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Phenol-Hypochlorite as
a Spectrophotometric
Reagent for the Estimation of
Methylamine
a
J. D. Carr & C. Dass
a
a
Department of Chemistry , University of Nebraska ,
Lincoln, Nebraska, 68588, USA
Published online: 24 Feb 2007.
To cite this article: J. D. Carr & C. Dass (1981) Phenol-Hypochlorite as a
Spectrophotometric Reagent for the Estimation of Methylamine, Analytical Letters,
14:11, 815-824, DOI: 10.1080/00032718108081436
To link to this article: http://dx.doi.org/10.1080/00032718108081436
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ANALYTICAL LETTERS, 14(All), 815-824 (1981)
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PHENOL-HYPOCHLORITE AS A SPECTROPHOTOMETRIC REAGENT
FUR THE ESTIMATION OF METHYLAMINE
KEY WORDS :
Methylamine, spectrophotometry,phenol,hypochlorite,
sodium nitroprusside, indophenol
J.D. Carr and C. Dass
Department of Chemistry
University of Nebraska
Lincoln, Nebraska 6 8 5 8 8 , USA
ABSTRACT
The use of phenol and alkaline hypochlorite has been investigated
for the spectrophotometric estimation of methylamine.
The rea-
gents react with methylamine forming an intensely blue colored dye,
the maximum absorption for which is observed at 630 nm.
of the reaction is slow.
The rate
Sodium nitroprusside is added to enhance
the sensitivity and rate of the reaction.
The rate is also en-
hanced by raising the temperature of the reaction.
The formation
of the indophenol dye is pH sensitive, maximum intensity being observed at pH 10.5.
The reaction mixture buffered at pH 10.5, is
thermostated at 37OC and its absorbance measured at 6 3 0 nm after
3 hours.
The system obeys Beer's law up to 15 ppm with slight de-
viation in the lower ranges.
Concentrations as l o w as 0.S ppm of
methylamine can be estimated in a 1 cm cell.
815
Copyright 0 1981 by Marcel Dekker, Inc
CARR AND DASS
816
INTRODUCTION
The formation of indophenol dye as a resuit of reaction between ammonia, alkaline hypochlorite and phenol is a well known
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method of wide applicability for the estimation of ammonia.
The
method popularly known as Berthelot’ method, after its discoverer
has been investigated by a number of workers. 2-5
It has been re-
ported6 that volatile amines either enhance or inhibit the formation of indophenol dye.
However, no attempt has been made to ex-
plore the possibility of using the formation of indophenol dye as a
method for the estimation of amines.
Our studies have shown that a
promising method can be developed using phenol and hypochlorite as
spectrophotometric reagents. A blue chromophore is formed as a result of reaction between methylamine, alkaline hypochlorite and
phenol catalysed by sodium nitroprusside.
We have investigated
various reaction variables and presented a simple method for the
estimation of methylamine in 0 . 5 to 15 ppm range.
EXPERIMENTAL
Reagents and Apparatus:
Approximately 2.5 ml of analytical grade liquid phenol was diluted to 2 5 0 ml with water.
diluting 2 ml of Purex
(6%
Hypochlorite solution was prepared by
of HOC1) to 2 5 0 ml with water. The cata-
lyst was prepared by dissolving 0 . 0 2 8 g of sodium nitroprusside in
100 ml of water.
A
stock solution of methylamine was prepared
from its 40% aqueous solution and standardized by titrating with
perchloric acid.
A
fresh working solution of amine was prepared
every day from this stock solution. All solutions were made in ammonia free deionized distilled water.
The phenol, hypochlorite and
stock methylamine solutions were stored in a refrigerator.
Solu-
tion pH was adjusted by phosphate or borate buffers and actual pH
was measured against commercial standard buffers.
ESTIMATION OF METHYLAMINE
817
All absorbance data were obtained on a Beckman Model 26 double
beam scanning spectrophotometer using the thermostated cell.
All
pH measurements were made with Corning Model 12 research pH meter
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and Corning triple purpose pH electrode and calomel reference electrode.
Procedure:
A 4 ml portion of methylamine solution in a 2 4 ml vial is
buffered to pH 10.5 by adding 5 ml of 0 . 6 M borate buffer.
