pH Meter- Definition, Principle, Parts, Types, Uses

The hydrogen ion concentration of any solution determines its pH value, and the pH value further tells about the power of hydrogen. Since the pH scale is designed by using the dissociation of water and the water dissociates into H+ and OH- components, thus to study the pH, it is important to study the relevance of the dissociation of water

H+ concentration in water is 1 x 10 power minus 7, which is neither acidic nor basic and thus known as neutral. Now, if the strength of H+ concentration is more than this, then it is acidic and if it is less than this, thus it is basic. This implies the relationship between the concentration of H+ ions and pH value which are inversely proportional to each other.

Thus, the pH scale ranges from 1 to 14 where 1 is a strong acid, 7 is neutral and 14 is a strong base. And, change in pH by 1 unit, implies a change in hydrogen ion concentration by 10 times. Thus the pH scale is logarithmic.

pH Scale
pH Scale

Conventionally, pH was measured using pH paper or pH indicators such as litmus paper but the measurement was not very precise and accurate. So, for precise and accurate reading of pH, a pH meter is designed that works on potentiometric analysis, which means that the pH meter measures the potential difference between two electrodes present in which one has a constant potential and the other changes its potential according to the solution, and due to flow of electron in both the solution, the current is generated which is dictated using a voltmeter, and this all combines to give the pH meter it’s reading.

pH Meter is an instrument used to measure the reading pH precisely and accurately.

Working principle of pH Meter

The pH meter is based on the Nernst equation, according to which there is a relation between electric voltage and ion concentration, and the pH meter, utilizes this relation to calculate the pH of the sample solution. Thus, the working principle of the pH meter is based on potentiometry analysis since the pH meter is made up of a reference electrode and a glass electrode. 

The potential of the reference electrode is constant and does not change with the change in the sample solution, but the glass electrode changes the potential with the change in the sample solution and is made up of a glass membrane that is sensitive to hydrogen ion concentration of test sample solution. Thus, the pH meter utilizes the difference in potential between the reference and the glass electrode, and after applying the Nernst equation to this, it calculates the difference to use to measure the hydrogen ion concentration, which indirectly gives the pH of the solution. And with the change in the potential difference between the glass electrode and the reference electrode, there is a moment in the electron which generates the current, and this is dictated by a voltmeter. 

Thus the combined potential difference, Ph, and generated current give the final pH of the solution. For example, the standard potential difference of 1 pH is 59.15 mV at 25 degrees Celsius, and this relationship gives the difference the value of 1 pH unit, which is 59.15 mV and hence used to measure pH.

Parts/components of pH Meter

  1. pH electrode: 
  • It is made up of special construction glass, which is used to sense the intensity of hydrogen ions.
  • The construction glass present in this is made up of alkali metal ions which have the occurrence of ion-exchange reaction between the alkali metal ions of the glass and hydrogen ions present in the solution, thus generating a potential difference.
  • pH probe is of two types, including pH electrode and reference electrode.
  • The solution in these contains hydrogen ions in free form, which helps in potential generations at the junction site.
  • Electrodes are designed in such a way to determine and measure the aqueous media.
  • There are junctions present in electrodes that release the internal fill solution of the measuring electrode into the solution that is needed to be measured. 
  • Types of electrodes: 
    • Glass electrode
    • Reference electrode
    • Combination gel electrode
  1. Reference electrode: It is a silver chloride electrode.
  2. Internal electrode: made up of silver chloride.
  3. Glass bulb: It is a sensor used for H+ ion concentration sensing and contains 0.1M HCL and is made from a special type of glass and membrane.
  4. Internal solution 1: It is also made up of silver chloride, which is immersed into a buffer solution of 0.1 mol/L KCL of pH 7.
  5. Internal solution 2: It is used to immerse the reference electrode and is made up of buffer solution containing 0.1 mol/L KCL of pH 7.
  6. Junction: aka ceramic junction or diaphragm, which allows the contact of sample solution and reference electrolyte. During this process, the electric connection between both the electrodes do not disturb.
  7. Filling hole: used for refilling the electrolyte.
  8. Electrode body: the body is made from non-conductive glass or plastic.
pH Meter Parts
pH Meter Parts. Image Source: Pharmaguddu.com

Types of pH meters

  1. Based on portability:
  • Portable pH meter: normal extensively used pH meter with additional compact DC power equipment.
  • Desktop pH meter: same as the previous one.
  • Pen pH meter: it is a single scale, traditional measurement range, easy to use, and handy equipment. 
  1. Based on usage:
  • Laboratory pH meter: multi-functional, higher accuracy and precision, wider measurement range
  • Industrial online pH meter: very stable, steady, environment-friendly, higher efficiency, digitalized equipment with upper and lower boundaries and control functions.
  1. Based on modernization:
  • Economical pH meter
  • Intelligent pH meter
  • Precision pH meter, which is further divided into digital and pointer pH meter.
Types of pH meters
Types of pH meters. Image Source: electricalfundablog.com

