Spectrophotometric Determination of Nitrite in Water

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double-beam spectrophotometer w t h 1 0-cm quartz cuvettes. * To whom ... growth Dilute thrs solution for use in the construction of the calibration graph.
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ANALYST JULY1988 VOL 113

Spectrophotometric Determination of Nitrite in Water P K Tarafder and D P S Rathore" Chemical Laboratory, Regional Centre for Exploration and Research, Atomic Minerals Division, Department of Atomic Energy, Nongmynsong, Shillong-793 0 12, India

A sensitive and selective colour reaction for the determination of nitrite is presented Nitrite reacts with p-aminophenylmercaptoaceticacid in the presence of hydrochloric acid to form a diazonium cation, which is subsequently coupled with N-(I-naphthy1)ethylenediaminedi hydrochloride in acidic medium to form a stable bluish violet azo dye The method ISsuitable for the determination of nitrite from 0 02 to 0 80 p p m in a 1 0-cm cuvette The observed molar absorptivity and Sandell's sensitivity of the azo dye are 465 x lo4 I mol-1 cm-1 and 0 001 pg cm-2, respectively The method is free from most interferences The method has been applied to polluted water samples and the results obtained compare favourably with those from standard methods

Keywords Spectrophotometry, nitrite determination, p aminophenylmercaptoacetic acid, N -(1-naphthyl)ethylenediamr ne dr hydrochloride

Owng to its important role as a precursor in the formahon of N-nitrosamines, many of which have been reported to be potent carmogens,1 and its importance in indicating the level of organic polluhon in water,2 the determinatmn of the exact concentrabon of nitrite in water is desirable There are many spectrophotometnc methods for the detemnation of nitrite and each has been clscussed bnefly m rewews 3 4 Most of these methods are based on the Griess reaction,%g i e ,the formation of an azo dye by &azohsabon of an aromatic amine mth nitnte and coupling of the diazonium cabon mth an aromatic amine or phenol Many of these methods grve good sensitiwty and selectmty but require close control of pH and temperature dunng the diazotisation step and a relahvely long couphng bme Recently, p-aminophenylmercaptoacetic acid has been used as a clazotisable amine together with 8-hydroxyquinoline and a-naphthol10 as couphng agents in the spectrophotometric determinabon of mtrite Although the diazotisation and coupling reactions are very fast in these methods the sensitiwty is not hgh and the methods are not completely free from interferences unless masking agents are used In this paper, we propose a method usmg p-aminophenylmercaptoacetic acid as a diazobsable amine together w t h N-(1-naphthy1)ethylenediamine &hydrochloride (NED) as a coupling agent in the sub-mcrodetermmabon of nitnte It is advantageous to use NED as a coupling agent because it couples in an acidic medium 11 p-Aminophenylmercaptoacetic acid contains a reactive amino group, a mercapto group and a carboxyl group, and is, therefore, capable of combiningin various polydentate modes wrth metal ions This probably facilitates chelation of these ions in solubon, resulting in an enhancement of the tolerance limts for these ions in the nitrite determinabon The proposed method 1s free from CuII, FeIII and sulphite interferences, which normally occur in other methods 12-18 The method is more selective and sensitme than recently reported methodslS22 and is comparable in sensitmty to the standard method using sulphanilarmde and NED 24

water,

Reagents

p-Aminophenylmercaptoacetic acid of 98 1% purity (Evans Chemebcs, New York) was used without further purificabon All other chemicals were of analytical-reagentgrade Standard nitrite solution, 1 mg ml-1 Prepare a stock solubon by Qssolving 0 3750 g of dried sodium nitnte in 250 ml of distilled water Add a pellet of sodium hydroxide to prevent liberation of nitrous acid and 1ml of spectroscopicgrade chloroform to inhibit bacterial growth Dilute thrs solution for use in the construction of the calibration graph p-Ammophenylmercaptoacetic acid, 0 1YO m/V Dissolve 0 50 g of p-aminophenylmercaptoacetic acid in hydrochlonc a a d (1 24) and dilute to 500 ml with the same aad N (1 Naphthy1)ethylenediarnine dihydrochloride, 1 0% mlV Dissolve 5 g of N-(1-naphthy1)ethylenediamine dihydrochlonde in distilled water containing 25 ml of concentrated hydrochloric acid and dilute to 500 ml with water Store this solubon in a dark bottle This reagent solution is stable for 1week

