The Detailed Theory and instrumentation of Both Amperometry and Biamperometric analysis is given with Titration curves and Applications.
Medha Thakur (M.Sc Chemistry)
Potentiometry is an electrochemical method of Analysis deals with the measurement of electric potential or emf of an electrolyte solution under the condition of constant current.
Potentiometry is the measurement of electrical potential of an electrolyte solution to determine its concentration.
The principle is based on the fact that the potential of the given sample is directly proportional to the concentration of its electro active ions or its activity (pH)
When the pair of electrodes is placed in the sample solution it shows the potential difference by the addition of the titrant or by the change in the concentration of the ions.
The theory of potentiometry is based on the nernst equation.It gives the basic relationship between the potential generated by an electrochemical cell and the concentration of the ions.
The potential E ( Half cell potential) of any electrode is given by nernst equation
Improvement of the electric power quality usingVikram Rawani
A control algorithm for a three-phase hybrid power
filter is proposed.It is constituted by a series active filter and a passive filter connected in parallel with the load.
The Detailed Theory and instrumentation of Both Amperometry and Biamperometric analysis is given with Titration curves and Applications.
Medha Thakur (M.Sc Chemistry)
Potentiometry is an electrochemical method of Analysis deals with the measurement of electric potential or emf of an electrolyte solution under the condition of constant current.
Potentiometry is the measurement of electrical potential of an electrolyte solution to determine its concentration.
The principle is based on the fact that the potential of the given sample is directly proportional to the concentration of its electro active ions or its activity (pH)
When the pair of electrodes is placed in the sample solution it shows the potential difference by the addition of the titrant or by the change in the concentration of the ions.
The theory of potentiometry is based on the nernst equation.It gives the basic relationship between the potential generated by an electrochemical cell and the concentration of the ions.
The potential E ( Half cell potential) of any electrode is given by nernst equation
Improvement of the electric power quality usingVikram Rawani
A control algorithm for a three-phase hybrid power
filter is proposed.It is constituted by a series active filter and a passive filter connected in parallel with the load.
The study made in this paper concerns the use of the voltage-oriented control (VOC) of three-phase pulse width modulation (PWM) rectifier with constant switching frequency. This control method, called voltage-oriented controlwith space vector modulation (VOC-SVM). The proposed control scheme has been founded on the transformation between stationary (α-β) and and synchronously rotating (d-q) coordinate system, it is based on two cascaded control loops so that a fast inner loop controls the grid current and an external loop DC-link voltage, while the DC-bus voltage is maintained at the desired level and ansured the unity power factor operation. So, the stable state performance and robustness against the load’s disturbance of PWM rectifiers are boths improved. The proposed scheme has been implemented and simulated in MATLAB/Simulink environment. The control system of the VOC-SVM strategy has been built based on dSPACE system with DS1104 controller board. The results obtained show the validity of the model and its control method. Compared with the conventional SPWM method, the VOC-SVM ensures high performance and fast transient response.
A robust state of charge estimation for multiple models of lead acid battery ...journalBEEI
An accurate estimation technique of the state of charge (SOC) of batteries is an essential task of the battery management system. The adaptive Kalman filter (AEKF) has been used as an obsever to investigate the SOC estimation effectiveness. Therefore, The SOC is a reflexion of the chemistry of the cell which it is the key parameter for the battery management system. It is very complex to monitor the SOC and control the internal states of the cell. Three battery models are proposed and their state space models have been established, their parameters were identified by applying the least square method. However, the SOC estimation accuracy of the battery depends on the model and the efficiency of the algorithm. In this paper, AEKF technique is presented to estimate the SOC of Lead acid battery. The experimental data is used to identify the parameters of the three models and used to build different open circuit voltage–state of charge (OCV-SOC) functions relationship. The results shows that the SOC estimation based-model which has been built by hight order RC model can effectively limit the error, hence guaranty the accuracy and robustness.
