6 VALUE OF K2 INDEPENDENT OF TRUE CONCENTRATION OF YELLOW COMPONENT

THE PRECISE CONCENTRATION of component Y was unknown since we do not know the chemical nature of the yellow intermediate compound and, hence, we could not know the precise spectral reflectance data to employ in the color-matching computations. Arbitrarily, the spectral reflectance curve or the yellow (unlaked) form of alizarin, 2,4-dihydroxyanthraquinone, was selected for our initial calculations, but we do not wish to imply that this is indeed the intermediate, Y. As the results in Table II indicate, however, the use of any one of three different yellow colorants in the color-matching program, even though each had slightly different spectral reflectance characteristics, had little effect on the values of k2. Consideration of equation vii will show why this is so: the intercept, k2 [that is, (dY/dt)/Y, when A/Y = 0] is not sensitive to uncertainties in Y. This is because any correction factor necessary to yield the true value of Y will appear in both the denominator (dY/dt) and numerator (Y). Thus, the effect on the intercept, k2 of a systematic factor of error in Y tends to cancel out.

Table II Specific Rate Constant for Fading of Yellow-Form, k2, Using the Spectral Reflectance Characteristics of Three Different Yellow Pigments in the Color-Matching Calculations When Computing the Concentration of the Yellow Component Vehicle: Vinac� B-7; Paint at Complete Hiding