Spectrophotometric Determination of Antimony in Water

Antimony is a toxic metalloid that poses potential risks to human health and aquatic ecosystems. Spectrophotometric methods offer a practical, cost-effective approach for routine antimony determination in water samples, particularly when advanced instrumentation such as atomic absorption spectrometry or inductively coupled plasma mass spectrometry is unavailable. 

The most widely employed spectrophotometric technique for antimony relies on the formation of a colored complex between antimony and a suitable chromogenic reagent.

Principle of the Method

The determination of antimony by spectrophotometry is commonly based on the reaction of antimony(III) with potassium iodide to form a yellow-orange complex, or more sensitively, on the reaction with rhodamine B or brilliant green after reduction to antimony(III). Among these, the rhodamine B method is frequently used due to its high sensitivity. 

In this procedure, antimony(V) is first reduced to antimony(III) using a reducing agent such as potassium iodide and ascorbic acid. Antimony(III) then forms an ion-association complex with rhodamine B in acidic medium, which can be extracted into an organic solvent such as benzene or toluene, and the absorbance is measured at a wavelength of approximately 550 to 565 nm.

Reagents and Apparatus

All reagents must be of analytical grade, and deionized water with low metal content should be used throughout. Standard antimony solutions are prepared from potassium antimonyl tartrate or antimony trichloride, with appropriate acidification to maintain stability. Reducing solution typically contains potassium iodide and ascorbic acid in dilute sulfuric acid. Rhodamine B solution is prepared as a dilute aqueous solution. Extraction solvent should be of high purity to minimize background absorbance. A spectrophotometer capable of measurements in the visible region, equipped with glass or quartz cuvettes, is required.

Sample Collection and Pretreatment

Water samples must be collected in acid-cleaned polyethylene or glass bottles and acidified to pH less than 2 with nitric acid or sulfuric acid immediately after collection to prevent adsorption of antimony onto container walls. Samples should be analyzed within 24 to 48 hours or stored at 4°C for short-term preservation. 

Prior to analysis, samples may require digestion to break down organic complexes and to convert all antimony species to a measurable form. A common digestion procedure involves heating the acidified sample with nitric acid and hydrogen peroxide until complete mineralization is achieved. 

After digestion, the sample is evaporated to near dryness, redissolved in dilute acid, and filtered if necessary to remove particulate matter.