pH meter Bushra mubarak

pH pH is defined as the negative logarithm of the hydrogen ion activity aH. Unit of measure which describes the degree of acidity or alkalinity of a solution. pH=-log[H+]

pH value The pH value of a substance is directly related to the ratio of the hydrogen ion and hydroxyl ion concentrations. If the H+ concentration is higher than OH- the material is acidic. If the OH- concentration is higher than H+ the material is basic.

The addition of acid to water increases the concentration of hydrogen ions and reduces the concentration of hydroxyl ions The addition of a base would increase the concentration of hydroxyl ions and decrease the concentration of hydrogen ions

Determination of pH A rough indication of pH can be obtained using pH papers or indicators, which change color as pH level varies. These indicators have limitations on their accuracy, and can be difficult to interpret correctly in colored or murky samples. More accurate pH measurements are obtained with a pH meter.

pH meter A pH meter is an electronic device used for measuring the pH (acidity or alkalinity) of a liquid (though special probes are sometimes used to measure the pH of semi-solid substances).

Parts of pH meter A high input impedance meter pH-sensitive electrode Reference electrode

pH-sensitive electrode Thin glass membrane whose outside surface contacts the solution to be tested. The inside surface of the glass membrane is exposed to a constant concentration of hydrogen ions (0.1 M HCl). Inside the glass electrode assembly, a silver wire, coated with silver chloride and immersed in the HCl solution, is called an Ag/AgCl electrode.

This electrode carries current through the half-cell reaction. potential between the electrode and the solution depends on the chloride ion concentration.

Reference electrode A common choice is to use another Ag/AgCl electrode as the reference. Ag/AgCl electrode is immersed in an 0.1 M KCl solution which makes contact with the test solution through a porous fiber which allows a small flow of ions back and forth to conduct the current. The potential created at this junction between the KCl solution and the test solution is nearly zero and nearly unaffected by anything in the solution, including hydrogen ions.

Working principle A pH meter is essentially a voltmeter which measures the voltage of an electrode sensitive to the hydrogen ion concentration, relative to another electrode which exhibits a constant voltage. This is translated into pH by the instrument and reading is displayed.

TWO POINT CALIBRATION also called bracketing calibration. Calibration with two buffer solution standards is usually performed every time a pH meter is used. One of the buffers has a pH of 7.01 (almost neutral pH) and the second buffer solution is selected to match the pH range in which the measurements are to be taken: usually pH for basic solutions and pH 4.01 for acidic solutions.

Multi-point calibration To achieve the best possible accuracy, the calibration should cover the range of the desired measurement values. More advanced pH meters will let the user calibrate at three, or five and even higher numbers of pH values. This will expand pH measurement range without the need of recalibrating

Types of pH meter