Determination of Water-Soluble and Acid-Soluble Zinc in Soils by Flame Atomic Absorption Spectrometry after Cloud Point Extraction

Abstract

A cloud point extraction (CPE) procedure has been developed for the determination of water-soluble and acid-soluble zinc (Zn) in soils by flame atomic absorption spectrometry. Deionized water and 0.1 mol L–1 hydrochloric acid (HCl) were selected as extracting agents. In the proposed approach, 2-(5-bromo-2-pyridylazo)-5-diethylam-inophenol (5-Br-PADAP) was used as a chelating agent, and polyethylene glycol octyl phenyl ether (OP) was selected as the surfactant. Some factors including the pH of analytical solution, concentrations of the chelating agent and surfactant, equilibration temperature and time, and salt effect, which would affect the extraction efficiency and subsequent determination of Zn, were studied and optimized. Under the optimized conditions, the calibration graph was linear in the range of 5.0 × 10–3 to 0.5 μg mL–1, and preconcentration of 20 mL sample solution gave an enhancement factor of 25. The detection limit was 4.93 × 10–3 μg mL–1. Recoveries in the range of 95.0–110% were obtained. Some metal ions including iron (Fe2+), cobalt (Co2+), and manganese (Mn2+) would interfere with the determination of Zn. The interference from these ions can be eliminated using thiourea (0.5% w/v) and triethanolamine (0.5% w/v) as masking agents. The proposed method was applied to the determination of water-soluble and acid-soluble Zn in soils, which were collected from the suburbs of Zhengzhou, and satisfactory results were obtained. To have more understanding of the soils, we determined the total content of Zn in soils. The results showed that the water-soluble and acid-soluble Zn contents in different soils are not correlated with the total content. For example, the total content of Zn for the soil from a farm in north loop was very low, but the percentage of water-soluble Zn was very high.