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The spectrophotometric determination of nitrite in water with 8-quinolinol
Chimica Acta. 111 (1979) 311-314 o Elsevier Scientific Publishing Company, Amsterdam Analytica
Printed in The Netherlands
Short Communication
THE SPECTROPHOTOMETRIC WATER WITH 8-QUINOLINOL
JAGADEESAN Department
(Received
NAIR
DETERMINATION
OF NITRITE
IN
and V. K. GUPTA*
of Chenzist~,
RatGshankar
Unilvzrsit~
Raipur
(M.P.)
(India)
19th April 1979)
Sttnzma~. The method is based on the formation of a purple azosine dye by coupling diazotized p-nitroaniline with 8-quinolinol. Beer’s law is obeyed at 550 nm in the range !+_28 fig NO; per 25 ml. The molar absorptivity and Sandell sensitivity are 3.88 X 10” 1 mol-’ cm-’ and 0.0012 ~1: cm-l, respectively_
Nitrite occurs in water as an inte~ediate during the nitrogen cycle; traces of nitrite in environmental samples give an excellent indication of the extent of pollution and eutrophication. Many spectropliotometric methods [l---7] have been reported for the determinat*ion of small amounts of nitrite. Most of these involve diazotization of an aromatic amine with nitrite and subsequent coupling of the azo compound with an aromatic amine or phenol to form highly coloured azo dyes. Many of these methods offer excellent sensitivity and selectivity, but quite often require close control of pH and temperature during the diazotization step as well as relatively long coupling times. The present communication deals with the use of S-quinolinol as a coupling reagent for the determination of nitrite in water. p-Nitroaniline is used as the diazotizing reagent_ An intense purple azoxine dye [8], extractable into organic solvents, is formed in alkaline medium. The colour react.ion is very sensitive and reproducible results can be obtained without rigorous control of experimental conditions.
Experimerttal Apparatus_
An ECIL spectrophotometer Model GS 565 or a Carl Zeiss Spekol was used with matched l-cm glass cells; pH measurements were made on an ECIL pH meter, Model PH 821. Standard sodium nitrite solution (1 mg NO, ml-‘). -4 stock solution of sodium nitrite was prepared by dissolving 150 mg of dried analytical-grade reagent in 100 ml of deaerated, doubly-distilled water. A little chloroform was added as stabilizer. Working standards were prepared by appropriate dilution. p-Nitroaniline solution_ The commercially available reagent was crystallized twice before use; a 1 X low3 M solution was prepared in 2 M hydrochloric acid.
312
Procedure_ Transfer an aliquot (not more than 15 ml) of the water sample containing Z-28 pg NO; to a 25-ml volumetric flask and add 2 ml of pnitroaniline reagent. Shake for 1 min and add 1 ml of ethanolic 0.2% (w/v) 8-quinolinol solution and 1 ml of aqueous 10% (w/v) disodium-EDTA solution_ Make alkaline (to about pH 12) with 2 M sodium hydroxide solution (ca_ 3 ml) and dilute to the mark with distilled water. Measure the absorbance at 550 nm against a reagent blank. Calculate the amount of nitrite from a calibration graph prepared from measurements done in the same way. Results and discussion The reactions involved 02NC,H,_NH2 O,N.C,H,.N+-
are
-I-MO; + 2H’ 7 02N_ChHJN+ = N -i- 2Hz0 N + C9H7 NO -+ 02N.C,H,.N
= N.C,H,NO
The final product in alkaline medium gives an intense purple colour. The coupling of the p-nitrophenyldiazonium ion may take place at either the 5or ‘i-position of %quinolinol, so that the formation of two isomeric dyes may be espected, but at ca. 30°C couplin g takes place only at the &position 19). Paper chromatographic esamination of the butanol extract of the dye confirmed that only one component is present. The azoxine dye has amasimum absorption at 550~555 m-n; the absorption of the reagent blank in this region is negligible_ Under the recommended conditions, the dye shows no appreciable change in absorbance