Instrumental Methods II Satish Pradhan Dnyanasadhana College, Thane Paper III Unit II Instrumental Methods II Instruments Based on Electrochemical Properties of the Analytes pH Metry By Dr. Bhushan Langi

Principle: pH meter is an instrument which is used to measure pH of the solution and gives a titration curve. During this titration the concentration of H+ ions changed. This change in concentration of H+ ions is measured in terms of pH. pH = - log [H+] A graph of pH against volume of titrant added is plotted and the equivalence point is determine from the curve. pH Volume of Titrant in cm3

There are three types of pH meter. Pocket-sized Testers Handheld Portable Meter Benchtop Meters Economical and Compact for routine, non-critical measurements Provide reportable pH accuracy for the field, manufacturing floor and laboratories Provide best accuracy, more strong data management

Glass Electrode: Principle: The most common electrode used for pH meter is glass electrode. Surface of the glass is protonated by both internal and external solution till equilibrium is reached. Both the side of the glass is are charged by the absorbed protons, this charge is responsible for potential difference. This potential difference is described by the Nernst equation and is directly proportional to the pH difference between solutions on both side of the glass.

Diagram: Construction: Most often used pH electrodes are glass electrodes. Typical model is made of glass tube ended with small glass bulb. Inside of the electrode is usually filled with buffered solution of chlorides in which silver wire covered with silver chloride is immersed. pH of internal solution varies - for example it can be 1.0 (0.1M HCl) or 7.0 (different buffers used by different producers). The majority of pH electrodes available commercially are combination electrodes that have both glass H+ ion sensitive electrode and additional reference electrode conveniently placed in one housing.

Working: Wash the electrode with distilled water and dry it. Connect it to pH meter. Dipped the bulb of the glass electrode in solution whose pH is to be determine take care the bulb is completely dipped in the solution, if not the add little amount of distilled water. pH of the solution is displayed on the screen of pH meter, record it.

Care of Combined Glass Electrode: The bulb of the glass electrode is very delicate hence it is always enclosed within the outer plastic jacket. The dry bulb does not function and hence it is always kept in hydrated by dipping the electrode in distilled water. The combined glass electrode is to be standardised every time before its use by dipping in a buffer solution of pH 7.0 , 4.2 and 9.2 respectively. As the pH of a solution varies with temperature, hence temperature of the solution must be maintained constant.

Application in titrimetry: (Strong acid – strong base) In the strong acid (HCl) against strong base (NaOH), the pH of a known volume of strong acid solution is measured. When Strong base (NaOH) is added to the solution of strong acid (HCl), H+ ions from the acid combine with OH- ions from the base producing unionised water. As a result, H+ ion concentration of the solution steadily decrease and pH gradually increase. However, near equivalence point, there is rapid decrease in H+ ion concentration. Hence the pH is increases rapidly. After the equivalence point, NaOH added remains unreacted which completely gives OH- ions. Hence, pH becomes gradual. Thus S shaped curve is obtained during the titration of strong acid with strong base.

The graph of pH against volume of NaOH solution added. Knowing the volume of alkali required for titration, strength of acid can be calculated. pH Volume of Titrant in cm3

Application in Biological and Environmental Analysis pH measurement of Blood: The pH of the blood is closely related to the actual physiological conditions of living beings. Hence accurate measurement of blood pH is essential to monitor physiological conditions of living being. Hence the pH of the blood is measured at 37oC. pH affects several biological and chemical process in water. Different organisms flourish in specific pH ranges. Most organism prefer pH range 6.5 – 8 for growth. Beyond this pH the physiological system of organism are affected and reproduction is reduced. At low pH, toxic metal and their compounds are taken up by aquatic plants and organisms. All types of acid-base titration can be done by pH meter. The dissociation constants Ka of weak acids like acetic acid can be accurately determined by pH meteric titration. pH meter is also useful for titration of coloured and turbid solutions.

Monitoring of safe water pH conditions: The ideal pH range for water is pH > 6 and pH < 8. A rise or fall in pH of water indicates chemical pollution. 6. Estimation of metal ions in water: Metal ions can be estimated in minute quantities in water by complexometric titration at a constant pH. For e.g. the hardness of water can be determined by titration with Na2EDTA to estimate Ca2+ and Mg2+ using Erichrome Black T indicator at pH =10. Enzymes which are complex proteins synthesised by living organisms catalyse chemical reaction at a particular reactions at a particular pH and at low temperature preferably below 50oC. pH of soil is determined by portable pH meters to estimate the soil acidity.