Lab pH Meter

Content
1. What is a lab pH meter?
     1.1. Definition
     1.2. Lab pH meter application
2. Lab pH meter parts
3. Working principle of lab pH meter
4. Types of lab pH meter
5. Lab pH meter specification
    5.1. Pen-type pH meter
    5.2. Benchtop pH meter
    5.3. Portable/Handheld pH meter
6. How to calibrate lab pH meter?
7. How to use lab pH meter?
8. Lab pH meter Precautions
9. How to order lab pH meter?

Lab pH meter, also known as lab pH tester, is generally speaking a common laboratory equipment, is mainly used in the laboratory for precision measurement of liquid medium pH value of the instrument.

Laboratory pH meter is widely used in microbiology, environmental protection, sewage treatment, scientific research, pharmaceutical, fermentation, chemical industry, breeding, tap water, and other fields, and it is also a necessary testing equipment for QS and HACCP certification.

The structure of the lab pH meter consists of three main structural components, which are a reference electrode; and an ammeter.
The following is a description of the main functions of each component.

The basic function of a reference electrode is to maintain a constant potential that serves as a control for measuring various deviations from the potential. Silver-silver oxide electrode is the most commonly used reference electrode in pH today.

The function of the glass electrode is to establish a potential difference in response to a change in the hydrogen ion activity of the solution being measured. By placing the pH-sensitive electrode and the reference electrode in the same solution, a primary cell is formed, so e cell = e reference + e glass, if the temperature is constant, the potential of this cell varies with the pH of the solution to be measured, it is difficult to measure the potential generated by the cell in a pH meter because its electric potential is very small and the impedance of the circuit is very large 1-100MΩ; therefore, the signal must be amplified to enough to drive a standard millivolt meter or milliammeter.

The function of the ammeter is to amplify the potential of the original battery several times, and the amplified signal is displayed through the meter, and the degree of deflection of the meter pointer indicates the strength of the signal driven by it.

The lab pH meter works on the principle of a primary cell, where the electric potential between the two electrodes is related to both the electrodes' properties and the concentration of hydrogen ions in the solution, according to Nernst's law. There is a correspondence between the electric potential of the primary cell and the hydrogen ion concentration, and the negative logarithm of the hydrogen ion concentration is the pH value.
In addition, lab pH testers are generally used to measure the activity of hydrogen ions in a solution.
The ionic activity is the effective concentration of ions involved in electrochemical reactions in an electrolyte solution. There is a quantitative relationship between ionic activity (α) and concentration (c) with the expression: α = γc where: α is the ionic activity; γ represents the activity coefficient of the ion, and c indicates the concentration of the ion.
γ is usually less than 1, the interaction between ions tends to zero when the solution is infinitely dilute when the activity coefficient tends to 1, the activity is equal to the actual concentration of the solution. In general, the concentration of H ions in aqueous solutions is very small, so the activity of H ions is equal to its concentration.
According to the Nernst equation, the ion activity is proportional to the electrode potential, so the relationship curve between the electrode potential and activity can be established for the solution, at this time the potential is measured, the ion activity can be determined, so in fact, we are measuring the potential to calculate the concentration of H ions.

With the continuous development of science and technology, the requirements for laboratory instruments are constantly improving, and the technology of laboratory pH meter is also constantly improving, so different kinds of pH testers are also born. At present, the laboratory pH meter in the market can be roughly divided into the following three categories:
* Pen-type pH meter
* Benchtop pH meter
* Portable/Handheld pH meter

Features:
* Pen-type pH meter is a pocket-size digital display acidity meter, small size handheld.
* High-cost performance, maintain excellent quality.
* Pocket digital display pH meter is a classic brand of pocket digital display acidity meter, with high stability, high reliability, easy to use, low price, and popular, favored by the majority of users.

Features:
* Benchtop pH meter adopts E201 type compound electrode, fast response, and good stability.
* Stable and reliable performance, easy to operate.
* It is suitable for precise measurement of acidity (pH) of solution in the laboratory, widely used in the electrochemical analysis in light industry, chemical industry, pharmaceutical, food, epidemic prevention, environmental protection and education, and scientific research departments.

