is there a way to test for chromium or nickel with the usual household chemicals? If there is nothing in there, can I pour the leftovers of my solutions down the drain after neutralizing? There should only be iron(II/III) sulfate and small amounts of manganese sulfate in there, right?

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    $\begingroup$ While there are many ways to test for contaminants, your process includes crystallization, which is fairly effective for removing some unwanted impurities (though nickel salts might co-precipitate with iron salts, it should not impact use for electroplating). As for home tests, look at those used for checking drinking water, e.g., chemetrics.com/product-category/test-kits/manganese $\endgroup$ Commented Nov 8, 2022 at 22:30

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Iron metal vs. sulfur

Your steel wool is of course mostly iron(0), i.e. metallic iron. This has a standard reduction potential of -0.44 V, or -440 mV.

$$\ce{Fe^2+ + 2 e− ⇌ Fe(s)}\;\;\;\;\;\;E_0=-440\;\textrm{mV}$$

The standard reduction potential of the sulfate / sulfide system is supposedly -220 mV.

$$\ce{SO4^2- + 8H+ + 8 e− ⇌ S^2- + 4H2O(aq)}\;\;\;\;\;\;E_0=-220\;\textrm{mV}$$

Thus, it is thermodynamically possible for the metal to reduce sulfate to sulfide. (In actual fact, redox equilibria with sulfur are very complicated, and varying degrees of sulfite, thiosulfate, dithionate, and sulfur(0) species may also be formed.) I'm not 100% sure if these reactions are kinetically hindered or not at room temperature in acid on the metal surface, but they are at least thermodynamically possible. It could potentially explain the whiff of sulfide you smelled. Since our noses are very sensitive to hydrogen sulfide, only minute amounts of this reaction would need to occur to explain the odor.

The blue color

Acidic solutions of iron(II) are blue-ish. Depending on the concentration, they have a very similar color to solid iron(II) sulfate. The blue color is not necessarily indicative of an impurity.

Crystallization

I agree with the comments though that crystallization should remove the vast majority of any such impurities.

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Nickel, chromium and manganese can be detected by the following tests.

Chromium. Some ammonia solution ($\ce{NH3}$) is added to the acidic solution containing $\ce{Fe^{2+}}$ ions and maybe $\ce{Cr^{3+}}$ ions. The obtained precipitate is filtrated, and introduced in a test tube with $3$ mL $0.2$same M $\ce{NaOH}$ solution. A tiny amount of $\ce{PbO2}$ is added. The test tube is heated to boiling for one minute : $\ce{Cr{OH}3¶ is oxidized into chromate ion. The obtained mixture is diluted with water and filtrated. The filtrate is neutralized by $1$M $\ce{H2SO4}$. 1 mL ether is added, then $3$ drops $3$% $\ce{H2O2}$. If the blue color (due to perchromic acid) appears in the ether phase, it is a proof of some Chromium in solution.

Nickel can be proved by the dimethylglyoxime reaction. But this is not an usual reagent.

Manganese. A small amount of the same precipitate as obtained with $\ce{NH3}$ in previous paragraph, is introduced in a test tube. $0.5$ g $\ce{PbO2}$ + $3$ mL concentrated $\ce{HNO3}$ are added. The test tube is then heated to boiling for $30$ seconds. The obtained mixture is dropped in another test tube containing $10$ mL water. Wait for decantation. The solution takes a purple or violet color (due to permanganate ion) if manganese is present.

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