Why is codeine's solubility higher in cold water!

[bigfootlives]

I'm sure most of you are familiar with this solubility chart that's found on nearly every CWE tutorial:

Solubility
| 31C water | 21C water
Aspirin | 1g / 100 ml | 1g / 300 ml
Acetaminophen | 1g / 70 ml | 1g / 150 ml
Codeine | 1g / 2.3 ml | 1g / 0.7 ml


Has anyone else ever wondered why it states that codeine is more soluble in cold water than hot water? Surely this must be a mistake, or does codeine phosphate's chemistry somehow "magically" make it more soluble in cold water than hot.

[resorcinol]

I'm not sure whether that data is correct, but it does depend on the salt. Some salts are more solube as temp goes up, some are more soluble as temp goes down. It has to do with how the stability of the ions lining up with the partial charges on the water molecules is affected by temperature (avg kinetic energy of the particles in the solution).

[bigfootlives]

Hmm I wonder if the Merck index has any info on codeine phosphate's solubility at different temperatures? I'm not gonna pretend to be familiar with the Merck index, but I've heard it's got lots of information pertaining to this type of thing right?

[resorcinol]

The Merck Index does have solubility info, but I'm not sure if they give solubility over a range of different temperatures and pressures, or just at STP (standard temp and pressure).

Solubility curves show you solubility of a particular salt over a wide range of temperatures.

Cerium sulfate nonahydrate
Ce2(SO4)3 * 9H20
^
That's an example of a salt that is less soluble in water as temperature increases.

Potassium nitrate
KNO4
^
Its solubility in water increases (quite dramatically) as temperature increases.

So there are definitly salts that have both types of behavior.

There are also salts that have erratic behavior; for example they may become more solube as temp goes up for a while, then at a certain temperature begin to become less soluble again as temperature continues to rise.

The reason for the amount of a solute that can dissolve (we'll only consider water at the moment since it's the most common solvent) in a solvent has to do with the polarity of both the solute and the solvent and the stability of the electrostatic interactions between the solute and solvent molecules at differing temperatures. Nature will try to create the least energetic systems possible, so this boils down to (no pun intended) what ratio of solute:solvent is most energetically favorable at any given temperature or pressure (pressure matters too for GASES dissolving in liquids and solids... that's why when you open a soda bottle CO2 begins to come out of solution... CO2 is less soluble in water at lower pressures (and higher temperatures, so high temp will make your soda go flat quicker).

Edit: I'm not sure how codeine phosphate behaves exactly. That data may be correct. I'm sure there's at least a value for codeine phosphate's solubility at STP in water in the Merck Index. I'm not sure if there's a solubility curve for codeine phosphate that could be found on google. I'll look.