Name: electrically generated reagents in titration
Uploaded: 2024-12-05T18:31:57.000Z
Duration: 1 min 18 s
Channel: JoVE

Кулонометрическое титрование измеряет количество аналита в растворе с помощью стандартного реагента, который взаимодействует с анализируемым веществом и генерируется электролитически постоянным током.
Гальваностат обеспечивает регулируемый ток, а предшественник стандартного реагента находится в избытке, обеспечивая 100% эффективность по току.
В типичной ячейке для титрования рабочий электрод генерирует реагент в исследуемом растворе, который затем вступает в реакцию с анализируемым веществом.
Вспомогательный или противоэлектрический электрод, который завершает электрическую цепь, остается изолированным от объемного раствора для предотвращения помех от продуктов его реакции.
В некоторых случаях вместо изоляции противоэлектрода реагент производится извне в изолированной генерирующей ячейке перед добавлением в ячейку титрования.
При кулонометрическом титровании электричество первоначально проходит через ячейку в течение длительных периодов времени, постепенно уменьшаясь по мере приближения к химическому эквиваленту, как будто все меньше и меньше жидкости выдается из бюретки.
Конечная точка определяется по реагенту, вступляющему в реакцию с цепью детектора, а количество анализируемого вещества рассчитывается с использованием тока и времени, необходимых для завершения титрования.

Управляемая токовая кулонометрия_Кулонометрическое титрование

Controlled-Current Coulometry: Coulometric Titration

Кулонометрическое титрование — это форма титриметрического анализа, при которой реагент генерируется электрически, а его количество оценивается на основе тока и времени генерации. Электрон служит стандартным реагентом. Процедура похожа на обычные титрования, такие как обнаружение конечной точки.

Основными требованиями к кулонометрическому титрованию являются (1) 100% эффективность в реакции электрода, генерирующего реагент, и (2) стехиометрическая и предпочтительно быстрая реакция между генерируемым реагентом и определяемым веществом. Реагент может генерироваться непосредственно в тестовом растворе или внешне в отдельном растворе.

Кулонометрическая титриметрия имеет несколько преимуществ, включая высокую чувствительность благодаря точному измерению небольших количеств электричества. Более того, она не требует стандартных растворов, может использовать нестабильные реагенты, позволяет генерировать небольшие количества титранта, избегать разбавления образца, обеспечивает более точные результаты посредством предварительного титрования и адаптируется к дистанционному и автоматическому управлению.

Кулонометрические титраторы требуют постоянного источника тока и интегратора для измерения произведения тока и времени. Ячейка титрования обычно состоит из стакана, генератора, вспомогательных и индикаторных электродов.

Общие процедуры кулонометрического титрования включают настройку электролизной ячейки с генераторными и индикаторными электродами, зарядку ячейки тестовым раствором, регулировку тока и проведение реакции. Конечная точка титрования оценивается на основе изменения тока во время титрования. Вспомогательный или противоэлектрод, который замыкает электрическую цепь, остается изолированным от основного раствора для предотвращения загрязнения. Иногда реагент генерируется извне перед добавлением в сосуд для титрования.

Конечные точки в кулонометрическом титровании можно обнаружить с помощью химических индикаторов, потенциометрических наблюдений или амперометрических процедур. Эти методы основаны на создании условий, которые заставляют либо определяемое вещество, либо титрант реагировать на индикаторном электроде, создавая ток, пропорциональный концентрации электроактивного вещества.

Ограничения кулонометрического титрования с внутренней генерацией титранта включают наличие веществ, которые могут реагировать на электродах генератора, и трудности определения точки эквивалентности амперометрическими методами для макромасштабных образцов. Эти ограничения можно преодолеть путем внешней генерации реагента при постоянном токе с эффективностью 100% во внешней ячейке генератора, который затем доставляется в ячейку титрования. Несмотря на свой незначительный недостаток, заключающийся в разбавлении содержимого ячейки титрования, эта процедура хорошо подходит для автоматического контроля и определения веществ, присутствующих в малых количествах.

Кулонометрическое титрование — это форма титриметрического анализа, при которой реагент генерируется электрическим путем, а его количество оценивается на основе тока и ...

controlled-current-coulometry-coulometric-titration

Education

JoVE Core

Analytical Chemistry

Аналитическая химия

Unit 1

Раздел 1

Electrochemical Analyses and Redox Titration

Электрохимический анализ и окислительно-восстановительное титрование

electrochemical-analyses-and-redox-titration

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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 quickly and in a stoichiometric way, meaning it reacts in the correct ratio, with the substance being tested. The reagent may be formed directly in the test solution or made in a separate solution first.

Coulometric titrimetry has several practical advantages. It is very sensitive because it measures small amounts of electricity with high accuracy. It does not require standard solutions, and it can use unstable reagents. It also allows very small amounts of titrant to be made, avoids sample dilution, improves accuracy through pre-titration, and can be adapted for remote or automatic control.

A coulometric titrator needs a constant current source and an integrator to measure current over time. The titration cell usually includes a beaker, generator electrodes, an auxiliary or counter electrode, and an indicator electrode. The general procedure is to set up the electrolysis cell, add the test solution, adjust the current, and let the reaction proceed. The endpoint is found from the change in current during titration. The counter electrode completes the circuit but stays isolated from the bulk solution to prevent contamination.

Endpoints can be detected with chemical indicators, potentiometric measurements, or amperometric methods. These approaches depend on making either the analyte or the titrant react at an indicator electrode so that a current is produced. That current is proportional to the concentration of the electro-active substance.

There are limits to internal reagent generation. Substances in the sample may react at the generator electrodes, and amperometric methods can make it difficult to locate the equivalence point in macro-scale samples. These problems can be reduced by generating the reagent externally at a constant current with 100% efficiency and then delivering it to the titration cell. This method slightly dilutes the contents of the cell, but it is useful for automatic control and for measuring substances present in small amounts.

Electrically Generated Reagents in Titration

electrically-generated-reagents-in-titration

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...

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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...

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Potentiometry and Electrochemical Cell Potential

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Reference and Indicator Electrodes in Potentiometry

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Membrane Electrodes in Ion Sensing

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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...

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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...

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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...

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Electrode Weighing in Electrogravimetric Analysis

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coulometry: overview

Coulometry and Electrical Charge Analysis

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Measuring Analyte Charge by Potentiostatic Coulometry

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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...

controlled-current coulometry: coulometric 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...

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Voltammetry for Detecting Redox Species

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voltammetry: factors affecting measurements

voltammetry-factors-affecting-measurements

Voltammetry and Faradaic Current

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Voltammogram Shapes and Redox Analysis

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voltammetric-techniques-cyclic-voltammetry

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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 coulometric titrations use electrically generated reagents, constant current, and endpoint detection to measure small amounts of a substance.

Electrically Generated Reagents in Titration | JoVE K12

analytical chemistry

JoVE Core Analytical Chemistry simplifies complex chemical principles by breaking them down into clear, engaging animations, making topics from SI Units to NMR spectroscopy more accessible and straightforward. Additionally, the scientist-in-action videos highlight the application of these concepts in real-world research conducted in laboratories globally.

Electrically Generated Reagents in Titration

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