Simultaneous determination of total nitrogen, phosphorus and sulphur by means of microwave digestion and ion chromatography

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Abstract

A method for the oxidation of nitrogen, phosphorus and sulphur to nitrate, phosphate and sulphate ions using 22% (v/v) hydrogen peroxide and closed-vessel microwave assisted digestion in two stages is described. Solutions of a variety of nitrogen-, phosphorus- and sulphur-containing compounds with formic acid added to prevent hydrolysis were used to test the efficiency of the procedure. The products of oxidation were determined by ion chromatography. Good recoveries of nitrogen, phosphorus and sulphur were obtained. The results for the NIST reference materials, oyster tissue and Buffalo River sediments agree well with the certified values.

Introduction

The nitrogen, phosphorus and sulphur cycles are of particular significance to a number of biological and non-biological processes in the environment [1]. Natural and anthropogenic effects can cause localised inter-related changes to the cycles. In order to assess the impact and extent of the changes, it is essential to develop analytical methods which allow the simultaneous determination of two or all of the three constituents in a wide variety of environmental samples.

In one of the first attempts at simultaneous determination, Ebina et al. [2] developed a method of oxidizing nitrogen and phosphorus to nitrate and phosphate, respectively using alkaline potassium peroxodisulphate. The composition of the oxidizing solution was carefully chosen so that its pH changed from basic to acidic during the oxidation step. The change in pH was necessary because oxidation with potassium peroxodisulphate of nitrogen and phosphorus occurs under basic and acidic conditions, respectively. The nitrate and phosphate ions were then determined colorimetrically.

In a different approach, Nygaard and Sotera [3] used inductively coupled plasma atomic emission spectrometry (ICP-AES) to determine water-soluble nitrogen and phosphorus in fertilisers. More recently, Matilainen and Tummavuori [4] investigated the application of ICP-AES to the determination of water soluble sulphur in fertilisers and reported on spectral and interelement effects.

To be able to analyse both bound and water soluble fractions, samples have to be digested. However, existing digestion methods are not easily adapted to simultaneous determinations because the use of oxidants such as nitric and sulphuric acids and potassium peroxodisulphate precludes the determination of one or more of the analytes.

The use of hydrogen peroxide avoids this problem. In addition, water is the main product when the oxidizing strength of hydrogen peroxide is spent, and as a result the digest is amenable to analysis by techniques such ion chromatography (IC) and ion selective potentiometry. UV-induced photoxidation using hydrogen peroxide has been applied successfully to the oxidation of nitrogen, phosphorus and carbon in sea water [5].

In this study, we report results from an investigation of the use of hydrogen peroxide at low pH in combination with closed-vessel microwave assisted digestion for the oxidation of various nitrogen-, phopshorus- and sulphur-containing compounds. The nitrate, phosphate and sulphate ions formed were determined by IC.

Section snippets

Apparatus

A Dionex QIC analyser ion chromatograph equipped with a Dionex AG4A guard column, a Dionex AS4A anion separation column, and a Dionex AMMS-II suppressor and conductivity detector was used. The sample was injected into the chromatograph via a 100-μl sample loop, and eluted with a solution of 1.8 mM sodium carbonate–1.7 mM sodium hydrogencarbonate at a flow-rate of a flow-rate of 1 ml min−1. A chart speed of 0.5 cm s−1, conductivity range setting of 30 μS, and conductivity suppressor solution of

Concentration of the oxidizing solution

Fig. 1 represents the effect in percent recovery by varying hydrogen peroxide concentrations on the conversion of urea, sodium pyrophosphate and l-cysteine in the presence of formic acid to nitrate, phosphate and sulphate, respectively. Formic acid was added to the samples in order to prevent the hydrolysis of the compounds. However, it has been suggested that the oxidizing power of hydrogen peroxide is enhanced when it is activated by either acid, metal ions or is exposed to UV light [6]. This

Conclusions

The proposed method for the oxidation of N, P and S followed by the determination of the nitrate, phosphate and sulphate ion by IC gave satisfactory results for the compounds tested. The effectiveness of this procedure is demonstrated by the good recoveries obtained for the two SRMs, oyster tissue and Buffalo River sediment. Current work is focused on the application of the method to more recalcitrant compounds where the N, P and S atoms are in ring systems.

Acknowledgements

M.C. is grateful for the financial support from CONICIT and CONDES-LUZ, Venezuela.

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1

Permanent address: Laboratorio de Quı́mica Ambiental, Facultad de Ciencias, Universidad del Zulia, Maracaibo 4011, Zulia, Venezuela.

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