A #GlassElectrode based #pHmeter helps potentiometric determination of #pH of a liquid by measuring the potential difference between a reference electrode and a measuring electrode i.e. the #GlassElectrode.
Image courtesy: pHmeter.info
Components
High resistance Glass tube: The #GlassElectrode is made up of a high-resistance glass tube that ends at the bottom in a low resistance thin-walled glass #bulb.
Glass bulb: This is filled with a standard electrolyte solution (0.1M #HCl) of constant H+ activity.
Reference electrode: An Ag-AgCl electrode ( #Platinum wire with Ag/ AgCl coating) possessing a definite electrode potential remains immersed in that electrolyte solution and acts as an internal reference that generates constant potential. Electrical connection is made with this electrode by means of a wire passing through the glass tube.
Principle
The #GlassElectrode which is joined to the reference electrode through a #SaltBridge is immersed in sample solution. The lattice work of #Aluminate and #Silicate ions in the membrane of the glass bulb allows only H+ ions to cross that membrane in exclusion of other cations and anions because of their inability to bind to the ion-exchange sites on the glass membrane.
Now, when the sample solution differs from the standard solution in H+ concentration, only H+ ions pass through the glass membrane and generates a #PotentialDifference between two sides of the #ReferenceElectrode. This potential difference is measured with reference to the #ReferenceElectrode using the nullifying #EMF of a #Potentiometer. For each 10-fold change in the H+ ion activity across the glass membrane ( equivalent to pH change of 1 of the sample solution) there will be a potential difference change of 59.2 mV across that membrane.
Merits
Can be used even with #turbid or #colored solutions containing #oxidants or #Reductants.
Requires only a #small volume of the sample solution.
Can be used even with semi-solid samples like #Cheese.
Accuracy with a few hundredths of a pH unit.
No need to #deoxygenate test solution.
Demerits
It can only measure pH from 1 to 11 units with precision within 0.05 pH units.
Presence of N (Normal) or higher concentrations of some metal ions like Na+ and K+ adversely effects the readings of strongly alkaline pH.
#Calibration needed against a series of buffers of known pH.
A vacuum-tube amplifying #Potentiometer is required because of small current through the glass membrane.
Applications
Measurement of pH of liquid to semi-solid solutions.
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