Indian Journal of Chemistry
Vol. 40A. June 2001, pp. 613-615
Determination of quantum yield for the
photolysis of aqueous solution of chloramine-B
C R Raju , K N Mohana, H S Yathirajan* & K S Rangappa
Department of Studies in Chemistry, University of Mysore,
Manasagangotri, Mysore 570 006, India
Received 16 Jun e 2000; revised 10 August 2000
Photolysis of aqueous solution of chl oramine-B (0.0 1-0.0002
mol dm.3) is studied with a UV light source and the products have
been identified. A suitable photolytic mechanism is suggested
based on the observed results. The photolytic decomposi ti on
obeys first order kinetics. Quantum yield for the photolysis of
chloramine-B is reported.
Chloramine-B (CAB, C 6 H5S0 2NC1Na.1.5H 20), the
sodium salt of N-chlorobenzenesulphonamide was
first proposed by Afanas'ev 1, as an oxidimetric
reagent. In recent years, CAB has received
considerable attention as an oxidant and analytical
reagent. The kinetics of oxidation of unsaturated
alcohols 2 ·3 and secondary alcohols 4 by CAB in acid
medium have been studied. The conductometric study
of the interaction of CAB with some metal ion
solutions have been reported 5 ' 6 • Chloramine-B is
employed as an analytical reagent to estimate various
organic and inorganic substrates 7' 8• The protonation
constant of CAB at pH< 3.3 by an ion-exchange
method has been studied by Subhashini et a/9 • Kinetics
of chlorine isotope exchange between CAB and
chlorine using tracer technique has also been
reported 10 . A survey of literature reveals that no
information is available on the photolysis of aqueous
CAB solution, but the photolysis of chloramine-T
(CAT) 11 - 13 , bromamine-B (BAB) 14 and bromamine-T
(BAT) 15 have been reported. It was therefore found to
be of interest to study the photolysis of aqueous
chlorami ne-B solution and hence the present work.
Experimental
Choramine-B was prepared 16 by the chlorination of
benzenesulphonamide in sodium hydroxide. An
approximately decimolar stock solution was prepared
and standardised by the iodometric method. All other
reagents were of AR grade. All solutions were
prepared in triply distilled water. UV irradiations
were carried out with a Philips (Holland) low pressure
E-mail: yath @chemi st.com
mercury vapour lamp ( 15 W) and the wave length of
the Iight 17 was 2537 A. The experimental set up has
been described e1sewhere 14 • Exactly 10 ml of CAB
solution were taken in quartz cells and exposed to UV
light radiation for various intervals of time. The
photolytic decomposition was studied for more than
two half-lives. The extent of photochemical
decomposition of CAB solutions were determined by
iodometric titration of the experimental solution and
comparison with a blank solution whereever
necessary. The intensity of the incident light (1 0 )
falling on the system was determined using the uranyl
oxalate actinometer 17 •
A
JASCO,
model
UVIDEC-610
UV-vis
spectrophotometer with 1.0 em matched quartz cells
was used to record the UV spectra. A direct reading
conductivity meter 304 (systronics) and a digital pH
meter, model LI-120 (Eiico) were used for
conductance and pH measurements respectively . FfIR spectra (KBr disc) were recorded on a BRUKER
IFS 66V FT-IR spectrometer. JEOL GSX400 MH z
NMR spectrometer was used to record the FT- 1H and
FT- 13 C NMR spectra.
Results and discussion
The photochemical decomposi tion kinetics of CAB
solutions has been studied in the range of 0.01-0.0002
mol dm-3. The rate constant k increases with dilution
from 0.256x 104 s- 1 to 13.818x l0 4 s- 1 in the [CAB ]
range of 0.01-0.0002 mol dm- 3, at l 0 =6.0x l0 15 quantals
and at a temperature of 30±0.5°C. A plot of log (VJV,)
against time t is linear passing through the origin, where
V0 and V, refer to titre values at zero time and various
intervals of time (s) respectively. Figure 1 shows a linear
plot of log k against log [CAB] (negative
slope = 0.9796), indicating inverse first order with
respect to [CAB] . The average quantum yield (<l>Av) for
the disappearance of CAB is 0.162 for 0.0002 mol dm-3 ,
0.198 for 0.001 mol dm- 3 and 0.190 for 0.01 mol dm-3
[CAB]. The quantum efficiency for bromamine- B
(BAB) photolysis 14 was 0.18 for 0.01 mol dm -3, 0.16 for
0.0025 mol dm-3 and 0.14 for 0.0005 mol dm- 3 [BAB].
Experiments between 300C and 400C show that the
temperature coefficient is 1.068 (1.05 for bromamineB), kw=2.972x10-4 s- 1 for [CAB] of 0.001 mol dm- 3.
This shows the absence of a thermal reaction in the
temperature range of 30-40°C.
INDIAN 1 CHEM, SEC. A, JUNE 2001
614
2.5 ,---- --
- - --
- -- --
-
'~ .
