3.2.9.5. Turbidity is Measured by Spectrophotometry with Long Light Path Flow Cell

This work confirms that absorbancies of suspensions of Formazin and other heterogenous solutions measured by spectrophotometry are well reproduced and form a strictly linear calibration line that correlates with NTU units. This is because loss of light in the Long Light Path flow cell follows the Beer-Lambert law that describes a linear relationship between concentration and logarithmic loss of light intensity along the light path length (l).

It is well known that when loss of light is due to absorption by molecules, the coefficient (ε) is molar absorptivity, which is dependent on wavelength. In comparison, when the light loss is due to scattering, the scattering coefficient (σ) is dependent on wavelength, particle sizes, shape, and color. As the experiments show, the loss of light intensity by scattering is logarithmic vs. concentration (P, T, Q), which fits the mathematical structure of Beer’s law, and therefore it is justified to define turbidity by scattering coefficient, which is σ 0.002 A for 1.0 NTU of Formazin measured at 550 nm and calculated for 1cm light path by using average slopes of calibration lines (P, T, Q and Appendix 2).     

Surprisingly, calibration obtained by SS dilution of Suspended Matter has the same slope of 0.02A NTU in a 10 cm light path. This is because the scattering spectra of Formazin (R, Z) are the same as the spectra of the Suspended Matter  (X, Z), and the loss of light is monitored at the same wavelength and length of the light path (10 cm) (AC). Considering the physical differences between Formazin and SM particles this finding is unexpected and unprecedented. But it is supported by the Law of Large Numbers. When the light beam passes through a sample containing a sufficiently large number of randomly formed particles, the random variations will cancel each other, and the light beam will experience a stable average scattering effect of the entire population. It is the unique geometry and construction of the Long Light Path flow cell which fulfills this condition and prevents scattered and stray light from reaching the detector. However, at low particle concentration reproducibility of monitored absorbance (R.S.D.) will decrease as differences in the size of particles will not be smoothed out. Obviously, more research is needed to establish the upper boundaries of linearity as well as the properties of Suspended Matter from other sources.