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Spectrophotometric determination of hydroxylamine alone and in the presence of monochloramine
Analytica Chimica Acta, 174 (1985) 365-368 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication SPECTROPHOTOMETRIC DETERMINATION OF HYDROXYLAMINE ALONE AND IN THE PRESENCE OF MONOCHLORAMINE MICHEL FERRIOL*
and JOSETTE
GAZET
Laboratoire de Physico-chimie min&rale II (Associl au C.N.R.S. no. 116), Universitb Lyon I, 43 Bd. du I1 nouembre 1918, 69622 Villeurbanne Cedex (France) (Received 4th January 1985)
Summary. Hydroxylamine (< 3 x low3 mol 1-l) is quantified by ultraviolet spectrophotometry after reaction with an excess of formaldehyde in buffered solution at pH 12.2. The absorbance of the formaldoxime produced is measured at 218 nm (E = 5480 1 mol-’ cm-‘) when hydroxylamine is alone but at 230 nm (e = 3230 1 mol-’ cm-‘) when monochloramine is present.
During a study of the degradation of monoehloramine in water, a problem arose in examining the kinetics of the reaction between chloramine and hydroxylamine. The main procedures [l-7] available for the determination of hydroxylamine were unsuitable because there was no way to stop the reaction with chloramine. A more convenient method, based on the reactivity of hydroxylamine with aldehydes and especially formaldehyde in alkaline medium [ 81, was therefore developed. The reaction produced formaldoxime which absorbs in the ultraviolet range NH20H + HCHO + CH,=N-OH
+ Hz0
To ensure complete formation of the oxime, a large excess of formaldehyde is needed. In the presence of monochloramine, another reaction takes place between chloramine and formaldehyde. The fast rate of this reaction in alkaline medium enables the reaction between chloramine and hydroxylamine to be stopped instantaneously [ 91. Figure 1 shows the ultraviolet spectrum of a mixture prepared from hydroxylamine and formaldehyde at pH 12.2. The absorbance is maximal at 218 nm. When hydroxylamine is alone, the absorbance is measured at this wavelength. In mixtures with monochloramine, hydroxylamine is determined at 230 nm because there is then negligible interference between the spectra of the products of the formaldehyde/hydroxylamine and formaldehyde/chloramine reactions. Experimental
A Cary 15 double-beam measurements.
spectrophotometer
was used for absorbance
366
0%
I if+-----
218
06-
0
I
I
06’
I
200
220
X(nm)
240
0
IO
20 Trne
30
(mm)
Fig. 1. Ultraviolet spectrum of a formaldehyde/hydroxylamine mixture at pH 12.2. Conditions: 1.30 x lo-’ mall-’ NH,OH, 1.84 x lo-’ mall-’ HCHO, ca. 20°C. Fig. 2. Evolution of maximum absorbance as a function of time for several sodium hydroxide concentrations: (1) 0.002, (2) 0.01, (3) 0.1 mol 1-l NaOH. Conditions: 1.73 X lO+ mall-’ NH,OH, 1.84 x lo-’ mol 1-l formaldehyde; measurement at 218 run.
