A simple and Sensitive Spectrophotometric Determination of Nitrate and Nitrite in Water Samples

 

María A. Jaworski¹, Guillermo J. Siri² and Mónica L. Casella^1,2

¹Centro de Investigacióny Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA) (CCT La Plata-CONICET, UNLP), Universidad Nacional de la Plata, Facultad de Ciencias Exactas, 47 Nº 257, 1900, La Plata, Argentina.

²Facultad de Ingeniería, Universidad Nacional de la Plata, 47 Nº257, 1900, La Plata, Argentina.

*Corresponding Author E-mail: casella@quimica.unlp.edu.ar

ABSTRACT:

The spectrophotometric method presented here was developed to provide information about the concentrations of nitrate and nitrite ions during the catalytic reduction of nitrate in drinking water. For the determination of nitrate, the absorbance is measured at 220 nm and HCl is used as the sole reagent. For the determination of nitrite, a method in which a peroxovanadate complex is decolorized by nitrite, was employed as basis for the quantitative determination of this ion in water samples. The calculated concentrations of NO₂^- and NO₃^- agree with the true concentrations within 5% for NO₃^- and 1% for NO₂^-, showing that the developed method is useful for the determination of both components. The results obtained with the proposed method agree well with those obtained by the standard IC method.

 

 

 

INTRODUCTION:

Despite the fact that most of the planet is covered by water, only a small amount of this water is available as fresh water. Almost 97.5% of the total is in oceans in the form of salty water and is not suitable for drinking, watering, or industrial use as is. The remaining 2.5% is fresh water. However, not even that small amount is easily accessible or exploited, because it is stored as ice on the poles and on mountaintops. Furthermore, a significant amount of the rest lies so deep in the ground that it is very difficult to extract.¹

At present, part of this water is contaminated with NO₂^- and NO₃^- in concentrations well above the maximum levels permitted by the U.S. Environmental Protection Agency (the Maximum Contaminant Level for nitrates has been set at 10 ppm, and for nitrites at 1 ppm).² While NO₃^- is part of the N₂ cycle and so is naturally present in water, soil and plants, its level in the environment, especially in groundwater, has been increasing due to human activity (leaching of nitrogen from farm fertilizer or from feedlots or from septic tanks).¹

 

On the other hand, NO₂^- does not typically occur in natural waters at significant levels, except under reducing conditions, in wastewater treatment plants and water distribution systems, for instance. Nitrite can enter the water distribution system through its use as a corrosion inhibitor in the textile, food and metal industries. In addition, many of its precursors are used in nitrogenous fertilizers.³

 

The level of NO₃^- and NO₂^- in water is an important indicator of water quality and its increase has been associated with the pollution by eutrophication and blooms.⁴ The intake of water with excess NO₃^-, which is rapidly converted to nitrite in the body, can cause serious health problems for people, due to two chemical reactions: induction of methemoglobinemia, especially in infants under one year of age ("blue baby syndrome"), and the potential formation of carcinogenic nitrosamides and nitrosamines.^5,6

 

The research and development of technologies to remediate contaminated water with NO₃^- have been steadily increasing over the past 20 years. Several treatment processes including ion exchange, biological denitrification, chemical denitrification, reverse osmosis, electrodialysis, and catalytic denitrification can remove nitrates from water with varying degrees of efficiency, cost, and ease of operation.⁷ In particular, nitrate removal in aqueous solution on a supported noble metal catalyst using H₂ as a reducing agent offers an economical alternative and it is potentially a very promising technique. In the procedure, first described by Vorlop et al. ^8-11, nitrates are selectively converted to nitrogen by means of hydrogen via intermediates in a two- or three-phase reactor operating under mild reaction conditions (e.g., T = 278–298 K, p(H₂) up to 7 bar). The main drawback of some of these catalytic systems is the formation of nitrite, which is also undesirable in drinking water.

 

To control the performance of such catalytic systems, it is necessary to monitor both the concentration levels of NO₃^- and NO₂^- during the course of the denitrification process. At present, to determine both anions, several techniques are used, such as ion-exchange chromatography^12-15, gas chromatography–mass spectrometry¹², fluorimetric methods^16,17, potentiometry^18-20 and capillary electrophoresis^21,22. However, most of them present some complications, such as the high cost of many of the reagents and equipment, and the fact that the chemicals employed are more toxic than the species being monitored.

