Vol.3, No.7, 566-572 (2011) Natural Science
http://dx.doi.org/10.4236/ns.2011.37079
A new approach for reduction of methylene green with
ascorbic acid by de-oxygenation through carbon
dioxide
Rafia Azmat1*, Noshab Qamar2, Raheela Naz2
1Department of Chemistry, University of Karachi, Karachi, Pakistan; *Corresponding Author:
2Department of Chemistry, Jinnah University for Women, Nazimabad Karachi, Pakistan.
Received 11 March 2011; revised 18 April 2011; accepted 25 April 2011.
ABSTRACT
Reduction kinetics of the methylene green (MG)
with ascorbic acid (AA) in acidic medium at

max
660 nm was monitored through visible spectro-
photomtry in absence and presence of sodium
carbonate. CO2 release through reaction of so-
dium carbonate and oxalic acid, created de-
oxygenated atmosphere for reduction of dye
which greatly boosted the reaction rate. Initially
slow reaction in presence of atmospheric oxy-
gen proceeded rapidly when sodium carbonate
was added. The reaction followed fractional or-
der kinetics with AA and zero order kinetics with
MG. The rate of reaction shows no linear de-
pendence on [H+] concentration as an acidic
medium. The rate of reaction is directly related
with the elevated concentration of salt, which
suggests that the two same charged species are
involved in the rate determining step. Secondary
reactions at elevated temperature showed com-
plex kinetics.
Keyw ords: Ascorbic Acid; Methylene Green;
Sodium Carbonate; Charged Species; Secondary
Reactions; Elevated Temperature
1. INTRODUCTION
Ascorbic acid (AA),a form of Vitamin C and water
soluble which comes primarily from fruit and vegetables
is an important micronutrient and plays many physio-
logical roles. It occurs as l-ascorbic acid (AA) and its
oxidized form, dehydro-l-ascorbic acid (DHAA), both of
which are biologically active. The formula of which is
C6H8O6, behaves as a vinylogous carboxylic acid, where
in the double bond (“vinyl”) transmits, electron pairs
between the hydroxyl and the carbonyl [1-2].
Cooper et al. [3] reported the photo electrochemical
analysis of ascorbic acid in aqueous solution at a plati-
num channel electrode, using the dissolved phenothiaz-
ine dyes methylene blue and methylene green. This is
achieved by measuring the current produced by immedi-
ate electro-oxidation of the reduced form of the dyes
produced during the 1:1 photoreaction between the dyes
and ascorbic acid induced by visible light of a wave-
length of 620 - 625 nm. Strizhak [4] propose a kinetic
scheme for reduction of methylene blue by ascorbic acid,
characterized by the existence of two pathways: outer-
sphere reduction of methylene blue according to a sec-
ond-order reaction, and reduction through formation of a
1:1 complex with ascorbic acid. Strizhak [5] reported the
reduction of methylene blue by ascorbic acid in the
presence of copper (II) ions. The introduction of copper
(II) ions increases the reaction rate, owing to the in-
creased concentration of ascorbic acid radicals in the
solution. It is shown that the inhibition of the reaction
that is observed with low concentrations of copper (II) is
a result of redox reactions proceeding in the system, in-
volving oxygen dissolved in the water. Yusuf and Giirel
[6] used methylene blue (MB), incorporated into tita-
nium phosphate for determination of AA and showed
that reaction can be helpful in some pharmaceutical
products and are for the detection of AA as the procedure
involving the reaction between triiodide and AA. AA has
been determined by photo bleaching of methylene blue
using continuous and FI systems, both procedures de-
termination of AA based on its photooxidation sensitized
by Toluidine Blue [7,8]. Cooper et al., [9] discussed the
photoelectrochemical analysis of AA and decolorizing
efficiency of AA to different synthetic dyes using Co/
H2O2 system [10].
Detail literature search showed that few reports were
available on the reduction of methlyene green with or-
ganic reductants but no reports were available on reduc-
tion of MG and AA in presence and absence of sodium
carbonate in acidic medium. This procedure is shown to
be a good alternative to routine vitamin C analysis in
pharmaceutical preparations, fruit juices and soft drinks.

