Table 1: pKa of frequently encountered sulfur species (in water at 25°C).

Acids/BasespKa (s)References

H2S/HS/S2−7.0/12.9[33]
H2S2/HS2/S22−5.0/9.7[34]
H2S3/HS3/S32−4.2/7.5[34]
H2S4/HS4−/S42−3.8/6.3[34]
H2S5/HS5−/S52−3.5/5.7[34]
H2S2O3/HS2O3/S2O32−0.6/1.7[35]
H2SO5/HSO5/SO52−<0/9.3[36]
H2SxO6/HSxO6/SxO62− (x = 3–5)?/−2.3[37]
H2SO3/HSO3/SO32−1.8/7.2[38, 39]
H2SO4/HSO4/SO42−−3.0/2.0[40, 41]
HSCN/SCN1.3[42]
S2O4H2/S2O4H/S2O42−0.3/2.4[33]