pH and solubility Flashcards
what is pH
PH = -log [H+]
what is pKa
pKa is a symbol to represent the negative logarithm of the acid dissociation constant
pKa= −log10Ka
Ka = [A-][H+]/[HA]
What is a strong acid and a weak acid ?
A strong acid is an acid which is completely ionised in water
A weak acid is an acid which partially dissociates in water
what is the henderson-hassslebalch equation ? (WEAK ACID)
The following equation, which relates the pH of an aqueous solution of an acid to the acid dissociation constant of the acid, is known as the Henderson-Hasselbach equation.
pH = pkA + log10[conjugate base]/ [weak acid]
or
pH = pkA + log10[A-] / [HA]
what is the henderson-hasselbalch equation ( WEAK BASE )
PH = pKa + log ([B]/ [BH+])
-log ka= - log[H+] - log[B] + log [BH+]
pKa= pH-log [B] + log [BH+]
pH= PKA + log [B]/[BH+]
What is an amphoteric drug ?
An amphoteric drug can act either an acid or a base.
at pH values below the isoelectric point, the molecules behave as a base
at pH values above the isoelectric point, the molecules behave primarily as an acid.
what is the relationship between pH and pKa ?
The pKa of a weak acid or base is the pH at which the acid/base is 50% ionised
summary of pH-solubility relationships
Solubility tends to be greater towards 100% ionisation
weak acids are more soluble at higher pH
weak bases are more soluble at lower pH
What is a buffer solution
Buffer solutions resist imposed changes in pH within certain limits
a buffer solution is typically an aqueous solution containing a mixture of weak acid and its conjugate base or vice versa
what is buffer range
Buffer range is the overall pH range within which a buffer will maintain a constant pH
what can buffer solutions be used for
controlled pH is needed for :
optimum stability of solution
optimum efficacy of preservative
compatible with tissue pH
what is buffer capacity
buffer capacity is a measure of how difficult it is to shift a buffer from its chosen pH value (the buffer will eventually change pH if enough strong acid or strong base is added to it
what are the requirements of a buffer solution
buffers should provide adequate pH stability over a suitable pH range
buffers should be biologically safe
buffers should help maintain solution stability
buffers should be compatible with and allow functioning of other excipients
How do buffer solutions work?
A mixture of a weak acid, acetic acid HA ,and its ionized salt, sodium acetate NaA , acts as a buffer:
pH= pKa + log [A-]/ [HA]
pH remains constant as long as “log [A-]/ [HA]” does not change.
Small amount of acid added in the solution, will convert some of NaA to HA.
But if the overall changes in concentrations of both A- and HA are relatively small then the change on pH will be negligible.
If large amounts of acid or base are added to it, the buffer will change pH.
