Determination of ammonia in wastewaters containing high concentrations of surfactants by flow injection potentiometry with on-line sample clean-up

doi:10.1016/S0003-2670(98)00168-8

Analytica Chimica Acta

Volume 367, Issues 1–3, 3 July 1998, Pages 193-199

https://doi.org/10.1016/S0003-2670(98)00168-8 Get rights and content

Abstract

It is demonstrated that surfactants lead to problems in the analysis of ammonia in gas-diffusion flow injection by wetting the membrane to allow transport of potentially interfering species to the detector. An on-line clean-up procedure, involving use of an activated carbon cartridge, is described for the removal of ionic and non-ionic surfactants (up to 1000 mg l⁻¹) and other organic compounds from aqueous samples. Ammoniacal nitrogen was determined in synthetic samples by gas-diffusion flow injection with a nonactin-based potentiometric sensor used as the detector for the ammonium ion. Analyses could be performed at a sampling rate of ca. 25 h⁻¹ and the cartridge was effective for at least 1000 injections of 50 μl samples in the concentration range 1–180 mg l⁻¹. The procedure was applied to the analysis of wastewaters and was found to be efficient in removing ionic and non-ionic surfactants from acidified (pH 1) samples.

Introduction

The widespread use of surfactants in domestic and industrial applications has stimulated studies of the influence of surfactants on the determination of many species using various methods, especially in the analyses of wastewater and sewage which may contain high concentrations of surfactants and other organics. For example, in potentiometric methods, surfactants 1, 2, 3, 4, 5and other organic species such as humic substances [6]have been found to have an adverse effect on the performance of ion selective electrodes (ISEs). The effects of surfactants seem to be very complex and depend on the type of surfactant and the electrode system used 1, 4. It is also difficult to determine the ionic species in complex sample matrices which contain surfactants and many other organic compounds.

Two of the most widely used methods for the determination of ammonia in natural and wastewater samples are based on flow injection (FI) employing a gas-diffusion (GD) cell or potentiometry with the ammonia probe. However, in both of these procedures, surfactants and other organics can lower the performance of gas-diffusion membranes either by wetting the hydrophobic membrane 5, 7, 8, 9or by blocking the minipores of the membrane [10].

To overcome surfactant and other organic interferences in potentiometry, several methods have been recommended in the literature. One of them involves headspace analysis, where the ammonia probe is suspended above the sample solution [5], but the detection limit is high (>1×10⁻³ M) and the time to reach equilibrium is long. Another method uses liquid–liquid extraction [9], but the procedure is tedious and time consuming. Alternative methods employing alternate washing [8]and standard addition [10]are also reported, but these methods do not eliminate completely the interferences from surfactants and other organic compounds.

This paper describes the use of activated carbon to remove surfactants and other organic substances on-line, prior to the determination of the ammoniacal nitrogen in wastewaters by gas-diffusion flow injection (GD-FI) using the nonactin-based ammonium ion sensor. The method is simple and reliable, with no interferences from surfactants and other organic compounds.

Section snippets

Reagents and chemicals

Ammonium chloride (BDH) was used to prepare standard solutions of the analyte. 0.1 M lithium chloride (Ajax) was used as the reference stream in the potentiometric detector. In the gas-diffusion systems, 0.5 M sodium hydroxide (Mallinckrodt) containing 0.05 M EDTA (BDH) was used as the donor stream to convert ammoniacal nitrogen to ammonia, and the receiving stream was a buffer of 0.02 M tris(hydroxymethyl)aminomethane (Aldrich) adjusted to pH 7.2 with hydrochloric acid (BDH). For on-line removal

Effect of surfactants on the nonactin-based ammonium ISE

In a recent paper, a differential pH procedure was described for the determination of ammoniacal nitrogen using a nonactin-based ammonium ion sensor in FI analysis without gas diffusion [12]. The method was able to compensate for interference from moderate concentrations of other cations. In an extension of this work, it was considered of interest to investigate the application of the differential pH procedure to the analysis of industrial wastewaters containing moderate to high concentrations

Conclusions

Injected surfactants wet the gas-permeable membrane in a GD-FI system so that non-gaseous species pass through the membrane and non-ionic surfactants wet the PTFE membrane more rapidly than ionic surfactants. Serious interference in the determination of the ammonium ion using a nonactin-based sensor can be caused by cationic surfactants present in the sample and by K⁺ transported across a membrane that is wetted by a surfactant. It is therefore essential that surfactants are removed from

Acknowledgements

We thank the Australian Research Council for financial support. HS is grateful to La Trobe University for the award of a Postgraduate Scholarship.

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Determination of ammonia in wastewaters containing high concentrations of surfactants by flow injection potentiometry with on-line sample clean-up

Abstract

Introduction

Section snippets

Reagents and chemicals

Effect of surfactants on the nonactin-based ammonium ISE

Conclusions

Acknowledgements

Analyst

Analyst

Analyst

Anal. Chim. Acta