1.5
.><
"'
[CAB]
(mol dm-3)
~
1
"'
0.5
0.5
1
1. 5
2.5
10·15
(quanta/s)
k x 104
(s-1)
R x 107
[R=k(CAB)]
2.875
3.873
4.902
6.000
1.270
1.731
2.213
2.783
1.270
1.731
2.2 13
2.783
lo X
0.001
0.001
0.001
0.001
~
+
Table I -Effect of 10 on the rate of photochemical
decomposition of aqueous CAB
-----,
photolytic decomposition of CAB solutions can be
represented by the equation,
4 +log [CAB)
Fig.
1-A plot of log k versus log [CAB]
The value of rate constant k slightly decreases and
remains almost constant by the addition of NaCI to
CAB solution indicating no side reactions. Table I
gives experimental values of rate constant k for the
photochemical decomposition of 0.001 mol dm- 3
CAB at various incident light intensities 10 • The last
column gives values of rate R, calculated from the
relation R = k [CAB]. A plot of log R against log 10 is
linear with slope= 1.02.
pH measurements showed that the solutions of
CAB becomes more acidic upon irradiation (from 6.67 to - 3.10) and a pale yellow colouration is
noticed after photolysis. The conductivity of the
solution increases after photolysis (- 0.42 mS to
- 1.75 mS). An attempt was made to identify the
photolytic products. No apparent change was noticed
in the UV spectrum of the product upon photolysis.
Using the TLC technique (chloroform: acetone:
benzene= 7:1 :5), only two spots were detected with
iodine as the spray reagent indicating the formation of
benzene sulphonamide (BS) ancll dichloramine-B
(DCB) in the irradiated solution of CAB. Further, the
ether extract of the photolysed CAB solution was
concentrated and the mixture separated by column
chromatography using silica gel as adsorbant and
chloroform: acetone (7: 1 v/v) as eluent. The first
fraction was found to contain DCB and the second
fraction BS. DCB and BS were further characterised
by the IR, 1H and 13 C NMR spectroscopy and the
spectra of DCB and BS agreed with the respective
standards. Liberation of free chlorine from the
18
photolytic solution was noticedi. Complex mixture or
19
resinous products were not observed.
The photochemical decomposition of CAB solution
resembles the photolytic decomposition mechanisms
of BAB and BAT, but does not resemble either (OCif
ion or CAT. Based on the present work and
experimental observations, the rate law for the
d [CAB] = k' _1_
0 _
dt
[CAB]
(l)
where k' is a constant, 10 is the intensity of the
incident light. Chloramine-B behaves as a strong
electrolyte similar to chloramine-T 20 -22 and dissociates
in aqueous solution as follows:
RNCl Na "-;::::=.====~ RNCI+Na+
.. . (2)
where R = C6HsS02
Our earlier conductometric and potentiometric
experiments 2 with CAB and HCI dicate equilibria
(3-5) in acidified CAB solutions.
RNCI + H+ ~==:::=: RNHCl
(3)
2RNHCI ~==== RNH2 + RNCh
(4)
RNHCI + H20 ~=== RNH2 + HOCI
(5)
"
With the above results for the photolysis of CAB
solutions and the equilibria of CAB in acid solutions,
the following photochemical reaction mechanism has
been proposed for the photolytic decomposition of
CAB solution. The first step is the activation process,
RNCl + hv
---7 (RNCI)*
.. . (6)
•
and the consecutive reaction is the formation of RNCl
as shown below:
(RNCI)*-*-->~ RNCl + eThe vartous
represented as,
(7)
propagation
reactions
could
be
H
I
•
RNCl + H-0-H -->""R-N-Cl + OH
... (8)
H
I
RNCI+R-N-Cl-->.;;.RNCh+RNH
... (9)
NOTES
H
I
R-N-CI-->-7 RNH + CI.
RNH + H-0-H
cr + em
> RNH 2
(10)
.
+ OH
(11)
(12)
> HOCI
and finally, the termination step can be written as,
Cl.+ Cl
> Ch
... (13)
The above photolytic mechanism is very similar
14
to bromamine-B
and bromamine-T and is well
supported by the fractional value of quantum yield
($) observed for the photochemical decomposition,
as not every excited species (RNCI) * will take part
in the reaction. As the CAB solution becomes more
dilute, there is a greater penetration of light .!_hrough
the system and also the deactivation rate [(RNCI) * +
RNCI --7 2RNCI] is less and this factor tends to
increase the rate with dilution. However, the
variation in the quantum yield (<!>) observed for the
photolysis of CAB in th e concentration range of 0.01
to 0.0002 mol dm- 3 is not significant. Probably, by
18
using a high intensity of li ght (> 10 quanta/s),
valuable information could further be obtained on
the photolysis of CAB solution. Also, it is found that
the rate of photolysis of CAB is nearly 1.5 times
greater than that of the BAB sol uti on. Various
experime nts were carried o ut in the presence of
uranyl ion and the results were unsuccessful for
photosensitization.
It may be finally concluded that the rate of
disappearance of CAB is proportional to first power
in 10 and inverse first order with respect to [CAB] in
the photolysis experiment. The reaction follows a free
radical mechanism and the products are CI 2, RNH 2
and RNCh.
615
Acknowledgement
One of the authors (CRR) thank the University of
Mysore for providing laboratory facilities to carry out
this research work .
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