If the solution of the acid sulphate is poured into a large volume of water, barium sulphate is immediately reprecipitated, the hydrogen-sulphate-ion being dissociated, in the dilute solution, into hydrogen-ion and sulphate-ion, SO_{4}^{2−}, whose barium salt is so difficultly soluble: Ba^{2+} + 2 HSO_{4}^{−} → Ba^{2+} + 2 H^{+} + 2 SO_{4}^{2−} → BaSO_{4} ↓ + 2 H^{+} + SO_{4}^{2−}. [p104] EXP. Finely divided barium sulphate is warmed for a moment with a few cubic centimeters of concentrated sulphuric acid in a test tube, the mixture is allowed to settle, and some of the clear acid is carefully decanted into a large beaker full of water. It may be added, that while the primary ionization of sulphuric acid does not yield an equilibrium constant for the ratio [H^{+}] × [HSO_{4}] / [H_{2}SO_{4}], even such a strong acid as is sulphuric acid is found to give a fairly good constant[189] for [H^{+}] × [SO_{4}^{2−}] / [HSO_{4}^{−}]. The value of this constant[189] is 0.03. «The Ionization Constants[A] of Acids» Acid. Equilibrium Ratio. K. Hydrochloric [H^{+}]×[Cl^{−}]/[HCl] (1) Hydrobromic [H^{+}]×[Br^{−}]/[HBr] (1) Hydroiodic [H^{+}]×[I^{−}]/[HI] (1) Nitric [H^{+}]×[NO_{3}^{−}]/[HNO_{3}] (1) Chromic[B] [H^{+}]×[HCrO_{4}^{−}]/[H_{2}CrO_{4}] (1) [H^{+}]×[CrO_{4}^{2−}]/[HCrO_{4}^{−}] 0.6E−6 Sulphuric[C][D] [H^{+}]×[HSO_{4}^{−}]/[H_{2}SO_{4}] (1) [H^{+}]×[SO_{4}^{2−}]/[HSO_{4}^{−}] 0.3E−1 Oxalic[E] [H^{+}]×[C_{2}O_{4}^{−}]/[H_{2}C_{2}O_{4}] 3.8E−2 [H^{+}]×[C_{2}O_{4}^{2−}]/[HC_{2}O_{4}^{−}] 0.5E−4 Phosphoric[F] [H^{+}]×[H_{2}PO_{4}^{−}]/[H_{3}PO_{4}] 0.1E−1 [H^{+}]×[HPO_{4}^{2−}]/[H_{2}PO_{4}^{−}] 0.2E−6 [H^{+}]×[PO_{4}^{3−}]/[HPO_{4}^{2−}] 0.4E−12 Arsenic[D] [H^{+}]×[H_{2}AsO_{4}^{−}]/[H_{3}AsO_{4}] 0.5E−2 Nitrous[B] [H^{+}]×[NO_{2}^{−}]/[HNO_{2}] 0.5E−3 Acetic[G] [H^{+}]×[CH_{3}CO_{2}^{−}]/[CH_{3}CO_{2}H] 1.8E−5 Carbonic[H][I] [H^{+}]×[HCO_{3}^{−}]/([H_{2}CO_{3}] + [CO_{2}]) 0.3E−6 [H^{+}]×[CO_{3}^{2−}]/[HCO_{3}^{−}] 0.7E−10 H_{2}S[J][K] [H^{+}]×[SH^{−}]/[H_{2}S] 0.9E−7 [H^{+}]×[S^{2−}]/[SH^{−}] 0.1E−14 Boric[B] [H^{+}]×[H_{2}BO_{3}^{−}]/[H_{3}BO_{3}] 0.7E−9 Hydrocyanic [H^{+}]×[CN^{−}]/[HCN] 0.7E−9 Arsenious [H^{+}]×[H_{2}AsO_{3}^{−}]/[H_{3}AsO_{3}] 0.6E−9 Water[L][C] [H^{+}]×[HO^{−}]/[H_{2}O] at 25° 0.2E−15 at 100° 0.9E−14