pH Meter Principle and Working: Types, Calibration, and Operation

pH Meter principle:

A pH meter is a scientific instrument that measures the hydrogen-ion activity in solutions, indicating its acidity or basicity (alkalinity) expressed as pH value. The principle of the pH meter is the concentration of hydrogen ions in the solution e.g. it is the negative logarithm of a hydrogen ion. The pH range of solutions varies between 1 to 14, where 1 is the highest in acidic nature, and 14 is the highest in alkalinity

Importance of pH meter in the pharma industry

pH meter is a unique instrument that is equally used in the Quality control and Production department of a pharmaceutical formulation company. In the laboratory, it is used for various purposes like checking the pH of raw materials in their analysis and measuring the pH of liquid oral finished products. It is also used to prepare various reagents. Similarly, a pH meter is also used in the production department of each company to maintain the pH of the product during the manufacturing process, for the preparation of some buffers for different purposes we need a reliable pH meter that provides factual readings.

Potentiometric pH meter:

The pH meter measures the difference in electrical potential between a pH electrode and the reference electrode, which is why the pH meter is sometimes referred to as a “potentiometric pH meter”.

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pH meter diagram:

Working of pH meters

A pH meter measures the potential difference between a reference electrode and a glass electrode that responds to the hydrogen ion concentration in a subjected solution. The glass electrode is designed in such a way that it has a very thin glass membrane through which ions present in the solution can pass, and generate an electrical potential. The instrument compares the electrical potential to the reference electrode that has a stable potential. The difference between the two potentials is directly proportional to the pH of the solution.

Preparation of Standard Buffer

Buffer Solution pH 4.00 (20^o C Transfer the content of the buffer capsule or tablet pH 4.00 into a 100 ml volumetric flask. Dissolve in about 80 ml of purified water, make up the volume to 100 ml with purified water & mix.

Buffer Solution pH 7.00 (20^oCTransfer the content of the buffer capsule or tablet pH 7.00 into a 100 ml volumetric flask. Dissolve in about 80 ml of purified water, make up the volume to 100 ml with purified water & mix.

Buffer Solution pH 9.20 (20^o C) Transfer the content of the buffer capsule or tablet pH 9.2 into a 100 ml volumetric flask. Dissolve in about 80 ml of purified water, make up the volume to 100 ml with purified water & mix.

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pH Electrode

pH electrode is a main device that is used to measure the pH of a solution by dipping it in the solution. It consists of a glass bulb filled with a certain type of electrolyte and is joined with an electrode supplying voltage to it. It has a sensitivity to feel the changes in hydrogen ion concentration present in the solution, then produce an electrical potential that is directly proportional to the pH of the solution.

Calibration of pH meter:

Operate the pH meter and electrode system according to the manufacturer’s instructions or according to the applicable SOPs. All measurements should be made at the same temperature of 20° to 25°. The apparatus is calibrated with the buffer solution of potassium hydrogen phthalate (primary standard) (buffer pH 4.0) and one other buffer solution of different pH, preferably buffer pH 9.2. The measured pH of a third buffer pH 7.0 must not differ by more than 0.05. A pH meter is a very sensitive instrument that requires a high level of care and maintenance. It also requires frequent calibration to ensure accurate and reliable pH readings. The calibration process is done by measuring the pH of some known pH buffer standard solutions. The pH meter is tuned to reference readings provided by the buffer solution.  There are different types of calibration methods used in industries. Different types of calibration methods are like single-point calibration, two-point calibration, and three-point calibration. The calibration process is performed on a regular basis. 

Calibration Procedure step by step:

• The instrument is calibrated to pH 4, 7, or 9.2 but remembers to calibrate with pH 7 first.
• Dip the electrode in a standard Buffer Solution of 7.00 pH value. 
• Measure the temperature of the solution and place the temperature knob accordingly.
• Bring the Function Switch of pH Mode.
• Adjust the “Calibrate” control so that the display reads 7.00
• Now again turn the Function Switch into Standby Mode.
• Remove the electrode from the 7 pH buffer solution and wash it with distilled water, soak & dry it.
• Put the electrode in 4 pH buffer solutions.
• Bring the Function Switch in pH Mode and Adjust the “Slope %” (Right side of the instrument) so
• that the display reads 4.00.
• Remove the electrode from the 4 pH buffer solution and wash it with distilled water.

