pH Meter: Parts, Principle, and Applications

A pH is a unit used to measure hydrogen ion concentration in a water-based solution. Simply, it is a scale that determines the acidity and basicity of a solution. The pH value of the solution ranges from 1 to 14, where pH= 7 means a neutral solution, pH<7 means an acidic solution, and pH>7 means a basic solution. A higher concentration of hydrogen ions in a liquid means the pH of the solution is lower. In contrast, higher pH implies the presence of less hydrogen ion concentration in a liquid. The pH of the solution is measured in two ways; a pH strip (pH indicator) and a pH meter. As the pH reading of the pH stripe is not very accurate, for more accurate and precise pH reading pH meter was discovered.

A pH meter is a piece of laboratory equipment used to measure hydrogen ion concentrations (i.e., acidity and alkalinity) in a water-based solution expressed as pH. It measures the difference in electrical potential between the pH electrode and the reference electrode. Thus, the calculated difference in electrical potential relates to the pH of the solution.

Parts of the pH meter

pH meter consists of an electrometer, glass electrodes, reference electrodes, or combination electrodes as their major component. Previously, pH meters were designed consisting of the glass electrodes and reference electrodes separately. Later, more user-friendly pH meters were designed with the glass electrode, and reference electrode in a single probe called a combined or combination electrode.

1. Probe: The probe, designed as a three-in-one combination, consists of a glass electrode, reference electrode, and temperature probe. One end of the probe consists of a sensitive tip glass that detects the solution’s acidity or alkalinity. In contrast, another end of the probe is connected to the electronic meter that measures and displays the pH value. The temperature detector in a probe helps to detect the temperature of the solution. The pH value of the test or buffer solution should be only measured at room temperature (i.e., 25°C).
2. Glass electrode: It is a narrow tube-like structure usually made up of glass consisting of a highly sensitive glass bulb at the end. It also includes a silver wire coated with silver chloride immersed in an electrolyte (potassium chloride pH 7) as an electrode. The glass electrode helps measure potential changes across the glass membrane.
3. Reference electrode: It is a glass tube-like structure consisting of a metallic conductor made of silver chloride immersed in an electrolyte solution, potassium chloride of pH 7. This electrode only comes in contact with the test solution through a ceramic junction. The reference electrode is used to provide a stable zero voltage connection and helps to complete the circuit. Therefore, it is insensitive to the solution’s pH. 
4. Electrometer/Voltmeter: It is attached to the pH electrode that can measure minimal differences in electrical potential in a circuit.
5. Filling hole: It is an opening used to refill the electrolyte.

Working Principle of pH Meter

The primary working mechanism of the pH meter is based on the exchange of ions from the sample solution to the inner solution (pH seven buffer) of the glass electrode through the glass membrane that generates electric voltage. Therefore, the result of the pH meter is based on the combined relation between electric voltage, ion concentration, and pH reading.

During pH measurement, when a pair of the glass electrode and reference electrode (or combined electrode) of a pH meter is immersed in a solution, hydrogen ions in a test solution move towards the glass electrode, replacing some metal ions inside the glass electrode. Meanwhile, some hydrogen ions also disperse into the solution being tested. This swapping of ions is known as the ion exchange phenomenon. This ion exchange phenomenon creates a small voltage (or electrical potential) across the glass electrode that is picked up by the silver electrode and passed to the electrometer/voltmeter. As a result, the voltmeter measures the voltage generated by the solution, which is equivalent to the pH value according to the Nernst equation. Apart from it, the potential difference in the reference electrode remains constant, that only helps to complete the circuit of the pH meter.

In conclusion, it infers that a decrease in voltage means fewer hydrogen ions, a reduction in acidity, an increase in alkalinity, and an increase in pH value. In the same way, an increase in voltage means more hydrogen ions, an increase in acidity, and a decrease in pH value.

↓ voltage = less H+ /more OH – = ↓ acidity = ↑pH                                         

↑Voltage= more H⁺/ less OH^– =  ↑ acidity =  ↓pH

Types of pH Meter

Based on portability

1. Pen tester pH meter:  These types of pH meters are designed with a pH meter, display, and electrode all in one form, making them easy to transport and use. They are pocket size and usually the cheapest pH meter.
2. Handheld/Portable pH meter: They are usually slightly larger than pen tester and consist of a separate electrode from the meter. Depending on pH measuring needs, its electrode can be changed.
3. Benchtop/Desktop pH meter: These types of pH meters have more accuracy than other pH meters. Therefore, they are well-suited to laboratories or professionals. Similarly, they can be fitted on a desk or wall.

Based on usage

1. Laboratory pH meter: This type of pH meter has a high measurement range and also gives a more accurate and precise reading
2. Industrial online pH meter: They are environment-friendly, have higher efficiency, and have digitalized functions.

On the basis of modernization

1. Economical pH meter
2. Intelligent pH meter
3. Precision pH meters are also of two types; digital and pointer pH meters.