R. Azmat et al. / Natural Science 3 (2011) 566-572 567
2. EXPERIMENTAL
2.1. Materials and Methods
All reagents of pure quality (E-Merk) were used
throughout the experiment. Dye stock solution of 3 ×
10–5 mol·dm–3 was prepared in 100 ml of deionized dis-
tilled water. Solution of dye was preserved in polyethylene
bottle. Stock solutions of ascorbic acid of 0.1 mol ·dm–3,
Oxalic acid of 0.5 mol·dm–3, Na2CO3 of 0.1 mol·dm–3,
KNO3 of 2 mol·dm–3, was prepared in distilled water
2.2. Kinetic Measurements
Five sets of mixture were prepared from stock solu-
tion. In each set one specie was varied while concentra-
tions of other four were kept fixed. Each component
were mixed together and portion was transferred to the
cuvette (1 cm) to record a change in optical density of
methylene green. Kinetic measurements were made with
visible spectrometer 180 A at λmax 660 nm of methylene
green at room temperature in thermostatic water bath
(FJPSO UK) [11,12].
2.3. Stoichiometry
The stoichiometry of the reaction mixture of MG and
AA were determined by mixing the reagents in molar
ratios of 2:1,1:1 and 1:2 with oxalic acid in excess.
Quick de-coloration was observed in the bottle contain
molar ratio in 1:2 as compared to other ratios showed
that MG react with AA in 1:2 ratios in aqueous medium
according to following equation [13].
N
N(C2H5)2
NO2
N(CH4)2
S
NNO2
N(CH3)2
H
(H3C)2N
+ 2AA+ DHAA
hv
methlyene greenleucomethylene green
3. RESULTS AND DISCUSSION
In the present work reduction of the methylene green
with ascorbic acid were studied with variable concentra-
tions of dye, ascorbic acid, and oxalic acid keeping one
parameter varied and other constant in acidic medium in
absence and presence of sodium carbonate [11,12,14].
The values of rate constant were determined through a
plot of lnA vs time for each parameter concentration
from the slope of line and reported in the Tables 1, 3-7.
A plot of log [dye] and log k were evaluated for each
variable. It was observed that rate of reaction was very
slow and took long time for complete de-colorization of
the dye in absence of sodium carbonate according to
following Eqs.1-3.
2
AHAHH fast

 (1)

MG HMGH
slow in absence of sodium carbonate

 (2)
MGH eMGH
h
A
 
 (3)
The above reactions were significantly enhanced in
presence of sodium carbonate with reduced de-colo-
ration time. The total de-coloration time reduced to 5
min. while at the same pH value, but in the absence of
sodium carbonate the total de-coloration was achieved in
a time more than 60 min. Addition of sodium carbonate
in reaction mixture results in the brisk effervescence of
CO2 (confirmed by using lime water which turn milky)
with rapid de-coloration of dye indicating that CO2 at-
mosphere helps in fast bleaching of the MG. The change
in concentration of sodium carbonate proves that release
of CO2 in the reaction mixture affected the reduction
kinetics of MG (Table 1) and showed that CO2 displace
the oxygen due to which equilibrium shift in forward
direction with the formation of leuco dye [15]. The de-
cline in the time of de-coloration due to the innovation
of CO2 during reaction by the addition of sodium car-
bonate throughout course of reaction is also supported
by the early study of Gest and Stokes [16] who reported
that the time required for the reduction of methylene
blue was greatly increased by CO2 removal when no
substrates were added. Spectral changes for reduction of
dye were recorded and presented in Figure 1. clearly
showed that dye reduced without shift in wavelength in
absence and presence of sodium carbonate.
Chemical oxygen demand (COD) decreases sharply
with evolution of CO2 which greatly influences the re-
duction of dye by providing the oxygen free atmosphere
for reduction (Table 2). Reduced COD showed that CO2
atmosphere helps in the reduction of MG through the
movement of H+ ion in the reaction mixture [16]. The