up to 30 h, and the absorbance remains constant in the temperature range 20-40°C. Effects of varying reactiorz conditions. The effect of acidity on the diazotization reaction was studied in the range O-4 XI hydrochloric acid. At least 0.02 M hydrochloric acid is necessary for complete diazotization; in 0.02-4 M acid, constant absorbance was observed_ The time necessary for complete diazotization was determined; the absorbance is constant for development periods of l-90 min_ The diazotization reaction is fast at 30°C and the p-nitrophenyldiazonium ion is stable under the esperimental conditions used. The time required for coupling the p-nitrophenyldiazonium ion with S-quinolinol was investigated; the minimum time for full colour development was 1 min. Although the colour started to form at pH 9, full colour development was obtained only above pH ll_ The effects of varying the molar ratios of p-nitroaniline and S-quinolinol were examined. For p-nitroaniline, the absorbance was constant at molar ratios of 1:l and above. For a lo-fold excess of p-nitroaniline, X,,, remained the same_ A molar ratio for S-quinolinol of 6:l or greater was needed for full colour development. There was no significant change in the absorbance on adding a very large excess of 8-quinolinol. Beer’s law, optimum range, sensitivity, molar absorptivity and reproducibility. The system obeyed Beer’s law in the range 2-28 ,ug of nitrite
313 TABLE
1
Spectrai
characteristics
of the dye
in various
sofvents
Solvent
Ama.. (nm)
C(X 10--l mol’.’ cm’ ‘)
Stability
Water n-Butanol n-Hexanol Chloroform
550 560 565 530
3.55 3.50 2.90 3.45
30 h 20 min 5 min Very unstable
555
3.50
10 min
Benzen~butanoI(1
TABLE Effects
+ 1)
2 of some
--
ions and compounds
.._ -_- _.__..-_._. ._..--
Ion (tolerance
on the determination
_.__ __-__._-______
of 0.4
---
ppm nitrite
limit in ppm)
PO;-(lOOO), NO; (10001, SOi-(lOOO), Bi- (400). I-11.61, SO:-(10”). K’(lOOO), Be’?+ (SO), Ca”’ (160b). hlg” (1Ob). Be” (200b). 0” (1.6). Cr’* (S), &IO’* (-fO), W\rr* (40), Mn2* (30), FeJ+ (-lOC), Co”’ (40b), Ni” (60”), Zn” (40b), Cd” (sob), Hg” (Sob), A13+ (60b). Pb” (80”). NH,‘(lOOO),SCN-(ZOO), anaiine (-IO). formaldehyde (40). phenoi (40) --_“In cIn
the presence the presence
added
after
Bi”’ (60’).
As”
(20),
Sn”’
(40). -.--
of 0.5 ml of 1.PA 1110, (20 vol). Yn the presence of 1 ml of lOF& EDTA. of 1 ml of 10% sodium potassium tartrate. -411 maskingagents were
S-quinolinol.
per 25 ml (O-OS--1.12 ppm) at 550 nm. The optimal concentration range, evaluated from the Ringbom curve [lo], \;rras6-24 pg per 25 ml, The Sandel sensitivity [ 111 and molar absorptivity cakulated from Beer’s law &&a were 0.0012 I_rgcm-?- and 3SS X 10’ 1 mol-’ cm-‘. respectively, The reproducibility of the method was checked by replicate analyses of a standard sodium nitrite solution over a period of 7 days. The standard deviation and relative standard deviation were kO.099 and rtO.9970 respectively (n = 7) for a solution containing 10.00 /lg of nitrite per 25 ml (mean value found, 9.99 ~8; range, 9.85-10.15). Extraction with organic soluents. After estraction into butanol, hesanol, chloroform, and benzene-butanol (I +l) the dyestuff was less stable than in water (stability > 30 h). A shift in A,,,, lvas also observed Jvith different solvents_ The data are given in Table 1. E=ffect of foreign ions. Because this method was developed for the analysis of water samples, the effects of the foreign ions commonly present in mater were studied. rMetal ions forming hydroxides in alkaline medium were expected to interfere but a large number of these ions were masked with EDT& Copper(D), iron(H) and suiphide ions caused serious interference. The tolerance limits of the foreign ions shown in Table 2 are the amounts that caused not more than 2% change in the absorbance during the determination of a fixed amount of nitrite.
