Unless otherwise prescribed, measure the absorbance at the prescribed wavelength using a path length of 1 cm and at 20 ± 1 °C. Unless otherwise prescribed, the measurements are carried out with reference to the same solvent or the same mixture of solvents. The absorbance of the solvent measured against air and at the prescribed wavelength shall not exceed 0.4 and is preferably less than 0.2. Plot the absorption spectrum with absorbance or function of absorbance as ordinate against wavelength or function of wavelength as abscissa.

Where a monograph gives a single value for the position of an absorption maximum, it is understood that the value obtained may differ by not more than ± 2 nm.

Apparatus. Spectrophotometers suitable for measuring in the ultraviolet and visible range of the spectrum consist of an optical system capable of producing monochromatic radiation in the range of 200 nm to 800 nm and a device suitable for measuring the absorbance.

Control of wavelengths. Verify the wavelength scale using the absorption maxima of holmium perchlorate solution R, the line of a hydrogen or deuterium discharge lamp or the lines of a mercury vapour shown in Table 2.2.25.-1. The permitted tolerance is ± 1 nm for the ultraviolet range and ± 3 nm for the visible range.

Table 2.2.25.-1. - Absorption maxima for control of wavelength scale

241.15 nm (Ho)

404.66 nm (Hg)

253.7 nm (Hg)

435.83 nm (Hg)

287.15 nm (Ho)

486.0 nm (Dß)

302.25 nm (Hg)

486.1 nm (Hß)

313.16 nm (Hg)

536.3 nm (Ho)

334.15 nm (Hg)

546.07 nm (Hg)

361.5 nm (Ho)

576.96 nm (Hg)

365.48 nm (Hg)

579.07 nm (Hg)

Wavelength (nm)

Specific absorbance a 1 per cent cm

Maximum tolerance

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