619146781-coulometry and the techniques involved in it.pptx

Coulometry and the techniques involved in the coulometry

1. Coulometry

2. Defintion of coulmetry Coulometry  is an  electrochemical method  in which the total charge (the number of coulombs) consumed in the redox conversion of an analyte at an  electrode  is measured. It differs from electrogravimetry , in that the total quantity of electricity (coulombs) required to cause the analyte to completely react is measured rather than the mass of the electrochemical reaction product. Coulometric procedures are; Rapid The tedious process of drying and weighing the electrode after each elecltrolysis is avoided. as accurate as conventional gravimetric and volumetric procedures readily automated easier and more accurate for small quantities of reagent, capable to detect analyte in a very diluted solution ( a problem related to volumetric analysis)   A further advantage is its inherent simplicity and therefore limited expense . This technique can be used for the quantitative analysis of various  inorganic and organic compounds .

3. Measurements in coulometry The quantity of electricity or charge is measured in units of coulombs (C). • A coulomb is the quantity of charge transported in one second by a constant current of one ampere. • Thus, for a constant current of I amperes for t seconds, the charge in coulombs Q is given by the expression: For a variable current i , the charge is given by:

4. Relation between the quantity of the analyte and quantity of the Charge Faraday's law relates the number of moles of the analyte (N) to the charge (Q). N =

5. Types of Coulometric Methods: 1. Controlled potential coulometry : 2. Controlled current coulometry :

6. Controlled potential ) Potentiostatic ) coulometry ): The working electrode will be kept at constant potential that analyte’s quantitative reduction or oxidation occurs without simultaneously reducing or oxidizing other species in the solution. The current flowing through the cell is proportional to the analyte’s concentration. So the current is initially high but decreases rapidly and approaches zero at the completion of the reaction. quantity of electricity is most commonly measured with an electronic integrator or from the reading of a coulometer in series with the cell.

7. Controlled Potential Coulometry ( Potentiostatic Coulometry ) Principle An analysis of this type is similar to electrogravimetric method, with the advantage that the weighing of a precipitate is avoided. Hence, controlled potential technique can therefore be applied to systems that yield deposits with poor physical properties (antimony) and to reactions that do not give solid products at all. For example, arsenic may be determined coulometrically by the electrolytic oxidation of arsenous acid (H3AsO3,) to arsenic acid (H3As04) at a platinum anode. Similarly, the analytical conversion of iron( lI ) to iron( lII ) can be accomplished with suitable control of the anode potential.

8. Instrumentation The instrumentation for potentiostatic coulometry consists of a potentiostat (to maintain a constant potential), an electrolysis cell ( composed of reference electrode, working and counter( auxilary )-electrodes, a chemical coulometer or an integrating device for determining the number of coulombs and placed in series with the working electrode.

9. Application of Controlled potential coulometric methods Inorganic As many as 55 elements of the periodic table can be determined by the cathodic reduction of metal ions to metallic state. Most of the metals (about two dozen element) can form amalgams with mercury, and hence controlled potential coulometry with mercury cathode is usually preferred. Micro analysis: This technique is especially useful for the determination of small amounts of analyte (0.01 – l mg) with an accuracy of (± 0.5 %). Advantageous over volumetric analysis Determination of several metal ions in the same solution is possible with controlled potential electrolysis using mercury pool cathode. Analysis of radioactive materials such as uranium and pluotinum Elctrolytic synthesis of new organic compounds Determination of n-values of the reaction which is a rout to deduce the kinetics and mechanism of the overall reactions.

10. Example A 0.3619-g sample of tetrachloropicolinic acid, C6HNO2CI4 (260.89 g/ mol ), is dissolved in distilled water, transferred to a 1000-ml, volumetric flask, and diluted to volume. An exhaustive controlled-potential electrolysis of a 10.00-mL portion of this solution at a spongy silver cathode requires 5.374 C of charge. What is the value of n for this reduction reaction? N numbers of moles of solute into 10 ml solution = (0.3619 / 260.89)* 0.01 = .0000139 moles

11. quiz The purity of a sample of picric acid, C 6 H 3 N 3 O 7 , is det e r m ined b y con t ro l l ed - p ot e n ti a l co u lo m e t r y , converting the picric acid to triaminophenol C 6 H 9 N 3 O ( reduction required 18 electrons). A 0.2917-g sample of picric acid is placed in a 1000- mL volumetric flask and diluted to volume. A 10.00- mL portion of this solution is transferred to a coulometric cell and diluted till the Pt cathode is immersed. The exhaustive electrolysis of the sample requires 21.67 C of charge. Report the purity of the picric acid.

