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The salts of the peroxydisulfate ion, S$_2$O$_8$$^2$$^-$, are strong oxidants like the perchlorates that are used in propellants for rockets.

In this answer on Chemistry stackexchange is a link to a thesis which describes the production of peroxydisulfuric acid by electrolysing concentrated sulfuric acid.

Because the clouds of Venus contain much concentrated sulfuric acid, could future missions to the atmosphere of Venus, using electrolysis of this acid to produce peroxydisulfuric acid, use this strong oxidizer to make a propellant to get back home to Earth ?

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The problem in making propellant on Venus, is surely in finding the fuel, rather than the oxidizer. Oxygen can be made by electrolyzing $\text{CO}_2$ if necessary and while $\text{S}_2\text{O}_8^{2-}$ is a very strong oxidizer, meaning that it will oxidize lots of things, the mass of the sulphur atoms means that pure oxygen is probably a better rocket fuel

Where electrolysis of sulphuric acid might be useful is as a way to make hydrogen, which is in short supply on Venus. This could be used as fuel, or combined with $\text{CO}_2$ to make methane and water. The water can then be electrolyzed to make more oxygen and get back some of the hydrogen.

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  • $\begingroup$ Thank you for this answer. It would be fine if you could add some links, for instance for the making of methane and water out of hydrogen and CO$_2$. $\endgroup$
    – Cornelis
    Aug 20 '19 at 16:39
  • $\begingroup$ Great links, accepted ! $\endgroup$
    – Cornelis
    Aug 21 '19 at 8:31
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Peroxydisulfates and acid itself would be a bad choice for rocket oxidizers for several reasons:

  • H2S2O8 is actually a solid with melting point of 65°C, therefore unsuitable for liquid fueled rockets,

  • In pure form acid is even more aggressive and corrosive than sulfuric acid, it's like sulfuric acid on steroids. It can explode in contact with organic materials and is never shipped in pure form, rather it's salts are used,

  • Ammonium salt (NH4)2S2O8 has a negative oxygen balance, sodium and potassium salts leave a lot of residue. Solid sulfates, and SO2,SO3 gasses are inevitable in exhaust, all heavy molecules. This will cause poor $I_s$$_p$ (specific impulse) and rocket performance.

  • Sodium and potassium salts have a very high pressure exponent, and they tend to detonate rather than deflagrate. Mixed with metal powders like Mg,Al, MgAl alloys they can detonate unconfined on sub-gram level.

But H2S2O8 can be used to produce some well known oxidizer, hydrogen peroxide H2O2. Actually hydrolysis of H2S2O8 was a baseline of previous industrial process for H2O2 production. This process suffered from heavy ions (Fe, Cr, Mn) contamination because acid used in process is very corrosive. Produced H2O2 was pretty unstable in higher concentrations (called HTP - high test peroxide) because heavy ions promote it's decomposition. Stabilizers like sodium stannate were used to bond heavy ions and improve HTP stability. Because of stability issues process was abandoned in favor of anthraquinone process which gives much more stable H2O2.

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  • $\begingroup$ Thank you for all the information, could you explain what ISP means ? $\endgroup$
    – Cornelis
    Feb 10 '20 at 9:26
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    $\begingroup$ Specific impulse of rocket fuel is abbreviated as Isp. $\endgroup$
    – WOW 6EQUJ5
    Feb 10 '20 at 10:08

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