Yes, HS⁻ (the hydrosulfide or bisulfide ion) is amphiprotic. It can donate a proton to act as an acid or accept a proton to act as a base, depending on what it reacts with. This makes it part of the same family of amphiprotic species as bicarbonate (HCO₃⁻), hydrogen sulfate (HSO₄⁻), and water itself.
Why HS⁻ Qualifies as Amphiprotic
An amphiprotic species is any molecule or ion that can both donate and accept a hydrogen ion (H⁺). HS⁻ meets both criteria. It still has one hydrogen it can give away, and it carries a negative charge that makes it willing to pick up another hydrogen from a nearby molecule.
When HS⁻ acts as an acid, it donates its proton to form the sulfide ion:
- HS⁻ → S²⁻ + H⁺
When HS⁻ acts as a base, it accepts a proton to re-form hydrogen sulfide:
- HS⁻ + H⁺ → H₂S
This dual behavior is the hallmark of amphiprotic species. The pattern is easy to spot: any ion that starts with “H” and carries a negative charge (like HSO₄⁻, HCO₃⁻, HSe⁻, or HS⁻) is almost always amphiprotic because it has a proton to lose and can also gain one back.
Which Role Dominates in Water
Just because HS⁻ can act as either an acid or a base doesn’t mean it does both equally. In water, HS⁻ preferentially acts as a base. It grabs a proton from a water molecule, producing H₂S and hydroxide (OH⁻). Dissolving sodium hydrosulfide (NaHS) in water produces a slightly alkaline solution, which confirms this. In one study, adding NaHS to drinking water nudged the pH from 7.60 up to 7.71.
The reason HS⁻ favors its basic role comes down to how difficult it is to lose that second proton. The first dissociation of H₂S (losing one proton to form HS⁻) has a pKa around 7.0, meaning it happens fairly readily near neutral pH. But the second dissociation, where HS⁻ loses its remaining proton to form S²⁻, is far harder. While older textbooks list that pKa₂ between 12 and 15, more careful measurements under oxygen-free conditions place it between 17 and 19. That makes HS⁻ an extraordinarily weak acid, so in practice it much prefers to accept a proton rather than donate one.
How HS⁻ Compares to Other Amphiprotic Ions
The three amphiprotic ions you’ll see most often in chemistry courses are bicarbonate (HCO₃⁻), hydrogen sulfate (HSO₄⁻), and water. HS⁻ follows the same logic but sits at an extreme end of the spectrum.
Hydrogen sulfate (HSO₄⁻) is a relatively strong acid. It readily donates its proton, so its acidic behavior dominates in water. Bicarbonate (HCO₃⁻) is more balanced but still slightly basic in water, which is why baking soda solutions feel slippery. HS⁻ is the most lopsidedly basic of the group. With a pKa₂ potentially as high as 19, its tendency to donate a proton is nearly negligible under normal conditions. It’s amphiprotic in principle, but in practice it almost always acts as a base.
Why the pKa₂ of H₂S Matters Here
If you’re working through a homework problem, you may only need to recognize that HS⁻ is amphiprotic because it has a proton to donate and can accept one. But if you need to predict the pH of an HS⁻ solution or determine which role it favors, the pKa values become essential.
The first dissociation constant (pKa₁ ≈ 7.0) tells you how easily H₂S gives up a proton to form HS⁻. The second (pKa₂) tells you how easily HS⁻ gives up its proton to form S²⁻. The enormous gap between these two values, roughly 10 to 12 pKa units, is why HS⁻ is such a reluctant acid. For comparison, the gap between the two pKa values of carbonic acid (H₂CO₃) is only about 4 units, which is why bicarbonate behaves more evenly as both an acid and a base.
It’s also worth knowing that many textbooks still cite the older, lower pKa₂ values (12 to 15) for H₂S. Those measurements were contaminated by polysulfides that form when HS⁻ reacts with oxygen at high pH. Careful experiments using oxygen-free solutions and techniques like Raman spectroscopy have revised the value upward to the 17 to 19 range. If your textbook uses the older number, your calculated pH for an HS⁻ solution will be slightly off, but the conclusion that HS⁻ is amphiprotic and predominantly basic remains the same either way.