 

Therefore, this work presents the development of an analytical spectrophotometric technique to quantify both anions differentially in a rapid and simple way, which is easily reproducible. For the determination of nitrate, the absorbance is measured at 220 nm and HCl is used as the sole reagent.²³ The analysis of nitrite is based on a technique recently published by Galil et al.²⁴

 

The spectrophotometric method presented here was developed to provide information about the concentrations of nitrate and nitrite ions during the catalytic reduction of nitrate in drinking water. The results of a typical catalytic experience involve a concentration range between 50 and 0 ppm for nitrate, and between 0 and 50 ppm for nitrite.

 

MATERIAL AND METHODS:

1. Apparatus

Cintra 20 UV–vis spectrophotometer with 1.0-cm silica quartz matched cells was used for measuring the absorbance.

 

2. Reagents

All the chemicals used were of analytical reagent grade and water used here refers to distilled water free of  NO₃^- and/or NO₂^-.

 

Standard potassium nitrate solution 1000 ppm

A weighed amount, 0.8152 g of potassium nitrate (previously dried at 120ºC overnight) was transferred to a 500 mL volumetric flask, it was dissolved and the solution was diluted up to the mark with water. The working solutions were prepared by appropriate dilution as and when required.

 

Standard potassium nitrite solution 470 ppm

Potassium nitrite (KNO₂) was previously dried in oven at 105ºC for 24h, then a weighed amount of 0.9347 g was transferred to a 500 mL volumetric flask, dissolved and the solution diluted up to the mark with water. This solution was transferred to a brown bottle and stored under diffused sunlight. The working solutions were prepared by appropriate dilution as and when required.

 

Ammonium metavanadate solution 0.01 M

A 0.585 g aliquot of ammonium metavanadate (NH₄VO₃) was transferred to a clean 50 mL beaker, dissolved with warm water, cooled and the solution poured into a 50 mL volumetric flask. The solution was diluted to the mark with water.

 

Hydrogen peroxide solution 0.025%

A volume of 0.5 mL of 100 Vol. hydrogen peroxide (H₂O₂) was transferred to a 50 mL volumetric flask and the solution was diluted to the mark with water. From this solution, 12.5 mL was transferred to a 500 mL volumetric flask and the solution was made up to the mark with water. The diluted H₂O₂ solution was further standardized by 0.016M standard potassium permanganate (KMnO₄) solution. The H₂O₂ content of the solution was found to be 0.025%.

 

Sulphuric acid solution 3.6M

This solution was prepared by suitable dilution of concentrated sulphuric acid (98%).

 

3. Recommended procedures

Determination of nitrate

To each one of a series of 50 mL volumetric flasks, 0.05, 0.1, 0.25, 0.5 and 1.25 mL aliquots of the 1000 ppm NO₃^- solution were added and diluted up to the top mark. Then 1 mL of HCl was added. The absorbances of each one of the solutions were read at 220 nm against distilled water. A standard curve was constructed by plotting NO₃^- absorbance against the concentration of the standards.

 

Determination of nitrite

A series of labeled 10 mL volumetric flasks were arranged. To each flask, 2 mL of 0.025% H₂O₂, 3 mL of 0.01M NH₄VO₃, 3 mL of 3.6 M H₂SO₄ and aliquots of the standard nitrite solution (470 ppm) of 0.1, 0.2, 0.3, 0.5, 0.7 and 1.0 mL were added. Then, the solution in each flask was diluted to the mark with water. The absorbance of each solution and that of the blank (containing all the reagents except KNO₂) were measured against water at 470 nm.

 

Another analytical method

For comparative purpose, nitrate and nitrite were determined chromatographically with an ion chromatographic instrument from Metohm (Switzerland) consisting of a 709 IC Pump, 733 IC Separation Center, a MSM Suppressor with a 752 Pump unit and a 732IC Conductivity detector. Anion separation was carried out in suppressor mode on a Metrosep A Supp 5 150 (Metrohm) column. A solution containing a mixture of 3.2 mM Na₂CO₃ and 1.0 mM NaHCO₃ at a flow rate of 0.7 mL min^-1 served as the eluent. A 200 mM H₂SO₄ solution was used as the regenerant. The volume of the sample injection loop was 20 μL.

