In case of suspensions:
- The concentration of drug in solution depends on its solubility
- When the drug decomposes in solution, more drug is released from suspended particles, so that the conc. Remains constant: (despite its decomposition by time).
- This conc. Represents the equilibrium solubility of the drug in a particular solvent at a particular temp.
- When there is no reservoir of solid particles: the equation of the solution will be first order: -dc/dt= KC
- Where, Kis the first order rate constant and Cis the remaining drug concentration
- When a reservoir is present: Cwill always be constant (solubility), then:
- -dc/dt= K0 , where, K0is the apparent zero-order rate constant.
- Hence , K0= K x equilibrium solubility.
First order reaction :
First order reaction :
- Reactions depending on the concentration of only one reactant raised to the first power. E.g. hydrolysis of hydrogen peroxide.
First order equation:
- dC/dt= KC or its exponential form:
- Ct = Cº e -kt
- By rearrangement: dc/C =-kdtand integration:
- The linear equation is: lnCt -lnCº = -kt
- In log form: log Ct = log Cº –kt/2.303
- Where Ct is the concentration remaining un-decomposed at time (t) and K is the first order rate constant.
-The half life of the reaction in case of first order kinetics is given by: t1/2= 0.693/k
If (a) is the initial conc. And (x) is the decrease in concentration:
If (a) is the initial conc. And (x) is the decrease in concentration:
- Log (a-x) = Log a –kt/2.303
- Shelf life time: time required for loss of 10 % of the original drug concentration (t 90%): t 90%=0.105/k