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Technical paper
Determination of Nitrite and Nitrogen Dioxide by Spectrophotometry After Solid Phase Extraction
Prachi Parmar,a Sunitha B Mathew,a Vinay Kumar Guptab and Ajai Kumar Pillaia*
a,* Department of Chemistry, Govt. V.Y.T. Autonomous College Durg (Chhattisgarh), 491001. Tel.:919425245612, Fax: 91-0788-2212030,
b School of studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, (Chhattisgarh),
India, 492001.
* Corresponding author: E-mail: drajaipillai@gmail.com
Received: 21-06-2007
Abstract
A simple method for the determination of nitrogen dioxide using alkaline Ferron as an absorbing medium as well as coupling agent is proposed. Nitrite ion diazotizes p-aminoacetophenone in the absorbing solution and the diazotised p-aminoacetophenone is subsequently coupled with ferron. The orange-red dye was extracted in an amino cartridge, which shows a maximum absorbance at 485 nm, obeys Beer’s law in the range of 0.04 to 0.36 µg mL–1 of nitrogen dioxide and has a molar absorptivity of 9.40 · 104 L mol–1 cm–1. Optimum reaction conditions for diazotization, full colour development and the effect of variables like temperature, time and pH have been studied. Detailed studies to check the collection efficiency and NO2 : NO2– stoichiometric ratio has been carried out. The method has been successfully applied for the analysis of nitrogen dioxide in cigarette smoke, scooter exhaust, and workroom air. The method has also been applied for the determination of nitrite in water, soil etc.
Keywords: Spectrophotometer, p-aminoacetophenone, ferron, solid phase extraction, nitrogen dioxide.
1. Introduction
Nitrogen dioxide is one of the most hazardous primary air pollutants present in the environment with considerable toxic effects. There are several natural and anthropogenic sources like lightning, coal combustion in a pulverized fuel combustion plant, steel making, transportation etc., which contribute to higher levels of nitrogen dioxide in the environment. Traffic is estimated to contribute as much as half of the total nitrogen dioxide emission.1, 2 It is also formed during the reduction of nitric acid, decomposition of nitrite and combustion of nitrogenous mate-rials.3 Cigarette smoke and auto exhaust is also reported to contain significant amount of nitrogen dioxide.4 Inhalation of nitrogen dioxide mainly affects the respiratory system, as affinity of hemoglobin for nitrogen dioxide is 300,000 higher than that for oxygen. This affinity drastically reduces the oxygen carrying capacity of the blood.5 Short-term industrial exposure to nitrogen dioxide may cause nausea, vomiting, irritation to eyes, nose, respiratory tract, cyanosis, cardiac dilatation and collapse.6 In-
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door nitrogen dioxide which is produced by gas or other fuels used for heating and cooking was found to exhibit increased respiratory symptoms, decreased pulmonary lung function, respiratory illness, increased incidences of chronic cough, conjunctivitis, bronchitis, and asthma exa-cerbation.7–8
The significance of nitrogen dioxide as a pollutant has culminated in the development of several analytical methods for its determination. These include GC,9 Liquid
Chromatography,10 Laser induced fluorescence,11 Field modulation laser spectroscopy,12 Tunable diode laser, Droplet method,13 amperometric,14 chromatomembrane cell,15 fiber optic spectroscopy,16 piezo-dosimeter,17 and
passive sampling devices18 etc. The collection efficiency has also been improved by modifying absorbing solution using sodium arsenite23, triethanolamine, 24 and guiacol.25 A large number of spectrophotometric methods are also reported in the literature for the detection and determination of nitrogen dioxide.19–22 The method is based on the interaction of electromagnetic radiations with matter. When a beam of light is allowed to pass through a transpa-
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rent medium, a part of it is absorbed by it. The relation between absorbance and concentration of solution follows Beer’s law which states that absorbance is directly proportional to the concentration. The present communication describes a method for the determination of atmospheric nitrogen dioxide using alkaline ferron as an efficient absorbing as well as coupling reagent. The diazonium salt formed as a result of diazotization is coupled with ferron in alkaline medium to give an orange- red dye. The dye was extracted by passing it through an amino cartridge, which leads to a ten fold increase in efficiency as compared to the original test solution.
