Method of determining the concentration ratio of two substances

Abstract

The concentration ratio of two components of a mixture of substances, having absorption bands which are adjacent each other in a manner such that the radiation absorption of the mixture of substances has a minimum between these absorption bands, is determined by measuring, in three adjacent, narrow spectral regions, of the radiation transmitted by the mixture of substances, a variable which is proportional to the ratio .DELTA..sup.1 I / .DELTA..sup.2 I wherein the numerator .DELTA..sup.1 I = (I.sub.3 - I.sub.1) is the difference between the intensities of radiation of the two outer spectral regions, and the denominator .DELTA..sup.2 I = (I.sub.3 - I.sub.2) -(I.sub.2 - I.sub.1) is the value by which the respective differences between the intensities of radiation of each outer spectral region and the middle spectral region differ from each other. The position, in the spectrum, of the three adjacent regions is adjusted so that they are located between the absorption maxima of the two components and where the numerator .DELTA..sup.1 I becomes zero at a definite concentration ratio which is preferably the ratio at which the highest accuracy of measurement is desired. Alternatively, the variable is proportional to the ratio I'/I', which is the ratio of the first derivative I' to the second derivative I' of the distribution of spectral intensity.