## Science

# Methods for Measuring log P

In the Sirius pH-metric method, a weighed sample (typically 0.5 – 1.0 mg on SiriusT3 is dissolved in a two-phase water-octanol system, and titrated over a pH range (typically 2 to 12 for bases and ampholytes,

12 – 2 for acids). Although the solution becomes opaque during stirring, the pH electrode continues to measure pH of the aqueous component of the solution. Results are obtained by a complex computational process. The pH of each point in the titration curve is calculated using equations that contain pK_{a} and P, and the calculated points are fitted to the measured curve by manipulating the P value. The value of P that provides the best fit is taken to be the measured P value, and is reported as the logarithm , i.e. log P.

As well as log P, the log D value as a function of pH is determined from the data.

In this example the log P of Lidocaine has been measured, with a result of 2.30.