The method proposed is rapid and free from rigorous control of experimental conditions, simple equipment, stability of colour, non toxic nature of reagents and its easy availability are its added advantages. As the method involves simple instrumentation, it can be used for routine analysis of nitrogen dioxide in air.
2. Experimental
2. 1. Apparatus and Reagents
A Toshniwal model TVSP 25 spectrophotometer was used for spectral measurements. pH measurements were made with systronics pH meter model 331. Fritted midget impingers (Diameter ?10 mm) of 35 mL capacity were used for air sampling. Flow rate adjustable calibrated rota-meters were used for measuring the airflow. All the chemicals used were of Analytical Reagent grade or the best quality available. SPE Amino, Cartridge (Alltech, Deerfield, IL, USA) was used for preconcentration of dye. Double distilled deionised water was used throughout the experi-
ment. A stock solution of nitrite containing 1000 µg mL1 of nitrite was prepared by dissolving 0.15 g of pre-dried sodium nitrite (Loba chemie, Mumbai) in 100 mL of nitrite free water. Few drops of chloroform was added as stabilizer. Working solution of nitrite was prepared by appropriate dilution of stock solution. 0.2% p-aminoacetopheno-ne (Loba chemie, Mumbai) was prepared in 1 M hydrochloric acid.0.25% of 8- Hydroxy 7-iodo 5-quinoline sulpho-nic acid [Ferron] (Merck, Mumbai) was prepared in aqueous solution. Absorbing solution was prepared by dissolving 25 mg of Ferron in 100 mL of 0.1 M sodium hydroxide. The solution stored at 4 °C was stable for ~2 weeks.1 M sodium hydroxide solution was prepared to obtain required alkalinity. Methanol (Merck, Mumbai) was used for clean up of Amino cartridge and elution of the adsorbed dye. A 10% w/v aqueous solution of disodium salt of ED-TA (Merck, Mumbai) was prepared for masking ions.
2. 2. Procedure
Known amount of nitrite solution was taken in an impinger kept in a water bath maintained at 60 °C, to which 2 M Hydrochloric acid was added drop wise to liberate nitrogen dioxide. The liberated nitrogen dioxide was drawn through the absorbing solution at a flow rate of 0.75 L min–1. The collection efficiency was improved by addition of 1 mL glycol. After sampling, the absorbing solution was transferred into a 25 mL measuring flask and then 1 mL PAAP was added. Acidity was maintained by adding ~ 0.4N Hydrochloric acid. The solution was shaken for five minutes followed by addition of ferron. The solution was made alkaline with sodium hydroxide resulting in the formation of orange-red dye. The absorbance
Step-1

y\ y-NH2 + no2
ir
p- aminoacetophenone    Nitrogen dioxide
NEN^
W   //
O
Acetophenone diazonium ion
Step-II
o
^ /
+                      O
-NENCF+ HO   S O
Diazonium ion
SO3H
Alkline medium OH-----------------^
Ferron
Orange Red dye Reaction scheme
The colour reaction involves two steps
Step- I p-aminoacetophenone reacts with nitrogen dioxide to form acetophenone diazonium chloride.
Step-II Diazonium ion is coupled with ferron to give orange-red coloured azo dye having X    485 nm.
 max
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of the dye was measured at 485 nm. Amount of nitrogen dioxide was deduced from the calibration graph prepared for 0.8 to 8 µg of nitrite after correction with stoichiome-tric factor i.e. 0.72.
Same method can be used for the determination of nitrite. Known amount of nitrite was taken in 25 mL volumetric flask to it was added 1 mL of p-aminoacetopheno-ne and the solution was kept for 2 minutes with occasional shaking to ensure complete diazotization. 1 mL of EDTA solution was added and then 1 mL of ferron solution was added for coupling and made alkaline with sodium hydroxide. The absorbance of the dye was measured at 485 nm.
Many reagent systems have been studied for the determination of nitrogen dioxide and nitrite such as p-aminoacetanalide, p-nitro aniline, p-aminoacetopheno-ne, 4-nitro 1-naphthyl amine, anthranilic acid, sulphanilic acid but p-aminoacetophenone was found to be the most appropriate reagent for coupling with ferron.
