Spectrophotometric Determination of Cadmium in Water: Principles and Procedure

Cadmium is a highly toxic heavy metal commonly found in industrial effluents and contaminated natural waters. Routine monitoring of cadmium concentrations is essential for environmental protection and public health. Among various analytical techniques, spectrophotometry—particularly the dithizone method—remains widely used in routine laboratories due to its simplicity, low cost, and adequate sensitivity for many regulatory applications.

Principle

The method relies on the formation of a coloured complex between cadmium ions and dithizone (diphenylthiocarbazone) under strongly alkaline conditions. In the presence of excess dithizone, cadmium reacts to produce a red cadmium–dithizone chelate, which is readily extractable into an organic solvent such as chloroform or carbon tetrachloride. The resulting organic phase exhibits maximum absorbance at a wavelength of approximately 518 nm. 

According to the Beer–Lambert law, the absorbance is directly proportional to the cadmium concentration within a certain range, provided that the measurement is performed against a reagent blank. This proportionality forms the basis for quantitative analysis.

Sample Collection and Preservation

Representative sampling is critical. Water samples should be collected in clean polyethylene or glass containers that have been pre‑washed with dilute nitric acid and rinsed with deionised water. For total cadmium determination, the sample must be acidified to pH < 2 with concentrated nitric acid immediately after collection to prevent adsorption onto container walls and to stabilise the metal species. 

If suspended matter is present, the sample should be filtered or subjected to appropriate digestion (e.g., with nitric acid and hydrogen peroxide) to release cadmium bound to particulates or organic matter.

Extraction and Colour Development

A measured volume of the pretreated water sample is transferred into a separating funnel. The solution is made strongly alkaline by adding sodium hydroxide or potassium hydroxide, which also helps to suppress interferences from certain other metals. A dithizone solution in chloroform is then added. 

The funnel is shaken vigorously for a defined period to ensure quantitative extraction of the cadmium–dithizone complex into the organic layer. After phase separation, the lower organic phase is collected. The extraction may be repeated to improve recovery. The combined organic extracts are then washed with a suitable alkaline solution to remove any co‑extracted impurities.

Absorbance Measurement and Quantification

The organic extract is placed in a spectrophotometric cuvette, and its absorbance is measured at 518 nm against a reagent blank (prepared identically but without the sample). A calibration curve is constructed using a series of cadmium standard solutions processed under the same conditions. The concentration of cadmium in the unknown sample is then derived by interpolation from this curve. For samples with cadmium levels above the linear range, dilution or a smaller aliquot should be used.

Interferences and Their Control

Many metal ions—including lead, zinc, copper, nickel, cobalt, and mercury—also form coloured dithizonates and can interfere. To minimise these effects, the extraction pH is carefully controlled so that cadmium is preferentially complexed. In addition, masking agents such as citrate, tartrate, thiosulphate, or triethanolamine are added to the aqueous phase before extraction. 

These reagents form stable, water‑soluble complexes with interfering metals, preventing them from being extracted into the organic layer. For samples with high concentrations of magnesium or calcium, increased amounts of masking agents may be necessary.