+

Procedure

To an aliquot of sample solution containing 0 50-20 pg of nitnte (0 02-0 80 p p m when diluted to 25 ml) in a 25-ml calibrated flask, add 1ml of 0 1% p-aminophenylmer captoacetic acid solution and 2 ml of the 1%N-(1 naphthyl) ethylenediamine dihydrochloride solution Set aside for a mnimum of 15 min for full colour development and dilute to the mark with distilled water, a bluish violet colour develops and remans stable for 48h Measure the absorbance at 565 nm in 1-cm cuvettes against a reagent blank prepared in the same manner but containing no nitrite Prepare a calibration graph for nitrite in this manner

Results and Discussion For the following experiments 10 pg of nitrite were taken in a final volume of 25 ml

Experimental Apparatus

Spectral measurements were made with a Varian 6344 double-beam spectrophotometer w t h 1 0-cm quartz cuvettes

* To whom correspondenceshould be addressed

Spectral Studies

Under the conditions of the preliminary investigabons in the proposed procedure, the coloured azo dye showed a m m mum absorbance at 565nm (Fig 1) The absorption of the reagent blank was found to be negligible, i e , at 565 nm it varied m the range 0 001-0 003 against distilled water in a 1 0-cm cuvette

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ANALYST, JULY 1988, VOL. 113 Table 1. Effect of foreign ions on the determination of 0.4 p.p.m. of nitrite

Permissible Permissible concentration,* concentration, * Interf erent p.p.m. Interferent p.p.m. Acetate . . . . Cadmium(I1) . . 100 120 Ammonia .. 100 Calcium(I1) . . 80 Cobalt(11) 120 Hydrogen Copper(I1) .. 100 carbonate . 120 Iron(I1) . . . . Carbonate 20 ,. 100 Iron(II1) . . . . 100 4000 Chloride . . . . Dichromate .. 25 80 Lead(I1) . . . . Lithium . . . . 40 120 Fluoride . . . . Magnesium(11) 100 Iodate .. .. 60 Iodide .. . . 20 Manganese(I1) . . 100 Nitrate . . . . 120 Mercury(I1) . . 100 Oxalate . . . . Nickel(I1) .. 100 80 Potassium Persulphate .. 4000 .. 100 Phosphate .. 120 Sodium . . . . 4000 Silicate . . . . Uranyl . . . . 300 500 Sulphate . . . . 4000 Vanadium(V) . . 80 Sulphite . . . . 100 Zinc(I1) . . . . 40

.

420

540

480

600

660

Wavelengthlnm

Fig. 1. Absorption spectrum of the azo dye containing 0.4 p.p.m. of nitrite against a reagent blank

Effect of Acidity on Diazotisstion

The hydrochloric acid concentration for diazotisation was varied from 1 199 to 1 9. A suitable acidity, as evident from the maximum absorbance and stability of the azo dye formed, was found to be given by 1ml of 1 24 to 1ml of 1 + 20 hydrochloric acid per 25 ml of final volume.

+

+

+

Effect of Acidity on Coupling

The hydrochloric acid concentration for the coupling reaction was vaned from 1 + 199 to 1 + 9. The best results were obtained with 2ml of 1 + 19 to 1 + 16 hydrochloric acid per 25 ml of final volume. Effect of p-AminophenylmercaptoaceticAcid Concentration

The effect of the p-aminophenylmercaptoaceticacid concentration on the colour intensity was studied following the proposed procedure and adding 1ml of 0.01-1.0% solutions of the acid in hydrochloric acid (1 + 24) to a series of nitrite solutions. The results showed that a 0.05% p-aminophenylmercaptoacetic acid solution gave satisfactory results. A higher concentration did not enhance the absorbance further, and a lower concentration did not yield satisfactory results. To allow for variations in the reagent purity, a 0.1% concentration of p-aminophenylmercaptoacetic acid is recommended in the proposed procedure. Effect of N-(1-Naphthy1)ethylenediamine Dihydrochloride Concentration

The effect of varying the concentration of NED on the colour intensity was studied using the proposed procedure and adding 0.05,O. 1,0.2,0.5, ...,5.0 ml of 0.053% NED solution in hydrochloric acid (1 + 19) to a series of nitrite solutions. It was found that a maximum intensity coupled with a better stability of the colour formed, was obtained with 2-3 ml of a 1% NED solution in each 25ml of final volume. This optimised NED concentration is sufficient for up to 10.0 p.p.m. of nitrite ion. Effect of Temperature on the Colour Stability

The bluish violet azo dye is stable for more than 48 h and its colour development is independent of temperature in the range 15-50 "C. Effect of Time Interval Between the Addition of Aromatic Amine and the Coupling Agent

The effect of the time interval between the addition of p-aminophenylmercaptoaceticacid and NED was studied by