Features:
* Portable pH meter adopts electrode interface, stable measurement, accurate data
* Battery-powered model, easy to replace, easy to solve the instrument power supply problems
* LCD screen display, clear data display
* Simple and clear operation panel, easy to operate, convenient to use.

The calibration period of a laboratory pH meter is generally 3 months, and the calibration method and steps are as follows:

(1) Preparation before calibration
①　Prepare two beakers, distilled water, low pH and high pH standard buffer solutions, and a 0~100℃ mercury thermometer.
②　Fill one beaker with enough low pH standard buffer solution and the other beaker with enough high pH standard buffer solution.
③　Remove the electrode system from the electrode chamber.

(2) Zero-point calibration
①　Wash the electrode (pH electrode) system with distilled water, dry it with filter paper, and immerse it in the standard buffer solution with pH 6.86.
②　When the electrode system equilibrates with the solution temperature and the transmitter (or converter) indicates stable, measure the solution temperature and find out the pH value of the solution at that temperature according to the "standard buffer solution pH-temperature comparison table".
③　Adjust the zeroing (or asymmetry) potentiometer of the instrument to make the instrument indicate the above pH value.

(3) Range (or slope) calibration (one point calibration)
Remove the electrode (pH electrode) system from the beaker, wash it with distilled water, dry it with filter paper and immerse it in a standard buffer solution of pH 4.01 or 9. 18. These two buffer solutions are selected in such a way that the pH value of the measured solution and the pH value of the standard buffer solution for calibration are close to each other.

(4)Range (or slope) calibration
①　Remove the electrode system from the beaker, wash it with distilled water, dry it with filter paper and immerse it in a standard buffer solution of high pH (pH4 or pH9).
②　When the electrode system equilibrates with the solution temperature and the transmitter (or converter) indicates stable, measure the temperature of the solution and find out the pH value of the solution at that temperature according to the "comparison table".
③　Adjust the range (or slope) potentiometer of the meter to make the above pH value of the meter (pH meter)

(5) Repeat calibration
Repeat the above zero and range calibration steps until the meter indicates accurately.

1. Preparation
* Turn on the power, and short press the "quit" button to open the PH meter to warm up.
*  Clean the electrode, use distilled water to clean the electrode more than three times.
*  Prepare the liquid to be measured, take an appropriate amount of liquid to be measured in a clean test tube.

2. Calibration method
Auto calibration: submerge the electrode below the level of standard buffer solution. Short press the "Calibration" key, and observe the upper left corner of the display, the calibration is finished when the Auto letter is no longer flashing. Short press the "Read/Confirm" key to the end.

3. Measurement Method
* After rinsing the electrode with distilled water more than three times, insert the electrode into the pre-prepared liquid to be measured and make sure the electrode is below the liquid level.
* Short press the "Read/Confirm" key and observe the upper left corner of the display, the measurement is finished when the word Auto is no longer flashing.
* After recording the number, remove the electrode and rinse it with distilled water more than three times. If you need to remeasure, repeat the first two operations.

4. Completion
After finishing, rinse the electrode with distilled water more than three times, then insert the electrode into the protection bottle. Press and hold "Exit" to turn off the machine, unplug the power, and put it back in the original place.

1. In general, the laboratory pH meter is in continuous use, once a day to calibrate; generally, within 24 hours, the instrument does not need to be calibrated again.

2. Before using the lab pH meter, pull down the rubber sleeve at the upper end of the electrode to expose the small hole at the upper end.

3. The buffer solution for calibration generally uses pH=6.86 solution, and the second time uses the buffer solution close to the pH of the measured solution. If the measured solution is acidic, the buffer solution should be pH=4.00; if the measured solution is alkaline, the buffer solution with pH=9.18 is chosen.

4. When measuring, the introduction wire of the electrode should be kept still, otherwise, it will cause unstable measurement.

5. The electrode of the lab pH meter should never be immersed in distilled water.

6. Must keep the electrode bulb moist, if found dry, it should be soaked in 3mol/L potassium chloride solution or slightly acidic solution for a few hours before use to reduce the asymmetric potential of the electrode.

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