PhAST Track Mathematics Module: Solubility

The solubility of a chemical is the maximum concentration of a solution which may be prepared with that chemical at a given temperature (usually 15 – 25^oC, although this can vary). The official term for this is “saturation solubility”.

Students often skip this step in practicals, which becomes frustrating (both for the student and the instructor) when the student spends ten or twenty minutes desperately trying to “dissolve” a powder which is not soluble in the vehicle they are using (or in the quantity of vehicle they are using). 

In practicals, we have been making you look up the solubility of any solids you are adding to your mixtures. By knowing the solubility of the solids in the mixture, you can ascertain whether the mixture will be a solution (all solid dissolved within the vehicle), or a suspension (solid particles dispersed within the vehicle). This is important for:

• Labelling of your product, and also for patient counselling
   
• As solutions are evenly distributed (as the powders are dissolved in the vehicle), patients do not have to shake/mix them before measuring out a dose. However, the powders in a suspension will slowly settle to the bottom of the bottle. If the patient doesn’t know to shake the bottle before measuring a dose, they will only be getting the vehicle in the first few doses, and then be overdosing at the end (when they get to the medicine powder at the bottom of the bottle!)
   
• Importantly for compounding practicals, this tells you how best to prepare your mixture (and when to stop stirring!)

There are two ways you will find solubility expressed:

1. Using pre-defined terms to define solubility level (used in the BP):
    
2. If a chemical’s solubility is > ~1 in 1000 parts of solvent, the solubility may be stated as number of parts of solvent required to solubilise one part (by weight or volume) of the chemical. This method of expressing solubility is sometimes used in the USP, and may be more useful in your dispensing practicals (as it gives you a numerical quantity of solvent required per gram of drug used)