Solutions. Formaldehyde solutions were obtained by diluting the reagentgrade commercial product (30% w/w). Hydroxylamine solutions were prepared from reagent-grade hydroxylammonium chloride. Monochloramine is unstable in water. Solutions were prepared just before use by reacting sodium hypochlorite and ammonia as described previously [lo] . The monochloramine concentration was determined by ultraviolet spectrophotometry at 243 nm (E = 458 1 mol-’ cm-‘). Procedure. The hydroxylamine solution (5 ml) of concentration less than 3 X 10e3 mol 1-l was added to a 50-ml volumetric flask containing 5 ml of a ca. 0.2 mol 1-l formaldehyde solution and the mixture was diluted to volume with buffer solution at pH 12.2 (0.042 mol 1-l sodium hydroxide/O.029 mol 1-l disodium hydrogenphosphate). After 20 min, the absorbance was measured at 218 nm. Results Influence of PH. The evolution of maximum absorbance with time was studied for sodium hydroxide concentrations of 0.002, 0.001 and of hydroxylamine and 0.1 mol 1-l with constant concentrations formaldehyde. Figure 2 shows the experimental graphs. The minimum time for recording the spectra was 2 min after the reagents had been mixed, thus
367
the curves were extrapolated to zero time. As can be seen, the absorbance at zero time increases with the sodium hydroxide concentration, and the trends of the curves differ according to the pH. With 0.002 mol 1-l sodium hydroxide, the absorbance decreases as a function of time; with 0.01 mol 1-l sodium hydroxide, the trend is much less pronounced and with 0.1 mol 1-l sodium hydroxide, the absorbance increases with time. In all three cases, the absorbance stabilizes after 20-30 min, but then decreases slightly (0.5-0.7%) after 90 min. Thus measurements must be made under strictly defined pH conditions to obtain reproducible results. A buffered medium at pH 12.2, in which equilibrium is attained in about 20 min, was selected for routine purposes. The procedure described above was used for establishment of the calibration graph and for the determination of unknown solutions. The calibration was linear for hydroxylamine concentrations in the range 0.4 X 104-2.3 X lo4 mol 1-l. The molar absorptivity was 5480 f 40 1 mol-’ cm-‘; the intercept was -2.09 X 10w3. These data were calculated by the least-squares method from the results of 15 independent measurements. Determination of hydroxylamine in the presence of monochloramine. As indicated above, absorbances were measured at 230 nm in this case. At this wavelength, the least-squares method gave a straight-line plot for the measured absorbance as a function of hydroxylamine concentration in the range 0.4 X 10d-2.3 X lOA mol 1-l. The molar absorptivity was 3230 f 80 1 mol-’ cm-’ and the intercept was -8.83 X 10b3. These results were obtained from 10 independent measurements. The method was tested by determining hydroxylamine in the presence of monochloramine in alkaline medium. For this purpose, standard solutions of hydroxylamine were mixed with different volumes of 6.00 X low3 mol 1-l monochloramine. Then, 5 ml of the mixtures were treated by the procedure described above, except that after 20 min the absorbance was measured at 230 nm. Table 1 gives the results obtained. There is very good concordance between the experimental and standard values for the hydroxylamine concentration. TABLE 1 Influence of monochloramine Concentrations
taken (lo-’
on the determination
mol 1-l )
NH,Cl
NH,OH
5.29 4.80 3.00 4.00 3.00
0.51 0.86 1.09 1.45 2.18
of hydroxylamine
A 130
NH,OH found (lo-’ mol 1-l)
0.170 0.290 0.355 0.465 0.695
0.53 0.90 1.10 1.44 2.15
368
Discussion Results obtained by Jencks [ll] show that the formation of oximes proceeds in two steps with formation of an intermediate carbinolamine: R,R,C=O + NHzOH + R,R,HC-NHOH
=+ R1R2C=N - OH + Hz0
The reaction is catalyzed by acids and bases, each step reaching equilibrium. The mechanism of the acid catalysis was studied extensively but that of the base catalysis was given less attention. It seems that it is the dehydration step of carbinolamine which is catalyzed [ 111. The evolution of the curves in Fig. 2 can be explained by assuming that the intermediate carbinolamine absorbs in the ultraviolet range with a molar absorptivity higher than that of the oxime. Thus, the recorded spectra would represent the sum of those of the oxime and carbinolamine. Their different courses as a function of time and pH would be due to different rates of the two steps in the reaction until equilibrium is reached. REFERENCES 1 N. P. Komar, T. G. Shapavaiova and A. N. Zots, Zh. Anal. Khim., 29 (1974) 829. 2 P. Pitta, A. Caiatroni and C. Zio, Analyst (London), 107 (1982) 341. 3 G. C. M. Bourke, G. Stedman and A. P. Wade, Anal. Chim. Acta, 163 (1983) 277. 4 T. K. Korpela and M. J. Makela, Anal. Biochem., 110 (1981) 261. 5 D. P. Johnson, Anal. Chem., 40 (1968) 646. 6 S. Shahine and R. Mahmoud, Mikrochim. Acta, (II) (1978) 431. 7 F. Dias, A. S. Olojola and B. Jaselkis, Talanta, 26 (1976) 47. 8 V. Grignard, Trait6 de Chimie Organique, Tome XV, Masson & Cie, Paris, 1958. 9H. Delaiu, These de Doctorat d’Etat &-Sciences, no. 77-29, Lyon, France, 1977. 10 H. Delaiu and R. Cohen-Adad, J. Chim. Phys., 76 (1979) 465. 11 W. P. Jencks, J. Am. Chem. Sot., 81(1959) 476.
