PRESENTED BY: E .NARENDRA

1 S T M.PHARM ,PHARMACEUTICAL TECHNOLOGY

Noyes- Whitney’s dissolution rate law

contents

1. Introduction2. Fick’s law of diffusion3. Noyes -Whitney’s equation4. Various steps involved in dissolution5. Limitations6. Sink and non sink conditions

Introduction:Dissolution is a process by which a solid

drug substance become dissolved in solvent.

It’s a dynamic property.

Drug dissolution is important before prior to systemic absorption.

Various factors effecting dissolution of solid oral dosage form includes ,

1. Physical and chemical nature of active drug substance.

2. Nature of excipients3. Method of manufacture4. Type of formulation

Permeation of a drug across gut wall (lipid membrane)is affected by

Partition coefficient between lipid membranes.

Temperature of the medium(invivo temp is 37ºc).

If temperature is increased then kinetic energy

of molecules increases and diffusion rate

constant D increases

Agitation rate should be kept constant. If it is increased then stagnant layer thickness is decreased .

This in turn allows more dissolution of the drug.

Ability of drug to diffuse

Fick’s second law of diffusion:It states that “Rate of change of

concentration of dissolved material with time is directly proportional to concentration difference between 2 sides of diffusion barrier”.

dc ‗ k.c

dt

dc/dt -Rate of change of dissolved drug conc with time.

c - Difference in concentration of solution at solid surface (Cs)and bulk of the solution(Cb).

Noyes Whitney equation:It is used to define dissolution from a from a

single spherical molecule.

dw/dt rate of dissolution D diffusion rate constant A Particle surface area Cs Concentration of drug in

stagnant layer Cb concentration of drug in bulk

solvent L Thickness of diffusion layer

.

According to this theory the stages involved in the dissolution are,

Stage 1:An interfacial reaction occurs which result in

liberation of solute molecules from solid phase.

Drug dissolution occur at the surface of the solid particle.

A saturated layer is formed around the particle i.e stagnant layer formation.

This stage is rapid

Stage 2:Solute molecules migrate through the

boundary layers surrounding crystal to bulk of solution at which its conc is Cb

This step involves in transfer of molecule away from solid liquid interface into bulk of liquid phase.

It is a slow step and hence it is a rate limiting step.

Limitation:1. This theory assumes that surface area of

dissolving solute remain constant during dissolution but practically it is not possible.

Hixon and Crowell modifies the above equation to explain the effect of change of surface area.

w01/3_W1/3=k.T

W0 is the initial powder weight

W is the powder weight at time t

k is the constant

t is the time

Non sink condition:

1. Invitro dissolution represent first order process for which driving force is concentration gradient.

2. It is true for invitro dissolution in a limited dissolution medium.

3. Dissolution slows down after some time due to increased drug concentration in bulk of the solution.

Sink condition:It is rapid than compared to invitro because

the moment drug dissolves it is absorbed into systemic circulation, so Cb is zero.

And Cs>>Cb so no concentration is build up in bulk of solution.

If the volume and surface area is kept constant then above equation reduces to

dc/dt= K

Dissolution is constant under sink conditions and follows zero order kinetics i.e yield linear plot

To obtain invitro and invivo dissolution rate correlation ,the invitro dissolution carried out under sink condition. It is achieved by,

1. Bathing the dissolving solid in fresh solvent from time to time.

2. Increase the volume of dissolution.

3. Partitioning of drug from aqueous phase into organic phase E.g: chloroform, heptane

Adding water miscible solvent like alcohol to dissolution fluid.

Adding selected absorbents to remove dissolved

drug.

To maintain Invitro sink condition Cb should always less than 10% of CS.

References

Biopharmaceutics and pharmacokinetics by brahmankar,B.jaiswal and shargel.

Remington -science and practice of pharmacy 21st edition

www.authorstream.comwww.4reen.com

Thank you all