* Amount of foreign ion causing an error of less than 2% in the determination. adding these reagents at different time intervals (10 s-30 min). The absorbance of the azo dye was found to be constant irrespective of such time intervals. Hence, the diazotisation was very fast at room temperature. A composite reagent system was also tried but this did not provide satisfactory results. Beer's Law, Sensitivity and Reproducibility of the Method

Beer's law holds from 0.02 to 10.0 p.p.m. of nitrite. More concentrated solutions can be diluted with distilled water to bring them within the operational range of the instrument. The apparent molar absorptivity (referred to nitrite) and Sandell's sensitivity in the region of least photometric error were found to be 4.65 X 1041 mol-1 cm-1 and 0.001 pg cm-2, respectively. Under the optimised conditions, the reproducibility of the method was checked by performing nine replicate determinations of 0.4 p.p.m. of nitrite over a period of nine consecutive days. The mean absorbance, standard deviation and relative standard deviation are 0.400, 0.002 and 0.61%, respectively. Hence the method appears to be reliable.

Effect of Foreign Ions

The effect of potential interferents which generally accompany nitrite has been studied at a nitrite concentration of 0.4p.p.m. Unlike the common interferences due to Cu" and FeI" in azo dye methods,l8 in the proposed method such interferences due to these ions are tolerated to a great extent (as is evident from Table 1). The high limit of tolerance of CuII and FeIII, in particular, can be attributed to the presence of the mercapto and carboxyl groups, which generally facilitate chelate formation of these ions. This results in the decreased availability of these ions in solution to interfere in the determination of nitrite. Almost all of the azo dye methods25 recommend that reductants such as Fe", SnII, iodide and sulphide and strong oxidants such as permanganate, chlorate, perchlorate, periodate and tungstate be absent in the samples while determining nitrite according to the prescribed procedure; this is also true for the proposed method. All other ions or radicals are appreciably tolerated in the proposed method.

ANALYST, JULY 1988, VOL. 113

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Table 2. Determination of nitrite in tap water (means of five analyses, 20-ml samples). Tap water gave no test for nitrite

Proposed method

Standard method Relative standard deviation,

Sample No.

Nitrite added per 25 ml/yg

Nitrite* found per 25 mllyg

Mean recovery, YO

YO

1 2 3 4 5

1.0 4.0 6.0 10.0 20.0

1.007 k 0.032 3.917 _+ 0.051 5.987 k 0.058 10.010 k 0.061 20.089 zk 0.155

100.7 97.9 99.8 100.1 100.4

3.18 1.30 0.97 0.61 0.77

Nitrite* found per 25 mVpg

Mean recovery, %

1.003 k 0.042 3.931 k 0.060 6.012 f 0.058 10,019 f 0.070 19.962 f0.203

100.3 98.3 100.2 100.1 99.8

Relative standard deviation, % 4.19 1.53 0.96 0.69 1.01

* Mean k standard deviation.

HOOC-HzC-SnNHz

+ 2H

+ NO2

-

HOOC-H2C-

a

N2+ + 2H20

H

Scheme 1. Proposed reaction mechanism

Determination of Nitrite in Polluted Water Samples In order to check the validity of the method for the determination of nitrite in polluted water, two sets of experiments were carried out. In the first experiment, the recovery of nitrite was checked by adding various amounts of nitrite to the sample solutions. The results obtained were linear and identical with those of the calibration graph obtained for water. In the second experiment, various amounts of sample solution were taken and diluted to a constant volume with distilled water. For all samples the determination of nitrite in polluted water was quantitative. Samples were treated with mercury(I1) chloride as preservative (4 mg per 100 ml of sample) and stored at 0 OC.26 The samples were analysed by the proposed method and by the standard method using sulphanilamide- NED. The results obtained are given in Tables 2 and 3 and show that the proposed method is comparable to the standard method. Proposed Reaction Mechanism (Scheme 1) In the presence of hydrochloricacid, nitrite forms nitrous acid, which reacts with the amino group of p-aminophenylmercaptoacetic acid to form immediately a stable diazonium cation. The diazonium ion subsequently couples with N-(1-naphthy1)ethylenediamine dihydrochloride at moderately high acidity to produce a bluish violet azo dye that can be used for the determination of microamounts of nitrite.

Conclusion The proposed method is a variant of the original Griess method. The sensitivity and selectivity of this method compare favourably with those obtained using the standard

Table 3. Determination of nitrite in polluted water (means of five analyses, 10-ml samples)

Concentration of nitrite, p.p.m. Samule Nd. 1

Proposed method 0.15

2 3 4 5 6 7

0.20 0.48 0.68 0.50 0.17 0.25

Standard method 0.15 0.20 0.49 0.69 0.51 0.17 0.26

method.24 The high sensitivity, simplicity, excellent reproducibility, freedom from pH effects, temperature independence, broad range of nitrite determination, high tolerance limit for a large number of foreign ions and the elimination of the extraction steps are the advantages of the proposed method. Table 4 shows the comparative performances of the spectrophotometric methods for the determination of nitrite. The selectivity of the proposed procedure allows the routine determination of nitrite in waters over a wide range of concentrations.

