Determination of pH using pH meter

 Determination of pH using pH meter

ELECTROMETRIC DETERMINATION OF pH

Many cases indicators are not a good tool for pH measurement. Such situations call for electrometric determination of pH. Its done usually by pH meter. pH meter consists of a glass electrode which when dipped into a solution develops an electrical potential depending upon the hydrogen ion concentration and this potential is read off the display which is calibrated into a pH scale.


Electrode Potentials

 When a strip of metal (electrode) is dipped into water. It tends to dissolve owing to its solution pressure. P. While the atoms of metal go into solution. they leave behind their loosely bound valence electrons as a negative charge on the electrode. Since the electrode has developed a negative charge. it now attracts the positively charged metal ions which have already gone into solution. Some of the positively charged metal ions might reattach themselves to the negatively charged electrode. The tendency of the atoms to leave the metal and the tendency of metal ions to reattach themselves finally becomes balanced and an equilibrium is reached. At equilibrium some positively charged metal ions remain in solution and the opposite charge of the electrode and the metal ions gives rise to a potential difference. Let us now alter the conditions a bit by dipping the metal into a solution of one of its salts. This is an altogether different situation because there are many metal ions already in solution. These metal ions will oppose the separation of more metal ions from the electrode and an equilibrium will be achieved early. The point of equilibrium in this case will be dependent upon the relative values of the two opposing forces: the solution pressure (P) of the metal and the osmotic pressure (p) of the metal ions in solution. Three possibilities arise

P = Solution pressure of the metal; p = Osmotic pressure of the metal.

(i)             P > p. Since the solution pressure is higher. the atoms from the metal strip continue to dissolve as positive ions till the accumulated charge is strong enough to oppose further dissolution. The metal strip in this situation acquires a negative charge as compared to the solution which becomes relatively positive. (ii) P < p. The opposite takes place. Since the osmotic pressure of metal ions in solution is higher. they attach themselves to the metal strip increasing the positive charge on the electrode. The solution becomes relatively negative. (iii) P = p. Since both the opposing forces are equal, metal ions neither leave the electrode nor the metal ions in solution get attached to the electrode. No potential develops.

Taking thermodynamic reasoning into consideration, the potential difference between a metal and a solution of one of its salts is given by Nerst equation

 The potential difference across the glass membrane (inside and outside) is calculated by Nernst (Nobel prize, 1920), the equation being RT/nF x 2.3log { [H+]i/[H+]o where R= gas constant; T= absolute temperature, n=valency, F= Faraday, [H+]i = concentration of hydrogen ions inside the cell and [H+]o = concentration outside, that of the unknown solution

 Measured pH is the difference in [H+] between the reference buffer inside the probe and the sample solution. An electrode potential is generated across a selectively permeable membrane separating two different concentrations of an ion.

Instrumentation

 

A pH meter is consisted of three different parts: an internal electrode, a reference electrode, and a high input impedance meter. Glass probe often contains the two electrodes -- internal electrode and reference electrode.

1. Reference electrodes

 

Various reference electrodes are in use . eg:  The Hydrogen Electrode , The hydrogen electrode consists of a piece of platinum foil dipped in IM solution of hydrogen ions. The solution used to provide this hydrogen ion activity is 1.18 MHCI. 

The Calomel Reference Electrode : The electrode consists of a strip of platinum sealed into glass and allowed to dip into mercury. A paste of calomel (Hg CI , 0.1 M.) is held against the mercury by means of a sintered glass plug or cotton wool. The whole electrode is filled with saturated KCI. Contact with an outside electrolyte is maintained through a porous ceramic plug.

The Silver/Silver Chloride Electrode: The internal electrode is a Silver wire covered with Silver Chloride (Ag/AgCl wire), and reference electrode is often made up of the same materials. It is immersed in a saturated potassium chloride solution.

2.     THE GLASS ELECTRODE

 The measurement of pH by glass electrode involves the use of two reference electrodes, separated by a glass membrane whose function is to establish an electrical potential depending upon the hydrogen ion activity of the solution being tested. It consists of a high resistance glass tube with a thin low resistance glass bulb fused at the bottom. The bulb is responsible for the pH sensitivity; the rest of the electrode is insensitive to [H+). The tube is filled up with 0.1 N solution of HCl. Dipping in this solution is a silver/silver chloride electrode which connects the solution to the input part of the instrument. The other reference electrode might be a calomel electrode. but in most current instruments this electrode also is an Ag/ AgCI electrode. When both electrodes are dipped into a sample. the resulting e.m.f. gives the pH of the solution. The glass electrode can be combined with an external reference electrode. In other words, the reference electrode can be built in the same unit.

 

Diagram of a Glass electrode

Diagram of a combination electrode of reference and glass electrodes


The pH measurements are made by comparing the pH reading of a sample solution to that of a reference solution with defined pH, such as buffers. Therefore, it is important to calibrate the instrument with appropriate buffer solutions before making any measurements. Standard buffer solutions are available for pH 4, pH 7 and pH 9.2. Calibration can be done either for acidic and neutral range or for neutral and basic range. Temperature of pH solutions should be at room temperature  while doing the measurements and the thermostat of pH meter also should be set at that pH.



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