Reaction of a Platinum Electrode for the Measurement of Redox Potential of Paddy Soil

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1 J. Jpn. Soc. Soil Phys. No. +*0, p.- +*,**1 Eh * ** Reaction of a Platinum Electrode for the Measurement of Redox Potential of Paddy Soil Daisuke MURAKAMI* and Tatsuaki KASUBUCHI** * The United Graduate School of Agricultural Science, Iwate University ** Faculty of Agriculture, Yamagata University, +,- Wakaba-machi, Tsuruoka, Yamagata, 331 2///, Japan Abstract Redox potential (Eh) of soil has been measured to know the redox state in paddy soil. However, it was di$cult to get reliable data because the Eh values in the field fluctuated largely. Although the measurement method had been examined to improve the problem since the +3/* s, it still remained to be analyzed. To improve the problem, we used the cyclic voltammetry (CV) to examine the reaction of the platinum electrode. The results obtained were as follows. The causes that fluctuated the data consisted of two factors. One was the chemical reactions of platinum electrode with the surrounding materials. The other was associated with the property of the soil. The platinum electrode induced reaction with oxygen in the oxygenated medium and made oxide layer in the surface of the electrode. This layer disturbed to make the Eh stable. Polishing the sensor could remove the layer. In the reductive state, the sensor adsorbed some materials in the reductive state around its surface. This also decreased the stabilizing time. In addition, the di#usivity of ions in the soil was too small because of the large surface area and electro double-layer of soil solid. These factors decreased the moving speed of electron or ion transport and non-equilibrium condition was formed in the soil. Key words : redox potential, measurement method, cyclic voltammetry, paddy soil, di#use double layer + Eh +3., ; +32, ; +32, +30* ; +32, +31* Eh Eh * ; * * : -*/ ** ** 331 2/// +,- :

2 4 +*0,**1 Eh Eh CV CV -,**, Eh Eh,, + LiC, mm,,,, + Eh, *./ mm, 2* mm, +, - ORP ODIC-1 Eh ORP Eh Eh mv,*0 *.1 T,/ + Fig. +, Eh A - CV B Two electrodes method of Eh (A) and three electrodes method of CV (B) T GreenKit22 ESD CR+*X ZoBell *.+ moll + + L + -** moll + Eh,/.,2 mv Starkey and Wight, L *.1 L +* : Eh : Eh :,

3 : Eh 5 + #+***, Eh : *./ mm,* mm -+ mm, 2* mm +,/ mm, -,* mm /** mm, - - Eh,,, CV *./ mm, 2* mm, + HAB-+/+, Eh,. h + V /* +**,** /** +,*** mvs + mv min + : KCl /* mmol L + CV :,. KCl /* mmol L + CV : Eh Eh +302 Eh Eh Eh - + +,, Eh Fig., Eh change just after the insertion of sensors (soil, ponded water) Eh Eh Eh , Eh -,

4 6 +*0,**1 - Eh Fig. - Change in Soil Eh by repetitive insertion / Eh Fig. / E#ect of surface area on soil Eh. Eh Fig.. E#ect of polishing on the change in Soil Eh /, -, cm Eh - + / Eh Eh Eh Eh Eh / Eh +32. Eh Eh CV -, CV CV

5 : Eh 7,**, CV -, + CV /* mmol L + KCl 0 0 CV Fig. 0 E#ect of polishing on CV curve -,, CV 1,. /* mmol L + KCl CV CV CV CV CV. CV 1 -, - CV KCl CV /* +,*** mv min + 2 CV CV Fig. 1 1 CV Change in CV curve by repetitive sweep (Numbers in the figure show the sweeping order.)

6 8 +*0,**1 Fig. 2 2 CV CV curves of the soil by the change in sweeping speed 3 2 Fig. 3 The relation between the oxidative peak of current and square root of sweeping speed from figure 2. *,**, 2

7 : Eh Eh Eh Eh KCl CV CV Eh, Eh. Eh + Eh CV Eh Eh CV,**, : p. 2* +32. : p , : Eh /- : /./ / : +3 : * : +0 Eh pp * +31* : / pp. / , : +0 : +*. ++0.

8 10 +*0,** : Eh -3 : /-/ /.,. +32, :. + pp Starkey, R.L. and Wight, K.M. (+3.0) : Anaerobic corrosion of iron in soil with particular consideration of the soil redox potential as indicator of corrosiveness, +*2, (New Brunswick, N. J.). Eh Eh, :,** :,**1-3

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