Five ml
of sodium hypochlorite is then added and the solution shaken for
complete mixing.
After about one minute, 1 ml of the catalyst and
5 ml of phenol are added in quick succession.
The mixture is
shaken vigorously and thermostated immediately at 37OC.
After
about 3 hours, the absorbance is measured at 630 nm against a reagent blank treated in a similar manner.
RESULTS AND DISCUSSION
The absorption curve of the blue chromophore formed as a result of the reaction of methylamine with phenol and alkaline hypochlorite is depicted in Figure 1.
For the sake of comparison, the
absorption curve of the corresponding ammonia reaction is also included.
From the similarity between the two curves, it can be con-
cluded that the end product of the reaction of methylamine with
phenol and alkaline hypochlorite is also a formation of an indophenol.
The reaction is highly pH sensitive, as shown in Figure 1.
No color was observed below pH 8 . 0 ;
increased as the pH was raised.
the color intensity, however,
The maximum absorbance was ob-
served at pH 10.5; the color intensity decreased with further increase in pH.
These observations are consistent with the results
of mechanistic studies of corresponding ammonia reaction by Patton
and Crouch. 2
CARR AND DASS
818
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1.o
0.8
0.2
0
450
500
550
600
650
700
750
A(=)
Fig. 1:
Absorption curves of indophenol formed between methylamine phenol and bypochlorite at various pH's (1) 8.10,
(2) 9.00, ( 3 ) 9.80, ( 4 ) 10.20, ( 5 ) 10.55, ( 6 ) 10.89, (7)
11.70 and (8) 12.00. Curve (9) shows the absorption
curve of indophenol formed with ammonia at pH 12.00.
Concentrations of methylamine and ammonia are
M and 2.86 X
M respectively. The
2.4 X
reaction temperature is 37OC.
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ESTIMATION OF METWLAMINE
ua
N
I
P
v)
1
0
r
0
N
\
819
w
'0
r
t
X
10
r
v)
z
' 0
X
0
P
ua
0
t
0
a
0
(Y
0
r
820
CARR
AND DASS
The rate of formation of indophenol dye from ammonia is kinetically very slow.
For this reaction to be of any significant
importance for analytical purposes, sodium nitroprusside is gen-
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erally incorporated as a catalyst.
The actual mechanism of its
catalytical action is still uncertain, although it has been suggested that the actual species responsible for catalytical activity
or aquopentacyanoferrate,2
may be sodium nitr it~pentacyanoferrate~
which are products of the alkaline solution of sodium nitroprusside.
Kinetic studies of the reaction of methylamine with phenol
and hypochlorite by us have revealed that this reaction is also
painfully slow, and the presence of sodium nitroprusside greatly
enhances the sensitivity and rate of the reaction.
A linear rela-
tion was observed when dA/dt was plotted against methylamine concentration as shown in Figure 2 .
This relationship shows a nega-
tive curvature for low concentrations at pH 10.5 which is absent at
pH 11.7 although scatter is worse at pH 11.7.
The rate of the
chromophore formation is further enhanced by increasing the reaction temperature. However, as the temperature is raised, the final
intensity of the blue chromophore decreases (Figure 3).
At 25°C
and pH of 10.5, it takes about 400 min for the reaction to reach the
equilibrium, while the corresponding equilibrium times at 37", 4 6 O
and 55°C are 180, 110 and 65 min respectively.
It was also ob-
served that at higher pH, the reaction is comparatively faster, but
much of the advantage of comparatively shorter. time is lost with
the loss in the sensitivity of the reaction at higher pH.
Hence, as
a compromise between the time required for the analysis and the
sensitivity that can be achieved, the reaction temperature of 37"
and pH 10.5 were selected as the optimum parameters for the estimation of methylamine.
Once the reaction had attained equilibrium,
the color intensity remained stable for a very long time.
ESTIMATION OF METHYLAMINE
821
1.01
,
I
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t
1
37'
0.6
0.4
0.2
0
40
80
160
120
200
240
280
TIME(mln)
Fig. 3:
Rate o f f o r m a t i o n of i n d o p h e n o l dye a t v a r i o u s temperat u r e s and a t pH 1 0 . 5 0 .
2.4
x
C o n c e n t r a t i o n of
methylamine i s
M.