Procedure for the operation of the pH meter

• Turn on the pH meter and wait for an adequate time for it to initialize (check your pH meter operating manual for the exact time).
• Remove the electrode from the storage solution and clean it with deionized water.
• After rinsing, blot the dry electrode with non-abrasive Kimwipes or Shurwipes to remove excess water.
• Maintain the temperature of the sample at 25°C for appropriate reading.
• Place the electrode in a test solution, stir the test solution and also press the measure button to measure the pH value.
• Then, wait until the reading gets stabilized for 1 to 2 minutes.
• Once the reading gets stabilized press the measure button and record the pH value and also the temperature.
• When done, remove the electrode, rinse it with deionized water, and blot dry with a Kimwipes.
• Lastly, immerse the probe in a 3M potassium chloride storage solution.

Procedure for calibration of pH meter

It is essential to calibrate the pH meter with the appropriate buffer before measuring each test solution. Mainly, more than one buffer solution is required to calibrate the pH meter. The first one will be a neutral buffer with pH 7 and the second one should be near the expected sample either pH 4 or 9.21. Buffer with higher pH (i.e; 9.21) are best for measuring bases whereas, buffer with lower pH (i.e; 4.0) are best for measuring acidic samples. The procedure to calibrate the pH meter includes the following;

• First, rinse the electrode with deionized water, and blot it to dry.
• Maintain the temperature of the buffer and solution at 25°C.
• Immerse the electrode in a buffer solution of pH 7 or 4. Make sure the entire electrode is immersed in a buffer solution.
• Slowly stir the buffer solution to create a homogeneous mixture.
• Enter the CAL/MEAS button to start the calibration.
• Wait for pH to stabilize and press HOLD/ENTER to stop reading for further fluctuation.
• Now the meter is calibrated.
• Finally, wash the electrode with the buffer, which is further used to determine the pH of the test sample.

Applications of pH meter

1. To measure the quality of municipal drinking water, swimming pools, and rainwater.
2. To measure the pH of biological fluid such as urine, blood, etc., to know the type of biological conditions.
3. To know the pH of the soil to increase productivity.
4. It is used in the chemical industry, neutralizing effluent in the paper, steel and pulp, pharmaceutical manufacturing, biotechnology, and petrochemical industries.
5. It is used to determine the pH value of food products to maintain a high level of safety and quality.
6. Also valuable for the determination of the concentration of substances by pH measurement.
7. It is also used to prepare the reagents of exact pH values in microbiology laboratories.

Advantages of pH meter

1. pH meters are portable and user-friendly.
2. If properly handled, it can be used for a longer duration of time.
3. pH meter provides highly accurate and more precise values than a pH strip.
4. Do not require high installation charges. Therefore, they are cost-effective.

Limitations of pH meter

1. Requires regular calibration that consumes an extra amount of time
2. Electrodes must be cleaned regularly to remove deposits.
3. It is equipped with a highly sensitive probe. Therefore, proper handling is required.
4. Fluctuation in reading might occur due to temperature variations and carbon dioxide absorption.
5. They are more expensive than pH strips.

Precautions

1. Proper pH meter calibration is required by using a buffer solution of known pH.
2. Always store the electrode immerging inside the storage buffer solution after use. Never leave the electrode exposed directly to the air; that might dehydrate it. If by chance the electrode gets dehydrated, soak it overnight in a storage buffer before use, but dehydration often decreases the electrode’s lifespan.
3. Never leave electrodes dirty after use.
4. Always clean the electrode with deionized water and wipe it gently before reading because excessive wiping of storage solution from the electrode might introduce a small charge in the glass electrode that will maximally fluctuate the task.
5. Never immerse the electrode in a sticky solution like milk, agar, etc., because such solutions deposit on the surface of the glass electrode, and the electrode will stop working.
6. Do not store the electrode in deionized water for a more extended time. Because the electrode has ionic properties, when dipped in water, ions leaving the electrode will try to maintain the ionic solution in water to create more equilibrium. This phenomenon will deplete the pH marinating property of the electrode. So in a shortage of storage solution, electrodes can be stored in deionized water but not for a long time.
7. The electrode should be entirely immersed in a storage buffer to avoid dehydration.
8. The pH reading should be done at an appropriate temperature because temperature variations can be inaccurate.
9. Constantly stir the electrodes in a test solution for proper homogenization of the mixture in order to obtain an accurate reading.

References

• pH meter-instrument. IOWA STATE UNIVERSITY. Retrieved on 10th October 2022. Retrieved from https://group.chem.iastate.edu/Holme/augmented-reality-in-educational-laboratories/ph-meter/
• What types of pH meters are there? instrument choice.com.au. Retrieved on 11th October 2022. Retrieved from https://www.instrumentchoice.com.au/news/what-types-of-ph-meters-are-there
• A guide to pH measurement. Retrieved on 9th October 2022. Retrieved from https://www.mt.com/mt_ext_files/Editorial/Generic/1/Guides_to_Electrochemical_Analysis_0x000248ff00025c9a00093c4a_files/guideph.pdf