- Reactions which depend on other agent than the concentration of reactants, e.g. photochemical decomposition of sulphadrugs and adsorption of some reactants on surfaces.
- i.e. the amount of products do not depend on the conc. Of reactants.
- For a substance having initial conc. Cº at zero time (t=0) and Ct at time t:
- The average change in concentration with change in time is given by:
- Ct-Cº /t = K or–ΔC/ Δ t = K by taking the limit function: -dC/dt= K where k is the reaction rate constant.
- The half-life (t1/2): time req……………
- When Ct = ½ Cº …..> t1/2= Cº/2K
- Units of k are: mole. Liter -1 sec-1
- When 10 % of the reactant concentration is lost att 90%. (shelf life).
- t90%= 0.1Cº/K
Apparent zero order reactions:
- In case of suspensions:
1- The concentration of drug in solution depends on its solubility
2- When the drug decomposes in solution, more drug is released from suspended particles, so that the conc. Remains constant: (despite its decomposition by time).
3- his conc. Represents the equilibrium solubility of the drug in a particular solvent at a particular temp.
4- When there is no reservoir of solid particles: the equation of the solution will be first order: -dc/dt= KC
5- Where, Kis the first order rate constant and Cis the remaining drug concentration
6- When a reservoir is present: Cwill always be constant (solubility), then:
7- -dc/dt= K0 , where, K0is the apparent zero-order rate constant.
8- Hence , K0= K x equilibrium solubility.