§ 796.1050 Absorption in aqueous solution: Ultraviolet/visible spectra.

(a) Introductory information—(1) Guidance information. (i) Molecular formula.

(ii) Structural formula.

(2) Standard documents. The spectrophotometric method is based on national standards and consensus methods which are applied to measure the absorption spectra.

(b) Method—(1)(i) Introduction, purpose, scope, relevance, application and limits of test. (A) The primary environmental purpose in determining the ultraviolet-visible (UV-VIS) absorption spectrum of a chemical compound is to have some indication of the wavelengths at which the compounds may be susceptible to photochemical degradation. Since photochemical degradation is likely to occur in both the atmosphere and the aquatic environment, spectra appropriate to these media will be informative concerning the need for further persistence testing.

(B) Degradation will depend upon the total energy absorbed in specific wavelength regions. Such energy absorption is characterized by both molar absorption coefficient (molar extinction coefficient) and band width. However, the absence of measurable absorption does not preclude the possibility of photodegradation.

(ii) Definitions and units. The UV-VIS absorption spectrum of a solution is a function of the concentration, c₁, expressed in mol/L, of all absorbing species present; the path length, d, of the spectrophotometer cell, expressed in cm; and the molar absorption (extinction) coefficient,ε_i, of each species. The absorbance (optical density) A of the solution is then given by:

For a resolvable absorbance peak, the band width λ is the wavelength range, expressed in nm = 10−9 m, of the peak at half the absorbance maximum.

(iii) Reference substances. (A) The reference substances need not be employed in all cases when investigating a new substance. They are provided primarily so that calibration of the method may be performed from time to time and to offer the chance to compare the results when another method is applied.