Apparent Zero Order-Suspensions
Pseudo zero order is a reaction, which may be a first order, but behaves like a zero order, depending on the experimental conditions. In suspensions, drug degradation is a chemical reaction and follows an apparent (or pseudo) zero order reaction. Here, the rate of degrada tion depends on solubility. The phenomenon of solubility limited degradation can be explained as follows: In suspensions, a part of the drug is in solution-phase and remaining part is present as undissolved solid (Figure 1-2). Degradation is possible only when the drug is available in solution-phase. As soon as the drug in solution degrades, suspended particles act as a reservoir and continuously release the drug into solution. Thus, the concentration of drug in solution will remain constant during this process. Therefore, the degra dation rate follows a zero order reaction.
The following arguments can be proposed to prove mathematically that suspensions follow zero order. When there is no reservoir of solid, the entire drug is in solution form and follows a first order pattern. In this situation, rate equation can be written as:
-d[A] = k¹[A] ------(1)
dt
where (A) is the concentration of undecomposed drug at time, t, and k, is the first order rate constant. When [4] is maintained constant due to reservoir of solids in the suspension, the rate equation (1) changes to
-d[A] = k¹×constant =k0 ------(2)
dt
In equation (2), the term 'constant' is equal to intrinsic solubility (or solubility) of the drug.
Equation (2) represents zero order. It is referred to as apparent zero order, owing to the presence of drug reservoir of suspended solids. Once all the suspended particles have been converted to solution, the system changes to first order reaction.
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