Always keep the Function Switch in standby Mode after measuring the pH value.

Frequency of calibration of pH Meter:

Calibration of the pH meter is a very important function that should be performed every day before
performing any test on the pH meter.

Procedure and operation of pH Meter

• Ensure the temperature of the Liquid being examined to 20^o-25^oC.
• Immerse the glass electrode in the liquid to be examined.
• Turn off the knobs to pH Checking & note.
• When measuring the pH above 10, ensure that the electrode is suitable for use under alkaline conditions & apply any correction that is necessary.
• Record the pH of the solution used to standardize the meter and electrodes at the end of a set of measurements. 
• If the difference between this reading and the original value is greater than 0.05, the set of measurements must be repeated.

Types of pH meters

• Benchtop pH meters
• Portable pH meters
• Pen-style pH meters
• Continuous pH meters
• Microprocessor-based pH meters
• Combination pH meters

Benchtop pH meters: 

Benchtop pH meters are the most commonly used pH meters in laboratories. They are the most accurate, and reliable, and have a wide range of measurements. They are designed in such a way that they can measure the temperature of the solution which is necessary for accurate pH readings. The pH at room temperature is known as its normal pH. They are bulky in size and require a high-voltage power source. They are very expensive in comparison to other types of pH meters. 

Portable pH meters:

As the name suggests the use of this type of pH meter can use anywhere and is easy to move. They are operated on battery so they are very handy for use. If you have fieldwork then Portable pH meters are best for you. However, these are not as accurate as Benchtop pH meters are. Their measurement range is not so wide, and they can not measure the temperature of the solution while measuring the pH. 

Pen-style pH meters:

Pen-style pH meters are compact in size and are handheld pH meters just like pens in shape, they are operated by a small in-built battery. They are very easy to use. For field work, they are ideal and easy for quick measurement. They are not as reliable as Benchtop pH meters. Their range of pH measurement is very limited. Pen-style pH meters are not suitable for temperature measurement of solutions. 

Continuous pH meters:

If you need regular checks of the pH of a solution during a process then continuous pH meters are the best choice for you. These pH meters continuously measure the pH of a solution. They are ideal for industrial use like in pharma manufacturing units. They can be integrated with a control system. They are extremely accurate and reliable in comparison of Pen-style and Portable pH meters. They are very costly and require a regular maintenance and calibration.

Microprocessor-based pH meter:

 A pH meter with a microprocessor in its hardware to perform calculations and the ability to store data. These pH meters are very accurate and reliable. These are very easy to use. Some of these pH meters also have an auto-calibration option. They have features of data logging and multiple measurement modes. However, these are very expensive in comparison to the rest of the types of pH meters.

Combination pH meters: 

 The pH meter with multiple functions like conductivity measurement dissolved oxygen measurement and temperature measurement is known as a Combination pH meter. They are very handy in high-tech laboratories and R&D centers. These are very high-cost instruments.

Usage of pH Meters in Different Industries:

pH meters are used in the following industries

• pH meters are used in the Pharmaceutical industry in both the Quality control and Production department.
• Food and Beverages to ensure shelf-life and product safety.
• In water treatment, the pH of water is very important to ensure the quality of water. 
• Agriculture, and its associated industries. In the agriculture industry, the pH meter is used to measure the pH of soil. 

Top 5 pH Meters Available in the Market

There are various pH Meters available in the Market, depending on the requirements of the industry. 

