The rate of dissolution is dependant on many
variables such as pH, viscosity, temperature etc.
Intrinsic dissolution rate is independant of these
How is it calculated?
The Noyes Whitney equation is rearranged
so that area variable is kept constant
within the equation
The equation can then be further simplified by
introducing the intrinsic dissolution rate constant (K)
K takes care of D/h, Units = cm/s
Simplied equation therefore:
1.) When calculating anything using the IDR equation, you
have to first decide whether or not sink conditions apply.
This is done by working out 10% of Cs and then seeing how
it compares with C. If C less than 10% then sink applies
2.) If sink conditions apply then Cs-C becomes simply Cs
3.) Then convert all of the units so that they are the
same or similar
4.) Plug the values into the equation. The equation may
need to be rearranged
Why do you measure IDR?
This is measured to know what the rate of dissolution would be if
all variables were kept at a certain constant. So that if the rate of
dissolution does change we know that at least one of the
variables has changed
This allows you to know how new drugs will
act when all variables are kept constant
What does it do?
This allows you to see how drugs will act
when in conditions similar to the human
body more specifically the small
Static Disc Method
Can also be rotating disc method
37 degree as this
is human body
Rotating paddle is used to make
the fluidity similar to that of the
Drug is compressed to ensure there are no
pores in the drug so solution cannot affect
Hixson-Cromwell Cube Root Law
Why is this used to the Noyes
The Noyes Whitney equation assumes that the
surface area of the particles remain constant but
in practice they dont. This equation takes into
account the changing surface area
What is the equation?
It can be arranged in such an
equation for a straight line
The equation says that the cube root of the undissolved
mass at time, t is the same as the cube root of mass at
the start minus cube root dissolution constant
For this question you fill out a table like so. first you
work out the undissolved using the concentration.
Then find the dissolved. Finally cube root the
dissolved to to get the final answer