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

La valoración coulométrica mide la cantidad de un analito en solución utilizando un reactivo estándar que interactúa con el analito y se genera electrolíticamente mediante una corriente constante.
Un galvanostato proporciona la corriente regulada y el precursor del reactivo estándar está en exceso, lo que garantiza una eficiencia de corriente del 100%.
En una celda de valoración típica, el electrodo de trabajo genera el reactivo dentro de la solución de prueba, que luego reacciona con el analito.
El electrodo auxiliar o contraelectrodo, que completa el circuito eléctrico, permanece aislado de la solución a granel para evitar interferencias de sus productos de reacción.
En algunos casos, en lugar de aislar el contraelectrodo, el reactivo se produce externamente en una celda de generación aislada antes de agregarse a la celda de valoración.
En la valoración coulométrica, la electricidad fluye inicialmente a través de la célula durante largos períodos, disminuyendo gradualmente a medida que se acerca a la equivalencia química, como si se dispensara cada vez menos líquido de una bureta.
El punto final se identifica mediante la reacción del reactivo con el circuito del detector, y la cantidad de analito se calcula utilizando la corriente y el tiempo necesarios para completar la valoración.

titulación coulométrica con corriente controlada

Controlled-Current Coulometry: Coulometric Titration

Las titulaciones coulométricas son una forma de análisis titrimétrico donde el reactivo se genera eléctricamente y su cantidad se evalúa en función de la corriente y el tiempo de generación. El electrón actúa como reactivo estándar. El procedimiento es similar a las titulaciones convencionales, como la detección de punto final.

Los requisitos fundamentales para las titulaciones coulométricas son (1) 100 % de eficiencia en la reacción del electrodo generador de reactivo y (2) una reacción estequiométrica y preferiblemente rápida entre el reactivo generado y la sustancia determinada. El reactivo se puede generar directamente dentro de la solución de prueba o externamente en una solución separada.

La titulación coulométrica ofrece varias ventajas, incluida una alta sensibilidad debido a la medición precisa de pequeñas cantidades de electricidad. Además, no requiere soluciones estándar, puede utilizar reactivos inestables, permite la generación de pequeñas cantidades de titulantes, evita la dilución de la muestra, proporciona resultados más precisos mediante la titulación previa y es adaptable al control remoto y automático.

Los tituladores coulométricos requieren una fuente de corriente constante y un integrador para medir el producto de la corriente y el tiempo. La celda de titulación consta típicamente de un vaso de precipitados, un generador, electrodos auxiliares e indicadores.

Los procedimientos generales para las titulaciones coulométricas implican configurar la celda de electrólisis con electrodos generadores e indicadores, cargar la celda con la solución de prueba, ajustar la corriente y permitir que la reacción continúe. El punto final de la titulación se evalúa en función del cambio de corriente durante la titulación. El electrodo auxiliar o contraelectrodo, que completa el circuito eléctrico, permanece aislado de la solución a granel para evitar la contaminación. El reactivo a veces se genera externamente antes de agregarlo al recipiente de titulación.

Los puntos finales en las titulaciones coulométricas se pueden detectar utilizando indicadores químicos, observaciones potenciométricas o procedimientos amperométricos. Estos métodos se basan en el establecimiento de condiciones que hacen que la sustancia que se está determinando o el titulador reaccionen en un electrodo indicador, produciendo una corriente proporcional a la concentración de la sustancia electroactiva.

Las limitaciones de la titulación coulométrica con generación interna del reactivo de titulación incluyen la presencia de sustancias que pueden reaccionar en los electrodos del generador y las dificultades para localizar el punto de equivalencia mediante métodos amperométricos para muestras a escala macroscópica. Estas limitaciones se pueden superar generando externamente el reactivo a una corriente constante con una eficiencia del 100 % en una celda generadora externa, que luego se suministra a la celda de titulación. A pesar de su pequeña desventaja de diluir el contenido de la celda de titulación, este procedimiento es adecuado para el control y la determinación automáticos de sustancias presentes en pequeñas cantidades.

Titulación coulométrica con corriente controlada

Las valoraciones coulométricas son una forma de análisis valorativo en la que el reactivo se genera eléctricamente y su cantidad se evalúa en función de la corriente y la ...

controlled-current-coulometry-coulometric-titration

Education

Educación

JoVE Core

Analytical Chemistry

Química Analítica

Unit 1

Unidad 1

Electrochemical Analyses and Redox Titration

Análisis electroquímicos y valoración redox

electrochemical-analyses-and-redox-titration

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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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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...

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Potentiometry and Electrochemical Cell Potential

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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...

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Membrane Electrodes in Ion Sensing

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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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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...

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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 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-factors-affecting-measurements

Voltammetry and Faradaic Current

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Voltammogram Shapes and Redox Analysis

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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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