ph measurement of plasma-exposed water using … · ph measurement of plasma-exposed water using...
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pH measurement of plasma-exposed water using colorimetric methodShogo Wakisaka, Kohshi Tsuda, Kazuhiro Takahashi and Kohki Satoh (Muroran Institute of Technology)
Division of Information and Electronic Engineering, Graduate School of Engineering, Muroran Institute of Technology,
27-1 Mizumoto, Muroran 050-8585, Japan
E-mail : [email protected]
Introduction
1.5
1.0
0.5
0.0
-0.5
po
wer
[M
W]
543210time [s]
Electric Power30
20
10
0
-10
vo
ltag
e [k
V]
543210time [s]
150
100
50
0
-50
curr
ent
[A]
Voltage
Current
VMax : 16.8 kV
IMax : 128 A
Pulse width : 510 ns
PMax : 1.09 MW
0.254 J/pulse
H.V. electrode
The needle electrode is made of
stainless steel, and the diameter
of the electrode is 4.0 mm.
Voltage, Current and Electric power waveforms
Ar
CH1
100Ω
GND
CH2
DigitalStorageOscilloscope
③ H2O+ immediately dissociates forming OH radical and H+.
H2O+ → H+ + OH(aq) (2) : H+ generation & pH decrease
Charge transfer from Ar+(g) to H2O(aq)
Ar +(g) + H2O (aq) → Ar + H2O
+ (1)
② Charge exchange reaction (Anderson et al. Plasma chem. Plasma Process. 36, 1393 – 1415 (2016))
ConclusionsPulsed discharge plasma is generated above water with positive and negative polarities, and then pH variation in the liquid is investigated by colorimetricanalysis using BTB.
At the positive polarity, value of pH in the liquid decreases from the liquid surface by H+ generation through the charge transfer from Ar+ to H2O(aq), while increases from
the earthed electrode by OH- generation through the electron transfer from the earthed electrode to the liquid.
At the negative polarity, value of pH in the liquid increases from the liquid surface by OH- generation through the electron irradiation from the plasma into the liquid,
while decreases from the earthed electrode by H+ generation through the electron transfer from the liquid to the earthed electrode.
The equal amounts of H+ and OH- are generated by reactions induced by the charge transfer across the gas-liquid and liquid-earthed electrode boundaries, since value of
pH in the plasma-exposed-liquid returns to neutral after stirring in both the polarities.
Ar10 mm
liquid
The color of plasma-exposed-liquid returns to green,
namely, neutral, after stirring; therefore, the amounts of
H+ and OH- generated in the liquid are found to be equal.
The 10th Asia-Pacific International Symposium on the Basics and Applications of Plasma Technology
Discharge propagation
Rotary GapSwitch
DCPowerSupply 1MΩ 400kΩ
Blumelein pulse generator(coaxial cable, 50 m×2, 100 pF/m )
Discharge propagation
Ar10 mm
liquid
A charged voltage of coaxial
transmission lines and a
pulse repetition rate are
±14.14 kV and 20 pps (pulse
per second), respectively.
Discharge plasma in contact with liquid generates active species in the liquid.
Reactive species, such as OH and ONOOH, areproduced in the liquid, and generation of thesespecies is promoted in the acidic liquid. (Lukes et al. :
Plasma Source Sci. Technol. 23 1000120 (2014))
pH variation in the liquid is caused by the chargetransfer from charged particles to watermolecules in the liquid. (Suggested) (Luckhaus et al. : J.
Am. Chem. Soc. 135 16264-16267 (2013))
It is well known that pH value in the liquid
decreases by the dissolution of HNOx, produced
by the discharge plasma in air.
Objective
To clarify the mechanism of pH variation induced by chargedparticles in liquid in Ar atmosphere using colorimetric analysis
Pulsed discharge plasma is generated above water with positive and negative
polarities in Ar atmosphere, and the spatiotemporal development of pH variation
in the liquid is visualized by colorimetric analysis using Bromothymol blue (BTB).
Experimental apparatus and conditions
Acrylic resin
Aluminum
80 mm
90 mm 4 mm
Fed gasAr gas is fed to the chamber
with a flow rate of 1 L/min.
