Amalia Cerdà

Sequential injection sandwich technique for the simultaneous determination of nitrate and nitrite

Abstract An automated sequential injection system for monitoring nitrate and nitrite in water samples is described. The method enables the simultaneous determination of both parameters with a single injection of sample. A sandwich arrangement, in which the sample is placed between two Griess reagent zones, is adopted. Nitrite is determined in one end of the sample zone by diazotization-coupling reactions and spectrophotometric detection of the azo dye formed. In the other end, nitrate is similarly determined after its on-line reduction to nitrite using a copperised cadmium column. The calibration is linear over the range 0.5–40xa0μmolxa0l−1 nitrite and 2–100xa0μmolxa0l−1 nitrate, the mid-range relative standard deviations being Under optimised conditions the system has been applied to the determination of nitrate in tap, mineral and sea waters (the nitrite concentration was below the detection limit) and the results have been compared with those obtained by ion chromatography and a flow injection system based on the same reactions.

Gas diffusion techniques coupled sequential injection analysis for selective determination of ammonium

The design of an analytical system based on the coupling of gas-diffusion separation and sequential injection analysis is described and applied to ammonium determination. The sample and an alkaline solution are sequentially aspirated using an automatic burette and mixed by flow reversal while they are being propelled to a gas-diffusion unit. There, the ammonia formed diffuses through an hydrophobic membrane into an acid-base indicator solution. The change in the absorbance of the acid-base indicator solution used as acceptor stream is measured and referred to ammonium content in the sample. The system requiring simple and commonly available instrumentation is automated and has been applied to the determination of ammonium in environmental samples.

Sequential injection analysis of nitrites and nitrates

Abstract A sequential injection system for the analysis of nitrate and nitrite is described. The sample and reagents are aspirated and mixed by flow reversal while being propelled to a reaction coil or to the detector. The determination of nitrite is based on the Griess-llosvay reaction. Nitrate is previously reduced by hydrazine in alkaline medium and analyzed as nitrite. The sequencing and overlap of the stacked zones are key parameters that have been studied together with concentrations of reagents, influence of temperature and potential interfering ions. Under the final optimal conditions up to 400 μM of nitrite and nitrate can be analyzed, the detection limits being 0.07 ppm and 0.2 ppm respectively. The method has been tested with prepared standard samples, atmospheric aerosol filter extracts and wastewater samples.

Monitoring of environmental parameters by sequential injection analysis

The use of sequential injection analysis (SIA), as applied to the control of environmental parameters, is reviewed. Its advantages and drawbacks are compared with those of flow injection analysis (FIA). FIA is simpler and easier to handle, has a higher sample throughput, and may be controlled manually, but SIA is more robust and versatile and better adapted for multiparametric and stopped flow techniques.

Evaluation of flow injection methods for ammonium determination in wastewater samples

A comparative study of flow injection methods for the determination of ammonium ion in wastewater samples is presented. It includes methods based on the Berthelot reaction and methods based on gas diffusion. Detection is either spectrophotometric or conductimetric. Analytical characteristics of each method are described in terms of reproducibility, sensitivity and selectivity. The performance of several gas permeable membranes to be used in gas-diffusion systems is discussed. The stopped flow approach is also investigated in order to improve the detection limits. Calibration graphs are obtained for each system, with and without that preconcentration step. Finally, a number of wastewater samples are analyzed using the described methods and the results are compared.

Speciation of nitrogen in wastewater by flow injection

A rapid method for the sequential determination of nitrite, nitrate and total nitrogen is proposed. Nitrite was determined directly by using the Griess reaction, which was also used to quantify nitrate after reduction to nitrite with hydrazine. For total nitrogen determination, the nitrogen-containing compounds (organic substances and nitrite and ammonium ions) were oxidized photochemically using a UV lamp and converted into nitrate, which was then reduced to nitrite and determined spectrophotometrically. Under the optimized conditions, up to 220 µmol l–1 N-NO2– and 240 µmol l–1 N-NO3– can be determined, the detection limits being 2 µmol l–1 N-NO2– and 8 µmol l–1 N-NO3–. The relative standard deviation for nitrite and nitrate are 1.5 and 2.3%, respectively. The photo-oxidation method for total nitrogen determination has a linear range of 30–1000 µmol l–1 N, with a relative standard deviation of 3%. The proposed method was applied to the determination of nitrate, nitrite and total nitrogen in wastewaters.

