The spectrophotometer is intended for measurement of spectral coefficients of a transmission of liquid substances in the field. The principle of measurement of two light streams relation is underlies the spectrophotometer performance: the one stream passes through a studied sample, while the other stream falls on it (or passes through a control sample).
The method of pK definition measures the ratio of an ion concentration and a neutral molecule of the studied compound dissolved in a series of solutions. This definition is conducted with an analytical wave length on which the greatest difference in values of optical density between an ion and a neutral molecule is observed. An ideal analytical wave length is such a length at which one particle strongly absorbs the light and the other is weak. As such conditions seem to occur very seldom, analytical wavelength is usually chosen if there is the greatest difference in optical density between both solutions. The second factor concerns the optical density of both solutions that only slightly changes with the change of the wavelength.
Analytical wavelength may also be of some preference when: a) the maximum of an absorption curve of one particle is over the minimum of an absorption curve of the other particle; b) the maximum of an absorption curve of one particle is over the maximum of the curve of the other particle provided that a difference in indications of optical density of both particles is not less, than 0,2; c) the maximum of an absorption curve of one particle is over a slope or a shoulder of an absorption curve of the other particle; d) the minimum of an absorption curve of one particle is over the minimum of an absorption curve of the other particle.
While determining the pK value it is necessary to prepare fresh solutions and measure the optical density of both particles with an analytical wavelength. As a result the obtained values A„ and A0 may be used further in pK calculations. At the stage of pK definition the concentration of solution is equal 10-4 M.
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