3.2.9. Flow Injection Nephelometry and Turbidimetry
M. Davis, J. Ruzicka, Ch. I. Measures and P. Chocholous August 2025 ®
- 3.2.9.1. Introduction
- 3.2.9.2. Nephelometry
- 3.2.9.3. Turbidimetry
- 3.2.9.4. Determination of Suspended Matter in Sea and Drinking Water
- 3.2.9.5. Turbidity is Measured by Spectrophotometry with Long Light Path Flow Cell
- 3.2.9.6. Beer’s Law Does Apply to Heterogenous Solutions
- 3.2.9.7. Conclusion
Preface
As our work on the automation of nephelometry and turbidimetry (3.2.9.3.) progressed, we realized that the results might be interesting not only for oceanographers but for a wider analytical community. Therefore, we asked our co-workers Chris Measures and his student Madeleine Davis to assent to publish a short Letter to the editor: "Why should Beer's Law be again called Lambert-Beer's Law”.
In the Letter, we pointed out that generations of chemists have been misled by being taught that Lambert-Beer's law does NOT apply to analysis of heterogeneous solutions, because scatter of light by suspended particles does NOT result in logarithmic loss of light intensity, and that this mistaken information is still being taught (Table 1). We also did not question the validity of this misinterpretation, until our experiments aimed at automation of turbidimetry by flow injection analysis have shown that the concentration of suspension of Formazin particles in water is in linear relation to absorptivity, which is what Lambert discovered by measuring loss of light in heterogenous samples. (3.2.9.6.)
But the joy of our accidental discovery did not last, because repeated submissions of the Letter to our favorite Journal were rejected in rapid succession. Reviewers unanimously pointed out that our experiments do not provide evidence broad enough to resurrect Lambert’s contribution, who designed the foundation of the Law, because our experiments with Formazin are too narrow in scope and unlikely to apply to a variety of heterogenous suspensions.
We returned with Madi and Chris to the bench and continued experimenting to find out not only how but also why one obtains reproducible results on materials with such diverse optical properties like particles of Formazin, of a mixture of Montmorillonite with Araldite suspended in sea water, and of colloidal hetero-molybdenum compounds, which are relevant to colorimetric analysis of phosphate and silica (3.2.9.4.). These experiments and their practical outcomes are summarized in the chapter on turbidity. This allows us to predict the interference of Suspended Matter on the detection limit when determining trace nutrients in seawater by spectrophotometry (Appendix 3). Also, by correlating absorbance with scatter in the unified Lambert-Beer law (3.2.9.6.) it will be easier to teach methods based on spectrophotometry and to explain to students why we can obtain a linear relationship between absorbance and concentration when we (more often than we realize) perform spectrophotometry on colloidal suspensions.
To resolve the issue of how to publish our work, we are choosing this Tutorial where we have the privilege of being authors, referees, publishers, and editors. It's not an easy way out; it is just a practical solution, because regardless of where our work is being published, it is we, the authors, who are ultimately responsible for the veracity of our findings and quality of our work.
Jarda Ruzicka and Petr Chocholouš
4/6/2026