Test liquid
200 mL of water containing 0.17
M(mol/L) of NaCl and 19 μM of
BTB is poured into liquid
container electrode.
To remove dissolved gases, Ar
gas is sparged into the liquid for
10 min.
9.56 mS/cmpH : 6.93
③ OH- generated by the electrolysis increases the pH value.
At liquid-electrode boundary
At gas-liquid boundary
H+, generated at the liquid surface, extends horizontally near
the earthed electrode.
Most of the liquid color is seemed to be yellow, but the
liquid color is partially changed to blue, indicating pH
increase, namely, OH- generation.
Liquid color is changed to blue by the discharge initiation,
and the coloration propagates downward.
This coloration indicates the pH increase, namely, OH-
generation.
② Electrolysis of water at the cathode
2H2O + 2e- → 2OH- + H2 (3)
Charge transfer
① Ar ions (Ar+) are transferred from the plasma to the liquid
surface.
① Electrons (e) are transferred from the earthed electrode into the
liquid by the electrolysis of water at the earthed electrode
(watching as cathode).
At liquid-electrode boundary
③ H+ generated by the electrolysis decreases the pH value.
② Electrolysis of water at the anode
2H2O → 4e- + 4H+ + O2 (6)Charge transfer
② Electron irradiation into liquid (Tochikubo et al. Journal of Physics Conf. Series. 565, 012010 (2014))
e → eaq (4)
Hydrated electron (eaq) is generated in the liquid.
③ Hydrated electron reacts with H2O to form H and OH-.
eaq + H2O → OH- + H (5) : OH- generation & pH increase
At gas-liquid boundary
Charge transfer
① Electrons (e) are transferred from the plasma into the liquid.
① Electrons (e) are transferred from the liquid into the earthed
electrode by the electrolysis of water at the earthed electrode
(watching as anode).
The same amount of charged particles are transferred across
the gas-liquid and liquid-earthed electrode boundaries to
satisfy the continuity of electric current.
These charge transfer induces the reactions of eqs. (1), (3),
(4) and (6).
As a result, the equal amounts of H+ and OH- are generated
at the gas-liquid and liquid-earthed electrode boundaries by
the reactions of eqs. (2), (3), (5) and (6).
pH variation induced by charge transfer
BeforeArpH variation
initial pH: 6.93
discharge area
OH-
OH-
earthed electrode
1. Spatiotemporal development of pH variation in liquid
Negative polarityPositive polarity
2. Mechanism of pH variation in liquid
Results and discussion
H2O+ is generated in the liquid.
Negative polarityPositive polarity Both polarities
Model of pH variation induced by the charge transfer
negativepositiveCurrentdirection
H+
e- e-e-
e- e-e-
Currentdirection
++ +
e- e-e-
OH-OH-OH-
H+H+
H+ H+ H+
OH- OH-OH-
pH variation
Ar
initial pH: 6.93
H+
OH-OH-
H+
stirring
earthed electrode
Before
H+
stirring
liquid
electrode
OH
H++ NOx-
H2O2
gas
plasma
NOe-
chargetransfer
H2O
H+
M+
OH-
HNOx
dissolve
H2O2
Reactive species
OHONOOH
e-
M+
:electron
:positive ion
This pH variation have not been demonstrated experimentally.
Liquid color is changed to yellow by the discharge initiation,
and the coloration propagates downward.
This coloration indicates the pH decrease, namely, H+
generation.
At 2 s
At 10 s
At 180 s
At 2 s
At 10 s
At 180 s
The color of plasma-exposed-liquid returns to green,
namely, neutral, after stirring; therefore, the amounts of H+
and OH- are equal.
Most of the liquid color is seemed to be blue, but the
liquid color in the vicinity of the earthed electrode is
changed to yellow, indicating pH decrease.
OH-, generated at the liquid surface, extends horizontally
near the earthed electrode.
discharge area Discharge plasma radially spreads
over the liquid surface.
(no polarity effect: branch number, length)
Discharge plasma radially spreads
over the liquid surface.
(no polarity effect: branch number, length)