If the deviation of calibration curve from linearity in spectrophotometry is generated by the simultaneous presence of a second complex in a stepwise complex system, it has been demonstrated theor. that a straight line can not be obtained by measuring at the wavelength of the isosbestic point of the two stepwise complexes. Because this wavelength only provides a condition, i.e. e1:2 = 2e1:1, where e is molar absorptivity, and according to the absorbance formula of a system in which both 1:2 and 1:1 complexes are present at the same time, a straight line can be obtained only when the requirement e1:2 = e1:1 + eL is met, where L is ligands. So, it is necessary to find another wavelength to meet this requirement. This wavelength was found theor. and used in the scandium-Xylenol Orange system. As a result, a calibration curve with satisfactory linear relationship was obtained.e

Key words: SPECTROPHOTOMETRY, COLORIMETRIC METHOD, COMPLEX COMPOUNDS, XYLENOL ORANGE, WAVE LENGTH, SCANDIUM, CURVE, LINEAR EQUATIONS, ION CONCENTRATION