The authors are grateful to Evans Chemetics, New York, for the gift of p-aminophenylmercaptoaceticacid. Thanks are due to Shri D. Narasimhan, in charge of the Chemistry Laboratory, Shri B. N. Tikoo, Head of the Chemistry Groups, and Shri A. C. Saraswat, Director of the Atomic Minerals Division, for providing the necessary facilities. In addition,

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ANALYST, JULY 1988, VOL. 113 ~~

~

Table 4. Comparison of the proposed method with some Griess modifications for the determination of nitrite

Reagent

Llax./

E/

nm 610 520 545 550 499 519 426 530 510 465 565

1 mol-1 cm-1

System Extractive Aqueous Aqueous Aqueous Aqueous Aqueous Aqueous Aqueous Aqueous Extractive Aqueous

Determination range, p.p.m. 0.035-O.123 0.1-3.0 0.1-0.8 0.08-1.1 0.1-1.5 0.1-1.3 0.1-1.2 0.5-2.0 0.03-0.8 0.0125-0.4 0.02-0.8

Potential Reference interferents* 5 Cu2+,Fe3+,S0327 Cu2+,Fe3+,S0329 Fe3+,S03214 Cu2+,Fe3+,S03217 Fe3+ 19 Fe3+ 20 Cu2+,Fe3+ 21 22 Fe3+ 23 Cu2+,Fe3+,NH4+ This work None

5.24 X 104 4-Nitroaniline . . . . . . . . . 1.47 X 104 4-Aminosalicylicacid . . . . . . . 4.6 x 104 p-Aminoacetophenone . . .. . . 3.8 X 104 4-Nitroaniline . _ . . . . . . . . 3.2 X 104 4-Aminobenzoicacid , . . . . . . . 3.5 x 104 4-Aminobenzoicacid . . . . . . . . 3.87 X 104 Orthanilicacid . . . . , . . , , . 9.5 x 103 Indole . . . . . . .. . . . . 3.91 x 104 4-Aminobenzotrifluoride . . . . . . 1.2 x 105 8-Aminoquinolinet . . . . . . . . 4.65 x 104 p-Aminophenylmercaptoaceticacid . . * Without the use of masking agents. t The reaction and coupling are time consuming. Also, the method is pH and temperature dependent and the extraction procedure is inefficient.

.

thanks are due to Shri V. N. Dwivedi of the Chemical Laboratory, NER, and Dr. H. P. S. Rathore, Senior Research Fellow of A.M.U., Aligarh, for their valuable suggestions during this work.

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13. Toei, K., and Kiyose, T., Anal. Chim. Acta, 1977,88, 125. 14. Nair, J., and Gupta, V. K., Anal. Chim. Acia, 1979,111,311. 15. Chaube, A., Baveja, A. K., and Gupta, V. K., Talanta, 1984, 31,391. 16. Sawicki, E., Stanley, T. W., and Elbert, W. C., Anal. Chem., 1962,34,297. 17. Bashir, W. A., and Flamerz, S., Talanta, 1981, 28, 697. 18. Norwitz, G.,and Keliher, P. N., Analyst, 1985,110,689. 19. Flamerz, S . , and Bashir, W. A., Microchem. J . , 1981,26,586. 20. Bashir, W. A,, Flamen, S., and Ibrahim, S. K., Int. J. Environ. Anal. Chem., 1983,15,65. 21. Rahim, S . A., Fakhri, N. A., andBashir, W. A., Microchem. J., 1983,28,479. 22. Amin, D., Analyst, 1986, 111,1335. 23. Foris, A., and Sweet, T. R., Anal. Chem., 1965,37,701. 24. “Standard Methods for Examination of Water and Waste Water,”Fourteenth Edition, American Public Health Association, Washington, DC, 1976. 25. Boltz, D. F., and Howell, J. A. ,“ColorimetricDetermination of Non-metals,” Wiley-Interscience, New York, 1978, p. 218. 26. Rainwater, F. H., and Thatcher, L. L., “Methods for Collection and Analysis of Water Samples,” Water Supply Paper 1454, US Geological Survey, Washington, DC, 1960, p. 20.

Paper Bl00057C Received December Ist, I987 Accepted March Bth, 1988