Measuring the solubility of liquid aldehydes 4-ethylbenzaldehyde; 3,5-dimethylbenzaldehyde and 3,4-dimethylbenzaldehyde in ethanol, methanol and THF.

During the course of these measurements I had opted purely to measure the solubility of solid aldehydes. Unfortunately a major error of judgement (I did not expect that liquids would be sold in grams) led me to choose 3 liquid aldehydes from the list compiled, that could be chosen from Sigma. I also did not refer to Chemspider for further property data, not really realising what chemspider was about.
Anyway, I have these three liquid aldehydes and I'm going to use Khalid's method of miscibility and density calculations to measure the solubility. I am going to make some changes to the amounts used to account for the very small amounts that I have of some chemicals.[That would be a place to start but even though all liquid solutes are miscible in methanol so far, that may not be the case in other solvents JCB]

I'm only doing the experiment for 3,4-dimethylbenzaldehyde now. I got two layers as you'll see in my method and I have stopped doing the others until I know that the rationale behind how I'm working out the results is sound - JH
I've kept the samples as this experiment used up all this aldehyde. If I need to repeat the experiment, I can collect up the samples and vac off the solvents to retrieve the aldehyde.

Method

100 µL ethanol, 100 µL methanol and 100 µL THF were added to 1.5 mL glass mass spec vials. 200 µL 3,4-dimethylbenzaldehyde (yellow-brown liquid) was added dropwise (from a Gilsen pipette) into each solvent. Each vial was swirled gently. It was difficult to see if there was one or two layers as the top layer could have been a meniscus. A further 100 µL 3,4-dimethylbenzaldehyde was added to each. There was still some difficulty over deciding the miscibility. A further 100 µL solvent was added to each sample. On seeking further advice, it was decided that it was two layers with a very slight colour difference and viscosity between the layers. The lower, main layer for each sample, which is a paler yellow in colour was pipetted carefully into a weighed Eppendorf. [Like with the solid solute you need to thoroughly mix by vortexing- if you do that I think you'll find miscibility JCB] [ok will vortex thoroughly in future - JH]

The mass of the solvents (200 µL) was calculated by measuring the masses of an empty eppendorf and the eppendorf with solvent in. The mass of the solvent was removed from the mass of the solvent + aldehyde to leave the mass of aldehyde in 200 µL solvent. This was processed by the normal means to calculate the solubility.

Results

The full calculations and figures can be found in the spreadsheet JennyHale-14

From the method I have used I have measured the solubility of 3.4-dimethylbenzaldehyde as 8.54 M in ethanol, 6.77 M in methanol and 7.74 M in THF. The result for methanol is lower than I would have anticipated, given the calculated solubility for the aldehyde in ethanol. On every compound I have measured so far, the solubility in methanol has been greater than in ethanol which is why I think the methanol result may be strange.
[Jenny I hope you would'nt mind me commenting here, I just think that it may not be possible to make a liquid solute more concentrated by dissolving it in any solvent. Based on the density (1.012g/mL from sigma aldrich) the concentration of 3,4-dimethylbenzaldehyde (98%) would be 7.54M]- Khalid
[oh thanks, well spotted. I was having a lot of trouble with this on Friday when I was working on it. It explains the weird number in relation to methanol. I just kept thinking that the methanol number was wrong and didn't consider it could be the ethanol at fault. The only thing that concerns me is that by using density calculations alone, each solute has the same solubility in any solvent and I would have anticipated there to be some differences in some solvents (this may involve me being thick and not thinking straight though!) - JH] [Jenny when we use the density of the pure solute to calculate the maximum concentration it just means that all concentrations up to that number can be made in a given solvent where it is miscible JCB][okay that makes sense - JH]

If we assume that the methanol was in fact fully miscible, the solubility can be calculated as 7.54 M. In both cases, the solubility is less than that calculated for ethanol.