potentiometry

pH is a Unit of Measurement

pH = Power of Hydrogen (H+) Defined as the Negative Logarithm of Hydrogen Ion Activity

pH = log (1/H+)

Used for Determining the Acidity or Alkalinity of an Aqueous Solution

Practical pH Scale for Industrial Instrumentation 0 - 14 pH

pH Hydrogen Ion (H+) Hydroxyl Ion (OH-)Acid 0 1 0.00000000000001

1 0.1 0.00000000000012 0.01 0.0000000000013 0.001 0.000000000014 0.0001 0.00000000015 0.00001 0.0000000016 0.000001 0.00000001

Neutral 7 0.0000001 0.00000018 0.00000001 0.0000019 0.000000001 0.00001

10 0.0000000001 0.000111 0.00000000001 0.00112 0.000000000001 0.0113 0.0000000000001 0.1

Alkaline 14 0.00000000000001 1

Potentiometry is a field of electro-analytical chemistry in which potential is measured under the condition of no current flow.

The measured potential may be used to determine the concentration of some component of the analyte solution.

The Measuring System consists of a pH Measuring Electrode (indicating electrode) and Reference Electrode

The Potential Difference Between the Two Electrodes is a Function of the pH Value of the Measured Solution.

The Analyte Solution Must Be Conductive and is Part of the Electrical Circuit.

• pH Measuring Electrode:Purpose is to Develop a Millivolt Potential Directly Proportional to the Free Hydrogen Ion Concentration in an Aqueous Solution.

• Reference Cell:-A half-cell with an accurately known electrode

potential, that is independent of the concentration of the analyte or any other ions in the solution.

-Purpose is to Maintain a Constant Reference Potential.

• Reference Cell Liquid Junction:-Purpose is to Maintain Electrical Contact

Between the Reference Electrode and the Measuring Electrode by way of the Solution.

-Potassium chloride is a nearly ideal electrolyte for the salt bridge because the mobilities of the K+ion and the Cl － ion are nearly equal

Ideal indicator electrode:

–responds rapidly and reproducibly to changes in the concentration of an analyte ion (or groups of analyte ions).

Three types:

Metal electrodes.Carbon electrode.Ion-selective electrodes

Combined PH glass electrodepH measurement system

Working principle of pH glass membrane electrode

Principle of pH meterPrinciple of pH meter

• The glass membrane itself is not limited to any configuration or shape; its only requirement is contact with solution.

• Therefore, different styles of glass electrodes have evolved to maximize pH sensing ability and extend longevity in some of the more difficult applications.

Is a technique similar to direct titration of a redox reaction.

No indicator is used, (instead the potential across the analyte) typically an electrolyte solution is measured.

To do this, two electrodes are used, an indicator electrode and a reference electrode.

The indicator electrode forms an electrochemical half cell with the interested ions in the test solution.

The reference electrode forms the other half cell, holding a consistent electrical potential.

It is used when the endpoints are very difficult to determine either when:

1- very diluted solution.

2-coloured and turbid solution.

3-absence of a suitable indicator.

PH meterReference electrode. Indicator electrode.Voltmeter which measures the potential difference.

Magnetic stirrer.hot plate ( use the stirrer and make sure heat is off ).magnet capsule .

•Glass (burrete and beaker)

of 0.1 N NaOH

Potentiometric titration curve of (0.1 N) NaOH against ??)N ( acetic acid:

Do not place the electrodes anywhere except attached to the electrode holder.

At the end of the experiment rinse the electrodes and place each one in its housing as required.

Never touch the glass part of the electrode with anything except soft tissue paper. While in use, the bulb of the glass electrode should be immersed in solution.

For short-term storage the combined glass electrode should be immersed in solution of 2 M KCl.

Buffer solutions of known pH are used for the pH calibration.

For high accuracy, calibration and measurements are to be performed at the same temperature.

IMORTANT TO KNOW

1-the outside surface of the glass membrane is in contact with the sample being measured.

2-and the inside surface contact the inner filling solution.

3-A complex mechanism at each glass liquid interface defines the potential of the pH glass electrode.

4-While the pH inner glass /filling solution potential is constant , the outside potential varies based on the hydrogen ion concentration in the sample .