Butyric acid can often be extracted from aq. sol. by phase separation assisted with 'salting out', what may be achieved by adding CaCl2 or KCl up to saturation. Alternatively, it may possibly be precipitated from the hot aq. sol. as calcium butyrate, by adding 'milk of lime' to the hot aq. sol. (~ 75 ºC).

Hi Carlos, thanks for your answer. What if my sample not only contain butyrate, but acetate and propionate also exist? Is it possible to just extract butyrate out? Tq. 

You can extract the acids into an organic solvent, then separate by vacuum distillation. A few theoretical stages is enough.  Be sure to reduce the pH to about 2 prior to extraction, to convert the acids from the salt to the protonated form.  Ethyl acetate is a good solvent, if you don't mind some transesterification to ethyl butyrate and ethyl propionate.  To avoid that, use MTBE or methylene chloride.  

Dear Sherlyn Tang:

The solubility of in water by 75 ºC ― a seemingly reasonable temperature that could be selected for the intended purpose ― would be 0.700 g/kgH2O for calcium butyrate, and 2.10 g/kgH2O (3-fold higher) for either calcium acetate dihydrate or calcium propionate (*). These solubilities would be reduced due to the common ion effect upon addition of 'milk of lime' to the hot aq. solution. Such addition would also increase the pH, what may also influence the solubility, but presumably to a lesser extent, comparing to the common ion effect. From this data we may expect a selectivity factor of around 3 for 'milk of lime' assisted precipitation from hot aq. solution. This may not seem a particularly high selectivity, but the simplicity of the method and economical considerations may help to support this option. Moreover, the method may be used to complement a seemingly possible prior extraction step of butyric acid, from the initial aqueous phase, by phase separation assisted with 'salting out' ― promoted by CaCl2 or KCl addition. By combining a first stage of phase separation assisted by 'salting out' with a second stage of precipitation from the previously extracted aq. sol., it seemingly could be possible to reach a satisfactory selectivity for the intended separation process.  

(*) Solubility data interpolated from "International Critical Tables" (Vol. IV, 1928, p. 230).

Dear Sherlyn Tang:

I would like to further complement my last answer, which tried to address your last comment:

Note that the boiling point of acetic acid occurs by 118.5 °C, of propionic acid at 141 °C, while for butyric acid we have 164 °C. Being the least volatile of the compounds considered (water included), butyric acid separation could be also promoted trough distillation in vacuum and/or steam-assisted distillation, since it would concentrate at the bottoms.

Hello Carlos

Good thought but there may be a problem with azeotrope formation.  According to https://en.wikipedia.org/wiki/Azeotrope_tables, butyric and propionic acid each form an 18% azeotrope with water at atmospheric pressure, with boiling points of 99.94 C and 99.98 C, respectively.  Of course the azeotropic compositions would be different under vacuum - I haven't had a chance to look for that information.  But assuming significant azeotropes still form and have similar volatility, separation by distillation would not be possible.  Prior solvent extraction (see my comment above) would get around this problem, and your suggestion of adding salt before extracting is a good one.

Hope this helps,

Alan

Dear Alan:

Thank you for calling my attention to azeotropes that butyric and propionic acid can form by 18% w/w with water at 100 ºC, at the normal atmospheric pressure; according to the tables you have quoted, while acetic acid would not form azeotrope. Actually, I did not expected that distillation could completely separate butyric acid from the other acids, but I still think that may in principle be considered as a possible method to «promote» such a separation, since butyric acid can be expected to «concentrate» at the bottoms. I did not mean that butyric acid could be isolated; just concentrated. Although I did not checked the vapour pressures and the possible azeotropes that might form, in vacuum, for the rather complex butyric acid ― propionic acid ― acetic acid ― water system, I still think that butyric acid can, in principle, be concentrated by vacuum and/or steam-assisted distillation. There are known ways to deal with the azeotropes that may form at distillation processes, but it seems useless to enter in such details without investigating the vapour - liquid equilibrium diagrams of this particular system.

Regards   

Suggest the vacuum distillation be tried in the lab.  Pretty simple to at least investigate the feasibility.  In theory several other of the ideas may work such as liquid extraction or salt out methods, but you use be able to generate phase separations, possibly by some moderate heat into or out of the mix. Probably will require some recycle stream or two.