12. Constant Current Coulometry In constant current coulometry , the current is maintained constant till the completion of the analytical reaction. The quantity of electricity required to attain the end point is derived from the amount of coulombs consumed. constant current coulometry has two advantages over controlled-potential coulometry : First, using a constant current  makes the process  more  rapid  since the current does not decrease over time . It is estimated that a  typical analysis time  for constant current   coulometry  is less than 10  min. On other hand,   controlled-potential  coulometry takes  30–60 min  for.   With  a constant current the total charge  is directly calculated from the the  product of current and time

13. Primary constant current coulometry : In this technique, the element to be estimated undergoes direct reaction at the electrode (oxidation or reduction) with 100% efficiency. Example: Constant current of 0.800 A (amps.) used to deposit Cu at the cathode and O 2 at anode of an electrolytic cell for 15.2 minutes. What quantity in grams is formed for each product? Sol ½ cell reactions: Cu2+ + 2e- Cu (s) (cathode) 2H2O 4e- + O2 + 4H+ (anode) To solve: Q = i.t Q = (0.800 A)(15.2 min) (60 sec/min) Q = 729.6 C ( amp.sec ) amount Cu produced in g = (729.6 / 2*96500)* 63.5 = 0.240 g Cu amount of O2 produced in g= (729.6 / 4*96500)* 32 = 0.0605 g O2

14. Secondary constant current coulometry coulometric titration      In a coulometric titration the titrant is quantitatively generated at an electrode which then stoichiometrically reacts with the analyte ion to be estimated.   Coulometric  titrations are in many ways similar to volumetric titrations:  the concentration of the titrant is equivalent to the generating current (electrons transferred) and the volume of the titrant is equivalent to the generating time . An example is the titration of halides by silver ions produced at a silver anode.

15. Argentometric titration of chloride Volumetric vs coulmetry . e.g. At the generator electrode (anode) Ag (s) Ag + + e - (oxidation of silver to silver ion) At the cathode: Possible reaction 2H + + 2e - H 2 (g) (hydrogen evolution) Therefore, need sintered glass to separate the hydrogen gas generated in thecathode , to prevent reactions with the “titration species” such as Ag + .

16. Typical coulometric titration cell an electrochemical cell consisting of a working electrode and a counter electrode. The working electrode is constructed from Pt, is also called the generator electrode since it is where the mediator reacts to generate the species reacting with the analyte . The counter electrode is isolated from the analytical solution by a salt bridge or porous frit to prevent its electrolysis products from reacting with the analyte . The other necessary instrumental component for controlled-current coulometry is an accurate clock for measuring the electrolysis time, te , and a switch for starting and stopping the electrolysis.

17. Method for the external generation of oxidizing and reducing agents in coulomtric titration

18. Comparison of Coulometric and Volumetric Titrations Both require a detectable end point and are subject to a titration error as a consequence regarding the apparatus and solutions employed: The timer and switch correspond closely to the buret , the switch performing the same function as a stopcock. • Coulometry , advantages is the elimination of problems associated with the preparation, standardization, and storage of standard solutions. (instability of Br, Cl and Ti ) • With coulometry , by proper choice of current, microquantities of a substance can be introduced with ease and accuracy the coulometric method adapts easily to automatic titrations, because current can be controlled quite easily.

19. Example The purity of a sample of Na2S2O3 was determined by a coulometric redox titration using I- as a mediator, and 13 - as the "titrant“. A sample weighing 0.1342 g is transferred to a 100-mL volumetric flask and diluted to volume with distilled water. A 10.00-mL portion is transferred to an electrochemical cell along with 25 ml, of 1 M KI, 75 mL of a pH 7.0 phosphate buffer, and several drops of a starch indicator solution. Electrolysis at a constant current of 36.45 mA required 221.8 s to reach the starch indicator end point. Determine the purity of the sample.

20. Sol of the example NA (number of moles)= Q/ nF Wt of Na2O3 = Q* FM Na2O3/ nF , n=1