Operating Procedure of pH Meter

  1. Switch on the pH meter. After turning it on, the MEAS annunciator and ATC indicator will display on the screen.
  2. Gently clean the electrode with distilled water and tap dry it. 
  3. Maintain the temperature of the sample at 25 degrees Celsius for appropriate reading.
  4. Immerse the electrode fully into the sample and simultaneously stir the sample for even distribution of it.
  5. Wait for the reading to get stabilized.
  6. After the reading top fluctuates, press the READY indicator, which will get activated. Now stop the reading from fluctuating by pressing the HOLD button and then press ENTER to save the reading.
  7. Record the reading and temperature.
  8. When done, again wash the electrode with deionized water and again store it in a storage buffer solution.

Procedure for calibration of pH Meter

  1. Wash the electrode with distilled water and gently tap dry it.
  2. Bring all samples and buffers at 25 degrees Celsius.
  3. First, immerse the buffer in a solution of pH 7 or pH 10. Make sure that the entire electrode is immersed in it. 
  4. Slightly stir the buffer solution to create the homogeneous mixture.
  5. Enter the CAL/MEAS button the start the calibration. 
  6. CAL sign will start displaying.
  7. The primary display will show the measured reading, and the smaller secondary screen will display the pH of the buffer solution.
  8. Wait for the pH to get stabilize.
  9. Press HOLD/ENTER to stop reading for further fluctuation.
  10. Now the meter is calibrated.
  11. Finally, wash the electrode with distilled water and store it in a storage buffer solution.

Uses/ Applications of pH Meter

  • To measure the pH of the soil, a pH meter is used in the agricultural industry.
  • Use to estimate the water quality for municipal water supplies like swimming pools.
  • It has applications in the chemical and pharmaceutical industries, where it is used to measure the pH of solutions.
  • Applications in the food industry where it is majorly used for dairy products like cheese, curds, yogurts, etc.
  • In the making of detergents.

Advantages of pH Meter

  • It is the fastest and most simple way to measure the pH.
  • High accuracy and precision.
  • It can be employed for various applications.
  • It covers both the acidic and alkaline range of pH from 1 to 14.
  • Easy calibration method without expertise using three standard buffer solutions such as pH 4, pH 7, and pH 10)
  • The pH meter is portable and user-friendly.
  • Do not require high installation charges.
  • Cost-effective and can be maintained by average-funded laboratories.
  • More accurate than pH strips and pH indicators.
  • Battery-operated pH meters are a great choice for a specific location with power issues.

Limitations of pH Meter

  • Need regular calibration.
  • Deposit of sticky material; can interrupt the process.
  • Equipped with the highly sensitive probe.
  • Require sophisticated handling.
  • Specific buffer solutions are required for calibration.
  • Readings may vary due to the fluctuation in temperature and absorption of carbon dioxide.
  • The fragile instrument thus should be handled with care.
  • Cost is higher than pH strips.

Precautions

  1. Never store the electrode without the storage solution containing the buffer, as it will dehydrate the solution. If, by chance, the electrode gets dehydrated, then soak it overnight in the storage buffer to regain hydration, but often dehydration decreases the lifespan of the electrode.
  2. Always cleanse the electrode with deionized water and gently tap dry it every time one wants to take the reading because excessive wiping of the storage solution from the electrode can introduce a small static charge on the surface of the electrode glass, which will deviate the reading maximally.
  3. Never leave your electrode dirty.
  4. Never immerse the electrode into sticky material such as agar, milk, fermenting wine, etc., because they will deposit on its outer surface, and the electrode will stop working.
  5. Storing electrodes into deionized water for a longer time. Because of the ionic properties of the electrode, the ion leaving the electrode will try to maintain an ionic solution in water, which can create more equilibrium. That will deplete the pH marinating properties of the electrode. If one will run out of storage solution, then it is advised to store the electrode in deionized water but not for a very long duration.
  6. The electrode should be calibrated often.
  7. Proper calibration of the electrode is most important. To properly calibrate the pH meter, the buffer solution of known pH is required. 
  8. The electrode should be fully immersed into the storage solution other not, proper immersion leaves a part of the electrode dehydrated, which will make it useless.
  9. Testing or reading should be taken at an appropriate temperature because temperature fluctuation can change the reading.

References

  1. https://microbiologynote.com/ph-meter-definition-principle/
  2. https://blog.thermoworks.com/thermometer/ph-meter-care-and-common-mistakes/
  3. https://in.omega.com/prodinfo/ph-meter.html
  4. https://biokimicroki.com/working-principle-and-construction-of-ph-meter/
  5. https://chrominfo.blogspot.com/2019/04/advantages-and-disadvantages-of-ph-meter.html

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