Online pH and acidity meteris a kind ofReal time and continuous monitoring of solution acidity and alkalinity (pH value)Automated instruments are widely used in industrial process control, environmental monitoring, biomedicine, and other fields. Its core function is to convert the activity (or concentration) of hydrogen ions in the solution into electrical signals, and output standardized pH values through circuit processing and calibration. The following will explain the working principle and application scenarios.

　　1、 Working principle of online pH and acidity meter​

The workflow can be divided intoElectrode induction → signal conversion → signal processing → output displayThe core of the four stages depends on the pH glass electrodePotential response characteristicsAnd subsequent circuit compensation technology.

　　1. Core components: pH electrode and measurement circuit​

The key components of an online pH acidity meter include:

　　pH combination electrodeIntegrated indicator electrode (glass electrode) and reference electrode (such as Ag/AgCl electrode), some models include temperature sensors (for temperature compensation).

　　glass electrodeThe sensitive membrane is made of special lithium silicate glass (with a thickness of about 0.1mm) and has selective response to H ⁺. When the glass membrane contacts the solution, the membrane surface will form a hydrated gel layer (about 10 8315 8315 8315 8315 8315 8315 8315 8315 8315 8315 8315 8315 8315 8315 8315 8315 8315; mm thick), and H 8315 8315; will diffuse between the gel layer and the solution, producingmembrane potential(According to the Nernst equation).

　　reference electrodeProvide a stable reference potential (such as the potential of Ag/AgCl electrode fixed by Cl ⁻ concentration), which together with the membrane potential of the glass electrode forms the electromotive force (EMF) of the battery.

　　measuring circuitThis includes a high impedance input amplifier (to prevent electrode current loss), a temperature compensation module (to correct the effect of temperature on pH), an A/D converter (to convert analog signals to digital signals), and a microprocessor (to perform calibration and linearization operations).

　　2. Potential response: application of Nernst equation​

After immersing the glass electrode and reference electrode in the solution, a primary cell is formed, and the relationship between its electromotive force E and the pH of the solution followsNernst equation：

E=E0​+ F2.303RT ​⋅pH

among which

E0: electrode standard potential (determined by the characteristics of the reference electrode and glass electrode);

R: Gas constant (8.314 J/(mol · K));

T: Thermodynamic temperature (K);

F: Faraday constant (96485 C/mol);

F2.303RT: Nernst slope (approximately 59.16 mV/pH at 25 ℃, meaning that for every 1 change in pH, the electromotive force changes by approximately 59 mV).

In practice, errors need to be eliminated through calibration due to factors such as the asymmetric potential of glass electrodes (differences between the inner and outer surfaces of the membrane) and the liquid contact potential of reference electrodes (potential differences at different solution interfaces).

　　3. Temperature compensation: correcting slope and offset​

Temperature directly affects two key parameters:

　　Nernstian slopeWhen the temperature increases, the slope increases (such as a slope of about 64.1 mV/pH at 60 ℃);

　　The pH of the solution itselfTemperature changes can alter the ion product (Kw) of water, leading to a shift in neutral pH (e.g. neutral pH at 25 ℃=7100 ℃, neutral pH ≈ 6.14).

The online pH and acidity meter compensates for temperature in two ways:

　　manual compensationThe user inputs the current temperature, and the instrument automatically adjusts the slope calculation;

　　automatic compensationIntegrated temperature sensor (such as Pt1000), real-time temperature acquisition and dynamic correction of slope and pH readings.

　　4. Calibration: Two or more point calibration​

To ensure measurement accuracy, regular use is requiredStandard buffer solution(Given pH values, such as pH 4.01, 6.86, 9.21) Calibrate the instrument:

　　Single point calibrationSuitable for scenarios with low precision requirements (such as only needing to determine acidity/alkalinity), adjust the zero point or slope with a standard solution;

　　two-point calibrationThe common method is to first calibrate the slope with pH 6.86 (close to neutral), and then calibrate the offset with pH 4.01 or 9.21 to eliminate the influence of asymmetric potential and liquid junction potential;

　　Three point calibrationUsed for high-precision scenarios (such as laboratory level online monitoring), covering acidic, neutral, and alkaline ranges, and fitting more accurate calibration curves.

