Indirect Determination of Chloride in Water by Atomic Absorption Spectrometry

Chloride is a non‑metal element whose resonance lines lie in the vacuum ultraviolet region, making direct determination by atomic absorption spectrometry (AAS) inherently difficult. To overcome this limitation, indirect methods have been developed—among which the silver chloride precipitation approach is the most widely adopted.

Principle of the Indirect Method

The method is based on a simple precipitation reaction: chloride ions in the water sample react with a known excess of silver ions to form silver chloride precipitate:

Cl⁻ + Ag⁺ (excess) → AgCl↓ + Ag⁺ (remaining)

After removing the precipitate, the remaining silver ions in the supernatant are measured by AAS. The difference between the total silver added and the residual silver corresponds to the amount consumed by chloride, from which the chloride concentration is calculated.

Two Principal Approaches

Two main variants of this indirect method exist. In the remaining silver approach, an excess of silver nitrate is added to the sample, the AgCl precipitate is removed by centrifugation or filtration, and the unreacted Ag⁺ in the supernatant is measured by AAS. In the precipitate dissolution approach, the AgCl precipitate is collected and dissolved in ammonia solution, and the silver content in the dissolved precipitate is measured. The former is more commonly applied in water quality analysis.

Analytical Procedure

A typical procedure begins with acidification of the water sample using nitric acid to suppress silver hydrolysis and maintain a suitable reaction medium. A precisely measured excess of silver nitrate standard solution is then added. The mixture is heated in a water bath (typically at 40±2°C for about 30 minutes) to ensure complete precipitation. Ethanol is added as a surface active agent to reduce the solubility of AgCl and promote more complete precipitation. 

After cooling, the solution is diluted to volume and centrifuged to separate the precipitate. The supernatant is then aspirated directly into the AAS flame for silver measurement at 328.1 nm, using a silver hollow cathode lamp. A reagent blank is run simultaneously, and the difference in absorbance (ΔA = blank absorbance – sample absorbance) is plotted against chloride concentration to construct a calibration curve.

Instrumental Conditions

For silver determination by AAS, the analytical line is typically set at 328.1 nm. Common operating parameters include a lamp current of 7.5 mA, spectral bandpass of 1.3 nm, an air‑acetylene flame, and a burner height of approximately 7.5 mm.