Supercapacitors and Battery power management for Hybrid Vehicle Applications ...Pradeep Avanigadda
This paper presents supercapacitors and battery association methodology for ECCE Hybrid
vehicle. ECCE is an experimental Hybrid Vehicle developed at L2ES Laboratory in collaboration with
the Research Center in Electrical Engineering and Electronics in Belfort (CREEBEL) and other French
partners. This test bench has currently lead-acid batteries with a rated voltage of 540 V, two motors
each one coupled with one alternator. The alternators are feeding a DC-bus by rectifers. The main
objective of this paper is to study the management of the energy provides by two supercapacitor
packs. Each supercapacitors module is made of 108 cells with a maximum voltage of 270V. This
experimental test bench is carried out for studies and innovating tests for the Hybrid Vehicle
applications. The multi boost and multi full bridge converter topologies are studied to define the best
topology for the embarked power management. The authors propose a good power management
strategy by using the multi boost and the multi full bridge converter topologies. The experimental and
simulation results of the two converter topologies are presented.
This paper was published by my former Supervisor and involves partly my calculations and the concepts used during my MSci Thesis at University College London.
This paper presents the three-phase CHB inverter fed induction motor suitable for renewable energy source applications. Normally, all present existing multilevel inverters produce multilevel output, but the number of components required is more, bulk in size, more in cost. Which are more burdens to small capacity renewable sources. These challenges are eliminated in CHB inverter. This CHB mainly consisting of one DC source, one capacitor and eight switches in each phase. To generate a five-level output in phase to ground voltage, it is required to maintain the capacitor voltage (V2) at fifty percent of the DC source voltage (V1). This capacitor voltage is regulated by a sensor less voltage regulating technique. The sensor less voltage regulation works without any sensor devices. We can implement this technique with very less cost compared to other techniques. The sensor less voltage regulation is realized by level-shifted sinusoidal pulse width modulation. The simulation results show a very good dynamic performance. Controller maintains the capacitor voltage at fifty percent of the source voltage irrespective of main source voltage changes and load changes. Inverter generates a five-level wave at the output from line to ground and seven-level wave from line to line with fewer Harmonic. It is implemented in matlab/simulink and showing good dynamic performance.
In this article, we have proposed a new control of a PV system connected to the grid. The goal is
to reduce current and voltage harmonicsfor increasing the quality of delivered energy. First, we have
modeled a PV panel. Then we have dimensioned the BOOST converter by finding L and C values. Next,
we have used Perturb and Observe (P&O) Maximum Power Point Control (MPPT) to improve energy
efficiency. Finally, We have developed a control of single-phase H-bridge inverter in order to eliminate the
3rd,5th,7th and 9th harmonics order, and added an LCLTo connect the PV inverter to the grid, an LCL
betweenthe inverter and the grid. Theperformance of the proposed system was tested by computing
spectrum and THD usingMatlab/Simulink software. The proposed architecture provides better Total
Harmonic Distortion (THD) which satisfy the EN50160 requirement the THD must be less than 4.66%. We
found that THD was decreased from 61.93% to 0.04%.
Certainly! The **basic principle of coulometry** involves passing a known electrical charge through a solution containing the analyte. Coulometry can be used to determine the amount of a substance in a solution, the purity of a compound, or the kinetics of an electrochemical reaction¹[3] ²[4]. It is a valuable technique in analytical electrochemistry for precision measurements of charge and is named after Charles-Augustin de Coulomb³[2]. One useful application of coulometry is determining the number of electrons involved in a redox reaction, which can be achieved through controlled-potential coulometric analysis using a known amount of a pure compound.
Auto tuning of frequency on wireless power transfer for an electric vehicleIJECEIAES
In these days, electric vehicles are enthusiastically researched as a countermeasure to air pollution, although these do not have practicality compared to gasoline-powered vehicles. The aim of this study is to transport energy wirelessly and efficiently to an electric vehicle. To accomplish this, we focused on frequency of an alternating current (AC) power supply, and suggested a method which determined the value of it constantly. In particular, a wireless power transfer circuit and a lithium-ion battery in an electric vehicle were expressed with an equivalent circuit, and efficiency of energy transfer was calculated. Furthermore, the optimal frequency which maximizes efficiency was found, and the behavior of voltage was demonstrated on a secondary circuit. Finally, we could obtain the larger electromotive force at the secondary inductor than an input voltage.
