reaction order

Reaction order tells you how the speed of degradation depends on how much drug is present. Picture water draining from a tub: if the flow stays steady regardless of depth that is one pattern; if it slows as the tub empties that is another. Order is the mathematical shape of that slowing-down.

For zero-order degradation the rate is constant and does not care about concentration, so the drug amount falls in a straight line over time — common in suspensions, where dissolved drug is continually replenished from solid. For first-order degradation the rate is proportional to the amount remaining, giving exponential decay and a straight line when the logarithm of concentration is plotted against time; this is the usual behaviour of a drug in solution. Second-order and pseudo-order kinetics also occur.

Identifying the order is the key that unlocks shelf-life prediction: it dictates which graph gives a straight line, hence which rate constant to extract, and how to extrapolate to the time at which the drug falls below specification. Get the order wrong and the predicted expiry date will be wrong too.

A practical subtlety: a reaction may appear to follow a simpler order than it really does. If water or oxygen is present in vast excess and barely changes, a genuinely second-order hydrolysis behaves like first-order — so-called pseudo-first-order kinetics, which keeps the maths manageable.