stability constant
/ stuh-BIL-ih-tee KON-stant /
Imagine measuring how tightly a clasp holds: a strong clasp almost never lets go, a weak one springs open easily. The stability constant is a single number that scores how tightly a metal and a ligand stay joined once they have formed a complex.
Formally, it is the equilibrium constant for forming a complex from a metal ion and its ligand. A large value means the complex strongly favours staying together; a small value means it falls apart readily. Because these numbers span an enormous range, they are usually quoted as their logarithm (log K).
It matters because the stability constant predicts whether a titration will work at all: the titrant must form a much more stable complex than the indicator does, or the end point will be vague. The honest caveat is that the bare stability constant ignores competing reactions like pH effects, so for real titrations chemists use the conditional version instead.
The calcium-EDTA complex has log K of about 10.7, while the magnesium-EDTA complex is weaker at about 8.7; the bigger constant tells you EDTA holds calcium more tightly than magnesium.
A larger stability constant means a more tightly held, less reversible complex.
'Stability constant' and 'formation constant' usually mean the same number. Its inverse is the instability or dissociation constant, which scores how easily the complex breaks up.