Petit Homme
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Registered: 6-5-2025
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Thermal behaviour of NaHSO4 as an H2SO4 substitute
As most of you know, NaHSO4 can sometimes be used as an OTC alternative to the now generally banned (for individuals) – yet essential building block
of civilization – sulfuric acid.
I'm looking specifically at the procedure to produce HNO3 from NaHSO4 and a Nitrate salt. As described in one of Nurdrage's many great videos, for
instance (see macckone's answer to my post inquiring about a procedure and detailed questions regarding the production of HNO3 specifically from
NaHSO4 + KNO3), this procedure calls for distilling by "heating to the highest temperature your hotplate can reach."
Since HNO3 azeotrope distills at 121C, and it tends to decompose at higher temperatures in the presence of impurities, I had to wonder why much hotter
than that since 120-140C would be fully sufficient to drive of HNO3 and thus carry the reaction forward?
Here are a few reasons that came to mind:
Speed up the reaction by increasing the reaction temperature (though the reagents involved don't strike me as something I'd expect to react very
slowly)
Speed up the reaction by increasing the concentration of reagents by driving off water held on to by some of the products / reagents
I then looked at the decomposition temperature of the reagents:
>121°C HNO3 gradually and exponentially, I expect, potentially unnecessarily decomposes vs temperature as catalyzed by (i.e. in the presence of)
some impurities
>280°C NaHSO4 starts to reversibly decompose into Na2S2O7
per 2 NaHSO4 → Na2S2O7 + H2O
>380C NaNO3 starts to reversibly decompose
per 2 NaNO3 ⇌ 2 NaNO2 + O2
>400C KNO3 starts to reversibly decompose
per 2 KNO3 ⇌ 2 KNO2 + O2
>460°C Na2S2O7 further decomposes into Na2SO4 and SO3
per Na2S2O7 → Na2SO4 + SO3
Hotplates generally go up to 350-550C (in the case of the most common 500-1000W plates), and a bit higher for slightly less common 2200-5000W plates,
so the margin between the HNO3 azeotrope's 121°C BP and "the highest temperature your hotplate can reach" is not trivial.
By looking at these decomposition thresholds (and partially ignorant about chemistry) I am left to wonder what level of heat is too much, or if (some
of) the decomposition are actually the very point in driving the reaction forward.
If we look at the >460°C level for instance: Na2SO4 formed from the decomposition of Na2S2O7 (itself forming from the decomposition of NaHSO4
starting >280°C) is a product in the formation of HNO3 from both NaNO3 and KNO3. Hence with respect to it alone this would be expected to
adversely impact the reaction. As per SO3, it is a gas so I'd expect it to be driven of unless immediately reacting in situ. It indeed does react with
H2O though little should be left >460°C. Anyways, these levels aren't very safe for glass so I'm not sure how advisable it is to push the heat
that high.
>400C KNO3 starts to reversibly form KNO2, which according to wikipedia reacts with acids to form nitrous oxides (which seems to me would decrease
yield in the context of producing azeotropic HNO3).
Apart from the potential in situ formation of H2SO4, I don't see much point in heating far above 280°C, which only increases the reaction yield by
increasing homogenization of the reagents via released water solvent and reaction speed by driving off the water from the reaction medium altogether,
at an (unknown) impact on yield.
Can it be said that NaHSO4's capacity to act as a substitute to H2SO4 increases with / necessitates very high temperatures (at first by driving off
H2O from it, lending it some of the dehydrating properties of H2SO4, and then by generating SO3 by forming H2SO4 in situ by reacting with water)?
I'm wondering what happens when the reaction is simply slowly carried out at 130C (maybe the above information explains why yields are limited by
going for the wet chemistry route in the production of HNO3 with NaHSO4 as H2SO4 subsitute?). Or if vacuum could benefit the reaction speed and yield
by facilitating the removal of HNO3 and facilitating the decomposition of NaHSO4 by driving off water from it at lower temperatures, both with much
less negative impact on yield from increased decomposition of HNO3 that occurs at otherwise higher temperatures required to achieve the same. Or if
that in situ generation of H2SO4 actually is of paramount importance to efficiently carry out the whole thing to completion / pushing the final
equilibrium / getting maximal yields.
Any thoughts?
[Edited on 13-10-2025 by Petit Homme]
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Radiums Lab
Hazard to Others
Posts: 279
Registered: 18-3-2025
Location: India
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Mood: Experiencing the elegance of science.
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People have made oleum with reasonable yeids using NaHSO4.
Water is dangerous if you don't know how to handle it, elemental fluorine (F₂) on the other hand is pretty tame if you know what you are doing.
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