International Journal of Engineering Research and DevelopmentIJERD Editor
Electrical, Electronics and Computer Engineering,
Information Engineering and Technology,
Mechanical, Industrial and Manufacturing Engineering,
Automation and Mechatronics Engineering,
Material and Chemical Engineering,
Civil and Architecture Engineering,
Biotechnology and Bio Engineering,
Environmental Engineering,
Petroleum and Mining Engineering,
Marine and Agriculture engineering,
Aerospace Engineering.
Palestine last event orientationfvgnh .pptxRaedMohamed3
An EFL lesson about the current events in Palestine. It is intended to be for intermediate students who wish to increase their listening skills through a short lesson in power point.
How to Make a Field invisible in Odoo 17Celine George
It is possible to hide or invisible some fields in odoo. Commonly using “invisible” attribute in the field definition to invisible the fields. This slide will show how to make a field invisible in odoo 17.
June 3, 2024 Anti-Semitism Letter Sent to MIT President Kornbluth and MIT Cor...Levi Shapiro
Letter from the Congress of the United States regarding Anti-Semitism sent June 3rd to MIT President Sally Kornbluth, MIT Corp Chair, Mark Gorenberg
Dear Dr. Kornbluth and Mr. Gorenberg,
The US House of Representatives is deeply concerned by ongoing and pervasive acts of antisemitic
harassment and intimidation at the Massachusetts Institute of Technology (MIT). Failing to act decisively to ensure a safe learning environment for all students would be a grave dereliction of your responsibilities as President of MIT and Chair of the MIT Corporation.
This Congress will not stand idly by and allow an environment hostile to Jewish students to persist. The House believes that your institution is in violation of Title VI of the Civil Rights Act, and the inability or
unwillingness to rectify this violation through action requires accountability.
Postsecondary education is a unique opportunity for students to learn and have their ideas and beliefs challenged. However, universities receiving hundreds of millions of federal funds annually have denied
students that opportunity and have been hijacked to become venues for the promotion of terrorism, antisemitic harassment and intimidation, unlawful encampments, and in some cases, assaults and riots.
The House of Representatives will not countenance the use of federal funds to indoctrinate students into hateful, antisemitic, anti-American supporters of terrorism. Investigations into campus antisemitism by the Committee on Education and the Workforce and the Committee on Ways and Means have been expanded into a Congress-wide probe across all relevant jurisdictions to address this national crisis. The undersigned Committees will conduct oversight into the use of federal funds at MIT and its learning environment under authorities granted to each Committee.
• The Committee on Education and the Workforce has been investigating your institution since December 7, 2023. The Committee has broad jurisdiction over postsecondary education, including its compliance with Title VI of the Civil Rights Act, campus safety concerns over disruptions to the learning environment, and the awarding of federal student aid under the Higher Education Act.
• The Committee on Oversight and Accountability is investigating the sources of funding and other support flowing to groups espousing pro-Hamas propaganda and engaged in antisemitic harassment and intimidation of students. The Committee on Oversight and Accountability is the principal oversight committee of the US House of Representatives and has broad authority to investigate “any matter” at “any time” under House Rule X.
• The Committee on Ways and Means has been investigating several universities since November 15, 2023, when the Committee held a hearing entitled From Ivory Towers to Dark Corners: Investigating the Nexus Between Antisemitism, Tax-Exempt Universities, and Terror Financing. The Committee followed the hearing with letters to those institutions on January 10, 202
Instructions for Submissions thorugh G- Classroom.pptxJheel Barad
This presentation provides a briefing on how to upload submissions and documents in Google Classroom. It was prepared as part of an orientation for new Sainik School in-service teacher trainees. As a training officer, my goal is to ensure that you are comfortable and proficient with this essential tool for managing assignments and fostering student engagement.
Model Attribute Check Company Auto PropertyCeline George
In Odoo, the multi-company feature allows you to manage multiple companies within a single Odoo database instance. Each company can have its own configurations while still sharing common resources such as products, customers, and suppliers.
