pH Meter

Introduction

         A pH meter is an electronic instrument used for measuring the pH (Acidity or Alkalinity) of a liquid.

F Arnold O. Beckman developed the commercial pH meter in 1934.

F In chemistry, pH (power of hydrogen) is a numeric scale used to specify the acidity or basic of an aqueous solution. 

F Solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic.

F Pure water is neutral, at pH 7 (25 °C), being neither an acid nor a base.

F pH measurements are important in medicine, biology,  chemistry, agriculture, forestry, food science, environmental science, oceanography, civil engineering, chemical engineering,  nutrition, water treatment and water purification, as well as in many other applications.

F The concept of pH was first introduced by the Danish chemist  Lauritz Sørensen in 1909.

F According to the Carlsberg Foundation, pH stands for "power of hydrogen"

Principle

The pH value of a substance is directly related to the ratio of the hydrogen ion (H+) and hydroxyl ion (OH-) concentrations.

If the hydrogen ion (H+) concentration is higher than hydroxyl ion (OH-) the solution is acidic.

If the hydroxyl ion (OH-) concentration is higher than hydrogen ion (H+) the solution is basic.

pH – 7.0 is neutral, <7.0 is acidic and >7 is basic.

Potentiometric pH meters measure the voltage between two electrodes and display the result converted into the corresponding pH value.

They comprise a simple electronic amplifier and a pair of electrodes, or alternatively a combination electrode, and some form of display calibrated in pH units. 

F It usually consists of three parts

A pH measuring electrode,

A reference electrode

High input meter.

F A pH measuring electrode is a hydrogen ion sensitive glass bulb

F The electrodes, or probes, are inserted into the solution to be tested.

Electrodes design

F Electrodes are rod-like structures - made of glass,

F Contains a sensor bulb like structure at the bottom.

F The glass electrode for measuring the pH has a glass bulb specifically designed to be selective to hydrogen-ion concentration.

F On immersion in the solution to be tested, hydrogen ions in the test solution exchange for other positively charged ions on the glass bulb, creating an electrochemical potential across the bulb.

F  The electronic amplifier detects the difference in electrical potential between the two electrodes generated in the measurement and converts the potential difference to pH units.

F The pH meter is calibrated with solutions of known pH, typically before each use, to ensure accuracy of measurement.

F A glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion.

F Both the electrodes are hollow bulbs containing potassium chloride solution with a silver chloride wire suspended into it.

F The silver chloride electrode is most commonly used as a reference electrode in pH meters

Applications of a pH meter

Ø  Medicine – diagnosis of various disorders in the human body

Ø  Agriculture

Ø  Brewing

Ø  Corrosion prevention

Ø  Dyeing

Ø  Jam and jelly manufacturing

Ø  Printing

Ø  Pharmaceuticals

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