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When the difference between successive pK values is less than about four there is overlap between the pH range of existence of the species in equilibrium. The smaller the difference, the more the overlap. The case of citric acid is shown at the right; solutions of citric acid are buffered over the whole range of pH 2.5 to 7.5.
Changing the level of acidity causes a shift in the equilibrium between two different structures that have different colors. In near-neutral or alkaline solution, the chemical has a sulfonate structure that gives the solution a purple color.
For optimal accuracy, the color difference between the two species should be as clear as possible, and the narrower the pH range of the color change the better. In some indicators, such as phenolphthalein, one of the species is colorless, whereas in other indicators, such as methyl red, both species confer a color. While pH indicators work ...
The neutral value of the pH depends on the temperature and is lower than 7 if the temperature increases above 25 °C. The pH range is commonly given as zero to 14, but a pH value can be less than 0 for very concentrated strong acids or greater than 14 for very concentrated strong bases. [2]
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] −, is practically the only species present. Around pH 9.8 (mid-way between the second and third pK a values) the monohydrogen phosphate ion, [HPO 4] 2−, is the only species ...
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 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 dissociation constant is commonly used to describe the affinity between a ligand (such as a drug) and a protein; i.e., how tightly a ligand binds to a particular protein. Ligand–protein affinities are influenced by non-covalent intermolecular interactions between the two molecules such as hydrogen bonding , electrostatic interactions ...