DE2510738B2 - ELECTRODE FOR AN ELECTROCHEMICAL MEASURING CELL FOR THE QUANTITATIVE CONCENTRATION MEASUREMENT OF SLIGHTLY SOLUBLE GASES IN LIQUID ELECTROLYTE - Google Patents

Description

The invention relates to an electrode for an electrochemical measuring cell for quantitative -io Concentration measurement of easily soluble gases in liquid electrolytes, consisting of a thin hydrophobic gas-permeable layer covered with a thin hydrophilic electrolyte-permeable catalyst layer connected is. r>

It is already a gas diffusion electrode with a liquid-permeable metal sintered body and a with this connected gas-permeable and liquid-impermeable, electrically non-conductive Layer known (DT-OS 2048879).

To an electrochemical measuring cell for quantitative concentration measurement of easily soluble gases The following requirements apply to liquid electrolytes:

a) Selectivity of the measurement, d. H. low cross-sensitivity towards others, in the measuring gas contained compounds,

b) fast display of measured values, ie low time constant of the xt function,

c) Relatively high and constant over time Emp- ho delicacy,

ti) Simplicity of the measuring equipment and thus the possibility of operation even by untrained personnel,
e) net / independent mode of operation and low μ maintenance effort.

For the quantitative measurement of the concentration of NO, NO ₂ , C ₂ H ₄ , H ₂ , SO ₂ , CO and C ₂ H ₂ , a device of several toes with gas diffusion electrodes has already been proposed (patent application P 2435813.2-52).

The invention is based on the object of providing a rapid display of measured values, a high and constant one To achieve sensitivity and independence from the water vapor content of the measuring gas.

According to the invention, this object is achieved in that the catalyst layer is matched There is a ratio of pore radius / thickness.

Appropriate further developments of the subject matter of the invention can be found in the subclaims.

The invention is described below with reference to an exemplary embodiment shown schematically in the drawing explained in more detail. It shows

Fig. 1 shows the design of the electrode,

2 shows an enlarged section of the catalyst layer of the electrode.

As can be seen from Fig. 1, the measuring electrode designed as a gas diffusion electrode comprises a hydrophobic layer 1 with gas pores 2, which consists for example of polytetrafluoroethylene and also an adjacent hydrophilic catalyst layer 3 with electrolyte pores 4, which consists for example of activated carbon. A measurement gas, for example SO ₂ , is directed to the free surface of the hydrophobic layer 1 and penetrates its gas pores 2, while an electrolyte, for example sulfuric acid, is directed to the free surface of the catalyst layer 3 and penetrates the electrolyte pores 4.

The thickness of the catalyst layer 3 depends on its pore structure (most common pore diameter as well Porosity). An electrolyte is required to ensure independence from the water content of the measuring gas used, the vapor pressure of which corresponds to that of the water vapor partial pressure of the measuring gas, or which has no hygroscopic properties with the same composition.

As can be seen from FIG. 2, the measuring gas SO ₂ flowing through the hydrophobic, electrolyte-free layer 1 enters the electrolyte at the liquid / gaseous phase interface, which is stable between the hydrophobic layer 1 and the hydrophilic active layer 3, and diffuses onto the Pore edges to the active centers and is made to react there with the formation of SO ₃ , protons and electrons. Since the diffusion path in the direction of the pore edge is significantly smaller than in the direction of the pore axis, all of the SO _{2 is} converted; the ratio of the two diffusion paths corresponds on average to the ratio of the most frequent pore radius / electrode thickness. With a frequently stone pore radius of, for example, 10 μm and an electrode thickness of 0.25 mm, a ratio results; from 1:25.

However, this is only the case because, with a simultaneously low adsorption capacity, a high number of catalytically active centers is present.

The active layer 3 is therefore fully wetted and there are many active centers in it for the implementation, from which the high sensitivity results, so that even with very small amounts of gas in the lower In the ppm range, relatively high currents can still be measured.

Sulfuric acid or phosphoric acid can be used as the electrolyte.

Electrode and electrolyte are used in a cell that is operated potentiostatically.

The following is the manufacture of the electrode

explained in more detail.

A 0.4 mm thick polytetrafluoroethylene film with a diameter of about 45 mm, a most common pore diameter of 1 μm and a porosity of 40% is clamped into a compression mold, onto which a preprepared active layer is only 200 kp / cm ² pressure is pressed. The raw electrode obtained in this way is then sintered at a temperature of about 165 ° C. for 35 minutes under 0.1 kp / cm pressure . Hi

The pre-preparation of the active layer is done in the following way.

Activated charcoal of the Norit RBIII type is ground, boiled in 2nH ₂ SO _{4 for} 5 hours, filtered, washed neutral, dried at about 200 ° C. and sieved to the i "fraction 45-90 μm. The electrode mixture containing 6% by weight of PTFE and Contains 12% by weight of polyethylene, is suspended in a dough-like manner in hexane and pressed in a filter press device at 50 kp / cm ² pressure to form a solid round rod with 34 mm O. Slices about 0.4 mm thick are cut from this molded body, which then then further processed as described above.

The thickness of the active layer of the electrode is about 0.25 mm, the most common pore diameter μΐη and the porosity 60%.

A measuring electrode prepared in this way exhibits optimal behavior with regard to the level of sensitivity of the measured value, the temporal constancy as well as the selectivity and the speed of the display. The following table reached technical values of a measuring electrode according to the invention are given ah.

Active layer: RBIII carbon with>% PTFE and 12% PE;

Electrode thickness: 0.25 mm;

Wettability: hydrophilic;

SOi sensitivity: 4.3 μΑ / ppm;

y ()% - time: 35 seconds;

H, O vapor sensitivity: <1 ppm SO, equivalent;

Temperature dependency: <0.2% / degree

The electrode shows no significant cross-sensitivity to the following substances: H, O, CO ,, H ₂ , N "O" O ₃ , HCl, NO, N ₁ O, C, H "CH" C, H ~. C ₍ , H, / C _: H ₅ SH, HCN, NH, etc.

For this purpose 2 sheets of drawings

Claims (6)

Patent claims: 1. Electrode for an electrochemical measuring cell for quantitative concentration measurement of easily soluble gases in liquid electrolyte, consisting of a thin hydrophobic gas-permeable layer with a thin hydrophilic electrolyte-permeable Kataylsatorschicht is connected, characterized in that the catalyst layer (3) has a matched pore radius / thickness ratio. 2. Electrode according to claim 1, characterized in that the hydrophilic catalyst layer (3) consists of activated carbon to which polyethylene is added. 3. Electrode according to claim 1, characterized in that the hydrophilic catalyst layer (3) Precious metal is added. 4. Electrode according to claim 1 to 3, characterized in that an electrolyte is used, whose vapor pressure corresponds to the water vapor par'ial pressure of the measuring gas. 5. Electrode according to claim 1 to 3, characterized in that an electrolyte is used, that neither absorbs nor releases water. 6. Electrode according to claim 1 to 3, characterized. That the electrolyte is an aqueous one Solution of sulfuric acid or phosphoric acid, the concentration of which is chosen so that you Vapor pressure corresponds to the water vapor partial pressure of the measuring gas.