2. 3. Procedure for Solid Phase Extraction
The orange-red dye was extracted by passing 25 mL of the dye through an amino cartridge that was pre-conditioned by passing in sequence, 3 mL each of methanol and water. Passing 2 mL methanol eluted the adsorbed dye, which was measured at 485 nm against a reagent blank, which gives negligible absorbance at this wavelength.
3. Results and Discussion
3. 1. Analytical Characteristic
The colour system shows maximum absorbance at 485 nm. (fig. 1) Beer’s law was obeyed in the range 0.04 to 0.36 µg mL–1; molar absorptivity and Sandell’s Sensitivity were found to be 9.40 · 104 L mol–1 cm–1 and 0.00050
Table 1. Spectral characteristics, precision and accuracy of the presented method.
Parameter
Results.
Stability of colour (hours)
? max (nm)
Limit of Beer's law (µg mL–1)
Molar absorptivity (L mol–1 cm–1)
Limit of detection (µg mL–1)
Limit of quantification (µg mL–1))
Sandell's sensitivity (µg mL–2)
Standard deviation(±)
Relative standard deviation (%)
Error
Regression equation
(Y = bx + a)*
Slope b
Intercept a
Correlation coefficient r**
~ 72
485
0.04 to 0.36
9.40 · 104
0.010
0.030
0.0005
0.0056
1.29
0.0016
2.1328 0.0074 0.9959
Fig 1: Absorption spectra of the dye
µg cm–2 respectively. Six replicate analysis of a 5 µg/25 mL nitrite solution following the proposed procedure gave a standard deviation and relative standard deviation of ±0.0056 and 1.29% respectively. Limit of quantification (LOQ) is evaluated by the relation 10 ?/s and the limit of detection is dy 3 ?/s, where ? is standard deviation of the blank with respect to water and s is the slope of the calibration curve. The limit of detection is well below the lower limit of Beer’s law. The slope, the intercept, and the correlation coefficient evaluated by least-squares regression analysis are also included. (Table 1) The calibration data for nitrogen dioxide prepared from standard nitrite solution (after correcting with stoichiometric factor) is given in table 2, fig. 2.
3. 2. NO2: NO2– Stoichiometric Ratio
The colour produced by absorbing a given amount of nitrogen dioxide can also be compared with that produced by adding an equimolar amount of nitrite. The ratio of their absorbance value is known as šstoichiometric factor’. The NO2 : NO2– stoichiometric factor for the proposed method has been evaluated and found to be 0.72. (table 2, fig. 2) The stoichiometric factor varied and was lowered if glycol was not used.
The following reaction takes place
2NO2 + 2OH —> NO2 + NO3 + H2O
0.75 Lit / min.
* Where x is the concentration in µg mL–1 ** n = 5
No. of	Absorbance	Absorbance	Stoichiometric
analysis	due to	due to	factor
	NO2 at 485 nm	NO2– at 485 nm	
1	0.093	0.129	0.720
2	0.176	0.244	0.721
3	0.260	0.360	0.722
4	0.350	0.479	0.730
5	0.431	0.599	0.719
6	0.520	0.715	0.727
7	0.611	0.839	0.728
8	0.690	0.949	0.721
9	0.775	1.064	0.728
Mean 72.46 %
Table 2. Evaluation of NO2: NO2– stoichiometric factor Flow rate =
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Fig 2: Stoichiometric factor NO2: NO2– Calibration curve for the determination of nitrogen dioxide (extractive system)
Every substance competing with NO2 for the OH radical will increase the NO2 : NO2– stoichiometric factor higher than 0.5.38 In the present investigation alkaline gly-col is used for the absorption of the NO2.
Fig 3: Calibration curve for the determination of nitrogen dioxide (non extractive system)
3. 3. Collection Efficiency
The collection efficiency of the absorbing solution was determined by drawing air containing nitrogen dioxide through two impingers containing absorbing solution, connected in series. After sampling, the NO2 content of each impinger was analysed separately by the recommended procedure. It was observed that the first impinger has a collection efficiency of 98–99% at a flow rate of 0.75 lit min–1 whereas the second impinger showed negligible absorbance.
3. 4. Effect of pH
The effect of pH on the absorbance of the dye was studied over the range 7.0–11.0. While complete diazotiza-tion required 0.1 M hydrochloric acid, the colour started appearing at pH 8. The dye had maximum and constant ab-
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sorbance across the pH range of 10–11. Below pH 7 no colour appeared and above pH 11.0 the colour was not stable.
