Partial components and introduction of dissolved oxygen meter electrodes
Dissolved oxygen meter electrode structure: generally composed of cathode, anode, electrolyte and plastic film.
Electrolyte: The formula of electrolyte is generally considered confidential and is not easily disclosed by merchants. The preparation of the electrolyte is very particular and requires the use of ion-free water. Some polluting ions will seriously affect the performance of the electrode. The pharmaceutical reagents used must be at least AR grade. Useful electrolytes include KOH; KCl, Pb(AcO)2 and other humidity sensor probes, stainless steel electric heating tubes, PT100 sensors, fluid solenoid valves, cast aluminum heaters, and heating rings.
Film: Polytetrafluoroethylene (F4) or polytetrafluoroethylene-polyhexafluoropropylene copolymer is generally used. Polypropylene chloride has also been used. Polyethylene film has high permeability to oxygen and low permeability to CO2.
Electrode Response Our simple analysis of the electrode performance shows that the electrode response is related to the electrode constant, k: k=π2D/d2. D is the diffusion coefficient of the film, and d is the film thickness.
The larger K is, the faster the response is. Of course, the structure of the electrode will greatly affect the performance of the electrode.
pressure compensation membrane
The electrodes used in the tank are generally equipped with pressure compensation membranes, and the electrodes used in small glass fermentation tanks usually adopt the pore balanced type. The pressure compensation membrane weight is needed to cope with the thermal expansion of the electrolyte during high-pressure explosion. Generally made of silicone.
working principle
The oxygen in the water must pass through the membrane and reach the surface of the cathode before it can be reduced by the electrode. Therefore, oxygen needs to overcome some resistance before diffusing to the cathode surface, the most important of which are the resistance of the liquid film close to the film and the resistance of the film itself. For galvanic cell type electrodes, it is very important that the main resistance should fall on the film, that is, the resistance of the film is much greater than the resistance of the liquid film, so that the impact of changes in resistance caused by the flow of the liquid being measured on oxygen diffusion can be minimized. . Therefore, it can be seen from formula (1) that measuring oxygen is essentially measuring the diffusion rate of oxygen.
IS = N FA (Pm/dm)P0 (1)
In the formula, IS is the output current, N is the number of electrons obtained by reducing oxygen, F is Faraday's constant, A is the cathode surface area, Pm is the diffusion coefficient of the plastic film, dm is the thickness of the film, and P0 is the partial pressure of oxygen in the liquid being measured. .
Based on this principle, when measuring DO in viscous fermentation broth, galvanic cell-type electrodes should try to use a thicker film, which can reduce the change in liquid film resistance and thus reduce the fluctuation of the output current. For polarographic electrodes, fluid movement has no effect on the output of the electrode.
Precautions
In fact, the dissolved oxygen meter electrode measures not the dissolved oxygen concentration, but the oxygen activity or oxygen partial pressure. Air or oxygen-free nitrogen is usually used to calibrate the 100% and zero points. The true dissolved oxygen concentration in a liquid can be determined chemically.
