The principles of pH meter

Answer 1

When you place the PH electrode in a PH unknown solution you'll find that an electrical potential difference is produced.

THIS IS DUE TO:

Difference of H+ ion concentration between the test solution and the glass bulb electrode (part of PH electrode that is sensitive to H+ ion concentration)

After that the potential difference of the reference electrode (another part of PH electrode) is measured and compared to the potential of the glass bulb electrode by means of a meter.

results are read from 0-14

The porous glass of the electrode prevents the H+ ions to diffuse from both sides, but allows the exchange of Na+ ions (contained in the silicates of the glass) with the external solution, for keeping the continuity ot this electrical system.

The H+ activity difference between the solution inside the electrode and the external one is measured as electric potential difference, then, converted in pH units (0-14) by the instrument.

Answer 2

pH metre is basically a scale that tells us whether a substance is an acid or a base,,,the pH scale is directly proportional to OH(-) ion and inversely proportional to H(+)ion if substance is acidic the its pH level will be less than 7 and if it is basic then it will have a pH more than 7

Answer 3

The pH meter measures the concentration of hydrogen ions (H+) in an aqueous solution that are responsible of its ACIDITY or ALKALINITY.

It's formed, in its simplest and most used version, by the so-called GLASS ELECTRODE, a tube made by a special semi-porous glass containing an HCl solution with a known and constant concentration (better named as ACTIVITY) of H+ and a silver (Ag) probe covered by AgCl (silver chloride) immerged in it, to keep constant the H+ and Cl- activities in this solution.

Just few mm outside this glass tube, there's another Ag probe, equally immerged in the solution to be measured for its H+ activity and linked with another .Ag/AgCl glass electrode that is the reference, with a constant electric potential.

The porous glass of the electrode prevents the H+ ions to diffuse from both sides, but allows the exchange of Na+ ions (contained in the silicates of the glass) with the external solution, for keeping the continuity ot this electrical system.

The H+ activity difference between the solution inside the electrode and the external one is measured as electric potential difference, then, converted in pH units (0-14) by the instrument.

This electrode is small, also few cm long, easy and fast to use and sensible (+/- 0.001 pH units) and the relation between the potential measured and the H+ activity (a) is expressed by the NERNST

EQUATION, very important in chemistry, where E is the electric potential (electrode/solution):

E = 0.059 log [a0(H+) / a1(H)]

where a0(H+) is the activity of H+ inside the electrode, constant and =1, while a1(H+) is the unknown activity of the solution to be measured.

So, the formula becomes:

E = 0.059 [1 / a1(H+)]

but, following the definition of pH (pH = log 1 /a(H+) ),

this formula is simply

E = 0.059 pH then, pH = E / 0.059

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Answer 4

The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.

The concentrations of hydrogen ions and indirectly hydroxide ions are given by a pH number (1-8). pH is defined as the negative logarithm of the hydrogen ion concentration. The equation is:

pH = - log [H+]

A pH meter is an electronic instrument used to measure the pH (acidity or alkalinity) of a liquid (though special probes are sometimes used to measure the pH of semi-solid substances). A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading.

For the electrode and electrode tip, the voltage (electrical potential) developed across the two electrodes can be measured by a voltmeter and can be related to the pH of the solution. However, because of the high electrical resistance across the glass membranes which produces only a very small current with which to measure the voltage, a rather sophisticated voltmeter must be used. The meter measures the difference between the pH and the reference electrodes or half cells, in millivolts DC. This millivolt reading is read by the unit and displayed as either mV or pH units.

Answer 5

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