R e a c t i o n s between methylamine and h y p o c h l q r i t e have been repr e s e n t e d by7
''
+ HOC1$CH3NHC1
CH3NHC1 + HOC1$CH3NC12
CH3NH2
2CH3NHC1*CH3NCl2
+
+
H20
+
H20
CH3NH2
C H ~ N H+~ O C ~ - & C H ~ N H C ~
+
OH-
(1)
(2)
(3)
(4)
822
CARR
Reactions ( 2 ) and
AND DASS
(3) are acid catalysed and reaction ( 4 )
takes place in basic media.
Consistent with these facts, the pH
dependence of the formation of the indophenol is not surprising.
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At lower pH, the stability of CH3NHCl in presence of excess HOCl
decreases due to reactions ( 2 ) and ( 3 ) , and at higher pH its rate
of formation is suppressed because of reaction ( 4 ) .
Based upon the
above facts and in accordance with the mechanism of corresponding
reaction of ammonia,
the following mechanism for the formation of
indophenol from methylamine is proposed:
CH3NHZ
+
OC1-
catai zed
'CH3NHC1 +
OH-
(5)
A
7
OH;
7
-0
0;
"\ /"=
However, this simple mechanism may not De the only process occurring.
This is reflected by the fact that the precision of the
method is far from satisfactory if the experimental conditions are
not rigidly adhered to.
Poor precision may also arise due to the
deterioration in the phenol and hypochlorite solutions.
For good
precision, the order and timing of mixing of the solutions should
be strictly followed and fresh reagent solutions should be prepared
every week.
The system was found to follow Beer's law up to 15 ppm of
methylamine
with
(Figure 4 ) .
As
slight
deviation
at
lower
concentrations
low as 0 . 5 ppm of methylamine can be estimated by
the present method using 1 cm cell, however, the lower limit can be
ESTTMATTOh’ OF METHYIAYINE
823
1.2
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1 .o
0.8
w
0
z
U
m
5 0.6
v)
m
U
0.4
0.2
1.0
0
Fig. 4 :
2.0
-4
4.0~10
3.0
M
plot of equilibrium absorbance vs methylamine concentration at pH 1 0 . 5 5 and 11.70. Reaction temperature is
A
37’C.
improved with the use of longer path length cell.
Thus the indo-
phenol method, which has been hitherto widely used f o r the estimation of ammonia can also be adopted f o r the estimation of methylamine with certain modifications.
The method is simple and reas-
onably sensitive to be of practical importance.
As
the rates of
corresponding reactions of methylamine and ammonia with phenol and
hypochlorite are quite different, a differential kinetic method can
824
CAM AND DASS
be developed for their estimation in the presence of each other.
Our preliminary results have not been very encouraging perhaps due
to the complicated kinetics of the formation of indophenol from
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methylamine and ammonia.
We, however, feel that a thorough
in-
vestigation of this system may result in a suitable method based on
differential kinetics. Further efforts are underwil in this direction.
REFERENCES
1.
M. Berthelot, Repertoire de Chemie Applique, 1, 284 (1859).
2.
C.J. Patton and S.R. Crouch, Anal. Chem., 49, 464 (1977).
3.
W.T. Bolleter, C.J. Bushman and P.W. Tidwell, Anal. Chem.,
33, 592 (1961).
4.
M.W. Weatherburn, Anal. Chem., 39, 971 (1967).
5.
H.B. Mark, T.E. Weichselbaum and J.C. Hagerty, Anal. Chem.,
41, 848 (1969).
6.
Makato Hayashi, Tsutomu Unemoto and Komei Miyaki, Chem.
and Pharm. Bull ( T o k y o ) , 7 , 65 (1959).
7.
J.C. Morris, “Kinetics of Reactions Between Aqueous Chlorine
and Nitrogen Compounds” in “Principles and Applications
of Water Chemistry”, S.D. Faust and J.V. Hunter, eds.,
Wiley, NY, 1964, pp. 23-53.
8.
E.T. Gray, Jr., W.M. Dale and R.P. Hauffman, A.C.S. Symposium
Series, No. 82, “Organometals and Organometalloids--Occurrence and Fate in Environment“, F.E. Brinckman and J.M.
Bellama, eds., 1978, pp. 264-77.
Received,March 13, 1981
Accepted,April 27, 1981