Kinetics of stability studies

chemical kinetics studies

1. CHEMICAL KINETICS STUDIES Presented By- ROHIT R.K.S.D college of pharmacy, Kaithal ( Hry ) M.Pharma 1 st year (Pharmaceutics)

2. CONTENTS • Introduction • Rates and order of reaction • Method for determining the order of reaction • Factor affecting rate of reaction • kinetics of drugs decompositions Applications

3. Definition Kinetics & Drug stability Drugs stability is defined as the pharmaceutical dosages form to maintain the physical, chemical, therapeutic and microbial properties during the time of storage and uses by the patient. Stability is defined as the capacity of a drugs substance to remain within the established specification to maintain its identity, strength, quality and purity throughout the retest or expiration during period. Chemical kinetics is the study of rate of chemical changes taking place during chemical reaction. Chemical kinetic: Chemical kinetic involve the study of the rate of chemical process . The rate of reaction can be understood by studying the time course change in concentration.

4. RATES AND ORDER OF REACTION The velocity with which a reaction or a process occurs is called as its rate, concentration of drugs influences the rate of reaction or process is called as the order of reaction or order of process.

5. The rate of forward reaction is expressed as : - dA / dt - ve sign = concentration of drugs A decreases with time. As the reaction proceeds, the concentration of the drugs B increases and the rate of reaction can also be expressed as: dB/ dt Experimentally, the rate of reaction is determined by measuring the decrease in concentration of drugs A with time.

6. If c is the concentration of drug A, the rate of decrease in c of drug A as it is changed to B can be described by expression as function of time t. dC / dt = - kcn (n is the power of concentration). Where, k= rate constant n=order of reaction If, n= 0 (zero order process)

7. The order of a reaction determines the way in which the concentration of a reactant or reactants influences the rate of a chemical reaction. Molecularity of Reaction The molecularity of a reaction refers to the numbers of molecules, atoms, or ions reacting in a elementary process to give the reactants. If only one type of molecules undergoes a change in to yield the product , the product is said to be unimolecular . If two molecules undergoes to change yield the product, the reaction is said to be bimolecular. Reaction that involves more than one steps ( complex reaction) may have different molecularity and order of reaction.

8. The three commonly encountered rate process: Zero order reaction First order reaction Second order reaction Types of order of reaction

9. Zero-order kinetics o Its is also called as constant rate process. o The reaction is said to be zero-order reaction , if the rate of reaction is independent of the concentration i.e. the rate of reaction can not be increased further by increasing the concentration of reactants. dc/ dt = - KoCo = - Ko equation.....1 Where Ko = zero-order rate constant (in mg/min) Rearrangement of equation 1 yields: dc= - Ko dt equation..........2 Integration of equation 2 gives: C - Co = - k0 t where Co = concentration of drug at t = 0, and C = concentration of drug yet to undergo reaction at time t.

10. First-order kinetics Whose rate is directly proportional to the concentration of the of drugs undergoing reaction i.e. greater the concentration , faster the reaction. First-order process is said to follow linear kinetics dC / dt = -KC Where K = first-order rate constant (per hour)

11. SECOND-ORDER REACTION • the rate of reaction is directly proportional to the square of the concentration of one of the reactants. • r = k [A]2

12. Factor affecting rate of chemical reaction Concentration of reactant Pressure Temperature Catalyst Nature of reactant Surface area Intensity of light Nature of solvent

13. Determination of Order of Reaction Use of rate equation – The data collected in a kinetic reaction should be substituted into the integrated form of equations of various orders. The process under test should be considered to be of that order where the calculated k value remains constant within limits of experimental error. Half life method – For a zero order or pseudo first order reaction, t . is proportional to initial concentration of reactant (Co ), t . for a first order reaction is independent of Co , .

14. Graphical method – For a zero order or pseudo first order reaction, plot of C vs. t is linear ; for first order reaction, plot of log ( Co-Ct ) vs. t is linear.

15. Thank you