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 : 16043070@mmm.muroran-it.ac.jp

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)