The Roman Empire A Historical Colossus.pdfkaushalkr1407
The Roman Empire, a vast and enduring power, stands as one of history's most remarkable civilizations, leaving an indelible imprint on the world. It emerged from the Roman Republic, transitioning into an imperial powerhouse under the leadership of Augustus Caesar in 27 BCE. This transformation marked the beginning of an era defined by unprecedented territorial expansion, architectural marvels, and profound cultural influence.
The empire's roots lie in the city of Rome, founded, according to legend, by Romulus in 753 BCE. Over centuries, Rome evolved from a small settlement to a formidable republic, characterized by a complex political system with elected officials and checks on power. However, internal strife, class conflicts, and military ambitions paved the way for the end of the Republic. Julius Caesar’s dictatorship and subsequent assassination in 44 BCE created a power vacuum, leading to a civil war. Octavian, later Augustus, emerged victorious, heralding the Roman Empire’s birth.
Under Augustus, the empire experienced the Pax Romana, a 200-year period of relative peace and stability. Augustus reformed the military, established efficient administrative systems, and initiated grand construction projects. The empire's borders expanded, encompassing territories from Britain to Egypt and from Spain to the Euphrates. Roman legions, renowned for their discipline and engineering prowess, secured and maintained these vast territories, building roads, fortifications, and cities that facilitated control and integration.
The Roman Empire’s society was hierarchical, with a rigid class system. At the top were the patricians, wealthy elites who held significant political power. Below them were the plebeians, free citizens with limited political influence, and the vast numbers of slaves who formed the backbone of the economy. The family unit was central, governed by the paterfamilias, the male head who held absolute authority.
Culturally, the Romans were eclectic, absorbing and adapting elements from the civilizations they encountered, particularly the Greeks. Roman art, literature, and philosophy reflected this synthesis, creating a rich cultural tapestry. Latin, the Roman language, became the lingua franca of the Western world, influencing numerous modern languages.
Roman architecture and engineering achievements were monumental. They perfected the arch, vault, and dome, constructing enduring structures like the Colosseum, Pantheon, and aqueducts. These engineering marvels not only showcased Roman ingenuity but also served practical purposes, from public entertainment to water supply.
Operation “Blue Star” is the only event in the history of Independent India where the state went into war with its own people. Even after about 40 years it is not clear if it was culmination of states anger over people of the region, a political game of power or start of dictatorial chapter in the democratic setup.
The people of Punjab felt alienated from main stream due to denial of their just demands during a long democratic struggle since independence. As it happen all over the word, it led to militant struggle with great loss of lives of military, police and civilian personnel. Killing of Indira Gandhi and massacre of innocent Sikhs in Delhi and other India cities was also associated with this movement.
Francesca Gottschalk - How can education support child empowerment.pptxEduSkills OECD
Francesca Gottschalk from the OECD’s Centre for Educational Research and Innovation presents at the Ask an Expert Webinar: How can education support child empowerment?
Unit 8 - Information and Communication Technology (Paper I).pdfThiyagu K
This slides describes the basic concepts of ICT, basics of Email, Emerging Technology and Digital Initiatives in Education. This presentations aligns with the UGC Paper I syllabus.
2. Coulometry and Electro gravimetric Analysis
Electrogravimetry and coulometry are related methods in which
electrolysis is carried out for a sufficient length of time to ensure
complete oxidn or redn of the analyte to a product of known
composition.
In electrogravimetry, the goal is to determine the amount of analyte
present by converting it electolytically to a product that is weighed as
a deposit on one of the electrodes.
In coulometric procedures, we determine the amount of analyte by
measuring the quantity of electrical charge needed to completely
convert it to a product.
Electrogravimetry and coulometry are moderately sensitive and
among the most accurate and precise techniques available to the
chemist.
1/3/2020 2
3. Cont…
Electrogravimetry require no preliminary calibration against
chemical standards because the functional relationship b/n the quantity
measured and the analyte conc can be derived from theory and atomic
mass data.
When there is a net current in an electrochemical cell. The measured
potential across the 2 electrodes is no longer simply the d/ce b/n the 2
electrode potentials as calculated from the Nernst equation.