3. 5. Effect of Time and Temperature
Diazotization and coupling required 2 minutes for completion. The resulting dye was stable for ?72 hours which makes the method versatile and useful for field measurement. Maximum absorption was obtained by maintaining the absorbing solution at 4 °C.
3. 6. Effect of Foreign Species
The effect of diverse ions commonly found with nitrogen dioxide was studied by adding known amount of diverse ions into the absorbing solution before sampling and then nitrogen dioxide was analysed as given in the procedure. Interferences from sulphur dioxide and heavy metals were masked with H2O2 and 10% EDTA respectively. Carbon monoxide, carbon dioxide, ammonia, formaldehyde and benzene which are present in air with nitrogen dioxide had no effect on colour development.
3. 7. Applications
To check the validity of the method it was applied for analysis of nitrogen dioxide and data were compared to those obtained by a reported method. In the reported method absorbing solution containing 8-hydroxyquinoli-ne and sodium hydroxide is proposed for the collection of nitrogen dioxide. The nitrite formed is diazotized with p-nitroaniline in acidic medium which is subsequently coupled with 8-hydroxyquinoline to give purple azoxine dye in alkaline medium.26
3. 7. 1. Determination of Nitrogen Dioxide in Cigarette Smoke
Absorbing system consisting of glass wool (to remove the particulate matter), followed by three midget impingers, the first two containing acidic potassium permanganate (2.5% KMnO4 (w/v) in 2.5% H2SO4) for oxidation of nitric oxide to nitrogen dioxide and third impin-ger containing 10 mL of absorbing solution for collection of cigarette smoke was set up. A cigarettes marked to a required length was fixed in a glass holder placed in the tip of suction hole of the first impinger. It was lighted and the air was sucked at a rate of 0.75 L min–1 till it burned to the marked length. The collected gas was analyzed for nitrogen dioxide as recommended in the procedure. (table 3)
3. 7. 2. Auto Exhaust
Auto exhaust gases from automobile was drawn through two midget impingers each containing 5 mL of absorbing solution attached to a suction pump. The gas
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was analysed for nitrogen dioxide by the proposed method and the results are shown in table 3.
3. 7. 3. Determination of Nitrogen Dioxide in Work Room Air
Air samples were drawn through two 35 mL calibrated midget impinger containing 5 mL of absorbing solution connected in series, at a flow rate of 0.75 L min–1. Amount of nitrogen dioxide was determined by the proposed method. (table 3)
3. 8. Applications for Determination of Nitrite 3. 8. 1. In Water Sample
River water, pond, and tap water samples were collected from the different regions. Samples were preserved by treating with 2 ml of mercuric chloride (4 µg /100 mL) and stored at 0 °C. They were filtered through whatman filter paper No. 41 before analysis. In order to check the applicability of the proposed method for the determination of nitrite in water, nitrite free tap water was fortified by adding known amount of nitrite. The sample were then
Table 3. Results of analysis of air
Source of	Volume	mg/m3	Nitrogen dioxide founda		F-test	± t-test	Errord
sample	of sample	present	Present	Reported			
	taken	methode	method	methode			
	(Lit)b		(\lg mL–1)	(|lg mL–1)			
	10	0.73	0.36(± 0.026)	0.35(±0.029)	1.24	0.63	0.005
Cigarette smoke	-	0.71	0.35(± 0.017)	0.35(±0.013)	1.71	0.00	0.000
(with filter)c	-	0.61	0.30(± 0.021)	0.31(±0.019)	1.80	0.87	0.004
	10	1.4	0.68(± 0.022)	0.67(±0.021)	1.10	0.81	0.004
Cigarette smoke	-	1.2	0.60(± 0.010)	0.62(±0.008)	1.56	3.82	0.001
