chromate, or oxalic acid by means of its calcium salt. Hydrochloric or nitric acid is used when decided acidity is required—as in the separation of groups by hydrogen sulphide (Chap. XI). «The Ionization of Bases.»—The same relations hold for bases as for acids: the weaker bases give ionization constants as [p106] do the weaker acids; the strong bases, again, as was mentioned above, do not give constants. The values for strong bases, stated in the following table, are bracketed, and refer to the ionization in 0.1 molar solutions of the bases. Polyvalent bases, like polybasic acids, ionize in stages, and the primary ionization is usually stronger than the secondary ionization, and so forth. For instance, a study of ferric chloride solution shows that the third hydroxide group of ferric hydroxide, Fe(OH)_{3}, must have the smallest ionization constant.[193] «The Ionization Constants[A] of Bases» Base. Ratio. K. Potassium hydroxide [K^{+}]×[HO^{−}]/[KOH] (1) Sodium hydroxide [Na^{+}]×[HO^{−}]/[NaOH] (1) Barium hydroxide [Ba^{2+}]×[HO^{−}]^2/[Ba(OH)_{2}] (0.03) Strontium hydroxide [Sr^{2+}]×[HO^{−}]^2/[Sr(OH)_{2}] (0.03) Calcium hydroxide[B] [Ca^{2+}]×[HO^{−}]^2/[Ca(OH)_{2}] (0.03) Ammonium hydroxide[C][D] [NH_{4}^{+}]×[HO^{−}]/ ([NH_{4}OH]+[NH_{3}]) 1.8E−5 Hydrazine[E] [N_{2}H_{5}^{+}]×[HO^{−}]/ ([N_{2}H_{5}OH]+[N_{2}H_{4}]) 0.3E−5 Aniline[F] [C_{6}H_{5}NH_{3}^{+}]×[HO^{−}]/ ([C_{6}H_{5}NH_{3}OH]+ [C_{6}H_{5}NH_{2}]) 0.5E−9 Water [H^{+}]×[HO^{−}]/[H_{2}O] at 25° 0.2E−15 at 100° 0.9E−14 [H^{+}]×[HO^{−}] at 25° 1.2E−14 at 100° 0.5E−11 TABLE NOTES: A. As explained above, the bracketed values under K for strong bases are not constants, but express the values of the ratios for 0.1 molar solutions only. (For the alkaline earths 0.05 molar solutions are referred to.) B. Estimated value. C. ‹Vide› page 160. D. Corrected value, from Bredig's data, ‹Z. phys. Chem.›, «13», 322 (1894). A. A. Noyes, ‹Carnegie Institution, Publication› No. 63, p. 178, finds the value 17.3E−6 for 18°. E. Uncorrected value, Bredig, ‹ibid.› F. Bredig, ‹ibid.›; Stieglitz and Derby, ‹Am. Chem. J.›, «31», 457 (1904).