Handbook of Solvents - George Wypych - ChemTech - Ventech!

15.1 Standard methods of solvent analysis 1061
Depending on the viscosity of liquid and its expected flash point range, one of the
above methods is chosen as described in detail elsewhere. 56 It should be additionally noted
that if the flash point method uses continuous heating, it is not suitable for testing mixtures
of flammable substances because their vapor concentrations are not representative of equilibrium
conditions. One of the weaknesses of flash point analysis is that the flame is well
above the liquid surface therefore full vapor concentration is not attained. Many cases exist
where a flash point cannot be detected but the material does form flammable mixtures. Before
a method is chosen and a data interpretation made full information on the test procedure
should be studied in detail and the proper authorities should be consulted to define safe
practices for a particular material.
15.1.18 FREEZING POINT
Freezing point apparatus consists of freezing tube, Dewar flasks to act as cooling and
warming baths, stirring mechanisms, absorption tubes, clamps and other auxiliary parts. 57
Freezing point can be obtained precisely from interpretation of time-temperature freezing
and melting curves. The determination is made by measuring the electrical resistance of
liquid which decreases on cooling and becomes constant when it freezes. This method in
conjunction with the testing details described in a separate standard 58 can be used to determine
the purity of many hydrocarbon solvents. The data given in the last standard 58 allow
for a precise determination of the purity of solvent in percent of pure compound. A simple
method was designed to determine solidification point of benzene based on visual
observation of formation of solid phase. 59
15.1.19 FREE HALOGENS IN HALOGENATED SOLVENTS
This simple qualitative test involves the extraction by water of free halogens, followed by
the reaction of the halogens with potassium iodide in the presence of a starch indicator. 60
The solution color changes to blue in the presence of free halogens.
15.1.20 GAS CHROMATOGRAPHY
Gas chromatography provides many tools for the analysis of solvents. In section 15.1.4, a
method was discussed which determines the distribution of boiling points of hydrocarbon
mixtures. Many uses of gas chromatography for the determination of purity of different solvents
are discussed in Section 15.1.25. In this section, some examples of of gas chromatography
are included to show its usefulness in the qualitative determination of solvents
mixtures 61 the analysis of solvent impurities, 62-64 the determination of solvents in a product
by direct injection, 65 and the generation of data to evaluate waste materials to determine
their hazardous content. 66
The relative distribution of aromatic hydrocarbons in xylene products can be quantitatively
determined by gas chromatography. 61 A flame ionization or thermal conductivity detector
is used with a capillary or packed column containing crosslinked polyethylene glycol
as the stationary phase. The peak area of each component is measured and the weight percentage
concentration is calculated by dividing the peak area of the component by the sum
of the areas of all peaks.
Ortho-xylene concentration and the concentrations of its admixtures and impurities
are measured using a flame ionization detector and a polar fused-silica capillary column. 62
An internal standard (iso-octane) is used to increase precision and a standard mixture is used
for calibration. A similar method is used with p-xylene but either n-undecane or n-octane
are used as the internal standards. 63 In both methods, peak areas are interpreted relative to
the peak area of internal standard. The main impurities in benzene are non-aromatics with
less than 10 carbons, toluene, 1,4-dioxane and aromatics containing 8 carbon atoms. The