R. Azmat et al. / Natural Science 3 (2011) 566-572 568
Table 1. Effect of Sodium Carbonate (Na2CO3) on Rate of
Reduction [MG +] = 1 × 10–5 mol·dm–3, [Ascorbic Acid] = 5 ×
10–3 mol·dm–3, [H2C2O4] = 0.18 mol·dm–3, [KNO3] = 0.2 mol·dm–3,
[Na2CO3] = (1 - 5) × 10–2 mol·dm–3, Temperature = 303 K.s.
Concentration
of Na2CO3 ×
102
mol· dm–3
Velocity
υ × 105
Specific rate
constant
(kobs) × 103 s–1
1/(kobs) %
decoloration
1.0 –10 .00 1.0 1000 55.0
2.0 –3.00 1.2 833.33 60.48
3.0 –3.00 1.2 833.33 73.134
4.0 –3.00 3.2 312.5 81.223
5.0 –3.00 2.9 344.828 90.415
Table 2. Chemical Oxygen Demand in presence of sodium
carbonate. [MG+] = 1 – 5 × 10–5 M Mixture, [MG+] = 1 × 10–5 M,
[Ascorbic Acid] = 6 – 9 × 10–3 M, [H2C2O4] = 0.06 – 0.3 M,
[KNO3] = 0.2 M, [Na2CO3] = 1 × 10–2 M, Temperature = 303 K.
[MG]  105
mol·dm–3 COD Ascorbic Acid × 103
mol·dm–3 COD
1.0 –307.2 5.0 585.6
2.0 –208 6.0 572.8
3.0 –195.2 7.0 572.8
4.0 –188.8 8.0 566.4
5.0 –256 9.0 579.2
Figure 1. Spectral scan of MG with ascorbic acid in presence
and absence of sodium carbonate. Where a = 5 × 10–3 mol·dm–3,
b = 6 × 10–3 mol·dm–3, c = 7 × 10–3 mol·dm–3, d = 8 ×
10–3 mol·dm–3, e = 9 × 10–3 mol·dm–3.
effect of change in concentration of oxalic acid on rate
of reaction showed no a linear dependence on [H+]. This
may be attributed with that the varied concentration of
oxalic acid as a medium, showed no effect on rate of
reduction (Table 3). This indicated that oxalic acid is
involved in the release of CO2 with sodium carbonate as
varied concentration of sodium carbonate decrease the
reduction time according to following equations no. (4 -
7).
2242 322422
HCONaCONaCO COHO (4)
2
2H/2e
MG AHMGHAH


 
fast (CO2 atmosphere) (5)
MG AHMGHA

 fast (6)
MGHeMGHA
h  

 fast (7)
The dye and ascorbic acid concentration were varied
to analyze the effect of reduction of MG on rate of re-
duction. Results showed that the reaction followed frac-
tional order kinetics with ascorbic acid and zero order
kinetics with methylene green (Tables 4 and 5). COD
increases with the increase in concentration of dye (Ta-
ble 2). The values of percent de-coloration decreases
with the increase in concentration of dye in reaction so-
lution (Table 4) [17-19]. Unexpectedly it was observed
that the rate constant of MG reduction appears to be the
same in concentration range of 2 × 10–5 to 5 × 10–5 mol·dm–3
also, when concentration of ascorbic acid was varied in
solution containing a fixed amount of the dye, the de-
-coloration percentage increases with the increase in the
concentration of dye. This reflects that the reaction de-
pends upon the concentration of ascorbic acid (Figure 2).
However, beyond certain limits the value of rate constant
decreases (Table 5). The hydrogen ion abstraction and
electron donation by ascorbic acid take place through
following reaction which leads to the formation of leuco
dye with dehydroxy –1- ascorbic acid (DHAA) (Eq.8 )
[1-3,19]..
Table 3. Effect of Oxalic Acid (H2C2O4) on Rate of Reduction
of MG With Ascorbic Acid. [MG+] = 1 × 10–5 mol·dm–3, [As-
corbic Acid] = 6 × 10–3 mol·dm–3, [H2C2O4] = (0.06 – 0.3) mol·dm–3,
[KNO3] = 0.2 mol·dm–3, [Na2CO3] = 1 × 10–2 mol·dm–3, Tem-
perature = 303 K.
Concentration
of H2C2O4
mol·dm–3
Velocity
υ × 105
Specific rate
constant
(k obs) × 103 s–1
1/(k obs) % decoloration
0.06 –3.0 1.3 767.23 77.241
0.12 –3.0 1.3 767.23 77.241
0.18 –3.0 1.3 767.23 77.241
0.24 –2.0 1.2 500.00 71.312
0.3 –3.0 1.3 767.23 77.241
Table 4. Effect of Methylene Green [MG+] on Rate of Reduc-
tion. [MG+] = (1 – 5) × 10–5 mol·dm–3, [Ascorbic Acid] = 6 ×
10–3 mol·dm–3, [H2C2O4] = 0.18 mol·dm–3 , [KNO3] = 0.2
mol·dm–3, [Na2CO3] = 1 × 10–2 mol·dm–3, Temperature = 303 K.
Concentration
of [MG+] × 105
mol·dm-3
Velocity
υ × 105
Specific rate
constant
(kobs) × 103 s-1
1/(kobs) % decoloration
1.0 –4.0 1.8 555.56 71.111
2.0 –4.0 1.0 1000 81.361
3.0 –5.0 1.0 1000 89.194
4.0 –5.0 1.0 1000 93.657
5.0 –7.0 1.0 1000 82.282

