Kinetics of degradation 2 Flashcards
What are zero order reactions?
If the loss of drug is independent of the concentration of the reactants and the rate is constant (i.e., 1 mg/mL/hour), the rate is called zero order.
Zero order equation!! need to know this:
Ct = C0 - kt
Where Ct is concentration of drug at given time
C0 is initial concentration of the drug. Units: g/100 mL or mg/mL or mole/L. Can be in seconds, minutes, hours, days, etc.
k has the units of concentration divided by time (rate constant). If C0 is in mg/mL and t in hours, the units of k are mg/mL/h = mg mL-1 h-1
The half-life equation for a zero-order reaction
t1/2 = (0.5)(C0 /k)
What slope does zero order have and what is the slope and intercept
Linear slope
the slope = - k
C0 is the intercept
what is a first order reaction
If the loss of drug is directly proportional to the concentration remaining with respect to time, it is called a first order reaction and has the units of reciprocal time, time-1.
What slope does first order reactions have
Negative slope
log : linear slope
LnC = lnC0-kt
What is a second order reaction
Rate depends on the product of two concentration terms.
When you have two components reacting with each other or one component reacting with itself.
What is the slope for a second order reaction
Positive slope
Arrhenius observed that most reaction rates depend on the following three factors:
Fraction of molecules possessing energy of activation (Ea)
Number of collisions per second
Fraction of molecules having the correct orientation
describe all aspects of the Arrhenius equation
k = rate constant
ae - pre exponential factor
ea - activation energy (j/mol or kJ/mol
r - universal gas constant
t = temperature (in kelvin)
increase the stability of pharmaceutical preparations containing drugs subject to hydrolysis SOLID ORAL DOSAGE FORMS
- Reduction / elimination of water from the pharmaceutical system – waterproof coating / sealed container
Liquid forms - how to increase stability
Use anhydrous vegetable oils / water substitute such as alcohol to reduce hydrolytic decomposition.
Unstable antibiotics - how to increase stability
Supply in dry form for reconstitution in solution just before dispensing
Refrigeration is advisable for most preparations considered subject to hydrolysis
what is photodegredation and how can molecules be protected from this
Photodegradation may occur not only during storage, but also during use of the product (in case of topical drugs).
Products can be protected from photodegradation by storage in amber coloured bottles and a dark place.
Tablets are protected by coating with polymer containing UV light absorbing substance.
