Name: controlled-current coulometry in redox analysis
Uploaded: 2024-12-05T18:31:57.000Z
Duration: 1 min 27 s
Channel: JoVE

קולומטריית זרם מבוקר, או קולומטריה אמפרוסטטית, משתמשת בזרם קבוע לניתוח מהיר ופשוט של המטען הכולל המיוצר במהלך אלקטרוליזה על ידי הכפלת הזרם בזמן האלקטרוליזה.
ההתקנה כוללת גלוונוסטט, תא אלקטרוכימי בעל שתי אלקטרודות, שעון למדידת זמן האלקטרוליזה, ומתג להתחלה ועצירה של התהליך.
תוצרי האנליזה והאלקטרוליזה על אלקטרודת הדלפק מופרדים באמצעות גשר מלח או פריט.
Galvanostats משמשים כי, במהלך אלקטרוליזה, הריכוז היורד של האנליטי יגרום לזרם לרדת כי פחות אלקטרונים נוצרים.
לכן, יש להגדיל את פוטנציאל התא כדי לשמור על זרם קבוע. עם זאת, תגובות אחרות עשויות להתחיל באלקטרודת הגנרטור, ותגובות כאלה יכולות להפחית את היעילות הנוכחית.
ניתן להשיג יעילות זרם של 100% על ידי הוספת עודף של מתווך, אשר מייצר יונים המגיבים כמותית עם האנליט הנותר.
לחלופין, ניתן להשתמש בריאגנטים מחמצנים או מחזרים שנוצרו מבחוץ.
נקודת הסיום של התגובה יכולה להיות מסומנת על ידי אינדיקטורים חזותיים או באמצעות מדידות פוטנציומטריות או מוליכות.

קולומטריה בזרם מבוקר: מבוא

Controlled-Current Coulometry: Overview

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קולומטריה בזרם מבוקר, המכונה גם קולומטריה אמפרוסטטית, היא טכניקה המשמשת בניתוח אלקטרוכימי למדידת כמות של חומר באמצעות העברת זרם מבוקרת. הטכניקה כוללת יישום של זרם קבוע לתא אלקטרוכימי המכיל את ההאנליט המעניין. כאשר הזרם זורם דרך התא, האנליט עובר תגובת חמצון-חיזור על פני שטח האלקטרודה, מה שמוביל להעברת מטען. על ידי מעקב אחר הזמן הנדרש להעברת כמות מסוימת של מטען, ניתן לקבוע את כמות האנליט על פי חוקי פרדיי של האלקטרוליזה.

המערכת כוללת גלוונוסטט, תא אלקטרוכימי דו-אלקטרודות, טיימר למדידת משך האלקטרוליזה ומתג להפעלה והפסקת התהליך. גשר מלח או מחיצה נקבובית מפרידים בין האנליט לבין תוצרי האלקטרוליזה. נעשה שימוש בגלוונוסטטים מכיוון שריכוז האנליט פוחת במהלך האלקטרוליזה—הזרם יורד בשל ייצור מופחת של אלקטרונים. לכן, יש להעלות את פוטנציאל התא כדי לשמור על זרם יציב. עם זאת, הדבר עלול לגרום לתגובות אחרות באלקטרודה הגנרטורית, מה שעלול להפחית את יעילות הזרם. כדי להשיג יעילות זרם של 100%, ניתן להוסיף כמות עודפת של מתווך, המייצרת יונים המגיבים באופן כמותי עם האנליט הנותר.

בקולומטריה בזרם מבוקר, יצירת הטיטרנט באופן חיצוני פותרת בעיות כמו הפרעות אלקטרודה וזרמים פרזיטיים, במיוחד בדגימות בקנה מידה גדול. הטיטרנט מיוצר בתא אלקטרוליטי נפרד ומועבר לכלי הטיטרציה, ומציע שליטה מדויקת.

מערכת טיפוסית כוללת תא אלקטרוליטי כפול עם אלקטרודות פלטינה סליליות בצינור U הפוך. האלקטרוליט זורם דרך כל זרוע, שם מתרחשת האלקטרוליזה. בטיטרציות חומצה, הקתודה מייצרת יוני הידרוקסיד הנייטרליים את החומצה. בטיטרציות בסיס, האנודה מייצרת יוני מימן לנטרול הבסיס.

בטיטרציות הכוללות ייצור חשמלי של יוד, היוד מיוצר על ידי אלקטרוליזה של תמיסת אשלגן יודיד. במקרה זה, תגובת האנודה יוצרת יוד, המועבר לתא הטיטרציה.

דוגמה נוספת כוללת יצירה חיצונית של יוני Ce^4+ באמצעות תמיסה מימית של Ce^3+. מגבלה אפשרית של שיטה זו היא דילול הטיטרנט במערכת ההולכה, אך ניתן למזער זאת על ידי שליטה בקצבי הזרימה.