2 additional phenomena, IR drop & polarization must be considered
when current is present.
Because of these phenomena, potentials larger than the
thermodynamic potential are needed to operate an electrolytic cell.
When present in a galvanic cell, IR drop & polarization result in the
development of potentials smaller than predicted.
1/3/2020 3
4. Cont…
Coulometric methods of analysis are based on an
exhaustive electrolysis of the analyte.
By exhaustive we mean that the analyte is quantitatively
oxidized or reduced at the WE or reacts quantitatively with a
reagent generated at the WE.
There are 2 forms of coulometry: controlled-potential
coulometry, in which a constant potential is applied to the
electrochemical cell, and controlled-current coulometry, in
which a constant current is passed through the
electrochemical cell.
The total charge, Q, in coulombs, passed during electrolysis
is related to the absolute amount of analyte by Faraday’s law
Q = nFN ----------------------------8.1
1/3/2020 4
5. Cont…
Where n is the # of es transferred per mole of analyte, F is Faraday’s
constant (96487 C mol–1), and N is the moles of analyte.
A coulomb is also equivalent to an A.s; thus, for a constant current, i,
the charge is given as
Q = ite -----------------------------8.2
Where te is the electrolysis time.
If current varies with time, as it does in controlled potential
coulometry, then the total charge is given by
---------------8.3
In coulometry, current and time are measured, and equation 8.2 or
equation 8.3 is used to calculate Q.
Equation 8.1 is then used to determine the moles of analyte.
To obtain an accurate value for N, therefore, all the current must
result in the analyte’s oxidation or reduction.1/3/2020 5
6. Cont…
o In other words, coulometry requires 100% current efficiency (or an
accurately measured current efficiency established using a standard), a
factor that must be considered in designing a coulometric method of
analysis. 8.1. Controlled-Potential Coulometry
oThe easiest method for ensuring 100% current efficiency is to
maintain the WE at a constant potential that allows for the analyte’s
quantitative oxidn or redn, without simultaneously oxidizing or reducing
an interfering species.
o The current flowing through an electrochemical cell under a constant
potential is proportional to the analyte’s conc.
o As electrolysis progresses the analyte’s conc decreases as does the
current.
oThe resulting current vs time profile for controlled-potential
coulometry which also known as potentiostatic coulometry, shown in
Fig.8.1.1/3/2020 6
7. Cont…
o Integrating the area under the curve (equation 8.3), from t = 0 until t
= te, gives the total charge.
Fig.8.1.current-time curve for controlled potential coulometry.
1/3/2020 7
8. Selecting a Constant Potential
In controlled-potential coulometry, the potential is selected so that
the desired oxidn or redn rxn goes to completion without interference
from redox rxns involving other components of the sample matrix.
To see how an appropriate potential for the WE is selected, let’s
develop a constant-potential coulometric method for Cu2+ based on its
redn to Cu metal at a Pt cathode WE.
---------------- 8.4
•The potential needed for a quantitative redn of Cu2+ can be calculated
using the Nernst equation
-----------------8.5
1/3/2020 8
9. Minimizing Electrolysis Time
The current-time curve for controlled-potential coulometry in Fig.8.1
shows that the current decreases continuously throughout electrolysis.
An exhaustive electrolysis, therefore, may require a long time.
Since time is an important consideration in choosing and designing
analytical methods, the factors that determine the analysis time need to
be considered.
The change in current as a function of time in controlled-potential
coulometry is approximated by an exponential decay; thus, the current at
time t is i = i0e–kte----------------------8.6
Where i0 is the initial current and k is a constant that is directly
proportional to the area of the WE & the rate of stirring and inversely
proportional to the volume of the solution.
For an exhaustive electrolysis in which 99.99% of the analyte is
oxidized or reduced, the current at the end of the analysis, te, may be
approximated as i =(10–4)i0----------------------------------8.7
1/3/2020 9
10. Cont…
Substituting equation 8.7 into equation 8.6 and solving for te gives
the minimum time for an exhaustive electrolysis as
te= -k-1ln(10-4) = 9.21xk-1
From this equation increasing k leads to a shorter analysis time.
