Kinetics of methylene blue reduction by ascorbic acid: Possible role of protonation

1
Kinetics of methylene blue reduction by ascorbic
acidPossible role of protonation

• Bill Simpson, Eric Dick, and the Fall 2000 Chem
  434 Class

2
The Reaction

• Vivid blue dye is reduced to colorless "leuco"
  form
• Will follow reaction with UV-Vis spectroscopy
• Catalyzed by HCl, according to literature
  catalysis mechanism not well determined
• In all experiments, MB is limiting, thus
  observed rate constants are psuedo constants
  first order observed

3
kp vs MB, ASC 0.025 M, HCl 0.3 M
0.12
0.1
0.08
0.06
kp / s-1
0.04
0.02
0
0
10-5
2x10-5
3x10-5
4x10-5
MB / M
Small variation in psuedo-first order rate
coefficient versus MB consistent with
first-order dependence on MB
4
kp vs asc, 0.2M HCl
0.14
0.12
0.10
0.08
kp / s-1
0.06
0.04
0.02
0.00
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
asc / M
Linear variation in psuedo-first order rate
coefficient versus asc consistent with
first-order dependence on asc
5
kp vs HCl, asc 0.045 M
0.25
0.2
0.15
kp / s-1
0.1
0.05
0
0
0.1
0.2
0.3
0.4
0.5
0.6
HCl / M
Non-linear variation in psuedo-first order rate
coefficient versus HCl indicates
catalysis is complex
6
Spectrophotometric titration
0.6
grows withpH decrease
shrinks withpH decrease
0.4
Absorbance / OD
0.2
0
200
300
400
500
600
700
800
wavelength / nm
pH varied 2.34 to -0.09 in this titration
Consistent with two absorbing form, MB and HMB2
7
Extracted spectra of MB and HMB2
0.800
A748
A665
0.600
MB
0.400
HMB2
Absorbance / OD
0.200
0.000
200
400
600
800
Wavelength / nm
Extracted assuming pKA -0.15 Can determine
degree of protonation by measuring A748 and
A665
8
kp versus protonation
0.25
0.2
0.15
kp / s-1
0.1
kp 0.6202( s
-1
)f
p
0.0479 s
-1
0.05
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
fraction protonated
Data essentially equally consistent with
protonation model
9
Conclusions
Reaction is first order in MB and also first
order in asc gt consistent with bimolecular
reaction between MB and asc Although the
reaction can occur in the absence of HCl, its
presence speeds reaction Dye is protonated by
HCl, and we can determine the degree of
protonation from the absorption spectrum of the
dye Making a mechanism where HCl protonates the
dye, and both protonated and unprotonated dye
react with ascorbic acid does not fit any
better than linear dependence on HCl But,
since we have observed that the dye protonates,
we tend to accept this mechanism, in which
case k (unprotonated dye)
1.0 M-1 s-1 k (protonated
dye) 14 M-1 s-1