(without filter)	-	1.3	0.65(± 0.022)	0.67(±0.017)	1.67	1.79	0.003
Auto exhaust	10	0.87	0.43(± 0.015)	0.44(±0.014)	1.12	1.20	0.002
	-	0.79	0.39(± 0.019)	0.39(±0.017)	1.24	0.00	0.003
	-	0.91	0.45(± 0.022)	0.46(±0.017)	1.67	0.89	0.003
Work room air	10	0.20	0.10(± 0.020)	0.11(±0.018)	1.25	1.91	0.003
(Domestic	-	0.59	0.29(± 0.011)	0.30(±0.010)	1.21	1.65	0.002
environment)	-	0.30	0.15(± 0.012)	0.12(±0.009)	1.78	4.96	0.002
Work room air	10	0.20	0.10(± 0.015)	0.11(±0.014)	1.12	1.21	0.002
(Laboratory)	-	0.2	0.09(± 0.021)	0.08(±0.020)	1.10	0.85	0.004
	-	0.2	0.09(± 0.020)	0.09(±0.016)	1.56	0.00	0.003
aMean of three replicate analyses, bVolume of air sample taken - 10 Liters, cSmoke of one cigarette analyzed, dThe F and t value refer to comparison of the proposed method with the reported method. Theoretical value at 95% confidence level t = 2.776.   eConversion factor ppm to mg/m3 mg/m3 = 46.01(Mol.wt of NO2) / 22.4 (at NTP) x C [ppm]
Table 4. Determination of nitrite in water and soil
Sample	Nitrite added	Proposed	method	Reported	method27
		*Nitrite found	Recovery	aNitrite found	Recovery
		(%)	(%)	(%)	(%)
bRiverwater	-	2.5(± 0.006)	-	2.7	-
(5 mL)	-	2.3(± 0.004)	-	2.2	-
		2.0(± 0.001)	-	1.9	-
Pond water	-	1.7(± 0.026)	-	1.4	-
(5mL)	-	1.5(± 0.015)	-	1.6	-
	-	1.2(± 0.011)	-	1.0	-
cSoil	-	0.8(± 0.001)	-	1.0	-
	-	1.5(± 0.010)	-	1.2	-
	-	2.0(± 0.015)	-	2.3	-
Tap water	4	3.90(± 0.026)	97.5	3.91	97.75
	6	5.70(± 0.021)	95.00	5.90	98.33
	8	7.94(± 0.020)	99.25	7.89	98.62
a Mean of three replicate analyses, b Samples were collected from different region of Shivnath River, which receives effluent from various industries, c 5 g of soil sample were taken.
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Table 5. Comparison table of nitrogen dioxide
S.     Reagents/Ref. No
Range           ?max
(µg mL–1)      (nm)
Remarks
1       Sulphanilicacid+
naphthylamine28
2       8-Hydroxyquiniline PNA19
3       o-Nitroaniline + 1-amino-naphthalene-2-sulphonicacid29
4       p-Aminoacetophenone +NEDA + oxalic acid20
5       Potassium iodide + HCl + LCV30
6       p-Aminoacetophenone + Ferron (proposed method)
0.05-1.20       520
0.07-0.49	570
0.08-0.64	545
0.12-0.96	548
0.004–0.04	590
0.04-0.36	485
Cu2+, Fe3+ and strong oxidants
Extractive, Cu2+, Fe3+ are interfere Extractive, less sensitive
Heavy metals and SO2 are interfering
Reagents costly
Simple, Sensitive, reagents cheap, common ions do not interfere.
Table 6. Comparison table of nitrite
S.     Reagents/Ref. No
Range (µg mL–1)
Remarks
(nm)
p-rosaniline + NEDA 31 p-Aminophenyl mercaptoacetic acid 32 Sulfathiazole + NEDA 33 p-Aminoacetophenone + NEDA 34 o-Nitroaniline +1–aminonaphthalene– 2-sulphonic acid 35 p-Nitroaniline + phloroglucinol 36 Leucocrystal violet 37 (proposed work) p-aminoacetophenone + Ferron
0.08-0.72 0.1-1.6 0.054–816 1-0.8
0.08-0.68
0.004–0.04
0.004–0.04
0.03-0.4
565      Fe3+, Cr6+ and s severely interfere
495      S and Sb3+ interfere
546      Less sensitive
545      Less sensitive
545      Less sensitive
420      Cu2+ and Fe3+ interfere above 75
590      Reagents costly
485      Simple, Sensitive, reagents cheap, common ions do not interfere.
analysed by the proposed and reported method.27 (table 4)
3. 8. 2. In Soil
Soil samples from farmland and roadside were taken, and dried at 55 °C in an oven for 12–16 hours. The dried sample was passed through a 2 mm mesh sieve. Sufficient water (containing 1 or 2 drops of concentrated sulphuric acid) was poured to soak the soil completely. After a few minutes it was filtered, and leached with water. The filtrate was made up to 100 mL and the amount of nitrite was analyzed by the proposed and reported methods. table 4.