For this reason controlled-potential coulometry is carried out in
small-volume electrochemical cells, using electrodes with large
surface areas and with high stirring rates.
A quantitative electrolysis typically requires approximately 30–60
min, although shorter or longer times are possible.
1/3/2020 10
11. Instrumentation
The potential in controlled-potential coulometry is set using a 3-
electrode potentiostat.
Two types of WE are commonly used: a Pt electrode manufactured
from Pt-gauze & fashioned into a cylindrical tube & an Hg pool
electrode.
The large over potential for reducing H3O+ at mercury makes it the
electrode of choice for analytes requiring negative potentials.
For example, potentials more -ve than –1 V vs the SCE are feasible
at an Hg electrode but not at a Pt electrode, even in very acidic so/ns.
The ease, with which mercury is oxidized, however, prevents its use
at potentials that are +ve with respect to the SHE.
Platinum WEs are used when +ve potentials are required.
The auxiliary electrode, which is often a Pt wire, is separated by a
salt bridge from the solution containing the analyte.
1/3/2020
11
12. Cont…
This is necessary to prevent electrolysis products generated at the
auxiliary electrode from reacting with the analyte and interfering in
the analysis.
A saturated calomel or Ag/AgCl electrode serves as the RE.
The other essential feature of instrumentation for controlled-
potential coulometry is a means of determining the total charge
passed during electrolysis.
One method is to monitor the current as a function of time and
determine the area under the curve (see Fig.8.1).
Modern instruments, however, use electronic integration to monitor
charge as a function of time.
The total charge at the end of the electrolysis then can be read
directly from a digital readout or from a plot of charge versus time
(Fig.8.3).
1/3/2020 12
14. 8.2 Controlled-Current Coulometry
A second approach to coulometry is to use a constant current in
place of a constant potential (Fig.8.4).
Controlled-current coulometry also known as amperostatic
coulometry or coulometric titrimetry, has two advantages over
controlled-potential coulometry.
First, using a constant current makes for a more rapid analysis since
the current does not decrease over time.
Thus, a typical analysis time for controlled current coulometry is
less than 10 min, as opposed to approximately 30–60 min for
controlled-potential coulometry.
Second, with a constant current the total charge is simply the
product of current and time (equation 8.2).
A method for integrating the current–time curve, therefore, is not
necessary.
1/3/2020
14
15. Cont…
o Using a constant current does present two important experimental
problems that must be solved if accurate results are to be obtained.
o1st, as electrolysis occurs the analyte’s conc &, therefore, the current
due to its oxidation or reduction steadily decreases.
o To maintain a constant current the cell potential must change until
another oxidation or reduction rxn can occur at the WE.
o Unless the system is carefully designed, these secondary reactions
will produce a current efficiency of less than 100%.
o The second problem is the need for a method of determining when
the analyte has been exhaustively electrolyzed.
oIn controlled-potential coulometry this is signaled by a decrease in
the current to a constant background or residual current (see Fig.8.1).
oIn controlled-current coulometry, however, a constant current
continues to flow even when the analyte has been completely oxidized
or reduced.
1/3/2020 15
16. Cont…
A suitable means of determining the end-point of the rxn, te, is
needed.
Fig.8.4.current-time curve for controlled-current coulometry.
Maintaining Current Efficiency
To illustrate why changing the WE’s potential can lead to less than
100% current efficiency, let’s consider the coulometric analysis for
Fe2+ based on its oxidation to Fe3+ at a Pt WE in 1 M H2SO4.
Initially the potential of the WE remains nearly constant at a level
near the standard-state potential for the Fe3+/Fe2+ redox couple.
1/3/2020 16
17. Cont…
As the conc of Fe2+ decreases, however, the potential of the WE
shifts toward more +ve values until another oxidation rxn can provide
the necessary current.
Thus, in this case the potential eventually increases to a level at
which the oxidn of H2O occurs.
Since the current due to the oxidation of H3O+ does not contribute to
the oxidation of Fe2+, the current efficiency of the analysis is less than
100%.