Total nitrogen determination by flow injection using on-line microwave-assisted digestion

Abstract A method for the determination of total nitrogen in wastewaters involving flow injection and on-line microwave-assisted digestion is described. The method is based on the oxidation of nitrogen-containing compounds to nitrate by means of peroxodisulfate using a microwave oven as a radiation source. Nitrate is determined by the Griess-Ilosvay reaction, after reduction to nitrite by hydrazine under alkaline conditions, using sulfanilamide and N -(1-naphthyl) ethylene diamine. A thorough study of the factors affecting the digestion process has been carried out using several nitrogen-containing compounds as test substances. Under the optimized conditions, the described method allows quantitative digestion of all the model compounds for concentrations up to 20 mg l −1 N. The method enables the analysis of 10 samples in triplicate per hour, the relative standard deviation (RSD) of 10 replicates being ca. 3%. The detection limit (3 × SD of blank) is 0.21 mg l −1 N. The system has been applied to the determination of total nitrogen in wastewaters and the results have been compared with those obtained by Kjeldahl digestion.

Sequential injection system for on-line analysis of total nitrogen with UV-mineralization

An automatic method for the determination of total nitrogen in wastewater by sequential injection analysis and mineralization with UV radiation has been developed. The method is based on the mineralization of the samples with sodium persulphate in basic medium under UV radiation. Small volumes of sample and reagents are firstly aspirated into a single channel and then propelled by flow reversal to the UV reactor and then to the detector. The organic and inorganic nitrogen compounds are oxidized to nitrate that is then measured at 226 nm. The sequential injection procedure has been optimized and the factors affecting the efficiency of the oxidation have been studied with a number of test substances with different chemical structures and properties. Solutions in the concentration range 1-56 gl(-1) of nitrogen can be analyzed with the described procedure. The sample rate is of 30-40 samples h(-1). The LOD is 0.6 mgl(-1) N and the reproducibility is 1.8% (28 mgl(-1) N). Organic carbon in the form of glucose was added to a number of test solutions to study the potential interference of organic matter. The method was compared with the Kjeldahl digestion method by analyzing 15 wastewater samples with both methods. The nitrate and nitrite content of the non-oxidized samples were subtracted from the corresponding nitrogen content determined after photo-oxidation and the value compared with the Kjeldahl nitrogen content.

Laboratory automation based on flow techniques

Flow techniques have undoubtedly aroused special interest in relation to many other automatic methodologies of analysis. Ever since segmented flow analysis (SFA) was developed by Skeggs in 1957, flow techniques have been in continuous evolution toward new develop-ments. There is no solid argument in favor of using any particular flow technique separately; rather, substantial advantages can be derived from their combination. Since flow-based methods are nonseparative tools, the advantages of combining flow techniques with separation techniques are noteworthy. High selectivity can be achieved by coupling them with liquid chromatography (LC), gas chromatography (GC), solid-phase extraction (SPE), or capillary electrophoresis (CE). Thus, a detailed description of flow techniques, their evolution, their hyphenation advantages, and a critical comparison between current developed methods exploiting flow techniques aimed at solving present analytical needs are reviewed in this article.

Determination of iodide in table salt by flow injection analysis using Pyrocatechol Violet

Abstract A spectrophotometric flow injection method for the determination of iodide based on the catalytic effect of this ion on the oxidation of Pyrocatechol Violet by potassium persulphate was developed. The method allows the determination of 0.5–5 mg litre−1 iodide at a rate of 60 samples per hour and is subject to very few interferences. It was successfully applied to the determination of iodide in table salt.