314
This method is satisfactory for the rapid determination of nitrite in water samples. The rapid colour development, excellent reproducibility, and independence of the colour intensity from pH, temperature, and reagent concentrations make the method versatile and useful. The reagents are cheap and easily avaifabfe, The authors express their thanks to the Head, Department of Chemistry, Ravisbankar University, Raipur, for providing laboratory facilities and J. Nt thanks the University Grants Commission, New Delhi, for the award of a feliowship, REFERENCES 1 Standard lMethods for the Examination of Water, Sewage and Industrial Waste, American Public Health Association, 13th edn., 1971. 2 I. M. Kolthoff and P. J. Eiving, Treatise on Analytical Chemistry, Part II, Vol. 5, Interscience, New York, 1961, p. 273. 3 C!_ A_ Streuli and P. R. AverelI, The Analytical Chemistry of Nitrogen and its Compounds, Part I, Wiley-Interscience, New York, 1970, p_ 121. 4 A. K. Babko and A. T. Pilipenko, Photometric Analysis, Methods of Determination of of Non-metals; translated from Russian by A. Rosinkin, Mir Publishers, Moscow, 1976, p_ 35. 5 F_ Cefardin, M, Marcantonatos and D. Monnier, Anal. Chim. Acta, 68 (1974) 61. 6 K. Toei and ‘I’. Kiyose, Anal. Chim. Acta, 88 (197’7) 125. 7 M_ Roman, A_ Fernadez-Gutierrez and M. C. Mahedero, Bull_ Sot. Quim. Peru, 43 (1977) 16. 8 J. S. Fritz, W. J_ Lane and A. S_ Bystroff, Anal. Chem., 29 (1957) 821. 9 A. Badrinas, Taianta, 10 (1963) 704. 10 A. Ringbom, Fresenius 2. Anal- Chem_, 115 (1938) 332. II E. 3. Sandeti, Calorimetric Determination of Traces of Metals, Interscience, New York, 3rd edn., 1959, p. 80.
Printed in The Netherlands
Short Communication
THE SPECTROPHOTOMETRIC WATER WITH 8-QUINOLINOL
JAGADEESAN Department
(Received
NAIR
DETERMINATION
OF NITRITE
IN
and V. K. GUPTA*
of Chenzist~,
RatGshankar
Unilvzrsit~
Raipur
(M.P.)
(India)
19th April 1979)
Sttnzma~. The method is based on the formation of a purple azosine dye by coupling diazotized p-nitroaniline with 8-quinolinol. Beer’s law is obeyed at 550 nm in the range !+_28 fig NO; per 25 ml. The molar absorptivity and Sandell sensitivity are 3.88 X 10” 1 mol-’ cm-’ and 0.0012 ~1: cm-l, respectively_
Nitrite occurs in water as an inte~ediate during the nitrogen cycle; traces of nitrite in environmental samples give an excellent indication of the extent of pollution and eutrophication. Many spectropliotometric methods [l---7] have been reported for the determinat*ion of small amounts of nitrite. Most of these involve diazotization of an aromatic amine with nitrite and subsequent coupling of the azo compound with an aromatic amine or phenol to form highly coloured azo dyes. Many of these methods offer excellent sensitivity and selectivity, but quite often require close control of pH and temperature during the diazotization step as well as relatively long coupling times. The present communication deals with the use of S-quinolinol as a coupling reagent for the determination of nitrite in water. p-Nitroaniline is used as the diazotizing reagent_ An intense purple azoxine dye [8], extractable into organic solvents, is formed in alkaline medium. The colour react.ion is very sensitive and reproducible results can be obtained without rigorous control of experimental conditions.
