Page 146 - Illustrated Pocket Dictionary of Chromatography

146 PETROLEUM ETHERS

petroleum ethers Alkyl hydrocarbons produced by taking
defined boiling range fractions from crude oil distillates. It should be
noted here that “ether” does not refer to the chemical class of com-
pounds, R—O—R¢, but rather to the high volatility of many of the
petroleum ether fractions. These fractions are defined by a boiling
range, a typical range used in GC work being 30–60°C. In this range
the fraction consists of mainly pentanes (boiling point ~35°C) and
hexanes (boiling point ~70°C). Fractions are also prepared over
narrower ranges and continue up to about 200°C.

pH The pH scale is a logarithmic measure of the hydronium ion, H ,
+
concentration in a solution:
pH =- log H [ + ]

In practice a common error made in the preparation of organic/aqueous
buffer solutions is that the pH is “measured” after mixing the solvents
rather than obtained in the aqueous phase alone (before mixing). Keep
in mind that glass pH electrodes are designed to respond to hydronium
ion concentration in water; the hydration layer in the electrode is
adversely and irreproducibly altered by the presence of the organic
modifier. Best practice is to prepare the buffer in the aqueous phase (at
a concentration high enough to compensate for the dilution with the
organic solvent) and then mix with the organic solvent.
phase A part of a system that is physically distinct and bounded,
that is, forms a definable interface with one or more other parts
(phases) of the system. Chromatographic systems utilize phases to
effect separation, being liquid-liquid (e.g., countercurrent chromatog-
raphy and liquid-liquid extraction), liquid-solid (e.g., HPLC and TLC),
and liquid-gas (e.g., GC and distillations).

b
phase ratio, b The volume of the mobile phase, V M, with respect
to the volume of the stationary phase, V S:

b= V M V S
For capillary GC open-tubular columns, b= (r - 2d f)/2d f, where r is
the column radius and d f is the thickness of the film (i.e., the station-
ary phase).
phenyl-bonded phase A phenyl-bonded phase has a phenyl func-
tional group covalently bonded to the surface of the support material.