| Summary: | 碩士 === 國立成功大學 === 化學工程學系 === 89 === Sulfuric acid is widely used in industry because of its important chemical and physical properties. It is of great important to monitor the water content of sulfuric acid in industry process. However, the study about water content sensor for sulfuric acid is few, there is only one spectroscopic sensor. The others are traditional methods including titration method, hydrometer method and Karl Fischer method. These methods are still time-consumed and cumbersome in operation for determination the water content of the concentrated sulfuric acid. And these methods are easily interfered in a high humidity atmosphere. Therefore, it is interest and important to develop a simple, low cost and faster response sensor for instantaneously detecting the water content of the concentrated sulfuric acid by an electrochemical method which can be in situ applied in the processes related to concentrated sulfuric acid.
The aim of this study is to develop an electrochemical sensor for detecting small amount water content of concentrated sulfuric acid. Initial screening of the test electrodes was done in the 96wt% sulfuric acid. Those electrodes which showed a tendency to dissolve in concentrated sulfuric acid within 24 hr were eliminated from further testing. Insoluble electrodes were then selected to do sensing test in concentrated sulfuric acid. The results show that Pt plate, sputtered Pt, Au, Pd, Ta, DSA (IrO2 + Ta2O5), glassy carbon, Si and Nafion®/metal electrodes have good prevention of corrosion in concentrated sulfuric acid. The results of Fe and graphite are fair.
100wt% sulfuric acid can be obtained by electrolysis. Above about 84wt% sulfuric acid, no free H2O molecules exist. Water molecules exist as H3O+ form. Nafion®/Pt electrode used Nafion® as isolated film, it can only let H3O+ pass through and react on electrode surface. The reaction formula was shown as follow:
2H3O+ + 2e H2 + 2H2O (I-1)
From the reaction (I-1), 2 moles of electron can react with 1 mole of water and carry away 2 moles of water; so 1 mole of electron can remove 1.5 mole of water theoretically. Water was condensed on the glass tube which outside the electrode during electrolysis. Water was vaporized by heat when applying high potential, and then condensed. This can prove the produce of water in equation (I-1). The efficiency of removing water was determined by initial water content of sulfuric acid. Because the drawing back rate of water was higher when higher concentration sulfuric acid was used. Moreover, the adding of hot gas or P2O5 dehumidity agent in glass tube can help to remove water.
The results of water content sensing performance in concentrated sulfuric acid show that Pt plate, Ta, Fe and Nafion®/Pt electrodes have higher sensitivity which are 10.58, 12.85, 12.16 and 27.84 (mA/cm2*wt%), respectively. DSA (IrO2+Ta2O5) electrode has the best linearity and graphite is worst. Furthermore, the effects of temperature and stirring rate were also discussed in this study. The increase of temperature will increase the mobility of ion in solution, so the conductivity will increase. And the sensing current will also increase. The results show that the effect of temperature is big, so temperature correction is necessary. The potential of Pt reference electrode in concentrated sulfuric acid drifts when temperature changes. So it is not suitable for application when temperature changes. The effect of stirring rate is very small.
In kinetic control system, the relationship between current and water content was derived theoretically as shown in equation (I-2) for platinum working electrode.
I1=AanF[K"C(H+)sol-K"'] (I-2)Among this equation, and are constant and the relationship between current and water content is linear. Experimental results are not exactly fitting the theoretically analysis, because the model does not discuss the effect of the change of conductance. In diffusion control system, the relationship between current and water content was derived theoretically into two parts including unsteady state and steady state. At unsteady state, the relationship between current and water content is
I(t)=[AanFD^(1/2)C(H3O+)]/[Pi^(1/2)t^(1/2)] (I-3)When n、F、D and are constants, the relationship between I and t-1/2is linear. Where the diffusitivity is obtained and to be 4.22*10-11 (m2/Sec). At steady state, the relationship between current and water content is
Ilim=[AanFDC(H3O+)]/L (I-4)
When n、F、D、and L are constants, the relationship between and is linear. The diffusion layer was obtained to be 1.42*10-5 (m).
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