Experimerttal Apparatus_
An ECIL spectrophotometer Model GS 565 or a Carl Zeiss Spekol was used with matched l-cm glass cells; pH measurements were made on an ECIL pH meter, Model PH 821. Standard sodium nitrite solution (1 mg NO, ml-‘). -4 stock solution of sodium nitrite was prepared by dissolving 150 mg of dried analytical-grade reagent in 100 ml of deaerated, doubly-distilled water. A little chloroform was added as stabilizer. Working standards were prepared by appropriate dilution. p-Nitroaniline solution_ The commercially available reagent was crystallized twice before use; a 1 X low3 M solution was prepared in 2 M hydrochloric acid.
312
Procedure_ Transfer an aliquot (not more than 15 ml) of the water sample containing Z-28 pg NO; to a 25-ml volumetric flask and add 2 ml of pnitroaniline reagent. Shake for 1 min and add 1 ml of ethanolic 0.2% (w/v) 8-quinolinol solution and 1 ml of aqueous 10% (w/v) disodium-EDTA solution_ Make alkaline (to about pH 12) with 2 M sodium hydroxide solution (ca_ 3 ml) and dilute to the mark with distilled water. Measure the absorbance at 550 nm against a reagent blank. Calculate the amount of nitrite from a calibration graph prepared from measurements done in the same way. Results and discussion The reactions involved 02NC,H,_NH2 O,N.C,H,.N+-
are
-I-MO; + 2H’ 7 02N_ChHJN+ = N -i- 2Hz0 N + C9H7 NO -+ 02N.C,H,.N
= N.C,H,NO
The final product in alkaline medium gives an intense purple colour. The coupling of the p-nitrophenyldiazonium ion may take place at either the 5or ‘i-position of %quinolinol, so that the formation of two isomeric dyes may be espected, but at ca. 30°C couplin g takes place only at the &position 19). Paper chromatographic esamination of the butanol extract of the dye confirmed that only one component is present. The azoxine dye has amasimum absorption at 550~555 m-n; the absorption of the reagent blank in this region is negligible_ Under the recommended conditions, the dye shows no appreciable change in absorbance up to 30 h, and the absorbance remains constant in the temperature range 20-40°C. Effects of varying reactiorz conditions. The effect of acidity on the diazotization reaction was studied in the range O-4 XI hydrochloric acid. At least 0.02 M hydrochloric acid is necessary for complete diazotization; in 0.02-4 M acid, constant absorbance was observed_ The time necessary for complete diazotization was determined; the absorbance is constant for development periods of l-90 min_ The diazotization reaction is fast at 30°C and the p-nitrophenyldiazonium ion is stable under the esperimental conditions used. The time required for coupling the p-nitrophenyldiazonium ion with S-quinolinol was investigated; the minimum time for full colour development was 1 min. Although the colour started to form at pH 9, full colour development was obtained only above pH ll_ The effects of varying the molar ratios of p-nitroaniline and S-quinolinol were examined. For p-nitroaniline, the absorbance was constant at molar ratios of 1:l and above. For a lo-fold excess of p-nitroaniline, X,,, remained the same_ A molar ratio for S-quinolinol of 6:l or greater was needed for full colour development. There was no significant change in the absorbance on adding a very large excess of 8-quinolinol. Beer’s law, optimum range, sensitivity, molar absorptivity and reproducibility. The system obeyed Beer’s law in the range 2-28 ,ug of nitrite
313 TABLE
1
Spectrai
characteristics
of the dye
in various
sofvents
Solvent
Ama.. (nm)
C(X 10--l mol’.’ cm’ ‘)
Stability
Water n-Butanol n-Hexanol Chloroform
550 560 565 530
3.55 3.50 2.90 3.45
30 h 20 min 5 min Very unstable
555
3.50
10 min
Benzen~butanoI(1
TABLE Effects
+ 1)
2 of some
--
ions and compounds
.._ -_- _.__..-_._. ._..--
Ion (tolerance
on the determination
_.__ __-__._-______
of 0.4
---
ppm nitrite
limit in ppm)
PO;-(lOOO), NO; (10001, SOi-(lOOO), Bi- (400). I-11.61, SO:-(10”). K’(lOOO), Be’?+ (SO), Ca”’ (160b). hlg” (1Ob). Be” (200b). 0” (1.6). Cr’* (S), &IO’* (-fO), W\rr* (40), Mn2* (30), FeJ+ (-lOC), Co”’ (40b), Ni” (60”), Zn” (40b), Cd” (sob), Hg” (Sob), A13+ (60b). Pb” (80”). NH,‘(lOOO),SCN-(ZOO), anaiine (-IO). formaldehyde (40). phenoi (40) --_“In cIn
the presence the presence
added
after
Bi”’ (60’).
