Measuring the solubilities of carboxylic acids in ethanol, methanol and THF

This experiment measures the solubilities of n-octanoic acid (sodium salt); 2-hydroxypropane-1,2,3-tricarboxylic acid monohydrate; 2-oxopentanedioic acid; 3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-enoic acid and 4-methylbenzoic acid.

Method

30 Eppendorfs were taken and split into two groups of 15. Group 1 were labelled with the planned contents and group 2 were labelled with the planned contents and weighed.

Saturated solutions of n-octanoic acid; 2-hydroxypropane-1,2,3-tricarboxylic acid monohydrate; 2-oxopentanedioic acid; 3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-enoic acid and 4-methylbenzoic acid were prepared by adding 0.75 mL of each solvent (ethanol, methanol and THF) to its correspondingly labelled Eppendorf and adding solid, mixing the contents by vortexing to ensure complete dissolution before addition of more solid until the solution was saturated. This gave 15 saturated solutions of n-octanoic acid in ethanol, methanol and THF; 2-hydroxypropane-1,2,3-tricarboxylic acid in ethanol, methanol and THF; 2-oxopentanedioic acid in ethanol, methanol and THF; 3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-enoic acid in ethanol, methanol and THF; and 4-methylbenzoic acid in ethanol, methanol and THF.

The saturated solutions were centrifuged for 1 minute at 13 200 rpm in order to ensure that the remaining solid was separate from the supernatant. 0.5 mL of each solution was transferred to it's corresponding weighed group 2 eppendorf.
The group 2 eppendorfs were transferred to the speedvac and dried under vacuum, initially for three hours. At this point, all the eppendorfs were weighed. The eppendorfs were returned to the speedvac to be dried to constant weight.
The eppendorfs were dried by speedvac overnight with heat and weighed again after overnight evaporation. the Eppendorfs were returned to the speedvac once again and dried under vacuum with heat for a further 2 hours. After this time eppendorfs had been found to be dried to constant weight.
Some eppendorfs had gained in weight overnight. I don't know why. I put them back on the speedvac too. I can only think that the balance is not stable in it's calibration of the fourth decimal place or I made an error reading the balance. These gains were lost after further speedvac evaporation. [There are other ways that the samples might actually have gained weight - especially if left for an extended period - the fact that the gains are lost after further evaporation suggest that it is real - how might this have happened? - CN]
The samples were left for an extended period overnight in the speedvac, which was why I was confused. If they had been in air, I could have understood it and would have said that the gains were made from picking up moisture in the atmosphere. The gains being real, I can only account for this as perhaps the speedvac is not going under vacuum as strongly as it could. I am currently at a loss to other reasons for a sample under a vacuum and heat to gain weight.

The experimental details and full analysis can be found at http://spreadsheets.google.com/ccc?key=pUtF1N4vVD6cvLnOmPczyOw&hl=en

Discussion

The slight changes in method to only use 0.75 mL of solvent made a big difference. The eppendorfs did not fill beyond their capacity and produced a more manageable volume.
The results showed that all five acids tested were soluble in the listed solvents to varying degrees. The n-octanoic acid was most soluble in methanol [Are you sure you were using octanoic acid? You report a white solid but it melts at 17C and EXP022 reports it is miscible in methanol JCB] and least soluble in THF. 2-hydroxypropane-1,2,3-tricarboxylic acid monohydrate was also most soluble in methanol and least soluble in THF. 2-oxopentanedioic acid was most soluble in methanol and least soluble in ethanol. 3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-enoic acid was most soluble in THF and least soluble in ethanol. 4-methylbenzoic acid was most soluble in THF and least soluble in methanol.

n-Octanoic acid

Khalid had contacted Jean-Claude to query a result of mine and I was contacted about it. The problem was that I had reported n-octanoic acid to be a white solid when it melts at 17°C. Obviously our lab is not likely to be so cold that it is still solid, as later experiments have measured the room temperaure of the lab to be 23°C. I have been back and found the mistake is mine. On the spreadsheet for the experiment, I have written that n-octanoic acid is also known by the name caprylic acid. On the experimental write up page, I only wrote the IUPAC name. I have been back and retrieved the pot of caprylic acid. In smaller writing underneath, it says sodium salt. I had not noticed this when I picked up the pot initially and carried out the experiment. This was extremely careless of me and does make a case for always photographing the chemical.
Here are pictures of the label on the pot and a sample of the contents:
PICT0015.JPG
PICT0017.JPG

I will check what details I have written down for the compound in the spreadsheet and amend accordingly if I have the RMM for n-octanoic acid written down.

Sorry for the confusion, and thanks to Khalid for flagging up my mistake.

I don't think it's very meaningful to report the final solubility to > 4 decimal places - RG. Fully agreed n this - experimental errors will easily swamp this declared accuracy. AJW. Have updated all spreadsheets to include sensible solubilities. Still arguing with googledocs as to how to format for set number of decimal places JH 26.11.08. Type "=round(cellnumber, 2)" JCB