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

Miareczkowanie kulometryczne mierzy ilość analitu w roztworze za pomocą wzorcowego odczynnika, który oddziałuje z analitem i jest wytwarzany elektrolitycznie przez stały prąd.
Galwanostat zapewnia regulowany prąd, a prekursor standardowego odczynnika jest w nadmiarze, zapewniając 100% wydajność prądową.
W typowej celi miareczkowej elektroda robocza wytwarza odczynnik w roztworze testowym, który następnie reaguje z analitem.
Elektroda pomocnicza lub przeciwelektroda, która uzupełnia obwód elektryczny, pozostaje odizolowana od roztworu masowego, aby zapobiec zakłóceniom powodowanym przez produkty reakcji.
W niektórych przypadkach, zamiast izolować przeciwelektrodę, odczynnik jest wytwarzany zewnętrznie w izolowanej komórce generacyjnej przed dodaniem do celi miareczkowej.
W miareczkowaniu kulometrycznym energia elektryczna początkowo przepływa przez ogniwo przez długi czas, stopniowo zmniejszając się w miarę zbliżania się do równoważności chemicznej, jak dozowanie coraz mniejszej ilości płynu z biurety.
Punkt końcowy jest identyfikowany przez odczynnik reagujący z obwodem detektora, a ilość analitu jest obliczana na podstawie prądu i czasu wymaganego do zakończenia miareczkowania.

kontrolowana kulometria prądowa miareczkowanie kulometryczne

Controlled-Current Coulometry: Coulometric Titration

Miareczkowanie kulometryczne to forma analizy miareczkowej, w której odczynnik jest generowany elektrycznie, a jego ilość jest oceniana na podstawie prądu i czasu generowania. Elektron służy jako odczynnik standardowy. Procedura jest podobna do konwencjonalnych miareczkowań, takich jak wykrywanie punktu końcowego.

Podstawowymi wymaganiami dla miareczkowań kulometrycznych są (1) 100% wydajność reakcji elektrody generującej odczynnik i (2) stechiometryczna i najlepiej szybka reakcja między wygenerowanym odczynnikiem a określoną substancją. Odczynnik może być generowany bezpośrednio w roztworze testowym lub zewnętrznie w oddzielnym roztworze.

Miareczkowanie kulometryczne oferuje kilka zalet, w tym wysoką czułość dzięki dokładnemu pomiarowi małych ilości elektryczności. Ponadto nie wymaga roztworów standardowych, może wykorzystywać niestabilne odczynniki, umożliwia generowanie małych ilości titrantów, unika rozcieńczania próbki, zapewnia dokładniejsze wyniki dzięki wstępnemu miareczkowaniu i jest dostosowane do zdalnego i automatycznego sterowania.

Titratory kulometryczne wymagają stałego źródła prądu i integratora do pomiaru iloczynu prądu i czasu. Komora miareczkowania zazwyczaj składa się z zlewki, generatora, elektrod pomocniczych i wskaźnikowych.

Ogólne procedury miareczkowania kulometrycznego obejmują ustawienie komory elektrolizera z elektrodami generatora i wskaźnikowymi, naładowanie komory roztworem testowym, dostosowanie prądu i umożliwienie przebiegu reakcji. Punkt końcowy miareczkowania jest oceniany na podstawie zmiany prądu podczas miareczkowania. Elektroda pomocnicza lub przeciwelektroda, która zamyka obwód elektryczny, pozostaje odizolowana od roztworu głównego, aby zapobiec zanieczyszczeniu. Odczynnik jest czasami generowany zewnętrznie przed dodaniem do naczynia miareczkowania.

Punkty końcowe w miareczkowaniu kulometrycznym można wykryć za pomocą wskaźników chemicznych, obserwacji potencjometrycznych lub procedur amperometrycznych. Metody te polegają na ustaleniu warunków, które powodują, że substancja oznaczana lub titrant reagują na elektrodzie wskaźnikowej, wytwarzając prąd proporcjonalny do stężenia substancji elektroaktywnej.

Ograniczenia miareczkowania kulometrycznego z wewnętrznym wytwarzaniem titranta obejmują obecność substancji, które mogą reagować na elektrodach generatora i trudności w zlokalizowaniu punktu równoważnikowego metodami amperometrycznymi dla próbek w skali makro. Ograniczenia te można przezwyciężyć, generując zewnętrznie odczynnik przy stałym prądzie ze 100% wydajnością w zewnętrznej celi generatora, która jest następnie dostarczana do celi miareczkowania. Pomimo niewielkiej wady rozcieńczania zawartości celi miareczkowania, procedura ta dobrze nadaje się do automatycznej kontroli i oznaczania substancji obecnych w małych ilościach.

Kontrolowana kulometria prądowa Miareczkowanie kulometryczne

Miareczkowanie kulometryczne jest formą analizy miareczkowej, w której odczynnik jest generowany elektrycznie, a jego ilość jest oceniana na podstawie prądu i ...

controlled-current-coulometry-coulometric-titration

Education

JoVE Core

Analytical Chemistry

Chemia analityczna

Unit 1

Jednostka 1

Electrochemical Analyses and Redox Titration

Analizy elektrochemiczne i miareczkowanie redoks

electrochemical-analyses-and-redox-titration

KEY TERMS AND CONCEPTS

KLUCZOWE TERMINY I POJĘCIA

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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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...

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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...

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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

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...

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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...

electrogravimetric analysis: overview

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

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...

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Voltammogram Shapes and Redox Analysis

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Polarography and Voltammetry with Mercury Drops

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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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