Procedure for Quantitative Determination Neutral Spectra

This procedure is followed for quantitative measurements of lignin in an unknown sample, such as for the determination of lignin concentration in water 

Accurately weigh 0.5 g (to 0.001 g) of the lignin sample into a 1000-ml volumetric flask and dilute to the mark with the solvent. For an aqueous solution, adjust the pH to 5 with acetic acid prior to dilution. 

Using a 1-ml pipet (0.01 subdivisions), a suitably-sized aliquot (e.g., 0.3 ml) of the above solution is transferred to a 1-cm path quartz curvette (=3-ml capacity) and diluted ten fold (i.e., to 3.0 ml) with the same solvent used to dissolve the sample (Note 1). 

Determine the UV light absorption in the 210 to 400 nm range in a doublebeam spectrophotometer against the solvent in a matching cuvet. The absorbance reading should be between 0.2 and 0.8 at the 280nm maximum (Note 2). 

Note 1. Adjust the concentration of the lignin solution as needed to ensure the absorbance reading being within the specified range. Too high a concentration may result in deviation from Beer s law (linearity between absorbance and concentration). 

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