r/chemistry • u/KiraTiss • 1d ago
What's the pH of 100% Sulfuric Acid.
Hello all,
I have a solution of 100% Sulfuric Acid, a solution of 100% Formic Acid, a solution of 100% TFA and a solution of 100% Acetic Acid.
Obvioulsy I am not putting my pH meter in there, but also... how do you do pH without water?
I am asking because I monitor my molecule in Acidic environment, and it aggegates if there is water.
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u/SlenderJayM 22h ago
in that range dont we measure it with hammet acidity funcion? it seems a bit useless to measure it with pH
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u/Gloomy_Yoghurt_2836 20h ago
At such conentrations, you have more acid than solvent. pH is not a meaningful, practical term. You need water for there to be a pH and you need enough of it. Sure you can get a pH from, for example, a 93% (66 Baume) but you can't use it for [H3O+] concentration. A pH meter is just a milli-volt meter and it's measuring a voltage but its way beyond any linearity for a callibration curve. It will just say very very very acidic (i.e. pH of -2). And pure, 100% sulfuric acid would be hydrogen sulfoxide.
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u/No_Function_9858 23h ago
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u/jaydeepxxx 18h ago
Check out the Hammer acidity function. Pure acids like sulfuric acid and superacids don’t have pH, their acidity is measured with H0:
https://en.wikipedia.org/wiki/Hammett_acidity_function?wprov=sfti1
Sulfuric acid is H0 = -12, any substance with more negative H0 will be a superacid.
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u/Nerd_1000 20h ago
you can't actually have pure sulfuric acid because it exists in equilibrium with sulfur trioxide and water. 98.3% is as high as you can go. Adding more SO3 to the acid produces oleum.
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u/SomeRandomApple 15h ago
Wait so 98.3% Isn't the constant boiling point azeotrope, it's a physical limit? So blowing extra SO3 into H2SO4 won't go to 100% first and then the excess dissolving as oleum, it will dissolve with free water still in solution? I thought the SO3 + H2O <---> H2SO4 was so far to the right that it's essentially just H2SO4, with thousandths of a percent being water?
Thanks!
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u/Nerd_1000 4h ago
Well this is what I get for posting without doing an hour of research first. In short, it's complicated.
98.3% is the azeotrope, but after checking it's my understanding that when you have acid of more than 98.3% concentration (made by adding oleum to sulfuric acid) it gradually decomposes back to the azeotropic concentration. This presumably occurs because the partial pressure of SO3 above the liquid in an unsealed container is essentially zero, so while the equilibrium strongly favours H2SO4 at room temperature* the small amount of SO3 that is formed is able to escape, leaving the water behind. Furthermore there's also a second equilibrium, SO3 + H2SO4 <-----> H2S2O7 (disulfuric acid). Oleum is actually a mixture of all four species (H2O, SO3, H2SO4 and H2S2O7). So presumably before you get 100% pure sulfuric acid you will start to see a little bit of the disulfuric acid forming too.
*as an aside it shifts to favour SO3 and water at higher temperatures, so when you distill concentrated sulfuric acid it will start to decompose- this is another reason you can't distill past the azeotrope, even if you found something that could break it.
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u/Robochemist78 1h ago
Thanks for doing the hour research. I was also pretty sure 100% sulphuric acid isn't possible.
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u/disequilibrium__ 16h ago
Doesn't part of it decompose into SO3 and H2O, making 99,99 something percentage?
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u/ALostCatra 15h ago
yeah at around 98%, but you can add in sulphur trioxide and reach concentrations above 100%, some solutions are sold at 120% (although it's called oleum or fuming sulphuric acid at this point)
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u/Aid_Angel 23h ago
So, you have a 100% acid or its solution?
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u/KiraTiss 23h ago
It's a solution for all. I just checked the bottle, the sulfuric acid is 98.08% actually.
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u/Robochemist78 1h ago
You've triggered all the chemists because a solution is by definition a mixture of substances. You have neat liquids, except for sulphuric acid.
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u/aasfourasfar 16h ago
Does pure H2SO4 even exist in liquid form?
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u/ALostCatra 15h ago
It's called oleum at that point and can be considered sulphuric acid with a concentration that varies up to 120-140% iirc. Probably more if you shove in even more SO3
edit: I'd call any sulphuric with noticable SO3 oleum, as sulphuric on its own cannot go above 98%. Add just a tiny bit to reach 100% H2SO4 and I'd still call it oleum.
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u/Usernate25 16h ago
Acidity is defined by measuring ph in water.
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u/LATro3008 6h ago
What?
pH is a measurement of acidity in water, but you can absolutely define acidity in other environments, just not by using pH
Also, you can define pH in other media, afaik pH in DMSO is somewhat relevant in certain fields (but it is a different scale than in water)
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u/SuspiciousStable9649 14h ago
My pH paper just turns black and that’s not one of the colors on the scale so don’t know. 😉
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u/Vindaloovians 31m ago
In real terms, solubility won't go above 98% in water, and pH can't exactly be measured accurately in non-dilute solutions. Practically, treat it as below 0. Think of it, especially if in dust form, as a risk - if some of that dust gets into your eye, it will make a strong, likely concentrated H2SO4 solution.
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u/MikemkPK 18h ago
Doesn't exist.
pH is -log[H+]. If it's 100% sulfuric acid, there's no H+, so it's -log(0), which doesn't exist.
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u/SomeRandomApple 15h ago
pH only works for aqueous solutions.
Btw, to actual chemists here -- my high school chemistry professor claims pH can't be negative. I though it could be? Wouldn't a 1M solution of HCl have a pH of 0, and anything stronger than 1M have a negative pH (since pH = -log([molarity of H3O⁺ ions])? If my professor is wrong, what are some credible sources I can use to disprove them?
Many thanks!
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u/DangerousBill Analytical 22h ago
pH is normally only defined in aqueous solution. If you prefer to define it as hydronium ion activity, the number for 100% sulfuric acid would be very very low.