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I am doing an investigatory project on studying the coloured reactions of d-block and f-block elements. I have included two reactions that require sulphur dioxide as a part of my study.

  1. $$\ce{5SO2 + 2KMnSO4(purple) + 2H2O -> 2H2SO4 + 2MnSO4(pale pink) + K2SO4}$$
  2. $$\ce{SO2 + K2Cr2O7(orange) + 3H2SO4 -> K2SO4 + Cr2(SO4)3(green) + 3H2O}$$

Now, my problem is I need an effective method to produce sulphur dioxide to be able to use for these in a school laboratory environment. I thought of using ferrous sulphate which on heating gives $\ce{SO2}$, $$\ce{2FeSO4 ->[{$\Delta$}] Fe2O3 + SO2 + SO3}$$ But this reaction also produces $\ce{SO3}$. Will that screw with any of the above reactions? Also, are all the above reactions feasible in a school laboratory? Do suggest if there are better methods and how I can make it to react with the other reactants.

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Note: Sulfur dioxide causes dyspnea. Prolonged inhalation of it may causes respiratory problems. So, reaction should be done under laboratory supervision.


The most common laboratory method of producing sulfur dioxide is the reaction of copper and sulfuric acid, both available in laboratories.

$$\ce{Cu(s) + 2H2SO4(l) → CuSO4(aq) + 2H2O(l) + SO2(g)}$$

Process: To a round bottom flask, 100 g of copper (small pieces, turnings, etc.) are placed. From a dropping funnel 100 ml of concentrated sulfuric acid is slowly added dropwise. The reaction flask is gently heated and obtained sulfur dioxide is dried by passing through a wash bottle with concentrated sulfuric acid. (Process taken from here. Read more to know how the reaction can be controlled and the precaution needed)

Alternatively, sodium sulfite can also be used.

$$\ce{Na2SO3 + H2SO4 -> SO2 + Na2SO4 + H2O}$$

Process: Sodium sulfite is placed in the bottom of the 300 ml Erlenmeyer flask of the apparatus. The dropping-funnel is filled with dilute sulfuric acid, made by pouring one volume of concentrated sulfuric acid into an equal volume of water and cooling the mixture. The glass elbow is connected with a gas washing-bottle containing concentrated sulfuric acid, and the dry gas is collected by displacement in two or three empty cylinders. On allowing the dilute sulfuric acid to drop slowly upon the sodium acid sulphite, a very regular evolution of sulfur dioxide is obtained.

enter image description here

For full setup, check here.

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  • $\begingroup$ Thank you, are normal lab conditions enough for the reactions 1,2 that I have mentioned in my question? And can you mention the downsides of using ferrous sulphate? $\endgroup$
    – MollyCooL
    Apr 5 '18 at 14:37
  • $\begingroup$ Probably yes. They are normal high school reactions and has been used for demonstration purposes. But take care of sulfur dioxide. It can be harmful if inhaled. $\endgroup$ Apr 5 '18 at 14:42
  • $\begingroup$ Okay then, I will either stay away and ask the teacher to do it or will undertake the necessary precautions. Thank you. $\endgroup$
    – MollyCooL
    Apr 5 '18 at 14:43
  • $\begingroup$ @MollyCooL ferrous sulfate decomposition also produce sulfur trioxide which is also a gas and can be difficult to separate from sulfur dioxide. Moreover, sulfur trioxide is way more nastier than sulfur dioxide. $\endgroup$ Apr 5 '18 at 14:44
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    $\begingroup$ @MollyCooL SO3 will not screw the reaction in any way. It is a part of products(gas) evolved. To separate SO3 and SO2 can be a hefty task. $\endgroup$ Apr 5 '18 at 14:52
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Wet chemistry can be so old fashioned! The gist of the reaction is the reduction of the Mn or Cr by tetravalent sulfur. How about bypassing the gas formation and introducing the tetravalent sulfur as bisulfite ion (from NaHSO3 solution)?

5SO2+2KMnSO4(purple)+2H2O --> 2H2SO4+2MnSO4(pale pink)+K2SO4

This scheme (plus a bit of added H2SO4) then becomes

5NaHSO3 + 2KMnO4 + H2SO4 --> 2Na2SO4 + NaHSO4 + 2MnSO4 +2H2O

The same color change will occur.

If the second reaction is properly balanced, it becomes

3SO2+K2Cr2O7(orange)+H2SO4 --> K2SO4+Cr2(SO4)3(green)+H2O

By replacing the SO2 with NaHSO3, this scheme then becomes

3NaHSO3 + K2Cr2O7 + 3H2SO4 --> K2SO4 + Na2SO4 + NaHSO4 + Cr2(SO4)3 + H2O.

The overall reaction is the same, but simpler. You have the advantage of using more available and much safer chemicals. And the NaHSO3 solution will still give you the fragrance of SO2, but without total dyspnea.

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  • $\begingroup$ Yes, I went ahead using sodium bisulphite but I got a mere tinge of green rather than a complete green solution! $\endgroup$
    – MollyCooL
    Apr 10 '18 at 11:55
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    $\begingroup$ The pH in both cases is different. Cr III oxide is green; its hydrate is violet or blue-gray green, according to CRC Handbook. Cr III sulfate is violet, red or blue violet. These color differences might be small shifts in an absorption band, depending on pH or impurities or who knows what! You could try measuring pH and recording color of solution. Interesting! $\endgroup$ Apr 10 '18 at 14:15

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