Introduction:

Some of the Acid-Base titrations are quite difficult to realize ordinarily via the utilization of visual indicators for one of various reasons. The analyst might be color-blind to a specific indicator color change; it could be that no appropriate color change exists for a specific kind of titration or the solutions themselves might be colored, opaque or turbid. It might be desired to automate a sequence of replicate determinations. In such conditions, Potentiometric titration, employing a glass hydronium ion selective electrode, an appropriate reference electrode and a sensitive potentiometer (a pH meter) might be beneficial.

Potentiometer:

Electro-analytical chemistry comprises of the field of chemistry which uses the relationship between the chemical phenomena that comprise charge transfer (example: redox reactions, ion separation and so on) and the electrical properties which accompany such phenomena for some analytical determination. The two generally employed instrument for making the potential measurement are potentiometer and pH meter.

A potentiometer is the instrument for measuring the potential in a circuit. Before the introduction of the moving coil and digital volt meters, potentiometers were employed in measuring the voltage, therefore the '-meter' part of their name. The process was illustrated by Johann Christian Poggendorff around the year 1841 and became a standard laboratory measuring method. Potentiometer is employed for measurements of low resistance circuits. The potentiometer operates via connecting a known voltage source to the cell whose voltage is to be computed by a sensitive galvanometer in between and adjusting the source voltage till it equivalents the cell voltage. This takes place whenever no current flows via the galvanometer. This is accomplished by means of a side wire that differs the fraction of the known source voltage applied to the cell. The potential can then be read from the known source voltage. The sensitivity of the potentiometer is associated to the sensitivity of the galvanometer and the cell resistance.

PH-Meter:

pH meter is the voltameter which transforms the unknown voltage to current which is later amplified and read out. The pH meter is a null-type device as is the potentiometer. The electrometer draws extremely small currents and is best appropriated for irreversible reactions which are slow to establish the equilibrium. They are as well needed for high-resistance electrodes, similar to glass pH or ion-selective.

Fig: Scheme of typical pH glass electrode

Modern pH probe comprises of an electrode that joins both the glass and reference electrodes to one body. The combination electrode comprises of the given parts as illustrated in the figure above:

1) A sensing portion of electrode, a bulb made from the specific glass.

2) Internal electrode, generally silver chloride electrode or calomel electrode.

3) Internal solution, generally a pH = 7 buffered solution of 0.1 mol/L KCl for pH electrodes or 0.1 mol/L MeCl for pMe electrodes

4) Whenever employing the silver chloride electrode, a small quantity of AgCl can precipitate within the glass electrode.

5) Reference electrode, generally the similar kind as internal electrode.

6) Reference internal solution, generally of 0.1 mol/L KCl

7) Junction with studied solution generally made up from ceramics or capillary by asbestos or quartz fiber.

8) Body of electrode, made up from non-conductive glass or plastics.

The bottom of a pH electrode balloons out to a round thin glass bulb. The pH electrode is best thought of as the tube in a tube. The inside most tube (that is, the inner tube) includes an unchanging 1 x 10^-7 mol/L HCl solution. As well inside the inner tube is the cathode terminus of the reference probe. The anodic terminus covers itself around the outside of the inner tube and ends by the similar sort of reference probe as was on the inside of the inner tube. This is filled by a reference solution of 0.1 mol/L KCl and consists of contact with the solution on the outside of the pH probe by way of a porous plug which serves up as a salt bridge.

Glass Membrane Electrode:  

Glass membrane electrode is a kind of electrode which is generally utilized for measuring the pH of a given solution. This is categorized into solid state membrane and liquid membrane electrode

Solid state membrane:

This electrode comprises of silver-silver chloride in the reference solution of hydrochloric acid include in a glass membrane. The membrane is made up of a special glass, generally a hydrated aluminosilicate having sodium or calcium ions. This electrode is selectively permeable to the hydrogen ions and the potential which develops across the membrane hydrogen ions of the tested solution compared by the reference acid solution in the electrode. The potential can be evaluated against the reference calomel electrode by employing a high sensitive voltmeter. In pH measurement, calibration of the instrument is significant and can be accomplished by either buffer whose pH has been formerly measured. However for measurement over a range of pH values, it is essential to standardize the instrument on at least two standard buffer solutions that covers the requisite range. The simplest solid state membrane is designed to compute the test ions. On the other hand, the test substance might comprise in one or two reactions on the surface of the electrode that change the activity of the mobile stage.

Table: Membrane materials and interfering ions:

Test ion     Membrane material    Major interfering ions

Fluoride                LaF                       I^- Br^- Cl^-

Chloride           AgCl/Ag₂S                   S^- I^-

Bromide           AgBr/Ag₂S                   S^- I^-                                                        

Iodide               AgI/ Ag₂S                     S^-                                                 

Sulphide           AgI/ Ag₂S                  Hg⁺ Ag⁺

Cupric              Ag₂S/CuS                  Hg⁺ Ag⁺

Lead                 Ag₂S/PbS                  Hg⁺ Ag⁺

Liquid-Membrane Electrodes:

Liquid membrane electrode is formed of an ion-selective material dissolved in the solvent which is not miscible by water. The liquid is held in the porous inert that allows contact between the test solution on one side and the reference electrolyte on the other. The ions from the reference solution will partition themselves among the two immiscible solvent, giving the electrode a specific potential. The presence of the test ion in the sample influences the activity of the reference ions in the membrane resultant in a change in the potential difference across the membrane.

Illustrations of liquid membrane electrode are:

Test ion               Membrane material

Potassium           Valinomycin in diphenyl ether

Ammonium          Macrotetrolides in tris phosphate

Calcium               Calcium dialkylphosphate

Potentiometric Measurements:

Potentiometric titration is a method identical to direct titration of the redox reaction. No indicator is employed; rather the potential across the analyte, usually an electrolyte solution is evaluated. To do this, two electrodes are utilized, an indicator electrode and a reference electrode. The indicator electrode makes an electrochemical half cell by the interested ions in the test solution. The reference electrode makes the other half cell, holding a consistent electrical potential. The total electric potential is computed as: 

E_cell = E_ind - E_ref + E_sol   

E_sol is the potential drop over the test solution between the two electrodes. E_cell is recorded at intervals as the titrant is added. The graph of potential against volume added can be drawn and the end point of the reaction is half way between the jumps in voltage.

E_cell based on the concentration of the interested ions by which the indicator electrode is in contact. For illustration, the electrode reaction might be

Mⁿ⁺ + ne^- → M

As the concentration of Mn⁺ modifies; the E_cell changes likewise. Therefore the Potentiometric titration comprises measurement of E_cell by the addition of titrant.

For direct Potentiometric measurement in which the activity of one ion is to be computed from potential of the indicating electrode, the potential of the reference electrode is illustrated by the equation:

E_cell = k - (2.303RT/nF) log a_red/a_ox

Here, 'k' is constant determined via measuring the potential of a standard solution in which the activity is recognized.

Whenever the ionic strength is maintained constant at the similar value, the activity coefficients of the test solution remains constant at all the concentration of the substance.

Then concentration can be found out from measured cell potentials.

E = E^o - (2.303RT/nF) log C_red/C_ox

Here, E⁰ = the standard electrode potential

R = the gas constant

F = the Faraday constant

n = the number of electrons involved

T = the absolute temperature

For measurements made up at 250C the equation is simplified as:

E = E^o - (0.059/n) log a

Kinds of Potentiometric titration: acid-base titration (that is, total alkalinity and total acidity), redox titration (that is, HI/HY and cerate), precipitation titration (that is, halides) and Complexometric titration.

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