A-Garrett and Carper method :
- Plot C ≠ t at 4 elevated temp.
-From straight lines, calculate K values from slopes.
- Plot Log K ≠ 1/T
- Extrapolate the resulting line to room temperature to calculate K 25 ˚C.
- Calculate the expiration date: t 90% = 0.1 C°/k25(zero-order) or =0.105/k25 (first order).
B-Free and Blythe method :
- Used for liquid products
- Plot log conc. Remaining ≠ time (if degradation is first order) at 40, 50, 60, 70 and 90 ˚ C.
- From curve calculate time required for 90 % remaining drug at different temp.
- Plot (t 90%) ≠ 1/T
- From straight line, find (t 90%) at room temperature.
C-Kenonmethod :
- Based on calculation of t90% at temp. (37, 45, 60 and 85 ˚ C) using standard kinetic equation at two activation energies (Ea= 10 and 20 Kcal/mole).
-A table is used to predict a 2 years expiry date at room temp. if the product retained 90 % or more of its potency after the time listed in the table.
D-Q10method :
- Q 10: is a factor by which the reaction rate increases for each 10 degrees rise in temperature:
For example: If Q10= 2 it means that if we raise the temp by 10 degrees K will be doubled=Q10: is a ration between K at (T+ 10) to K at T:
- Q10= KT+10/KT
From Arrhenius equation :
- And substituting: T1 =T, T2 = T + 10 and Ea = 12-24 K cal/mole, Q10 = 2-4 (average3).
-Applications: comparative calculation of (t 90%) at two temp. T1 and T2:
- Assuming that (t 90%) = (a)/k where (a) depends on reaction order, then:
- (t 90%)T1 = a/k1 and (t 90%)T2 = a/K2……> (t 90%)T1/ (t 90%)T2= K1/K2 = Q10= KT+10/KT
So, (t 90%)T2= (t 90%)T1/ Q10 (ΔT/10)
- This equation: can be used foe estimation of shelf-life if reaction order is not known.