 

Determination of nitrate and nitrite during the catalytic denitrification of water

1. Preparation of the catalysts

To test the proposed method, a PdCu catalyst was prepared, similar to those suggested in the literature for the denitrification of drinking water process.²⁵ To do this, a γ-Al₂O₃ (Cyanamid Ketjen, BET surface area: 260 m² g^-1; total pore volume 0.64 m³ g^-1) support was calcined for 2 h at 500°C and then it was impregnated using H₂PdCl₄, obtained treating a solution of  PdCl₂ in HCl (pH=1) in order to obtain a catalyst having 1wt.% Pd. After a contact time of 24 h, the solid was dried at 100ºC and then it was impregnated with an aqueous solution of Cu(NO₃)₂ in order to obtain 0.3 wt% Cu in the resulting solid. The catalyst obtained was dried at 100ºC and finally calcined at 400ºC.

 

2. Catalytic test

Before being submitted to the catalytic reaction, the PdCu catalyst was pretreated under a H₂ flow (100 mL min^-1) at 500ºC with a heating rate of 10ºC min^-1 for 2 h. Then, a stirred Pyrex batch reactor was loaded with 90 mL of degassed distilled water, 200 mg of catalyst, and 10 mL of the 1000 ppm NO₃^- solution. Subsequently, a H₂ flow of 400 mL min^-1 was fed to the reactor. Small samples        (1.5 mL) were taken at different times (75, 183 and 1440 min) from the reactor, filtered in order to separate any rest of catalyst and submitted to the determination of NO₃^- and NO₂^-.

 

Analysis of the samples containing both anions

A volume of 0.5 mL of the reaction sample was diluted to 10 mL, 0.2 mL of HCl was added, and its absorbance at 220 nm was measured in order to determine NO₃^- and NO₂^- concentrations. Another 1 mL of the sample was treated with H₂O₂, NH₄VO₃ and H₂SO₄ to determine NO₂^- by measuring the absorbance at 470 nm. Also, the concentration of both anions was obtained chromatographically.

 

RESULTS AND DISCUSSION:

Spectrophotometric analysis of NO₃^- and NO₂^-

At first, it was necessary to identify the optimum wavelengths for NO₃^- and NO₂^- determination in order to develop the analytical method proposed.

 

Figure 1 shows the absorption spectrum of a NO₃^- in deionized water, measured against the solvent. All absorbance measurements were made at 220 nm for NO₃^- (λ_max). Under optimal experimental conditions, the absorbance of various concentrations of nitrate was measured at 220 nm.

 

Figure 1. Absorption spectrum of a 50 ppm NO₃^- solution

 

Beer’s law holds well in the concentration range 0–50 ppm NO₃^- (Figure 2). The value for the molar extinction coefficient (ε) of NO₃^- at 220 nm was 3760 L mol^-1 cm^-1_. At this same wavelength, the molar extinction coefficient (ε) of NO₂^- was found to be 820 L mol^-1 cm^-1.

 

Figure 2. Beer's Law plot for nitrate ion solutions having contrations between 0 and 4.0 10^{-4 M.}

 

For the determination of nitrite in water samples, a method proposed by Galil et al.²⁴ was taken as the basis. Ammonium metavanadate reacts with hydrogen peroxide forming a peroxovanadate complex in 3.6M sulphuric acid medium. The complex (with λ_max at 470 nm as shown in the absorption spectrum, Figure 3.) is decolorized by nitrite, which is the basis for the quantitative determination of this ion in water samples.

 

Figure 3. Absorption spectrum of a solution containing 50 ppm nitrite ion + 2 mL of 0.025% H₂O₂ + 3 mL of 0.01 M NH₄VO₃ + 3 mL of 3.6 M H₂SO₄.

 For NO₂^- determination, a good linear relationship was found between the absorbance of the complex and the concentration of NO₂^-. Beer’s law was obeyed in the concentration range employed in this work, as can be see in Figure 4.

 

Figure 4. Absorbance of peroxovanadate complex in the presence of different amounts of nitrite ion measured at 470 nm in a 1.0 cm length path cell.

 

Development of the proposed method

The effect of NO₃^- on the NO₂^- determination through the absorption of the complex has been studied (Table 1).

 

Table 1. Absorbance (measured at λ = 470 nm) of peroxovanadate complex at different concentrations of nitrite. Effect of the addition of 30 ppm NO₃^- to the solutions

Nitrite concentration (mg L-1)	Absorbance	Absorbance (30 mg L-1 NO3- added)
0	0.128	0.129
4.52	0.111	0.108
9.04	0.094	0.091
13.56	0.077	0.074
18.08	0.059	0.060

 

The absorbances at 470 nm for the solutions with and without NO₃^- agreed within 5%, so it can be concluded that the complex is not decolorized by NO₃^- anion and that the presence of NO₃^- does not interfere in the determination of NO₂^‑.

