Quote from Chief Pharmacist on February 20, 2026, 5:37 am

Short answer: No.

 

And USP <1236> is very clear on this point.

 

Apparent solubility and Equilibrium solubility are two different measurement classes answering two different questions.

 

Apparent solubility asks: 𝘞𝘩𝘢𝘵 𝘤𝘰𝘯𝘤𝘦𝘯𝘵𝘳𝘢𝘵𝘪𝘰𝘯 𝘥𝘰𝘦𝘴 𝘵𝘩𝘦 𝘥𝘳𝘶𝘨 𝘳𝘦𝘢𝘤𝘩 𝘥𝘶𝘳𝘪𝘯𝘨 𝘵𝘩𝘦 𝘥𝘪𝘴𝘴𝘰𝘭𝘶𝘵𝘪𝘰𝘯 𝘵𝘦𝘴𝘵 𝘸𝘪𝘯𝘥𝘰𝘸?

 

Equilibrium solubility asks: 𝘞𝘩𝘢𝘵 𝘪𝘴 𝘵𝘩𝘦 𝘵𝘩𝘦𝘳𝘮𝘰𝘥𝘺𝘯𝘢𝘮𝘪𝘤 𝘴𝘰𝘭𝘶𝘣𝘪𝘭𝘪𝘵𝘺 𝘭𝘪𝘮𝘪𝘵, 𝘢𝘵 𝘪𝘯𝘧𝘪𝘯𝘪𝘵𝘦 𝘵𝘪𝘮𝘦, 𝘸𝘪𝘵𝘩 𝘦𝘹𝘤𝘦𝘴𝘴 𝘴𝘰𝘭𝘪𝘥 𝘱𝘳𝘦𝘴𝘦𝘯𝘵?

 

So, the real question isn’t which one is correct - it’s which question are you trying to answer.

 

Here’s a practical example:

• 𝗗𝗼𝘀𝗲: 100 mg
• 𝗗𝗼𝘀𝗮𝗴𝗲 𝗳𝗼𝗿𝗺: IR tablet
• 𝗧𝗮𝗿𝗴𝗲𝘁 𝗱𝗶𝘀𝘀𝗼𝗹𝘂𝘁𝗶𝗼𝗻 𝘁𝗲𝘀𝘁 𝘁𝗶𝗺𝗲: 30 – 45 minutes
• 𝗜𝗻𝘁𝗲𝗻𝗱𝗲𝗱 𝗺𝗲𝗱𝗶𝘂𝗺 𝘃𝗼𝗹𝘂𝗺𝗲: 900 mL

You generate both equilibrium solubility (EQS) and apparent solubility (APS) data:

 

• Medium pH 1.2: EQS = 5 mg/mL, APS = 6 mg/mL
• Medium pH 4.5: EQS = 0.3 mg/mL, APS = 1.2 mg/mL
• Medium pH 6.8: EQS = 0.05 mg/mL, APS = 0.4 mg/mL

What this tells you:

• Apparent solubility is much higher than equilibrium solubility
• Based on equilibrium solubility alone, sink conditions aren’t met in pH 6.8.
• Yet apparent solubility shows that for 30-45 minutes, pH 6.8 can temporarily solubilize the dose.

 

That almost 8x difference in APS vs EQS in pH 6.8 tells us that supersaturation exists and precipitation is delayed.

 

This explains why tablets often release fast even when equilibrium solubility is low.

 

So yes, what we seen in the dissolution profile is driven by apparent solubility.

 

The why do we still bother with equilibrium solubility?

 

You’d see 90% release in 30 minutes at pH 6.8 and conclude ‘𝘚𝘪𝘯𝘬 𝘤𝘰𝘯𝘥𝘪𝘵𝘪𝘰𝘯𝘴 𝘢𝘳𝘦 𝘮𝘦𝘵. 𝘔𝘦𝘵𝘩𝘰𝘥 𝘪𝘴 𝘢𝘥𝘦𝘲𝘶𝘢𝘵𝘦.’

 

But when apparent solubility exceeds equilibrium solubility, it means the system is metastable. So small changes in the conditions could flip the results leading to divergent profiles or erratic results.

 

Equilibrium solubility tells you upfront that you would expect this sensitivity, therefore design the method and acceptance criteria accordingly and that results should not be over-interpreted.

 

Takeaway:

• Apparent solubility explains the dissolution outcome.
• Equilibrium solubility explains the risk behind that outcome.
• Both are essential- just not interchangeable.