Xylene impurities, analysis

Impurities in benzene, toluene, xylene, naphthas, and industrial aromatic hydrocarbons are determined by a simple colorimetric analysis of an acid wash. A solvent is washed with sulfuric acid and the color of the acid layer is determined by a visual comparison with color standards prepared Irom solutions of cobalt chloride and ferric chloride. 

Analysis of product xylenes is by ASTM D3797 (o-xylene) or D3798 (p-xylene). These procedures are similar, and satisfactory columns are given in Table 13.7. The concentration of impurities is measured by internal standard calibration and the purity of the o- or p-xylene is determined by subtracting the percent impurities from 100%. 

TABLE 13.7 Typical Columns for Analysis of Impurities of Xylenes... 

The range and parity of solvents have improved considerably over recent centuries. At the beginning of the twentieth century, it was difficult (and expensive) to obtain acetone without water or toluene without sulphur contamination (Thorpe, 1913). These contaminants would have short- and long-term effects on objects and would create different behaviours to that expected now of nominally the same materials. All solvents contain impurities. Solvents bought as chani-cals can be obtained at very high purity, up to 99.999% for critical applications such as pesticide analysis. For many industrial purposes, high purity (>95%) is not necessary. However, the purity of solvents used on objects should be at least 99%, and there should not be any non-volatile impurities or any that might react with or remain in the object (Khanjian et al., 2004). Many solvents are not sold as pure chemicals but to conform with performance specifications, particularly ethanol and hydrocarbon solvents. For instance, specifications for Xylene S do not explicitly state that xylene is present (Shell, 2007) and commercial xylene often contains ethyl benzene as part of the mixture. 

This test method covers the analysis of normally occurring impurities in, and the purity of, oxylene by gas chromatography. Impurities determined include nonaromatic hydrocarbons, benzene, toluene, p- and m-xylenes, cumene, styrene, and ethylbenzene. 

A known amount of internal standard is added to the sample. A gas chromatograph equipped with a flame ionization detector and a polar-fused silica capillary column is used for the analysis. The impurities are measur relative to the internal standard. To calculate o-Xylene purity subtract the impurities found from 100.00 %. 

This test method covers the determination of known hydrocarbon impurities in, and the purity of p-xylene by gas chromatography (GC). It is generally meant for the analysis of p-xylene of 99 % or greater purity. Impurity concentrations that can be measured range from 0.001 to 1.000 weight %. 

Purity is commonly reported by subtracting the determined expected impurities from 100.00. However, a gas chromato phic analysis cannot determine absolute purity if unknown components are contained within the material being examined. Refer to Specification DS136 for determining other chemical and physical properties of p-xylene. 

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