R. Azmat et al. / Natural Science 3 (2011) 566-572 569
Table 5. Effect of Ascorbic Acid on Rate of Reduction of MG.
[MG+] = 1 × 10–5 mol·dm–3, [Ascorbic Acid] = (6 – 9) ×
10–3 mol·dm–3, [H2C2O4] = 0.18 mol·dm–3, [KNO3] = 0.2
mol·dm–3, [Na2CO3] = 1 × 10–2 mol·dm–3, Temperature = 303
K.
Concentration of
Ascorbic acid × 103
mol·dm–3
Velocity
υ× 105
Specific rate
constant
(k obs) × 103 s–1
1/(kobs) %
decoloration
5.0 –10.00 2.7 370.37 81.223
6.0 –4.00 1.8 555.56 71.111
7.0 –10.00 5.5 181.82 57.667
8.0 –20.00 14.3 69.93 67.025
9.0 –9.00 121.0 8.265 49.738
Figure 2. A plot of 0
ln t
A
AA A



 vs Time for reduction
of MG with variable concentration of AA. [MG+] = 1 × 10–5 M,
[Ascorbic Acid] = 5 – 9 × 10–3 M, [H2C2O4] = 0.18 M, [KNO3]
= 0.2 M, [Na2CO3] = 1 × 10–2 M, Temperature = 303 K.
OO
HO OH
H
HO
HO OO
HO OH
H
HO
HO
-H +
OO
OOH
H
HO
HO
N
N(C2H5)2
NO2
N(CH4)2
S
NNO2
N(CH3)2
H
(H3C)2N
CO2
Atmosphere ___(8)
(MG) LMG DHAA
The general effect of an added electrolyte on the ob-
served rate constant of a reaction in solution is deter-
mined only by the ionic strength which showed that the
rate of reaction is directly related with the elevated con-
centration of salt, suggested that the two same charged
species are involved in the rate determining step [11,14].
The kinetic salt effect of added salt electrolyte was de-
termined through Debye-Huckel limiting law which
gives relationship between ionic strength and rate con-
stant. The equation may be given as
0
loglog1.02 AB kk ZZ

 (9)
for the determination of reactive specie in the rate deter-
mining step. This equation shows that a plot of log k vs

should be linear, with the slope and intercept equal
to 1.02 ZAZB and log k0 respectively. The slope repre-
sents the product of charges on the species involved in
the rate timing stop. If the rate limiting step is between
the species of like charges, a positive slope is expected.
When the reaction is in between opposite charges, it re-
sults in a negative slope. In present work effect of salt
was studied at constant concentration of dye, reductant
and acid by varying concentration of salt which showed
that the rate constant increases with increase in an ionic
strength of the reaction (Table 6) indicating that the re-
action is in between ions of like charges in rate deter-
mining step. The plot of log k vs

is linear with
1.5012 slope [20]. The positive sign of the slope

R. Azmat et al. / Natural Science 3 (2011) 566-572 570
Table 6. Effect of Ionic Strength on Rate of Reduction of MG. [MG+] = 1 × 10–5 mol·dm–3, [Ascorbic Acid] = 6 × 10–3 mol·dm–3,
[H2C2O4] = 0.18 mol·dm–3, [KNO3] = (0.2 – 1) mol·dm–3, [Na2CO3] = 1 × 10–2 mol·dm–3, Temperature = 303 K.

mol·dm–3
Concentration of
KNO3
mol·dm–3
Velocity
υ × 105
Specific rate constant
(kobs) × 103 s-1 1/(kobs) % decoloration
0.8809 0.2 –4.0 1.8 555.56 71.111
0.9879 0.4 –3.0 1.1 909.091 77.931
1.0844 0.6 –3.0 1.3 767.23 83.448
1.1730 0.8 –10.0 3.4 294.118 87.805
1.2554 1.0 –20.0 5.5 181.818 81.034
indicate that the species involve in the rate determining
step has like charges. If ZA and ZB have the same sign,
then ZA Z
B is positive and the rate constant increases
with ionic strength as observed in the present investiga-
tion. The value of ZA and ZB was obtained from the slope
of the plot of logk vs