לבסוף, ניתן לקבוע את נקודת הסיום של התגובה באמצעות אינדיקטורים חזותיים או באמצעות מדידות פוטנציאליות וקונדוקטומטריות.
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קולומטריית זרם מבוקר, הידועה גם בשם קולומטריה אמפרוסטטית, היא טכניקה המשמשת באנליזה אלקטרוכימית למדידת כמות ...

Education

JoVE Core

Analytical Chemistry

כימיה אנליטית

Unit 1

יחידה 1

Electrochemical Analyses and Redox Titration

אנליזות אלקטרוכימיות וטיטרציית חמצון-חיזור

electrochemical-analyses-and-redox-titration

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

controlled-current coulometry in redox analysis

Controlled current coulometry is an electrochemical method used to measure the amount of a substance by passing a steady current through a cell. It is also called amperostatic coulometry. The analyte, or substance being analyzed, reacts at the electrode surface as charge moves through the system.

The amount of analyte is found by measuring how long it takes for a known charge to pass. This calculation follows Faraday’s laws of electrolysis. A typical setup includes a galvanostat, a two-electrode electrochemical cell, a timer, and a switch to start and stop the process. A salt bridge or porous frit keeps the analyte and the electrolysis products separated.

A galvanostat is needed because the analyte concentration drops during electrolysis. As fewer electrons are produced, the current tends to decline. To keep the current constant, the cell potential must be increased. This can sometimes cause other reactions at the generator electrode and lower current efficiency.

To reach 100% current efficiency, an excess mediator can be added. A mediator is a species that produces ions which react completely with the remaining analyte. Controlled current coulometry can also use external titrant generation to reduce electrode interference and parasitic currents, especially with large samples. In this case, the titrant is made in a separate electrolytic cell and sent to the titration vessel for better control.

A common setup uses a double-arm electrolytic cell with platinum spiral electrodes in an inverted U-tube. The electrolyte flows through both arms, where electrolysis takes place. For acid titrations, the cathode makes hydroxide ions that neutralize the acid. For base titrations, the anode makes hydrogen ions that neutralize the base. Iodine titrations use potassium iodide, where the anode forms iodine that moves to the titration cell. Another example is the external generation of Ce4+ ions from an aqueous Ce3+ solution.

One limitation of this method is dilution of the titrant in the delivery system. That effect can be reduced by controlling the flow rate. The endpoint of the reaction can be detected with a visual indicator or by potentiometric and conductometric measurements.

Controlled-Current Coulometry in Redox Analysis

controlled-current-coulometry-in-redox-analysis

electrochemistry: overview

Electrochemistry and Redox Reactions

Electrochemistry connects electrical measurements with chemical reactions, especially oxidation and reduction. It focuses on how electron movement helps explain reaction behavior. Oxidation is the loss of electrons from a substance, and reduction is the gain of electrons. A species with strong electron affinity is an oxidizing agent, or oxidant. A species that donates electrons is a reducing agent, or reductant. These redox steps are often described with half-reactions. When electrons move...

electrodes: overview

Types of Electrodes in Electrochemistry

Electrodes are conductive materials in an electrochemical cell. They allow electron transfer reactions to happen at their surfaces. Electrochemical measurements use them to control and measure current and potential. The working electrode, also called the indicator electrode, is the main sensing electrode. Its potential changes with the analyte concentration and with the activity of the electroactive species. Common indicator electrodes include inert platinum electrodes, silver electrodes that...

interfacial electrochemical methods: overview

interfacial-electrochemical-methods-overview

Static and Dynamic Electrochemical Methods

Static and dynamic electrochemical methods study what happens at the boundary between an electrode and a solution. These interfacial methods focus on the electrode-solution interface rather than the overall properties of the liquid. They are useful in electroanalytical chemistry because they can detect very small changes in analyte concentration. These methods are grouped by whether a nonzero current flows through the cell. Static methods, such as potentiometry, measure the cell potential...

potentiometry: overview

Potentiometry and Electrochemical Cell Potential

Potentiometry measures the potential difference between two electrodes in an electrochemical cell. It does this without drawing significant current, so the solution’s composition is not altered. The method uses an indicator electrode, which exchanges electrons with the analyte solution, and a reference electrode, which keeps a constant potential. Each electrode is placed in a solution that makes up a half-cell. In a common setup, the reference electrode is the anode, or left-hand electrode,...

potentiometry: types of electrodes

Reference and Indicator Electrodes in Potentiometry

Potentiometric measurements use reference electrodes and indicator electrodes to track changes in a solution. A reference electrode gives a stable baseline, while indicator and working electrodes respond to changes in the solution’s composition. The Standard Hydrogen Electrode, or SHE, is a common reference electrode. It keeps a zero potential at all temperatures. However, it needs a continuous supply of hydrogen gas, so it is not practical for routine use. The Saturated Calomel Electrode, or...