3. 7. 4. Comparison With Other Reported Methods
The method has been compared with other reported methods. The advantages of the present method over other methods are summarized in table 5 and 6.
4. Conclusions
In the proposed work a simple, selective and inexpensive solid phase extraction method coupled with spec-trophotometry has been developed for the determination
of nitrite and nitrogen dioxide. Although a number of sophisticated techniques like HPLC, GC, CE are available for determination of these pollutants at trace levels, factors such as low cost of instrument, easy handling and almost no maintenance have caused spectrophotometry to be popular technique, particularly in laboratories of developing countries. The sensitivities of various reagents used for the spectrophotometric determination of nitrite and nitrogen dioxide are compared (table 5 and 6) and the method was found to be quite sensitive. Use of solid phase extraction further lead to a ten folds increase in sensitivity.
5. Acknowledgement
The authors are thankful to Department of Chemistry, Govt. V. Y. T. PG. Autonomous College Durg (C. G.) India.
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Povzetek
Predlagana je enostavna metoda za dolo~evanje du{ikovega dioksida z uporabo Ferrona kot absorpcijskega sredstva in reagenta. Nitritni ion reagira s p-aminoacetofenonom v alkalni absorpcijski raztopini, tako diazotirani p-aminoacetofe-non pa nadalje reagira s Ferronom. Nastalo oran`no-rde~e barvilo z maksimumom absorbance pri 485 nm in molarnim ekstinkcijskim koeficientom 9,4 · 104 L mol–1 cm–1 skoncentriramo z ekstrakcijo na trdnem nosilcu. Odvisnost absor-bance od koncentracije du{ikovega dioksida je linearna v obmo~ju 0,04 do 0,32 µg mL–1.
Optimizirali smo reakcijske pogoje diazotiranja in prou~ili vpliv spremenljivk kot so temperatura, reakcijski ~as in pH. Podrobno smo raziskali u~inkovitost absorpcije in stehiometri~no razmerje pretvorbe du{ikovega dioksida v nitrit. Metodo smo uspe{no uporabili za analizo du{ikovega dioksida v cigaretnem dimu, izpu{nih plinih motorjev in delovnih prostorih ter za dolo~evanje nitrita v vodah, vzorcih tal itd.
Parmar et al.:  Determination of Nitrite and Nitrogen Dioxide ...
Acta Chim. Slov. 2008, 55
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Acta Chim. Slov. 2008, 55
ERRATA
ACSi 2007, 54, 325-335, and 2007, 54, 868-881
In two papers published in the previous issues of Acta Chimica Slovenica, written by Leokadia Strzemecka in Acta Chim. Slov. 2007, 54, 325-335, and 2007, 54, 868-881, the correct name of studied compounds in the title and throughout the text should be: N-allyl-5-(pyridin-2-yl)-1,3,4-thiadiazol-2-amine for compound 1, and N-cin-namyl-5-(pyridin-2-yl)-1,3,4-thiadiazol-2-amine for compound 2 (see Figure 1).
Figure 1
A
N-allyl-5-(pyridin-2-yl)-l,3,4-thiadiazol-2-amine (1)
N-cinnamyl-5-(pyridin-2-yl)-l,3,4-thiadiazol-2-amine (2)
In all Figures where the tautomeric and resonance structure of compounds 1 and 2 with one electron (radical) appears, the biradical structure should be readed (see Figure 2 for example), and single arrow (›) linking different tautomeric structures should be replaced with two reverse arrows (^).
__N                      y---------.
Figure 2
biradical of N-allyl-5-(pyridin-2-yl)-l,3,4-thiadiazol-2-amine (1)
Corrections of type indicated in Figure 3 (see Figure 6 in Acta Chim. Slov. 2007, 54, 868-88, for example) and of similar types should be carried out in all other schemes as well.
Figure 3
2-methylpyridyl radical
N
N
N
S