A non-linearity criterion applied to the calibration curve method involved with ion-selective electrodes

Some rational functions of the Padé type, y=y(x; n,m), were applied to the calibration curve method (CCM), and compared with a parabolic function. The functions were tested on the results obtained from calibration of ion-selective electrodes: NH4-ISE, Ca-ISE, and F-ISE. A validity of the functions y=y(x; 2,1), y=y(x; 1,1), and y=y(x; 2,0) (parabolic) was compared. A uniform, integral criterion of nonlinearity of calibration curves is suggested. This uniformity is based on normalization of the approximating functions within the frames of a unit area.