Potentiometery - Pharmaceutical Analysis 1 B. Pharma 1st semester

Potentiometery

CONTENTS

• Indicator electrodes

• Quinhydrone electrode

• Antimony electrode

• Glass electrode

• Potentiometric titrations

• Apparatus and requirements

• Method of detecting end point

• Applications

• Potentiometry

• Principle involved

• Components of potentiometry

• Reference electrodes

• Saturated calomel electrode

• Silver/Silver Chloride electrode

• Standard hydrogen electrode

OBJECTIVE

By the end of this session, students will be able to:

• Explain the construction and working of quinhydrone electrode

• Explain the construction and working of glass electrode

• Discuss the pros and cons of potentiometric titrations

• Brief the applications of potentiometry

• Describe Electrochemistry

• Explain the principle involved in potentiometric titrations

• Outline the ideal characteristics of ideal reference electrode

• Explain the construction and working of saturated calomel electrode

• Explain the construction and working of standard hydrogen electrode

Indicator Electrodes

• To measure potential of a solution, an appropriate electrode is needed

• Indicator electrode measures potential when connected to a suitable reference electrode

• Electrode which is used to measure potential or pH of a solution is called as indicator electrode

Types of indicator electrode used are:

• Hydrogen electrode

• Quinhydrone electrode

• Antimony electrode

• Glass electrode

Hydrogen Electrode

• Construction is similar to Normal Hydrogen Electrode (NHE)

• Electrode is similar to NHE

• Electrode is responsive to hydrogen ion concentration

• Can be used for pH measurement

• Platinum of electrode do not take part in electrochemical reaction

• Only acts as site for transfer of electrons

• Potential of hydrogen electron is

• E H+, H2 = E0(H+, H2= a) – 0.0591/n (H+)

                    = 0.0591 pH, as E0(H+, H2= a) = 0

Quinhydrone Electrode

• Electrode is used in measuring pH of a given solution

• Consists of a bright platinum wire dipping into a solution saturated with quinhydrone

• In solution, quinhydrone dissociates into equimolecular quantities of quinone and hydroquinone

• pH of a solution can be determined by a redox system

• Because reduction to hydroquinone or oxidation to benzoquinone involves H+ ions

Pros

• Attains equilibrium rapidly

• Can be used where hydrogen electrode is unsuitable

• Gives accurate results

Cons

• Cannot be used in more alkaline solution, whose pH is above 8

• Gets readily oxidized by air in alkaline medium

Antimony Electrode

• Used in pH measurements

• Consists of rod coated or covered with antimony trioxide

• Electrode is prepared by placing a stick or a rod of antimony metal covered with thin crust of its oxide

• It is dipped into a solution whose pH is to be measured

• Electrical connection is made with reference electrode like saturated calomel electrode

• Reaction involved is

• Sb + H2O === SbO+ + 2H+ + 3e

• Sb2O3 + 6H+ + 6e === 2Sb(s) + 3H2O

Pros

• Can be used for measuring pH from 2 to 8

• Can be used in viscous or turbid solvents

• It is sturdy- very useful where continuous pH recording are made

• Do not get readily poisoned

Cons

• Cannot be used in measuring pH below 3, as oxide gets dissolved

• Cannot be used in the presence of strong oxidizing and complexing agents

• Cannot be used in the presence of metals such as Cu, Ag, Au which are more noble than antimony (i.e., below in electromotive series)

• It suffers from salt error

Glass Electrode

• Very useful electrode for determination of hydrogen ion (or pH) of a solution

• It involves no electron exchange but allows transfer of H+ ions through its membrane

• Consists of very thin bulb made from glass membrane having high electrical conductivity

• Bulb contains hydrochloric acid solution of definite concentration and or Ag/AgCl wire to make electrical contract with it

• Thin membrane of bulb is prepared from special type of glass, sensitive and permeable to H+ ions

• Glass membrane is made from soft soda-lime glass containing lithium silicate with lanthanum and barium ions added to it

• Sensitivity  of  membrane  of  protons  and  other  cations  depend  upon  the composition of glass membrane

• Corning glass contains 22% Na2O, 6% CaO and 72% SiO2

• Membrane of glass is specific to H+ ions upto pH 9.0 above which it is somewhat responsive to Na+ and other charged cations

• Potential of glass electrode is given by

• E = k + 0.0592 (pH1-pH2) at 25 0C

• Where k = constant for the electrode

• pH1 = pH of the solution inside the bulb

• pH2 = pH of the test solution (outside)

• E = k + 0.0592 pH1 – 0.0592 pH2

• E = K – 0.0592 pH2

• Since pH1 = constant, k + 0.0592 pH1 is also another constant

Pros

• Gives a rapid response

• It is chemically resistant to oxidizing and reducing agents, dissolved gases, colloids and salts, etc

• Can be used over the entire pH range when lithia-silica glasses are used

Cons

• Extremely fragile- contains a very thin bulb

• Minute abrasions on the surface of tip, damages electrode

Note

• Never be allowed to remain dry

•  All  glass  electrodes  must  be  conditioned  by  soaking  in  water  or  saturated potassium chloride or dilute acid buffer solution