Short Communication SPECTROPHOTOMETRIC DETERMINATION OF HYDROXYLAMINE ALONE AND IN THE PRESENCE OF MONOCHLORAMINE MICHEL FERRIOL*
and JOSETTE
GAZET
Laboratoire de Physico-chimie min&rale II (Associl au C.N.R.S. no. 116), Universitb Lyon I, 43 Bd. du I1 nouembre 1918, 69622 Villeurbanne Cedex (France) (Received 4th January 1985)
Summary. Hydroxylamine (< 3 x low3 mol 1-l) is quantified by ultraviolet spectrophotometry after reaction with an excess of formaldehyde in buffered solution at pH 12.2. The absorbance of the formaldoxime produced is measured at 218 nm (E = 5480 1 mol-’ cm-‘) when hydroxylamine is alone but at 230 nm (e = 3230 1 mol-’ cm-‘) when monochloramine is present.
During a study of the degradation of monoehloramine in water, a problem arose in examining the kinetics of the reaction between chloramine and hydroxylamine. The main procedures [l-7] available for the determination of hydroxylamine were unsuitable because there was no way to stop the reaction with chloramine. A more convenient method, based on the reactivity of hydroxylamine with aldehydes and especially formaldehyde in alkaline medium [ 81, was therefore developed. The reaction produced formaldoxime which absorbs in the ultraviolet range NH20H + HCHO + CH,=N-OH
+ Hz0
To ensure complete formation of the oxime, a large excess of formaldehyde is needed. In the presence of monochloramine, another reaction takes place between chloramine and formaldehyde. The fast rate of this reaction in alkaline medium enables the reaction between chloramine and hydroxylamine to be stopped instantaneously [ 91. Figure 1 shows the ultraviolet spectrum of a mixture prepared from hydroxylamine and formaldehyde at pH 12.2. The absorbance is maximal at 218 nm. When hydroxylamine is alone, the absorbance is measured at this wavelength. In mixtures with monochloramine, hydroxylamine is determined at 230 nm because there is then negligible interference between the spectra of the products of the formaldehyde/hydroxylamine and formaldehyde/chloramine reactions. Experimental
A Cary 15 double-beam measurements.
spectrophotometer
was used for absorbance
366
0%
I if+-----
218
06-
0
I
I
06’
I
200
220
X(nm)
240
0
IO
20 Trne
30
(mm)
Fig. 1. Ultraviolet spectrum of a formaldehyde/hydroxylamine mixture at pH 12.2. Conditions: 1.30 x lo-’ mall-’ NH,OH, 1.84 x lo-’ mall-’ HCHO, ca. 20°C. Fig. 2. Evolution of maximum absorbance as a function of time for several sodium hydroxide concentrations: (1) 0.002, (2) 0.01, (3) 0.1 mol 1-l NaOH. Conditions: 1.73 X lO+ mall-’ NH,OH, 1.84 x lo-’ mol 1-l formaldehyde; measurement at 218 run.
Solutions. Formaldehyde solutions were obtained by diluting the reagentgrade commercial product (30% w/w). Hydroxylamine solutions were prepared from reagent-grade hydroxylammonium chloride. Monochloramine is unstable in water. Solutions were prepared just before use by reacting sodium hypochlorite and ammonia as described previously [lo] . The monochloramine concentration was determined by ultraviolet spectrophotometry at 243 nm (E = 458 1 mol-’ cm-‘). Procedure. The hydroxylamine solution (5 ml) of concentration less than 3 X 10e3 mol 1-l was added to a 50-ml volumetric flask containing 5 ml of a ca. 0.2 mol 1-l formaldehyde solution and the mixture was diluted to volume with buffer solution at pH 12.2 (0.042 mol 1-l sodium hydroxide/O.029 mol 1-l disodium hydrogenphosphate). After 20 min, the absorbance was measured at 218 nm. Results Influence of PH. The evolution of maximum absorbance with time was studied for sodium hydroxide concentrations of 0.002, 0.001 and of hydroxylamine and 0.1 mol 1-l with constant concentrations formaldehyde. Figure 2 shows the experimental graphs. The minimum time for recording the spectra was 2 min after the reagents had been mixed, thus
367
