The basic principle of pH electrode and dissolved oxygen electrode and its application in biological fermentation

August 27, 2020

The basic principle of pH electrode and dissolved oxygen electrode and its application in biological fermentation

The basic principle of the pH electrode:
The electrode used in pH measurement is also called the primary battery. A primary battery is a system whose function is to convert chemical energy into electrical energy. The voltage of this battery is called electromotive force (EMF). This electromotive force (EMF) consists of two half cells. One of the half-cells is called the measuring electrode, its potential is related to the specific ion activity; the other half-cell is the reference half-cell, usually called the reference electrode, which is generally connected to the measuring solution and connected to the measuring instrument. . The voltage between the two electrodes follows the Nernst formula:

Where: E - potential E0 - standard voltage of the electrode R - gas constant (8.31439 joules / mole and °C)
T——Kelvin absolute temperature (example: 20 °C = 273=293 Kelvin) F——Faraday constant (96493 coulombs/equivalent)
n - the valence of the measured ion (silver = 1, hydrogen = 1) aMe - the activity of the ion

The standard hydrogen electrode is the reference point for all potential measurements. The standard hydrogen electrode is a platinum wire which is electroplated (coated) with platinum chloride and filled with hydrogen gas (fixed pressure of 1013 hpa). Since the hydrogen electrode as a reference electrode is difficult to implement in practice, a second type of electrode is used as a reference electrode. The most commonly used one is the silver/silver chloride electrode.

The most familiar and most commonly used pH indicator electrode. It is a glass tube that is blown on a pH-sensitive glass membrane. The tube is filled with a certain concentration of buffer solution. The potential difference reflecting the pH on both the inner and outer surfaces of the glass film was derived using an Ag/AgCL conduction system. This potential difference also follows the Nernst equation.

The basic principle of dissolved oxygen electrode:
Dissolved oxygen ( DO), short for Dissolved Oxygen, is a parameter that characterizes the concentration of oxygen in an aqueous solution. A dissolved oxygen electrode is a current-type electrode for determining oxygen dissolved in a liquid based on the polarographic principle.

1. Classification of dissolved oxygen electrodes There are various methods for determining DO: such as chemical Winkler method, electrode method, mass spectrometer, etc. Here we mainly introduce the electrode method. The dissolved oxygen electrode was first invented by Clark (1956). It is a current-type electrode covered by a gas permeable film.
DO electrodes can be divided into two categories: primary cells (Galvanic) and polargrafic.

2. Principle of DO electrode measurement Primary battery type: generally consists of a noble metal such as platinum, gold or silver; the anode is composed of lead. PbCl 2 or Pb(AcO) 2 is formed in the presence of an electrolyte such as KCl or lead acetate , and the galvanic electrode does not require an applied voltage.
The Polargrafic type electrode requires an additional polarization voltage of 0.6-0.8V. The cathode is generally composed of a noble metal such as platinum or gold; the anode is composed of silver. The polarographic electrode needs to be applied with a constant voltage, and the electrolyte participates in the reaction, so the electrolyte must be replenished at certain time intervals.

pH electrodes, DO electrodes applied on the fermentation

1. Fermentation control system for fermentation control system requires a powerful, scalable, flexible operation, reliable, and low failure rate. The basic control parameters: temperature, stirring speed, the PH (Hamilton OXYFERM Switzerland, Mettler INPRO6800), DO (Switzerland Hamilton EASYFERM, Mettler INPRO3100), intake air flow rate, pressure tank, fed-time quantitative, continuous fermentation , level, a variety of automatic feeding mode, defoaming. Special analysis and control are: such as CO2, density, sugar, automatic sterilization, etc.

2. The control system consists of 1) RTU-6600 as the control core, completes the PID control and fuzzy control tasks of multiple loops; at the same time communicates with the 10.4" man-machine interface to achieve seamless integration with the embedded configuration software. 2) 10.4" Man-machine interface, built-in WinCE.NET operating system and embedded configuration software, complete communication with RTU-6600, upper display, settings, alarms, report output and historical data storage.

3. Communication mode 1) Modbus/TCP, communicates with the man-machine interface through RTU-6600 Ethernet. This communication method is carried out via Ethernet, using the standard Modbus/TCP protocol. 2) Modbus/RTU (485) communicates with the human-machine interface through RS-485 of RTU-6600. This communication method is carried out by RS-485, using the standard Modbus/RTU protocol.

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