How to Calculate the pH Valve? Basics & Working of pH Sensor

Defining pH

pH is the numeric representation of gram-equivalent per liter of hydrogen ion concentration in any solution. It varies between 0 to14. It is the logarithmic measurement of moles of hydrogen ions per liter of solution. The solutions having a pH value between 0 to 7 are acidic solutions with a large concentration of hydrogen ions whereas solutions having pH values between 8 to 14 are basic solutions with small hydrogen concentration. The solutions having a pH value of 7 are neutral solutions. Measuring the pH gives the measure of alkalinity or acidity of a solution.

Why is pH measurement essential?

  • To monitor the pH level of blood, which must be between 7.35 and 7.45
  • To monitor the pH level of soil for optimal growth of crops according to the requirements.
  • To monitor the pH of rain so that we can detect the pollutants in the air, if the rainwater becomes more acidic.
  • To monitor the pH of many other daily used products like milk, shampoo, etc.

Three Ways to measure the pH of a solution

  • Using an indicator strip which when placed in a solution, changes its color accordingly. The strip is then taken out and its color is matched with a color on the color chart to decide the corresponding pH value.
  • Using a pH indicator fluid where the unknown solution is added to the fluid and the changed color of the fluid is matched with an already available color on the color wheel to decide the pH value.
  • Using a pH sensor where a probe can be simply inserted inside the solution and pH reading can be done.

5 Advantages of using pH meter over other methods

  • They give more accurate measurements.
  • They can be easily used.
  • The pH reading is easier comparatively.
  • They give more precise measurements as they can measure up to 1/100th of pH unit.
  • They are reusable.

Principle of pH Meter or pH Sensor

pH meter basically works on the fact that an interface of two liquids produces an electric potential which can be measured. In other words, when a liquid inside an enclosure made of glass is placed inside a solution other than that liquid, there exists an electrochemical potential between the two liquids.

pH Sensor Components

It is basically an electrode consisting of 4 components:

  • A measuring electrode: It is a tube made up of glass and consists of a thin glass bulb welded to it, filled up with Potassium Chloride solution of known pH of 7. It also contains a block of silver chloride attached to a silver element. It generates the voltage used to measure pH of the unknown solution.
A Measuring Electrode
A Measuring Electrode
  • A Reference Electrode: It is a glass tube consisting of a potassium chloride solution in intimate contact with a mercury chloride block at the end of the potassium chloride. It is used to provide a stable zero-voltage connection to complete the whole circuit.

    A Reference Electrode
    A Reference Electrode
  • Preamplifier: It is a signal conditioning device and converts the high impedance pH electrode signal to a low impedance signal. It strengthens and stabilizes the signal, making it less susceptible to electrical noise.
  • Transmitter or Analyzer: It is used to display the sensor’s electrical signal and consists of a temperature sensor to compensate for the change in temperature.
Transmitter or Analyzer
Transmitter or Analyzer

Working of pH Sensor:

Working of pH Sensor
Working of pH Sensor

The electrode is placed inside the beaker filled with a solution whose pH is to be measure. The glass bulb welded at the end of the measurement electrode consists of lithium ions doped to it which makes it act as an ion-selective barrier and allows the hydrogen ions from the unknown solution to migrate through the barrier and interacts with the glass, developing an electrochemical potential related to the hydrogen ion concentration. The measurement electrode potential thus changes with the hydrogen ion concentration. On the other hand, the reference electrode potential doesn’t change with the hydrogen ion concentration and provides a stable potential against which the measuring electrode is compared. It consists of a neutral solution that is allowed to exchange ions with the unknown solution through a porous separator, thus forming a low resistance connection to complete the whole circuit. The potential difference between the two electrodes gives a direct measurement of the hydrogen ion concentration or pH of the system and is first preamplified to strengthen it and then given to the voltmeter.

     U = EpH – Eref

EpH – Voltage potential of the measurement electrode

Eref – Voltage potential of a reference electrode

The pH is calculated based on the Nernst equation which states that change in the total potential for every change in pH is

U = -kTpH

k- Boltzmann’s constant, T- temperature.

Precautions in Measurement of pH:

  • The electrode potential of the measurement electrode tends to vary with temperature, which may affect the pH measurement. For this reason, a temperature compensation technique is to be provided. It is achieved either by manually by making a separate temperature measurement and enter the value in the pH meter or by using an automatic temperature compensation, which consists of a temperature sensor fed to the pH meter.
  • Since glass is actually a bad conductor of electricity, the measurement electrode actually provides a high resistance compared to that of the reference electrode, causing a large voltage drop which affects the output measurement. A solution to this problem is by using an amplified meter with high resistance or by using a null balance voltage measurement setup.
  • Hydrofluoric acid should not be used in any solution as it dissolves glass

Modern pH Sensors Available in the Market:

  • PHE-45P: It can measure pH in the full range with a sensitivity of 0.02pH and works in the temperature range of -5 to +95⁰C. It consists of a temperature compensator Pt1000 RTD.
  • WQ201 pH Sensor: It is measured by Global Water Instrumentation Inc and are mounted on 25ft of marine grade cables of length up to 500 ft , with an output of 4-20mA.
WQ201 pH Sensor
WQ201 pH Sensor

Applications Involving pH Sensor

pH Control

pH measurement plays an important role in many industrial applications like controlling chemicals in industrial scrubbers, measuring sulfur dioxide in sugar refineries, and optimizing coagulation in water clarification. It provides a control point for neutralizing acids and bases.

The pH control system is used to measure the pH of the solution and also controls the addition of a neutralizing agent to maintain the solution at a neutral defined pH. It consists of pH analyzer and two or more pH sensors.

pH Control
pH Control

Now that we had a basic knowledge of the pH sensor and its application in pH control, let us know about some other applications of this sensor. If you have furthermore queried on this topic or on the electrical and electronic projects leave the comments below.

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