which is 1.50. The pseudo
first-order rate constant (k') with respect to dye concen-
tration was found to non significantly correlated (R2 =
0.5717) as a function of ionic strength. From the data in
Table [4] it can be observed that higher rate constants are
attained at higher ionic strengths. This can be related to
alterations of ionic atmosphere and changes in the
charge densities around anions involved in the rate-de-
termining step. The formation of a single highly charged
ionic complex from two less highly charged ions is fa-
vored by a high ionic strength because the new ion has a
denser ionic atmosphere [20].Using rate constant data at
different ionic strengths, a graph was obtained employ-
ing the Brønsted–Bjerrum equation, based on the limit-
ing law of Debye–Hückel (Figure 3).The rate constant
at infinite dilution (k0) is 4.2 M [the product of charges
ZA ZB is 1.5. Considering the extended law of De-
bye–Hückel (Figure 3), k0 is 4.2 mol·dm–3 ZAZB is 1.50.
This value is close to the expected value of 2 considering
the proposed mechanism, which involves a collision
between a monovalent and a biivalent cation in the
rate-determining step [20].
The effect of temperature on rate of reaction indicated
that elevation in temperature results in the complex ki-
netics which may be due to the some secondary reac-
tions operating in the reaction mixture at a given tem-
perature [19, 21-29]. It was observed from the Table 7
that there is no linear relation in between the temperature
elevation and rate constant which showed that tempera-
ture elevation cause secondary reactions in which dye
molecule or reductant may be break down due to which
black particles were appear in the reaction mixture. The
values of activation parameters like entropy of activation,
enthalpy of activation and energy of activation support
secondary reaction predominantly degradation of dye
molecule instead of reduction [14,23].
4. REACTION MECHANISM
Reaction mechanism of the reduction of MG proposed
in which role of CO2 was considered to displace the
oxygen and provide deoxygenated atmosphere to in-
crease the rate of reduction. Ascorbic acid existed in
reaction mixture as follows (equation) and reduction
take place by abstraction of H from AA (Eqs.10 and 11).
1
1
2
AHAH H
k
k



 (10)
2
MG HMGH
k



 (11)
3
+
MGH +2AHMGH+A
k

Rate law for abstraction of H to convert dye into col-
orless form is as follows

2
d[MG] MG AH
dt



(12)



2
dMGMG H
dk t


 (13)
Rate of formation of active species of reductant ac-
cording to Eq. 2.
Figure 3. A plot of logk vs

. [MG+] = 1 × 10–5 M, [Ascor-
bic Acid] = 6 × 10–3 M, [H2C2O4] = 0.18 M, [KNO3] = (0.2 – 1)
M, [Na2CO3] = 1 × 10–2 M, Temperature = 303 K.

R. Azmat et al. / Natural Science 3 (2011) 566-572 571
Table 7. Effect of Temperature on Rate of Reduction. [MG+] =
1 × 10–5 M, [Ascorbic Acid] = 5 × 10–3 M, [H2C2O4] = 0.18 M,
[KNO3] = 0.2 M, [Na2CO3] = 1 × 10–2 M, Temperature = 303 –
323 K.
Temperature
(K)
Velocity
υ × 10–5
Specific rate
constant (kobs)
× 103 s–1
1/(kobs) %
decoloration
303 –3.00 1.6 625 65.87
308 –3.00 1.5 666.667 66.25
313 –8.00 3.3 303.03 88.22
318 –4.00 3.2 312.5
323 –5.00 2.5 400

mol· dm–3 ∆E≠
kj·mol–1
∆H≠
kj·mol–1
∆S≠
kj·mol–1
∆G≠
kj·mol–1
0.8809 28.337 258.765 380.392 143.506



2
12
dAHAH
dk t
 (14)
At equilibrium rate of formation of H = rate of reduc-
tion of dye
Hence




+
122
AHMG Hkk

 (15)



+
1
22
MG H
AH
k
k


 (16)
Taking k1/k2 = K



+
2
MG H
AH
K


 (17)

+
2
H
AH
K


 (18)
According to Eqs.1 and 2 reaction is in depended
upon H ion concentration



22
dMG
d
AH
t
Kk





+
2
dMG
d
MG H
t
k






(19)



22
dMG
d
AH
t
Kk

 (20)
2
2
d[M G]
d
[AH ]
t
kk

 (21)
The above Eq.21 corresponds to our results that re-
duction is in depended upon concentration of dye and
fractional order with respect to ascorbic acid Tables 4
and 5.
5. CONCLUSIONS
It was concluded from above investigation that dehy-
drogenation of ascorbic acid with methylene green in the
presence of sodium carbonate decolorizes the dye more
rapidly in acidic medium which could be more eco-
nomical technique in future and has a significant advan-
tage over other studies reported in the chemical literature
for the dehydrogenation of ascorbic acid or in other
words it may be used for the detection of AA in different
fruit samples in very diminutive quantities.
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