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When the difference between successive pK values is about four or more, as in this example, each species may be considered as an acid in its own right; [27] In fact salts of H 2 PO − 4 may be crystallised from solution by adjustment of pH to about 5.5 and salts of HPO 2− 4 may be crystallised from solution by adjustment of pH to about 10 ...
In the case of multiple pK values they are designated by indices: pK 1, pK 2, pK 3 and so on. For amino acids, the pK 1 constant refers to its carboxyl (–COOH) group, pK 2 refers to its amino (–NH 2) group and the pK 3 is the pK value of its side chain.
Values are at 25 °C and 0 ionic strength. The pK a values are the pH values where the concentration of each species is equal to that of its conjugate bases. At pH 1 or lower, the phosphoric acid is practically undissociated. Around pH 4.7 (mid-way between the first two pK a values) the dihydrogen phosphate ion, [H 2 PO 4] −
The hydration equilibrium constant at 25 °C is [H 2 CO 3]/[CO 2] ≈ 1.7×10 −3 in pure water [12] and ≈ 1.2×10 −3 in seawater. [13] Hence the majority of carbon dioxide at geophysical or biological air-water interfaces does not convert to carbonic acid, remaining dissolved CO 2 gas.
The difference between successive pK a values is sufficiently large so that salts of either monohydrogen phosphate, HPO 2− 4 or dihydrogen phosphate, H 2 PO − 4, can be prepared from a solution of phosphoric acid by adjusting the pH to be mid-way between the respective pK a values.
The equilibrium is not complete because the acidity difference between guanidinium and water is not large. The approximate pK a values: 13.6 vs 15.7. Complete deprotonation should be done with extremely strong bases, such as lithium diisopropylamide. C(NH 2) + 3 Cl − + Li + N(C 3 H 7) − 2 → HNC(NH 2) 2 + HN(C 3 H 7) 2 + LiCl
The higher the proton affinity, the stronger the base and the weaker the conjugate acid in the gas phase.The (reportedly) strongest known base is the ortho-diethynylbenzene dianion (E pa = 1843 kJ/mol), [3] followed by the methanide anion (E pa = 1743 kJ/mol) and the hydride ion (E pa = 1675 kJ/mol), [4] making methane the weakest proton acid [5] in the gas phase, followed by dihydrogen.
In terms of its acid–base properties, chloride is a weak base as indicated by the negative value of the pK a of hydrochloric acid. Chloride can be protonated by strong acids, such as sulfuric acid: NaCl + H 2 SO 4 → NaHSO 4 + HCl. Ionic chloride salts react with other salts to exchange anions.