potentiometry: membrane electrodes

Membrane Electrodes in Ion Sensing

Membrane electrodes measure ions by using a selective membrane that creates a potential difference across it. This membrane potential is called the asymmetry potential, and it can remain nonzero even when analyte concentrations are the same on both sides of the membrane. The signal usually depends on all ions in the sample that can interact with the membrane, not just one analyte. Selectivity is described by a selectivity coefficient. This value compares the response to an interferent with the...

redox titration: overview

Redox Titration Endpoints and Indicators

Redox titration uses electron transfer in a redox, or reduction-oxidation, reaction to find the concentration of an unknown solution. The titrant has a known concentration, and it is added gradually to the analyte, which is the solution being tested. The endpoint shows when the reaction between the two substances is complete. Before a redox titration begins, the analyte should be in a single oxidation state. If needed, this is done by pre-oxidizing or pre-reducing the sample with an auxiliary...

redox titration: iodimetry and iodometry

Using Iodine to Find Unknown Concentrations

Iodometry and iodimetry use iodine to measure unknown concentrations in redox titrations. These methods help determine the amount of an oxidizing or reducing agent in a sample. The key idea is that iodine changes color and reacts in a known stoichiometric ratio with the analyte. In iodometric titrations, the sample usually contains an oxidizing agent. The solution is acidified and then treated with an excess of iodide ions, which releases an equivalent amount of iodine in equilibrium with...

redox titration: other oxidizing and reducing agents

redox-titration-other-oxidizing-and-reducing-agents

Choosing Oxidizing Titrants in Redox Titration

Redox titrations can use oxidizing titrants other than iodine. Common choices include potassium permanganate (KMnO4), cerium(IV), and potassium dichromate (K2Cr2O7). The best titrant depends on stability, cost, analyte strength, and the reaction rate between the analyte and the titrant. Potassium permanganate is a strong oxidizing titrant in highly acidic solution. It reduces from Mn(VII) to Mn(II) while it oxidizes the analyte to a higher oxidation state. KMnO4 also works as a self-indicator,...

potentiometric titration: overview

Endpoint Detection in Potentiometric Titration

Potentiometric titration is a quantitative analytical method used to find the concentration of an analyte by measuring the potential difference between two electrodes in a solution. The endpoint is the point where this potential changes sharply. It usually appears when the stoichiometric reaction between the analyte and the titrant is complete. To find the endpoint, the measured potential difference is plotted against the volume of titrant added. This graph is called a titration curve. A...

end point prediction: gran plot

Gran Plot for Titration Endpoint Prediction

A Gran plot helps predict the endpoint of a potentiometric or acid-base titration before the endpoint is reached. It uses titration data collected only up to a point that is still below the equivalence volume. The data are then transformed into a straight-line format. To build the plot, the line is extended to the x-axis. That x-intercept shows the titrant volume needed to reach the equivalence point. In potentiometric titration, the Gran plot uses the volume-corrected base-10 antilogarithm of...

electrogravimetric analysis: overview

Electrode Weighing in Electrogravimetric Analysis

Electrogravimetric analysis measures an analyte by weighing the material that deposits on an electrode. A pre-weighed working electrode is placed in solution, and a potential is applied. The target substance then deposits by reduction at the cathode or by oxidation at the anode. After electrolysis, the electrode is weighed again. The mass increase gives the weight of the analyte in the solution. To check that deposition is complete, the fresh electrode surface is exposed to the solution while...

coulometry: overview

Coulometry and Electrical Charge Analysis

Coulometry is an analytical method that finds the amount of an analyte by measuring the electrical charge needed for its complete electrolysis. It is known for being rapid, accurate, and precise. Unlike many other methods, it does not require analytical standards. The total charge passed during electrolysis is linked to the amount of analyte through Faraday's laws of electrolysis. For correct coulometric measurements, the charge supplied must match Faraday's constant multiplied by the number...

controlled-potential coulometry: electrolytic methods

controlled-potential-coulometry-electrolytic-methods

Measuring Analyte Charge by Potentiostatic Coulometry

Controlled-potential coulometry measures analyte amount by carefully controlling electrode potential during electrolysis. It is also called potentiostatic coulometry. A potentiostat keeps the working electrode at a set potential in a three-electrode system. The working electrode choice depends on the potential range. Platinum working electrodes are used for positive potentials. Mercury pool electrodes are preferred for very negative potentials. The platinum counter electrode is kept away from...

controlled-current coulometry: overview

Controlled current coulometry is an electrochemical method used to measure the amount of a substance by passing a steady current through a cell. It is also called amperostatic coulometry. The analyte, or substance being analyzed, reacts at the electrode surface as charge moves through the system. The amount of analyte is found by measuring how long it takes for a known charge to pass. This calculation follows Faraday’s laws of electrolysis. A typical setup includes a galvanostat, a...

