Why the product absorbance is decreased in long standing condition

Why the product absorbance is decreased in long standing condition

The dissolved material may precipitate, change color or dissolve properly with time.

Thank you sir for your reply.

But actual my doubt is time factor may effect the absorbance or not.
Even the solution is not precipitated.

Yes as the time or storage increased there is the chance of decreased of potency of drugs. So absorption decreased . This may be due to any reason such as inadequate storage condition, Leaching and sorption in case of Plastic container, light etc