Question

what are the advantages and disadvantages of determining the actanol-water partition coefficient of compounds by oscillation method?

Answer 1

The classic and most reliable method of determination of log P is the agitation bottle method, which consists in the dissolution of some of the solute in question in a volume of octanol and water, and the subsequent measurement of the concentration of the solute in each solvent . The most common method for measuring solute distribution is by UV / VIS spectroscopy. There are some pros and cons in this method:

Pros:

It is the most accurate method.

Precise for the widest range of solutes (applicable to neutral and charged compounds)

The chemical structure does not have to be known in advance.

Cons:

Consumption of time (> 30 minutes per sample)

The octanol and the water must be previously mixed and balanced (it takes at least 24 hours to balance)

Complete solubility must be achieved, and it can be difficult to detect small amounts of undissolved material.

The answer vs. concentration UV-Vis response must be linear over the entire concentration range of the solute. (See the law of Lambert-Beer)

If the compound is extremely lipophilic or hydrophilic, the concentration in one of the phases will be extremely small, and therefore difficult to quantify.

In relation to chromatographic methods, large amounts of material are required.

As an alternative to UV / VIS spectroscopy, other methods can be used to measure the distribution, one of the best is the use of a radioactive tracer free of drag. In this method (which is very suitable for the study of metal extraction), a known quantity of radioactive material is added to one of the phases. Then the two phases are put in contact and mixed until equilibrium is reached. Then the two phases are separated and the radioactivity of the two phases is measured. Using a dispersive energy detector (such as a high purity germanium detector) allows the use of several different radioactive metals at once, while the simplest gamma detectors only allow a radioactive element to be used in the sample.