　　2、 Application range of online pH and acidity meter​

Online pH and acidity meterReal time, continuity, automationIts characteristics make it a "must-have equipment" for industrial process control and environmental monitoring, mainly applied in the following fields:

　　1. industrial process control​

　　chemical production: Monitor the pH of the reaction kettle and neutralization tank (for example, adjust the pH in sulfuric acid production to prevent equipment corrosion, and control the monomer hydrolysis rate in polymerization reaction).

　　power industryPH monitoring of circulating water and boiler water in power plants (to prevent scaling or corrosion, such as controlling the pH of cooling water between 8-9 to reduce metal oxidation).

　　Metallurgy and ElectroplatingThe pH of the plating solution affects the quality of the coating (for example, if the pH of the copper plating solution is too high, it will cause the precipitation of copper hydroxide, and if it is too low, the current efficiency will decrease); Regulate pH to precipitate heavy metal ions in metallurgical wastewater treatment.

　　Food and BeverageFermentation process (such as yeast activity dependent on pH 4.0~5.0 in beer brewing), fruit juice sterilization (pH<4.6 can inhibit microbial growth), dairy product processing (yogurt fermentation solidifies when pH drops to 4.6).

　　2. Environment and sewage treatment​

　　Surface water/groundwater monitoringAssess the degree of water acidification (such as acid rain pollution causing lake pH<5.6), in compliance with environmental regulations (such as the "Surface Water Environmental Quality Standards" requiring pH 6-9).

　　sewage treatment plantAdjust the pH of the aeration tank and sedimentation tank (such as maintaining pH 6.5-8.5 to ensure microbial activity in activated sludge process), control the amount of acid/alkali added, and reduce the consumption of chemicals.

　　Garbage leachate treatmentLeachate contains high concentrations of organic acids and ammonia nitrogen, with large pH fluctuations (2-12), and needs to be monitored in real time to avoid pipeline corrosion or treatment failure.

　　3. Biomedical and Laboratory​

　　pharmaceutical industryThe pH of bioreactors (such as vaccine production) needs to be strictly controlled (such as mammalian cell culture pH 7.2-7.4), otherwise it will affect protein expression or cell survival.

　　clinical laboratory testingMonitoring the pH of body fluids such as blood and urine (such as arterial blood pH within the normal range of 7.35-7.45, deviation indicates acidosis or alkalosis).

　　laboratory researchChemical reaction kinetics experiments, enzyme catalyzed reactions, etc. require real-time tracking of pH changes and optimization of experimental conditions.

　　4. Agriculture and Aquaculture​

　　soil improvementMonitor the pH of soil extraction solution (lime adjustment is required for acidic soil pH<5.5, sulfur is required for alkaline soil pH>8.5), and guide fertilization.

　　aquacultureThe pH of fish pond water affects fish respiration and algae growth (such as freshwater fish being suitable for pH 6.5-8.5, while shrimp are sensitive to sudden pH changes), and needs to be adjusted in real time to avoid flooding.

　　3、 Key technical features and challenges​

　　anti-interference capabilityThere are strong electromagnetic interference and high viscosity/high particle solutions (such as mud and slurry) in industrial sites, which require shielded cables and anti blocking designs for flow tanks (such as ultrasonic cleaning and mechanical scraping).

　　long-term stabilityThe sensitive membrane of glass electrodes is susceptible to contamination (such as oil and protein adhesion) or aging (such as dehydration causing slow response), and requires regular maintenance (such as activation by soaking in 3M KCl solution).

　　Intelligent expansionModern online pH and acidity meters support 4-20mA analog signals, Modbus/Profibus digital communication, and can be connected to DCS (distributed control system) or cloud platforms for remote monitoring and data analysis.

　　summary​

Online pH and acidity meterPotential response of glass electrode+Nernst equation conversion+temperature compensation and calibrationReal time and accurate measurement of solution pH has been achieved. Its application covers key fields such as industry, environment, healthcare, and agriculture, and is a core tool for ensuring stable production processes, meeting environmental quality standards, and maintaining the health of biological systems. With the development of sensor technology (such as solid-state pH electrodes) and the Internet of Things, online pH monitoring is moving towardsLow maintenance, high integration, and intelligenceDirection evolution.