the curves were extrapolated to zero time. As can be seen, the absorbance at zero time increases with the sodium hydroxide concentration, and the trends of the curves differ according to the pH. With 0.002 mol 1-l sodium hydroxide, the absorbance decreases as a function of time; with 0.01 mol 1-l sodium hydroxide, the trend is much less pronounced and with 0.1 mol 1-l sodium hydroxide, the absorbance increases with time. In all three cases, the absorbance stabilizes after 20-30 min, but then decreases slightly (0.5-0.7%) after 90 min. Thus measurements must be made under strictly defined pH conditions to obtain reproducible results. A buffered medium at pH 12.2, in which equilibrium is attained in about 20 min, was selected for routine purposes. The procedure described above was used for establishment of the calibration graph and for the determination of unknown solutions. The calibration was linear for hydroxylamine concentrations in the range 0.4 X 104-2.3 X lo4 mol 1-l. The molar absorptivity was 5480 f 40 1 mol-’ cm-‘; the intercept was -2.09 X 10w3. These data were calculated by the least-squares method from the results of 15 independent measurements. Determination of hydroxylamine in the presence of monochloramine. As indicated above, absorbances were measured at 230 nm in this case. At this wavelength, the least-squares method gave a straight-line plot for the measured absorbance as a function of hydroxylamine concentration in the range 0.4 X 10d-2.3 X lOA mol 1-l. The molar absorptivity was 3230 f 80 1 mol-’ cm-’ and the intercept was -8.83 X 10b3. These results were obtained from 10 independent measurements. The method was tested by determining hydroxylamine in the presence of monochloramine in alkaline medium. For this purpose, standard solutions of hydroxylamine were mixed with different volumes of 6.00 X low3 mol 1-l monochloramine. Then, 5 ml of the mixtures were treated by the procedure described above, except that after 20 min the absorbance was measured at 230 nm. Table 1 gives the results obtained. There is very good concordance between the experimental and standard values for the hydroxylamine concentration. TABLE 1 Influence of monochloramine Concentrations
taken (lo-’
on the determination
mol 1-l )
NH,Cl
NH,OH
5.29 4.80 3.00 4.00 3.00
0.51 0.86 1.09 1.45 2.18
of hydroxylamine
A 130
NH,OH found (lo-’ mol 1-l)
0.170 0.290 0.355 0.465 0.695
0.53 0.90 1.10 1.44 2.15
368
Discussion Results obtained by Jencks [ll] show that the formation of oximes proceeds in two steps with formation of an intermediate carbinolamine: R,R,C=O + NHzOH + R,R,HC-NHOH
=+ R1R2C=N - OH + Hz0
The reaction is catalyzed by acids and bases, each step reaching equilibrium. The mechanism of the acid catalysis was studied extensively but that of the base catalysis was given less attention. It seems that it is the dehydration step of carbinolamine which is catalyzed [ 111. The evolution of the curves in Fig. 2 can be explained by assuming that the intermediate carbinolamine absorbs in the ultraviolet range with a molar absorptivity higher than that of the oxime. Thus, the recorded spectra would represent the sum of those of the oxime and carbinolamine. Their different courses as a function of time and pH would be due to different rates of the two steps in the reaction until equilibrium is reached. REFERENCES 1 N. P. Komar, T. G. Shapavaiova and A. N. Zots, Zh. Anal. Khim., 29 (1974) 829. 2 P. Pitta, A. Caiatroni and C. Zio, Analyst (London), 107 (1982) 341. 3 G. C. M. Bourke, G. Stedman and A. P. Wade, Anal. Chim. Acta, 163 (1983) 277. 4 T. K. Korpela and M. J. Makela, Anal. Biochem., 110 (1981) 261. 5 D. P. Johnson, Anal. Chem., 40 (1968) 646. 6 S. Shahine and R. Mahmoud, Mikrochim. Acta, (II) (1978) 431. 7 F. Dias, A. S. Olojola and B. Jaselkis, Talanta, 26 (1976) 47. 8 V. Grignard, Trait6 de Chimie Organique, Tome XV, Masson & Cie, Paris, 1958. 9H. Delaiu, These de Doctorat d’Etat &-Sciences, no. 77-29, Lyon, France, 1977. 10 H. Delaiu and R. Cohen-Adad, J. Chim. Phys., 76 (1979) 465. 11 W. P. Jencks, J. Am. Chem. Sot., 81(1959) 476.