To maintain a 100% current efficiency the products of any
competing oxidation rxns must react both rapidly and quantitatively
with the remaining Fe2+.
This may be accomplished, for example, by adding an excess of
Ce3+ to the analytical solution (Fig.8.5b).
When the potential of the WE shifts to a more +ve potential, the first
species to be oxidized is Ce3+.
1/3/2020 17
18. Cont…
The Ce4+ produced at the WE rapidly mixes with the so/n, where it
reacts with any available Fe2+.
---------------8.8
Combining these rxns gives the desired overall rxn of
In this manner, a current efficiency of 100% is maintained.
Furthermore, since the conc of Ce3+ remains at its initial level, the
potential of the WE remains constant as long as any Fe2+ is present.
This prevents other oxidation rxns, such as that for H2O, from
interfering with the analysis.
A species, such as Ce3+, which is used to maintain 100% current
efficiency, is called a mediator.
1/3/2020 18
19. Cont…
Instrumentation
Controlled-current coulometry normally is carried out using a
galvanostat and an electrochemical cell consisting of a WE and a
counter electrode.
The WE, which often 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.
Alternatively, oxidizing or reducing the mediator can be carried out
externally, & the appropriate products flushed into the analytical so/n.
A so/n containing the mediator flows under the influence of gravity
into a small-volume electrochemical cell.
1/3/2020 19
20. Cont…
The products generated at the anode & cathode pass through separate
tubes, and the appropriate oxidizing or reducing reagent can be
selectively delivered to the analytical solution.
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.
Analog clocks can read time to the nearest ±0.01 s, but the need to
frequently stop and start the electrolysis near the end point leads to a net
uncertainty of ±0.1 s.
Digital clocks provide a more accurate measurement of time, with
errors of ±1 ms being possible.
The switch must control the flow of current and the clock, so that an
accurate determination of the electrolysis time is possible
1/3/2020 20
21. Coulometric Titrations
Controlled-current coulometric methods commonly are called
coulometric titrations because of their similarity to conventional
titrations.
We already have noted, in discussing the controlled-current
coulometric determination of Fe2+, that the oxidation of Fe2+ by Ce4+ is
identical to the reaction used in a redox titration.
Combining equations 8.1 and 8.2 and solving for the moles of
analyte gives
------------------8.9
Compare this equation with the relationship b/n the moles of strong
acid, N, titrated with a strong base of known concentration.
N = (M base) (V base)
The titrant in a conventional titration is replaced in a coulometric
titration by a constant-current source whose current is analogous to the
titrant’s molarity.
1/3/2020 21
22. Cont…
oThe time needed for an exhaustive electrolysis takes the place of the
volume of titrant, and the switch for starting and stopping the
electrolysis serves the same function as a burette’s stopcock.
Quantitative Applications
o Coulometry may be used for the quantitative analysis of both
inorganic and organic compounds.
Controlled-Potential Coulometry
o The majority of controlled-potential coulometric analyses involve
the determination of inorganic cations and anions, including trace
metals and halides.
oThe ability to control selectivity by carefully selecting the WE’s
potential, makes controlled-potential coulometry particularly useful
for the analysis of alloys.
o For example, the composition of an alloy containing Ag, Bi, Cd, and
Sb can be determined by dissolving the sample and placing it in a
matrix of 0.2 M H2SO4. 1/3/2020
22
23. Cont…
A platinum WE is immersed in the solution and held at a constant
potential of +0.40 V versus the SCE.
At this potential Ag (I) deposits on the Pt electrode as Ag and the
other metal ions remain in solution.
When electrolysis is complete, the total charge is used to determine
the amount of silver in the alloy.
The potential of the Pt electrode is then shifted to –0.08 V vs the
SCE, depositing Bi on the WE.
When the coulometric analysis for bismuth is complete, antimony is
determined by shifting the WE’s potential to –0.33 V vs the SCE,
depositing Sb.
Finally, Cd is determined following its electrodeposition on the Pt
electrode at a potential of –0.80 V vs the SCE.
