An order determination method in direct derivative absorption spectroscopy for correction of turbidity effects on COD...

In chemical oxygen demand (COD) measurements based on ultraviolet-visible spectroscopy, turbidity interference represents a difficult technical problem. Common methods need the spectrum of real turbidity particles as the baseline, but it is inconvenient and vain to obtain because it changes with perturbation. Due to lack of theoretical guidance, the direct derivative method with no need of baseline was rarely used. Here in, we studied the absorption spectra of turbidity particles in series of samples and proposed an order determination method in the` direct derivative. In turbidity interference handling, the absorbance derivative of turbidity at each wavelength fell to zero by determining the appropriate derivative order. Two parameters were presented for order determination, which represented the overall size of particles and its scattering type based on the spectral shape and profile. Experiments showed that the formazine solution, as an extreme situation, had the largest parameters, so the derivative order was determined depending on it to ensure effective turbidity correctionin various real water samples. Through comparing the correction effects and COD prediction results of first-to-fourth order derivative spectra, the proposed method was verified to provide theoretical basis of direct derivative method and reduce blindness in the order determination.