# How to measure solubility?

pashafer | 02:06 Sun 20th Feb 2005 | Science
How can I measure just how well something is disolved? Is there a definition?

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Usually measured in mass of solute per volume of solvent.

eg g/dm^3

Remember that solubility of solids varies with temperature, and gaseous solubility varies inversely with temperature.

Classically to measure solubility in water you first mix water and octanol together, the two are immiscible and the mixture forms two layers. Add the sample to this and determine the concentration of sample in each of the layers (by HPLC for example). This gives you the hydrophobicity of the sample. In addition you need to know the lattice strength of the sample (how well it sticks together) you can find this out by measuring the melting point. These two measurements form part of the 'general solubility equation' which will give you a good estimate of a compounds solubility in water. Techniques such as Nephelometry can be used to assess the amount of undissolved material in a solution, although this is somewhat innacurate. Without having a sample there are some methods for using either 2D or 3D structure of the sample in tandem with regression or neural networks / genetic algorithms to assess solubility. Full quantum mechanical models of solubility are available but time consuming. Solubility is important in assessing the efficacy of potential drug compounds and is routinely assessed in the pharmaceutical industry.
Sorry roder but that is the biggest load of rubbish I have read for a long time.

Also, brachiopod, some solids become less solubly as the temperature increases.

Generally speaking at least for solids such as table salt (NaCl) for example, if you cannot see the solid, it is probably dissolved.  Therefore, a good way to test the solulbility of a substance,would be to keep adding the solid to the solution until the solid is no longer being dissolved so that you could see the solid phase present.  The presence of a solid phase in a solution that is in equilibrium suggests that the solution is saturated with respect to that solid.  This method might work with NaCl, but for other solids, it might take quite a long time for the solution to come to equilibrium, so it might not be very practical.  You can calculate the solubility of different subsances.  For a salt where the reaction is the dissasociation of two or more ions:

NaCl ->  Na + Cl

(Na)(Cl)=K(sp)

Basically, at equilibrium the product of the activities of Na and Cl will equal a constand K(sp), which can be looked up in almost any chemistry text.  For back of the envelope calculations, you can use molar concentrations (moles/liter) instead of activities.  The constant will be for a specific temperature (I believe 25 deg C), so this calculation will be approximate.  In this case the mole ratio of Na to Cl is one, so if you solve the following equation for x:

x^2=K(sp)

this molar concentration will be the solubility for NaCl.

I hope this helps.  You could probably find a more complete explanation on the web quite easily.

make a saturated solution buy dissoving so much material that no more will dissolve....filter it. let a known amount of the liquid evaopurate and measure the mass of solid left. Subtract the amount of solid left from letting the solvent evapourate before you added the solid and now you know how much material will dissolve in how much solvent therefore the solubility.
Question Author

Thanks all. Given that this was to help my 15 year old daughter in a high school science project, I am afraid most of the answers will go over her head. Not sure about her head, but its way past me.

Thanks guys.

since i am also a 15 year old doing an experiment such as this i would say that decanting or evaporating the liquid will be the easiest way as long as you have the measurement of the solvent before you add it to the liquid.

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