controlled-current coulometry: coulometric titration

controlled-current-coulometry-coulometric-titration

Electrically Generated Reagents in Titration

Coulometric titrations use an electrically generated reagent to measure the amount of a substance in a sample. The electron acts as the standard reagent, and the result depends on the current and the time used to generate it. The method follows the same basic idea as a regular titration, including endpoint detection. Two conditions are essential for this type of titration. First, the electrode reaction that makes the reagent must be 100% efficient. Second, the generated reagent must react...

voltammetry: overview

Voltammetry for Detecting Redox Species

Voltammetry measures current in an electrochemical cell as the applied potential changes. It is an electroanalytical method used to study redox-active species. The current response is shown on a graph called a voltammogram. A voltammetric cell uses three electrodes: a working electrode, a reference electrode, and an auxiliary electrode. Redox reactions happen at the working electrode, and a potentiostat controls its potential. The reference electrode, such as a saturated calomel electrode or a...

voltammetry: factors affecting measurements

voltammetry-factors-affecting-measurements

Voltammetry and Faradaic Current

Voltammetry measures faradaic current, which is the current created by redox reactions of an analyte at the working and auxiliary electrodes. The current from reduction is called cathodic current and carries a positive charge. The current from oxidation is called anodic current and has a negative charge. The applied potential at the working electrode controls the flow of faradaic current. The size of that current depends on how fast the oxidation or reduction reaction occurs at the electrode...

voltammograms: overview

Voltammogram Shapes and Redox Analysis

Voltammograms are current plots made against applied potential. They help show how an electrochemical system behaves during a redox reaction. The shape of the plot depends on how the current is measured and on whether convection, or fluid movement, is present. When convection is present, as in hydrodynamic voltammetry, the voltammogram usually has a sigmoid shape. In this case, the current rises from a residual current to a limiting current at the analyte’s oxidation or reduction potential.

voltammetric techniques: linear-scan (e vs time)

voltammetric-techniques-linear-scan-e-vs-time

Polarography and Voltammetry with Mercury Drops

Polarography and hydrodynamic voltammetry are voltammetric techniques used to study electrochemical reactions. They track the current that flows as the electrode potential changes. In polarography, the key working electrode is a dropping mercury electrode, or DME. A DME uses a capillary tube filled with mercury. A tiny mercury droplet forms at the tip and then drops away. This gives a fresh electrode surface for each measurement. During the test, the potential applied to the DME changes...

voltammetric techniques: pulse voltammetry

voltammetric-techniques-pulse-voltammetry

Pulse Voltage Methods for Electrochemistry

Pulse voltage methods in electrochemistry use small, timed changes in voltage to measure current. Differential-pulse voltammetry, or DPV, adds short potential pulses to a linear sweep across an electrochemical cell. These pulses are usually in the millivolt range and appear at regular intervals. Each pulse lasts only a few milliseconds. DPV works by creating faradaic current at the electrode surface. Faradaic current comes from redox reactions, where electrons move between the electrode and...

voltammetric techniques: cyclic voltammetry

voltammetric-techniques-cyclic-voltammetry

Reading Redox Peaks with Cyclic Voltammetry

Cyclic voltammetry measures how current changes as the potential at an electrode is varied. It is used to study the redox properties of a chemical species, meaning how easily it is oxidized or reduced. The setup uses three electrodes in an electrolyte solution. The working electrode is where the redox reaction of interest takes place. The reference electrode provides a stable potential, and the counter electrode completes the circuit for current flow. The process starts at a chosen initial...

voltammetry: stripping methods

Trace Analysis by Stripping Voltammetry

Stripping voltammetry is an electrochemical method used to detect trace amounts of analytes in solution. It works by applying a potential to an electrode and measuring the current that results. The current is then used to estimate the analyte concentration. Anodic Stripping Voltammetry, or ASV, is used for metals and metalloids at very low levels. It has two main steps: deposition and stripping. First, a negative potential reduces analyte ions and deposits them on the working electrode surface.

amperometry: overview

Measuring Analytes with Amperometric Sensors

Amperometry measures specific analytes in a solution by tracking the electric current from an electrochemical reaction. A constant potential is applied between a working electrode and a reference electrode. The current that results is proportional to the analyte concentration. The Clark oxygen electrode is a common example of amperometry. It uses a cathode and an anode sealed inside a gas-permeable membrane. The cathode is typically platinum or gold, and the anode is silver. When a potential...

key terms and concepts

Learn how controlled-current coulometry uses constant current, Faraday’s laws, and generated titrants to measure analytes in redox and acid-base titrations.

Controlled-Current Coulometry in Redox Analysis | JoVE K12

analytical chemistry

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Controlled-Current Coulometry in Redox Analysis

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