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Is water an acid or a base? (mander.xyz)

submitted 22 hours ago by [email protected] to c/[email protected]

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[–] [email protected] 5 points 19 hours ago* (last edited 19 hours ago) (1 children)

Also I’m pretty sure it’s only coincidentally 7. The calculation for pH isn’t based on any property of water.

[–] [email protected] 10 points 19 hours ago* (last edited 19 hours ago) (2 children)

Well, yes and no. The pH scale follows the hydrogen ion concentration, but specifically in aqueous media. The reason 7 is in the "middle" of the scale is because the natural dissociation of water sits at equilibrium at 10^-7 M H+ at 298K, IIRC. So perturbations naturally just displace that specific equilibrium, so it absolutely is normative to water.

[–] [email protected] 2 points 14 hours ago

Interestingly enough, in other solvents a neutral pH is going to be a different value. IIRC, ammonia has an autoionization constant of 10^-30, so a neutral pH would be 15

[–] [email protected] 1 points 19 hours ago (1 children)

By that definition, it can’t be exactly 7 then either. 10^-7 is just an estimate that we’ve agreed works fine. To my knowledge we haven’t really tried to improve this accuracy either?

[–] [email protected] 8 points 18 hours ago

The exact value varies with temperature, so it's a "good enough for the typical variations in temperature experienced by most aqueous solutions" estimate.