As”
(20),
Sn”’
(40). -.--
of 0.5 ml of 1.PA 1110, (20 vol). Yn the presence of 1 ml of lOF& EDTA. of 1 ml of 10% sodium potassium tartrate. -411 maskingagents were
S-quinolinol.
per 25 ml (O-OS--1.12 ppm) at 550 nm. The optimal concentration range, evaluated from the Ringbom curve [lo], \;rras6-24 pg per 25 ml, The Sandel sensitivity [ 111 and molar absorptivity cakulated from Beer’s law &&a were 0.0012 I_rgcm-?- and 3SS X 10’ 1 mol-’ cm-‘. respectively, The reproducibility of the method was checked by replicate analyses of a standard sodium nitrite solution over a period of 7 days. The standard deviation and relative standard deviation were kO.099 and rtO.9970 respectively (n = 7) for a solution containing 10.00 /lg of nitrite per 25 ml (mean value found, 9.99 ~8; range, 9.85-10.15). Extraction with organic soluents. After estraction into butanol, hesanol, chloroform, and benzene-butanol (I +l) the dyestuff was less stable than in water (stability > 30 h). A shift in A,,,, lvas also observed Jvith different solvents_ The data are given in Table 1. E=ffect of foreign ions. Because this method was developed for the analysis of water samples, the effects of the foreign ions commonly present in mater were studied. rMetal ions forming hydroxides in alkaline medium were expected to interfere but a large number of these ions were masked with EDT& Copper(D), iron(H) and suiphide ions caused serious interference. The tolerance limits of the foreign ions shown in Table 2 are the amounts that caused not more than 2% change in the absorbance during the determination of a fixed amount of nitrite.
314
This method is satisfactory for the rapid determination of nitrite in water samples. The rapid colour development, excellent reproducibility, and independence of the colour intensity from pH, temperature, and reagent concentrations make the method versatile and useful. The reagents are cheap and easily avaifabfe, The authors express their thanks to the Head, Department of Chemistry, Ravisbankar University, Raipur, for providing laboratory facilities and J. Nt thanks the University Grants Commission, New Delhi, for the award of a feliowship, REFERENCES 1 Standard lMethods for the Examination of Water, Sewage and Industrial Waste, American Public Health Association, 13th edn., 1971. 2 I. M. Kolthoff and P. J. Eiving, Treatise on Analytical Chemistry, Part II, Vol. 5, Interscience, New York, 1961, p. 273. 3 C!_ A_ Streuli and P. R. AverelI, The Analytical Chemistry of Nitrogen and its Compounds, Part I, Wiley-Interscience, New York, 1970, p_ 121. 4 A. K. Babko and A. T. Pilipenko, Photometric Analysis, Methods of Determination of of Non-metals; translated from Russian by A. Rosinkin, Mir Publishers, Moscow, 1976, p_ 35. 5 F_ Cefardin, M, Marcantonatos and D. Monnier, Anal. Chim. Acta, 68 (1974) 61. 6 K. Toei and ‘I’. Kiyose, Anal. Chim. Acta, 88 (197’7) 125. 7 M_ Roman, A_ Fernadez-Gutierrez and M. C. Mahedero, Bull_ Sot. Quim. Peru, 43 (1977) 16. 8 J. S. Fritz, W. J_ Lane and A. S_ Bystroff, Anal. Chem., 29 (1957) 821. 9 A. Badrinas, Taianta, 10 (1963) 704. 10 A. Ringbom, Fresenius 2. Anal- Chem_, 115 (1938) 332. II E. 3. Sandeti, Calorimetric Determination of Traces of Metals, Interscience, New York, 3rd edn., 1959, p. 80.