pH Measurement theory and Electrodes

pH Measurement
The measurement of pH using a pH Electrode is an age old method that centres around
the perception of a substance as acidic or alkaline and is dependent on the concentration
of the hydrogen Ion (H+) within the substance.

The use of a pH electrode allows this measurement to be expressed in a meaningful way.
Derived from the Sorensen Equation the pH value is defined as a negative logarithm of the H+ concentration in a given solution.
A high H+ Concentration equals: 1mol/L = 10o pH = 0 (ACIDIC)
A low H+ Concentration equals: 10-14 mol/L pH=14 (ALKALINE)
It is therefore simple to measure the pH of a substance and compare it with other substances. pH 0 is extremely acidic, pH 14 is extremely alkaline and pH 7 neutral.
Measuring pH Values
Measuring the pH of a substance requires the use of a pH Electrode and a Reference Electrode. The pH Glass at the end of the electrode acts as the sensing part of the circuit.

The second part of the circuit is the reference electrode. This is a stable point that has a defined potential and is independent of the solution to be measured. The reference electrode fig is made up of a reference element that is commonly a Silver/Silver Chloride wire encased in a known electrolyte. The reference electrode then has a junction, which is the contact between the stable internal reference electrode and the solution to be measured. This is commonly a porous ceramic pin.

The evolution of pH electrodes came with the joining of the two separate electrodes to produce the COMBINATION pH ELECTRODE. The formation of the combination electrode can be seen below. The formation of the combination electrode still has the pH Glass membrane acting in the same way. The reference electrode is continually encased around the pH electrode. As the pH Glass comes into contact with an aqueous substance to measure, a gel layer forms on the membrane. This also happens on the inside of the glass layer. The pH value of the aqueous solution will either force Hydrogen Ions out of the Gel layer or into this layer. The Internal buffer in the glass electrode has a constant pH value and this keeps the potential at the inner surface of the membrane constant. The membrane potential is therefore the difference between the inner and outer charge. If you then factor in the reference electrode with its stable potential you have a combination pH electrode that encapsulates the measuring electrode and reference electrode.

P&R Labpak are able to offer a huge range of electrodes and through it’s electrode supplier Sentek can offer their equivalents which are less expensive and yet are the same or better quality.  Contact us on 0870 034 2055 or e-mail us at sales@prlabs.co.uk with your electrode enquiries.

 
How the pH Electrode Works
As the pH Glass comes into contact with an aqueous substance to measure, a gel layer forms on the membrane.
This also happens on the inside of the glass layer.
The pH value of the aqueous solution will either force Hydrogen Ions out of the Gel layer or into this layer.
The Internal buffer in the glass electrode has a constant pH value and this keeps the potential at the inner
surface of the membrane constant.

The membrane potential is therefore the difference between the inner and outer charge.
If you then factor in the reference electrode with its stable potential you have a combination
pH electrode that encapsulates the measuring electrode and reference electrode.

Visit www.prlabs.co.uk for more information or to contact us.  Check out our news pages for special offers – www.prlabs.co.uk/news

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Posted on February 6, 2012, in Useful Information and tagged , , , , , . Bookmark the permalink. Leave a comment.

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