Another area where controlled-potential coulometry has found
application is in nuclear chemistry, in which elements such as
uranium and polonium can be determined at trace levels. 1/3/2020
23
24. Cont…
o For example, microgram quantities of uranium in a medium of
H2SO4 can be determined by reducing U (VI) to U (IV) at a Hg WE.
o Controlled-potential coulometry also can be applied to the
quantitative analysis of organic cpds, although the number of
applications is significantly less than that for inorganic analytes.
o One example is the six-electron reduction of a nitro group, –NO2, to
a primary amine, –NH2, at a mercury electrode.
Solutions of picric acid, for instance, can be analyzed by reducing to
triaminophenol.
o Another example is the successive reduction of trichloroacetate to
dichloroacetate, and of dichloroacetate to monochloroacetate
Cl3CCOO–(aq) + H3O+(aq) + 2e– ↔Cl2HCCOO–(aq) + Cl–(aq) +
H2O(l)
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25. Application Cont…
Cl2HCCOO–(aq) + H3O+(aq) + 2e– ↔ClH2CCOO–(aq) + Cl–(aq) +
H2O(l)
Mixtures of trichloroacetate and dichloroacetate are analyzed by
selecting an initial potential at which only the more easily reduced
trichloroacetate is reduced.
When its electrolysis is complete, the potential is switched to a more
negative potential at which dichloroacetate is reduced.
The total charge for the first electrolysis is used to determine the
amount of trichloroacetate, and the difference in total charge between
the first and second electrolyses gives the amount of dichloroacetate.
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26. Controlled-Current Coulometry
The use of a mediator makes controlled-current coulometry a more
versatile analytical method than controlled-potential coulometry.
For example, the direct oxidn or redn of a protein at the WE in
controlled-potential coulometry is difficult if the protein’s active
redox site lies deep within its structure.
The controlled-current coulometric analysis of the protein is made
possible, however, by coupling its oxidn or redn to a mediator that is
reduced or oxidized at the WE.
Controlled-current coulometric methods have been developed for
many of the same analytes that may be determined by conventional
redox titrimetry.1/3/2020 26
Application Cont…
27. Application Cont…
Coupling the mediator’s oxidn or redn to an acid–base,
precipitation, or complexation rxn involving the analyte allows for the
coulometric titration of analytes that are not easily oxidized or
reduced.
For example, when using H2O.
If the oxidn or redn of H2O is carried out externally using the
generator cell then H3O+ or OH– can be dispensed selectively into a
solution containing a basic or acidic analyte.
The resulting rxn is identical to that in an acid–base titration.
Coulometric acid–base titrations have been used for the analysis of
strong & weak acids and bases, in both aqueous & non-aqueous
matrices.
In comparison with conventional titrimetry, there are several
advantages to the coulometric titration.
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28. Cont…
One advantage is that the electrochemical generation of a “titrant”
that reacts immediately with the analyte allows the use of reagents
whose instability prevents their preparation & storage as a standard
so/n.
Thus, highly reactive reagents such as Ag2+ and Mn3+ can be used in
coulometric titrations.
Because it is relatively easy to measure small quantities of charge,
coulometric titrations can be used to determine small quantities of
analyte that cannot be measured accurately by a conventional
titration.
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29. Cont…
Example1. The purity of a sample of Na2S2O3 was determined by a
coulometric redox titration using I– as a mediator & I3
– as the “titrant.”
A sample weighing 0.1342 g is transferred to a 100-mL volumetric
flask & 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 & several drops of a starch indicator
so/n. 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.
SOLUTION The coulometric titration of S2O3
2– with I3
– is Oxidizing
S2O3
2– to S4O6
2 –requires 1e per S2O3
2– (n = 1). Combining equations
8.1 & 8.2, and making an appropriate substitution for moles of
Na2S2O3 gives
1/3/2020 29
30. Cont…
represents the amount of Na2S2O3 in a 10.00-mL portion of a 100-mL
sample, thus 0.1325 g of Na2S2O3 is present in the original sample.
The purity of the sample, therefore, is
Note that the calculation is worked as if S2O3
2– is oxidized directly at
the WE instead of in so/n.
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