Potentiometric Titrations

• Performed for the solutions which show changes in potential or pH by addition of a reagent or titrant

• Here the absolute value of potential with respect to standard half-cell is not required to be known

• Changes occurring during the course of addition of titrant are sufficient

• Equivalence point of reaction is shown by sudden change in potential on a plot of emf readings against the volume of titrant being added

• Knowledge  of  actual  potential  of  reference  electrode  need  not  be  known accurately

• Any electrode which will furnish a constant potential is useful as a reference electrode

• Provided its potential remains constant throughout the titrations

Apparatus and Requirements

• Can be done manually or under automation

• If it is manually, a beaker with stirrer and a pipette are sufficient

• In case of automated models,

• A sample cell which can hold a pair of electrode

• Inlet for titrant

• Stirrer for mixing the solution are essential

• Pair of electrodes depends on the type of titration i.e., acid-base or redox

• In most titrations, saturated calomel electrode is used as reference electrode

• Indicator electrode varies with respect to titrant

Potentiometric Titrations

• Suitable indicator electrode is used

• For example, glass electrode for acid-base titrations

• Platinum for redox titrations

Pros

• Colored solutions, dilute solutions or turbid suspensions can be titrated

• Actual potential of reference electrode need not be known

• Titration can be automated

• Inexpensive method with more accuracy

Method of Detecting End Point

• When indicator method is not suitable, we use potentiometric method for determining end point

• A normal titration curve i.e. a plot of

• emf vs volume of titrant or

• pH vs volume of titrant

• First derivative curve i.e. a plot of ΔE/ Δv or ΔpH/ Δv vs volume of titrant

• Second derivative curve i.e. a plot of Δ2E/Δv2 or Δ2pH/Δv2 vs volume of titrant

• In a potentiometric titration, at the end point

• Rate of change of potential is maximum

Applications

• Acid-base titrations

• Redox titrations

• Diazotisation titrations

• Precipitation titrations

• Complexometric titrations

• Dead stop end point technique

Applications of Potentiometry

• Analysts make more potentiometric measurements than any other chemical instrumental measurement

• Manufacturers measure the pH of many consumer products

• Clinical laboratories determine blood gases as important indicators of disease states

• Industrial and municipal effluents are monitored continuously to determine pH and concentration of pollutants

• Oceanographers determine carbon dioxide and other related variables in sea water

• Also used in fundamental studies to determine equilibrium constants such as Ka, Kb

• Thousands of applications for potentiometric measurements are available

Electrochemistry

• Analyst always ask questions such as

• What is it?

• How much of it is present?

• How fast does it change?

• Electrochemistry is ideal analytical tool for answering these questions

• Can be termed as electroanalysis

Potentiometry

• Determines the potential of electrochemical cells- usually at zero current

• Potential of electrode responds to change in concentration of species under study

• With respect to standard reference electrode

• Most common potentiometric methods used by analyst employ pH meters

• Relatively cheap to perform

• But can be slow and tedious unless automated

• Electrochemical cell comprises of two electrodes

• Indicator electrode- voltage of which solely depends on concentration of one specific component in the solution

• Reference electrode- voltage of which must be absolutely independent of the nature and composition of solution

Nernst Equation

• The potential of a metal electrode at 25 0C immersed into a solution of its own ions is given by

E = E0 + 0.0592/n log c

• Where E0 – standard potential of the metal

• n- valency of ions

• c- concentration of ions

Components of a Potentiometric Cell

1.   Reference electrode

2.   Salt bridge

3.   Analyte

4.   Indicator electrode

Potentiometry

• Electrodes are relatively free from interferences

• Provides rapid, convenient and nondestructive means for quantitative determination of numerous important anions and cations

• Placing together of these two electrodes on a solution gives rise to an electrochemical cell

•  From  this  voltage  generated  across  the  electrodes  may  be  determined  by connecting to a potentiometer

• Instrument should be sensitive to measure ± 0.2mV

• Under these experimental parameters, the e.m.f. of the cell may be expressed as

• Ecell = E+ - E- + Ej

• Ej is the e.m.f. at the liquid junction

Reference Electrodes

Ideal reference electrode-

• Has a potential that is accurately known and constant

• Completely insensitive to composition of the analyte solution

• Electrode should be rugged

• Easy to assemble

• Should maintain a constant potential while passing minimal currents

• For example, calomel reference electrodes

• Consists of mercury in contact with a solution that is saturated with mercury chloride (calomel) and also contains a known concentration of potassium chloride

Calomel Reference Electrode

• Calomel half-cells can be represented as

• Hg|Hg2Cl2 (saturated), KCl(xM)||

• X represents the molar concentration of potassium chloride in the solution

• Electrode potential for this half-cell is determined by the reaction

• Hg2Cl2(s) + 2e- ó 2Hg + (l) + 2Cl-(aq)