1. Hanna Instruments HI98128 pH ep 5.:
2. Milwaukee MW102 PH/Temperature Meter
3. Oakton Eco Testr pH 2 Waterproof pH Tester
4. Lab Junction pH Meter, Digital pH Meter MINI 2 POINT CALIBRATION LJ-141 
5. GABY INSTRUMENTS Plastic Digital TDS/EC/PH/Salinity/Temperature Combo Meter
6. Multi-Functional Water Quality Tester

Acid-base Equilibrium:

It is known as the balance of concentrations between Hydrogen ions (H+) and Hydrogen ions(-OH) in the solution. The referred balance measures the pH of the solution and decides the nature of the solution whether it is acid or base.

Troubleshooting for pH Meters:

If you find an error in the pH meter while performing tests then you must know the ways of troubleshooting for pH meters. Usually, some common problems in pH meters include electrode contamination, some calibration errors, and meter malfunctions. Some troubleshooting instructions are listed below:

• Check the electrode: First of all check the electrode, if it is found dirty, clean it with tissue paper or a smooth cloth piece. Then clean it with distilled water. Check if it is damaged or cracked then replace it. Check for any air bubbles or moisture in the electrode.
• Check the buffer solutions: Buffer solutions are used to calibrate the pH meters and to get accurate results. If the buffer is contaminated and found expired life then replace it with freshly prepared buffer solution. 
• Check the temperature: The temperature of a solution is a very big factor in the output readings of pH.If the temperature is not in range then it can result in wrong readings. Use the thermometer to check the temperature of the solution then adjust the temperature to room temperature.
• Check the connection: If the instrument is not responding, then check the cables and connectors and make sure that they are connected properly. If the connections are loose and damaged they can result in inaccurate readings.
• Check the power source: All pH meters require a constant power source to perform tests. If the meter does not get on or behaves erratically then check the batteries or power supply. 
• Check the reference electrode: The reference electrode is a very important part of a pH meter. If the reference electrode is contaminated or broken, it can cause misleading readings. Clean or replace the reference electrode if it is necessary.
• Check the calibration: If none of the above steps resolve your problem, then you should recalibrate the instrument with fresh buffer solutions. For more instructions follow the manual for calibration.

Frequently Asked Questions:

SUMMARY:

pH meter is an essential part of the chemical industry, Food Industry, Pharmaceutical Industry, and
Agriculture sectors. It allows us to know the nature of the solvent and therefore provides insights to take
further actions accordingly. If a product has low pH then the manufacturer can raise its pH with some
alkali solutions. However, the pH meter looks like a very simple scientific instrument but its functionality
puts it at the top position of Chemical-physics instruments. pH meter is a physics instrument but its
working is chemistry oriented. If you find useful the pH meter principle working, kindly share the article.

References:

1. https://www.bbc.co.uk/bitesize-What useful products can be made from acids?
2. Instrumentchoice.com.au–How to use soil pH meters(and why it matters)?
3. Harris, D. C. (2010). Quantitative chemical analysis (Vol. 7). Macmillan.
4. Skoog, D. A., West, D. M., Holler, F. J., & Crouch, S. R. (2013). Fundamentals of analytical chemistry. Cengage Learning.
5. Atkins, P. W., & de Paula, J. (2010). Atkins’ physical chemistry. Oxford University Press.
6. Svehla, G. (1999). Vogel’s textbook of quantitative chemical analysis (Vol. 5). Longman.
7. Sawyer, D. T., Sobkowiak, A., & Roberts, J. L. (1995). Electrochemistry for chemists. Wiley.
8. Bard, A. J., & Faulkner, L. R. (2000). Electrochemical methods: fundamentals and applications (Vol. 2). Wiley.
9. Harris, D. C. (1997). Quantitative chemical analysis (Vol. 3). WH Freeman.
10. Bakker, E. (Ed.). (2006). Analytical electrochemistry (Vol. 18). Springer.
11. Dickert, F. L. (Ed.). (2012). Chemical sensors: an introduction for scientists and engineers. Springer Science & Business Media.
12. Mikkelsen, S. R. (Ed.). (1995). Handbook of biosensors and electronic noses: medicine, food, and the environment. CRC press.

Video pH Meter: working and Principle By Dr. Sanjay Swami