Appendix VIII O. 2-Ethylhexanoic Acid

Examine by gas chromatography (2.2.28), using 3-cyclohexylpropionic acid R as the internal standard.

Internal standard solution Dissolve 100  mg of 3-cyclohexylpropionic acid  R in cyclohexane  R and dilute to 100  ml with the same solvent.

Test solution To 0.300  g of the substance to be examined, add 4.0  ml of a 33  per cent  V/V solution of hydrochloric acid  R. Shake vigorously for 1  min with 1.0  ml of the internal standard solution. Allow the phases to separate (if necessary, centrifuge for a better separation). Use the upper layer.

Reference solution Dissolve 75.0  mg of 2-ethylhexanoic acid  R in the internal standard solution and dilute to 50.0  ml with the same solution. To 1.0  ml of the solution add 4.0  ml of a 33  per cent  V/V solution of hydrochloric acid  R. Shake vigorously for 1  min. Allow the phases to separate (if necessary, centrifuge for a better separation). Use the upper layer.

The chromatographic procedure may be carried out using:

• — a wide-bore fused-silica column 10  m long and 0.53  mm in internal diameter coated with macrogol 20  000 2-nitroterephthalate  R (film thickness 1.0  µm),

• — helium for chromatography  R as the carrier gas at a flow rate of 10  ml/min,

• — a flame-ionisation detector,

with the following temperature programme:

Inject 1  µl of the test solution and 1  µl of the reference solution.

The test is not valid unless the resolution between the peaks corresponding to 2-ethylhexanoic acid (first peak) and the internal standard is at least 2.0.

Calculate the percentage content of 2-ethylhexanoic acid from the expression: