Research and design of an online monitoring device for hydrogen peroxide concentration

To address the issues of manual measurement, inaccurate instrument measurement, and frequent manual intervention required for hydrogen efficiency, oxygen efficiency, and raffinate in the production of hydrogen peroxide using the anthraquinone process, a hydrogen peroxide concentration online monitoring device was designed. The device includes a sampling module, extraction module, temperature control module, detection module, and display module. The detection module is designed based on the amperometric analysis method, using a constant potential instrument with a three-electrode system for current detection, establishing a relationship model between current and hydrogen peroxide concentration. It also includes calibration and detection modes to reduce detection errors generated during long-term operation. The device enables online analysis of hydrogen peroxide concentration through online sampling, pretreatment, and testing analysis, with a detection range of 0.05-15 g/L. The device has demonstrated good accuracy and reliability and has been tested and applied in industrial settings.


Introduction
The compound of hydrogen peroxide and water is called hydrogen peroxide, which has strong oxidizing properties [1].Its decomposition products are hydrogen gas and oxygen gas.It is an important industrial product in the fields of bleaching, cleaning, and oxidation.The anthraquinone process is the main method for producing hydrogen peroxide, which includes hydrogenation, oxidation, extraction purification, and post-treatment processes [2].
The industrial site usually uses titration analysis, which requires manual intervention and is timeconsuming.Existing hydrogen peroxide analyzers are limited by pH, temperature, and detection range, requiring frequent manual intervention such as cleaning the measuring container and replacing the measuring electrode.Spectrophotometry, ultrasonication, and other methods mostly have the disadvantages of long operation time, requiring manual intervention, and easy contamination of testing equipment [3][4][5][6].The electrochemical method of potential analysis requires maintaining the stability of the chemical system, a long detection period for a single test, and the need to import instruments.The Coulomb analysis method is easily affected by other components during electrolysis and the degree of electrolysis is difficult to control.The amperometric analysis has the advantages of high sensitivity, fast response, stable chemical system, wide detection range, and short single detection cycle time [7][8][9][10][11].

Principle of Measurement
The core of this monitoring device is based on the amperometric analysis method to detect the concentration of hydrogen peroxide.During electrolysis in a three-electrode system, hydrogen peroxide molecules in the electrolyte diffuse to the surface of the anode, where they undergo oxidation.Because the rate of electron transfer at the interface of the hydrogen peroxide molecule and the electrode is greater than the diffusion rate, the entire electrode process conforms to a diffusion-controlled mechanism.As a result, the strength of the anodic current is directly proportional to the concentration of hydrogen peroxide in the solution.
As shown in Figure 1, the three electrodes consist of the working electrode (WE), reference electrode (RE), and counter electrode (CE).The electrochemical reaction occurs at the WE.A constant voltage is maintained between the WE and RE, and there is a high impedance between the two electrodes, so the current does not flow between the WE and RE.The current is generated between the WE and CE, and when the hydrogen peroxide concentration in the solution varies, the magnitude of the generated current also varies.Within a certain range, there is a linear relationship between the current and the concentration.By conducting tests, a standard curve relating the current to the hydrogen peroxide concentration can be established.Based on this standard curve, the measured current can be used to estimate the concentration of hydrogen peroxide.

Measurement process flow design
This device is primarily composed of multiple peristaltic pumps, solenoid valves, liquid level meters, syringe pumps, air pumps, stirrers, LCD touch screens, constant potential output controls, and temperature control modules based on PT100 platinum resistance and Peltier, as shown in Figure 2. The peristaltic pump is controlled by PWM to achieve the movement of milliliter-level solutions.The syringe pump is controlled via serial communication to achieve the movement of microliter-level solutions.When conducting current detection, temperature changes can affect the size of the current generated during the electrochemical reaction of the solution.Therefore, a temperature control module is designed to ensure temperature stability during each concentration measurement.The stirrer needs to be activated during detection to prevent transient substances generated by chemical reactions from adsorbing on the electrodes.A liquid level meter is used to accurately measure the volume of the electrolyte.The electrolyte used is a potassium nitrate solution with a neutral pH, which does not participate in the electrochemical reaction of hydrogen peroxide.The peristaltic pump, solenoid valve, and syringe pump are used to control the entry and discharge of liquids into the respective reaction chambers.
In addition to the detection pool, the device also includes two intermediate pools.The extraction pool is used for sample extraction, the buffer pool is used to temporarily store pre-treated samples or standard hydrogen peroxide solutions, and the detection pool is used for determining the standard curve of hydrogen peroxide concentration or for online monitoring of hydrogen peroxide concentration.
The device is divided into calibration mode and detection mode.In calibration mode, the standard curve of hydrogen peroxide concentration is determined, using a known concentration of hydrogen peroxide standard solution.In detection mode, samples are first extracted, with the oil-phase sample and water-phase electrolyte added to the extraction pool, and extraction is achieved through air pumping.After a period of aeration, the oil phase and water phase separate, and the hydrogen peroxide molecules in the oil phase enter the water phase.The water phase solution is then extracted into the buffer pool by using a peristaltic pump.These steps are referred to as pre-processing, and subsequent detection involves the pre-processed sample.

Hardware device design
The device is designed with a microprocessor as the core for hardware design.The peristaltic pump uses the Cammer KFS series peristaltic pump, and control of the solenoid valve, peristaltic pump, and stirrer is achieved through GPIO output and level conversion of the microcontroller, while the syringe pump is controlled through serial communication.A constant potential module is designed to provide a constant voltage to the three-electrode system.A current-voltage conversion circuit is designed to convert the current signal generated during the reaction into a voltage signal.The voltage signal is amplified through a gain-adjustable signal amplification module composed of CD4052 and AD623, and the voltage signal is sampled through the ADC channel embedded in the microcontroller.The measurement results are transmitted to the touch screen through serial communication and stored on an SD card.The touch screen is selected from Guangzhou Dacai's M-series LCD serial screen, and the VisualTFT software of this company is used for development and design.The system composition is shown in Figure 3.

Main control core
Choosing the STM32F103RET6 processor as the core processor of the entire device, which has 64 pins, 5 serial ports, and 3 12-bit ADCs, each with a maximum of 16 external input channels, can meet the requirements for controlling external devices and detecting signals.

Three-electrode potentiostat detection system
The three-electrode potentiostat detection system consists of a constant voltage module and a currentvoltage conversion module.The constant voltage module selects the DAC7512 digital-to-analog converter to the output voltage.The DAC7512 is a 12-bit voltage output digital-to-analog converter that uses an SPI communication interface, with an output voltage range of 0-3.3 V.During current detection, the three-electrode system requires positive and negative potential activation to stabilize the electrochemical properties of the electrodes.The positive and negative activation potentials are provided by a subtraction circuit composed of the CA3140 operational amplifier.The CA3140 is an eight-pin single operational amplifier, powered by ±12 V, with a 2.5 V reference voltage, resulting in a voltage output range of -0.7 V to 1.5 V. Due to the direct contact of the constant voltage module of the threeelectrode system with the solution, which promotes the reaction, it requires a certain load capacity.Therefore, a two-stage voltage follower was designed by using the CA3140 to provide a constant voltage output between -1.5 V and 0.7 V to the working electrode and reference electrode.When outputting a constant voltage for concentration measurement, a current of 10-3 A-10-5 A is generated between the working electrode and the counter electrode.The current-voltage conversion module converts this weak current signal into a differential voltage signal output through sampling resistors.The schematic diagram is shown in Figure 4.

Signal amplification module
The hydrogenation, oxidation, and extraction processes correspond to different ranges of hydrogen peroxide concentrations in the samples.Therefore, a signal amplification module with adjustable gain was designed.
The AD623 was selected to amplify the voltage signal.The AD623 is an integrated instrumentation amplifier.Considering the voltage signal range obtained from the device, it is powered by ±5 V, with +1.2 V as the reference voltage.The CD4052 was chosen as the gain resistor selection chip for the AD623.When detecting different processes, different gain resistor values are selected through analog switches to provide the AD623 with different amplification factors.The amplified voltage signal ranges from 0-3.3 V.The signal amplification principle is shown in Figure 5.

Temperature measurement and control module
The temperature has a significant impact on the detection of hydrogen peroxide concentration, a temperature measurement and control module has been designed.This module uses a PT100 platinum resistance thermometer as the temperature sensor and utilizes the REF200 chip to provide a constant current of 500 uA.The temperature is measured by using the built-in ADC of the STM32 to measure the differential voltage output from the PT100, which has been amplified by the AD623, thus achieving temperature monitoring.To control the temperature, a Peltier module is employed.The Peltier module is a type of semiconductor cooling device, and when the direction of the current is reversed, the hot and cold sides of the Peltier module also reverse.Since the Peltier module is a high-power device, an L298N driver circuit has been designed.This circuit, combined with PWM and PID algorithms, enables the control of the temperature of the sample under test, as shown in Figure 6.

Software device design
The software of the hydrogen peroxide concentration monitoring device mainly includes calibration mode software, detection mode software, and touchscreen software.The device is controlled through a human-machine interface provided by the touchscreen, enabling peripheral control for calibration and detection functions.
Additionally, the device is equipped with an automatic mode, allowing for timed calibration, timed detection, current concentration curve display, SD card storage, concentration information display, and other functions.

Calibration mode
The measurement process in calibration mode is as follows: the standard solution is added to the buffer tank, and 15 mL of electrolyte is injected into the buffer tank.Temperature control is initiated, stabilizing the temperature at 25℃, and then the stirrer is activated to commence calibration.The microcontroller controls the ADC to sample the current signal at a rate of 100 Hz and stores it on the SD card.Every 30 seconds, 100 ul of standard solution with concentration c is added from the buffer tank to the detection tank using a syringe pump, and this process is repeated n times.By calculation, the slope k of the standard curve for current and standard solution concentration can be obtained.
The calibration curve for hydrogen peroxide concentration is plotted with current I (in mA) on the y-axis and hydrogen peroxide concentration c (in g/L) on the x-axis.Figure 7 the standard curve for hydrogen peroxide concentration.At the beginning of calibration, the initial current in the potassium nitrate solution in the detection tank is measured, corresponding to a hydrogen peroxide concentration of 0. The current size after each addition of the standard solution and the corresponding hydrogen peroxide concentration are recorded.Additionally, the slope and intercept values of the fitted line are calculated by using the least squares method.The concentration c of the standard solution and the number of additions n can be changed as preset parameters of the instrument, and different concentration range standard curves can be obtained after modification.

Detection mode
The measurement process in detection mode is as follows: the electromagnetic valve and peristaltic pump are controlled to add the sampling sample to the extraction tank, and 15 mL of electrolyte is injected into the extraction tank.The air pump is turned on to initiate the extraction process.After a certain period, the extraction is completed, and the extracted sample is added to the buffer tank along with the electrolyte.Temperature control is initiated, stabilizing the temperature at 25℃, and then the stirrer is activated to commence the concentration detection process.
The microcontroller samples the current signal at a rate of 100 Hz and records it on the SD card.Every 30 seconds, 100 ul of sample solution is added from the buffer tank to the detection tank by using a syringe pump, and this process is repeated n times.By substituting the current values obtained after n additions into the standard curve, the concentration of the sample can be calculated.

Accuracy testing for hydrogen peroxide concentration detection
On September 10, 2023, the instrument was installed and tested on-site at the Yueyang Baling Petrochemical Company in Hunan.The recorded and analyzed data is available.Table 1 provides a comparison of the standard curve results for hydrogen peroxide concentration, measured on September 13, using a 0.25 g/L hydrogen peroxide solution as the standard solution.The average value of the slope is 0.104712, with a minimum of 0.103424 and a maximum of 0.105787.The error range is within -1.23% to +1.03%.The determination of the hydrogen peroxide concentration standard curve during instrument testing at the industrial site yielded good results.The measured standard curve was stable, and the error in the slope k was within ±5%, meeting the requirements for industrial on-site monitoring.It can be used for subsequent detection processes.
On September 14, the hydrogen peroxide concentration in samples taken from the industrial site was measured.A comparison with the results of manual titration is shown in Table 2.The device's measurement results ranged from a maximum of 6.552 g/L to a minimum of 6.250 g/L, with an error range of -1.7% to +3.0%.The current detection model demonstrates good practicality at the industrial site, with errors within ±5%, meeting the requirements for industrial on-site monitoring.

Conclusions
This article describes a hydrogen peroxide concentration monitoring device.In contrast to traditional manual detection methods, this device enables real-time online monitoring of hydrogen peroxide concentration at industrial sites without the need for manual intervention, except for replenishing the electrolyte and cleaning solution.Additionally, the device is designed with an RS-485 module to facilitate remote data communication with the DCS system.After testing, it was found that this device can be installed on the anthraquinone method hydrogen peroxide industrial production line to meet the real-time online monitoring requirements for hydrogen peroxide concentration in industrial settings.

Figure 2 .
Figure 2. The liquid control scheme for the detection process.

Figure 3 .
Figure 3.The hardware composition structure of the device.

Figure 4 .
Figure 4. Schematic diagram of the three-electrode potentiostat detection system.

Figure 5 .
Figure 5. Schematic diagram of the voltage amplification module.

Figure 7 .
Figure 7. Standard curve of hydrogen peroxide concentration.

5 .
Concentration measurement accuracy and reliability testing5.1.Device operational reliability testingThe physical device of the hydrogen peroxide concentration monitoring system, which meets the explosion-proof requirements, is shown in Figure8.It underwent simulated testing in a laboratory environment from August 14, 2023, to September 5, 2023.During this period, the device operated continuously without interruption, following a calibration during the night and detection during the day.The detection cycle was 1 hour, and no faults occurred.

Figure 8 .
Figure 8. Physical diagram of the device.

Table 1 .
Comparison of standard curve determination results.

Table 2 .
Comparison of concentration results for single sample determination.