 

Both NO₃^- and NO₂^- anions absorb radiation at 220 nm, so it was necessary to check whether their absorbances were additive before going on with the development of the method. The absorbances of known amounts of NO₃^- (entries 1 and 2, Table 2) and NO₂^- (entries 3 and 4, Table 2) were measured separately at 220 nm. Then, those same amounts of NO₃^- and NO₂^- were put together in the same flask and the absorbance at 220 nm was measured. The results are presented in Table 2 and, from this table it can be seen that the observed absorbance for the solution corresponding to entry 5 (1.629) agrees within 1.5% with the absorbance of this mixture calculated as the sum of the absorbances of entries 1 and 3 (1.605). Analogously, the absorbance of entry 6 (0.771) and the sum of the absorbances of entries 2 and 4 (0.780) agree within 1.2%. Hence, it is demonstrated that the absorbances of NO₃^- and NO₂^- anions are additive at 220 nm.

 

Table 2. Analysis of the additivity of absorbances of nitrite and nitrate ions at 220 nm

Entry	Nitrate concentration (ppm)	Nitrite concentration (ppm)	Absorbance measured	Absorbance expected
1	25	0	1.523
2	10	0	0.608
3	0	4.52	0.082
4	0	9.04	0.172
5	25	4.52	1.629	1.605
6	10	9.04	0.771	0.780

 

 

For the simultaneous determination of NO₃^- and NO₂^- the following expressions derived from Beer’s law have been employed:

 

In equation (2), A₀⁴⁷⁰ represents the absorbance of the peroxovanadate complex before the addition of nitrite solution. Combining equations (1) and (2) and taking into account that b = 1 cm throughout the experiments, NO₃^- concentration can be obtained as:

 

 

 

Table 3. Determination and recovery of nitrite and nitrate ions by the proponed method. Ion chromatographic results are also presented for comparison.

Table 3 gathers the results obtained when applying the developed method to a series of synthetic mixtures containing nitrate and nitrite anions. The molar absorptivities previously obtained together with the absorbances measured at 220 and 470 nm were used to calculate the concentrations of NO₂^- and NO₃^- in the prepared solutions with equations (2) and (3). The calculated concentrations of the two components agree with the true concentrations within 5% for NO₃^- and 1% for   NO₂^-, showing that the developed method is useful for the determination of both components using these two wavelengths. For comparative purpose, NO₃^- and NO₂^- concentrations of these samples were also calculated through a standard ion chromatographic (IC) procedure. The results are presented in Table 3 and, as can be seen the results obtained with the proposed method agree well with those obtained by the IC method.

 

Analytical application

The proposed method was tested in the analysis of samples taken during the catalytic denitrification of water, in order to investigate its applicability.

 

The catalytic reduction process for the removal of NO₃^- from contaminated waters is based on the use of bimetallic catalysts such as Pd-Cu/g-Al₂O₃ that can reduce NO₃^- to N₂ in water. During the NO₃^- reduction process besides N₂, NH₄⁺ and specially NO₂^- can be formed; that is why it is important to develop a simple and economical procedure for the detection of both anions.

 

The results obtained for NO₃^- and NO₂^- concentrations both by the proposed method and the standard IC method are given in Table 4 for 3 samples taken from the reactor at different times (75, 183 and 1440 min). The results of the proposed method agree well with those of the chromatographic one.

 

Table 4. Determination of nitrate and nitrite anions in samples taken at different times during the catalytic denitrification reaction.

Time (min)	Proposed method		Cromatographic method
	Nitrate found (ppm NO3-)	Nitrite found (ppm NO2-)	Nitrate found (ppm NO3-)	Nitrite found (ppm NO2-)
75	85	12.6	85	13
183	65	26	64	27
1440	20.1	50.4	19	51

 

CONCLUSIONS:

The proposed new method was found to be rapid and simple and allowed the simultaneous determination of nitrate and nitrite anions spectrophotometrically.

 

It is simple because it does not involve complicated steps such as solvent extraction nor critical control of pH and temperature. It employs only a simple instrument, a UV-Vis spectrophotometer, available in any laboratory.

 

The proposed method resulted as effective as the standard IC method, as indicated by the results when applied to the analysis of samples taken during the course of the NO₃^- catalytic reduction process.

 

ACKNOWLEDGEMENTS:

The authors gratefully acknowledge CONICET (PIP 0185, Argentina) and ANPCyT (PICT 25827/04, Argentina) for their financial support.

 

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Received on 04.04.2011        Modified on 12.05.2011

Accepted on 30.09.2011        © AJRC All right reserved