Spectrophotometric TOC Determination

Spectrophotometric determination of total organic carbon (TOC) in water is not a single technique but a family of approaches united by one principle: converting organic carbon into a light-readable signal. Unlike instrument-based methods that rely on nondispersive infrared detection or conductivity cells, spectrophotometric methods use colour or absorbance changes to quantify the carbon originally present in the sample.

Two principal pathways exist. The first borrows the logic of chemical oxygen demand testing: potassium dichromate oxidises organic carbon under strongly acidic conditions, and the Cr(VI) reduced to Cr(III) is measured around 585 nm. The absorbance of Cr(III) is proportional to the organic carbon consumed. This route tolerates high concentrations and particulate matter without filtration, but it measures only the fraction of carbon that dichromate can oxidise, which may leave recalcitrant organics under-reported.

The second pathway oxidises organic carbon to carbon dioxide, which then diffuses across a hydrophobic membrane into an acceptor solution containing a pH indicator such as phenolphthalein or cresol red. Dissolved CO₂ lowers the pH, shifting the indicator’s acid–base equilibrium and changing its absorbance. 

The magnitude of that change is proportional to the CO₂ transferred, and therefore to the organic carbon oxidised. This route is less dependent on the chemical nature of the organic matter, because the signal is tied to CO₂ generation rather than to a specific oxidation reaction.

In either pathway, inorganic carbon must be removed before oxidation. Acidifying the sample to roughly pH 2–3 and purging with an inert gas strips carbonate and bicarbonate as CO₂, preventing them from being counted as organic carbon. The reliability of spectrophotometric TOC measurement therefore rests less on the colour reaction itself than on complete inorganic carbon removal and a clear understanding of what the chosen pathway can—and cannot—oxidise.