• Calomel electrode is commercially available as

• H-shape body of the electrode is made of glass

• Right arm of electrode contains a platinum electrical contact

• Small  quantity  of  Hg|Hg2Cl2  paste  in  saturated  potassium  chloride  and  few crystals of potassium chloride

• The tube is filled with saturated KCl to act as a salt bridge through a piece of porous Vycor (thirsty glass) sealed in the end of left arm

Saturated Calomel Electrode

Silver/Silver Chloride Reference Electrode

• Most widely marketed reference electrode

• Consists of a silver electrode immersed in a solution of potassium chloride that has been saturated with silver chloride

• Represented as Ag|AgCl(saturated), KCl(saturated)||

• Electrode potential is determined by the half-reaction

• AgCl(s) +  e- ó Ag(s) + Cl-

• Electrode is prepared with either a saturated or 3.5M potassium chloride solution

• It is a glass tubing with narrow opening at the bottom connected to a Vycor plug for making contact with analyte solution

• Tube contains a silver wire coated with a layer of silver chloride that is immersed in a potassium chloride solution saturated with silver chloride

• Advantage- can be used at temperature greater than 60⁰C, while calomel electrodes cannot

• Mercury ions react with fewer sample components

• Such reactions can lead to plugging of the junction between electrolyte and the analyte solution

Standard Hydrogen Electrode

• Hydrogen gas electrode

• Pt(H2(1 atm), H+ (1M)

Construction:

• Consists of a glass tube having holes at its bottom

• Inside this tube is another glass tube having a platinum or copper wire with a platinum foil

• Surface of platinum foil is coated electrically with platinum black

• Pure hydrogen gas at 1 atmospheric pressure is passed through opening of glass tube

• And it escapes through small holes at the bottom of the electrode

• Electrode is dipped in a solution of standard acid like hydrochloric acid of unit activity (1.8 M of HCl at 25 0C)

• Some hydrogen gas is absorbed by platinum black of electrode and it permits the exchange from gaseous to ionic of hydrogen

• Also reverse the process to occur without any obstacle

•  Under fixed conditions of pressure of hydrogen gas passed and hydrogen ions in solution  with  contact  of  electrode,  hydrogen  electrode  possess  a  definite potential

• By convention, this potential is taken as zero at all temperature

• When normal hydrogen electrode is connected to any electrode through salt bridge, potential of that electrode can be measured

• For example, zinc electrode (zinc metal rod in contact with a solution of zinc ion) is connected by potassium chloride salt bridge, can be represented as

• Pt, H2 |H+ (a=1)|| Zn+2 || Zn

• Where a is unknown

• Cell reaction is

• H2 + Zn⁺² à  2H+ (a=1) + Zn

• Similar way, we can measure the potential of any electrode

• Half-cell reactions of electrode are written as

• Mn+ + ne- === M or

• Zn+2 + 2e- === Zn, for zinc E0 = 0.76 volts

• Electrode potential E is equal to standard potential E0, when activity of Mn+ is equal to unity (1M solution)

• Potential of a hydrogen electrode depends upon the pH value of a solution which it is contact

• pH  value  of  a  given  solution  can  be  calculated  by  combining  with  normal hydrogen  electrode  (NHE)  or  with  any  standard  reference  electrode  like saturated calomel electrode

• Combined cell reaction can be represented as,

• Pt, H2 (1 atm), H+_unknown ||KCl_(sat)Hg2Cl2_(s) , Hg

• E of above reaction is E_cell = E_{calomel (sat}) - E_hydrogen

Advantages:

• Fundamental electrode and is used as standard in pH measurements

• Can be used over wide pH range

• Exhibits no salt error

• Establishes equilibrium rapidly and gives accurate results

Disadvantages

• Cannot be used in solutions containing strong oxidizing or reducing agents

• Cannot  be  used  in  solutions  containing  metal  ions  that  below  hydrogen  in potential series

• Interaction  with  hydrogen  will  occur  and  the  metal  will  be  deposited  on electrode surface

• Gets readily poisoned by number of substances like proteins, tannins, mercury salts, etc

• Cumbersome to prepare and use in routine analysis

• Because of its cumbersomeness, it is replaced by other standard electrodes

SUMMARY

• Types of indicator electrodes used

• Hydrogen electrode, Quinhydrone electrode, Antimony electrode, Glass electrode

• Equivalence point of reaction is shown by sudden change in potential on a plot of emf readings against the volume of titrant being added

• In most titrations, saturated calomel electrode is used as reference electrode

• When indicator method is not suitable, we use potentiometric method for determining end point

• Analysts make more potentiometric measurements than any other chemical instrumental measurement

• Determines the potential of electrochemical cells- usually at zero current

• Potential of electrode responds to change in concentration of species under study

• Most common potentiometric methods used by analyst employ pH meters

• Relatively cheap to perform

• Indicator electrode- voltage of which solely depends on concentration of one specific component in the solution

• Reference electrode- voltage of which must be absolutely independent of the nature and composition of solution

• Nernst equation-  E = E0 + 0.0592/n log c