I'm not an expert on pharmacology as I did my masters in chemistry. However, I'm helping a friend out with developing a new formula so I was wondering what the possible ways of measuring the saturation solubility of a drug are.
Add an excess amount of drug to distilled water then leaving it stirred or shaken until equilibrium is established for 48 or 72 hours. Then filter this suspension, discard the first 2 ml of filterate, then analyze the drug in the filterate. The concentration is considered the saturation or equilibrium solubility of drug.
Proceed according Ramy Elsergany answer, but dont forget to maintain temperature constant during all procedure. The measured solubility is valid for the recorded temperature
Equilibrium based methods are the best, but will be slow. A straightforward way is to collect the filtrate according to the above answers and carefully register its mass, then let the solvent (water?) evaporate and record the mass of the dry material. Careful treatment (good balance, temperature control all the time, many repeats, etc.) is essential for good result.
Then there are dynamic methods. E.g., making a slurry and slowly heating it while measuring the turbidity, which will normally register a sharp drop when the crystals vanish. If the heating rate is low enough and equilibrium established reasonably fast, it can give a good approximation.
In our experience dry mass was not the best results, with a wide spread of data due sometimes of liquid trapping by the solid phase. In our lab we check the equilibrium time measuring one property of the solution, like refraction index of the liquid phase, that is considered in equilibrium after 3 readings spaced 1/2h each. Then we decide for 24, 36 or 48 hs.
Normally we made a quantititave analysis fo the liquid phase or use a standart refraction index curve against concentration at a higher temperature yhan the measured.
I have also encountered cases where the results of a gravimetric measurement are not satisfactory, either due to trapped liquid, solvate formation or sublimation. In all such cases the spread between repeat measurements will be high, providing a clue to the problem.
polythermal and gravimetric method are commonly reported for solubility study
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