Name: direct, back, and displacement titrations
Uploaded: 2024-04-04T10:51:16.000Z
Duration: 1 min 8 s
Channel: JoVE

滴定法は、分析化学で使用される定量法です。
例えば、容量滴定では、分析対象物と完全に反応するために必要な既知の濃度の滴定液の体積を測定します。
一方、重量滴定では、分析物が滴定剤と反応して沈殿物を生成し、この沈殿物を分離して秤量します。
あるいは、電量滴定では、電気化学反応を完了するために必要な電荷を測定します。
滴定の戦略には、滴定液が終点に到達するまで滴定物と反応する直接滴定が含まれます。
反応が遅い場合、または直接滴定反応がない場合は、他の戦略が使用されます。
逆滴定では、分析種の消費に使用される標準溶液の過剰量を、2つ目の標準溶液で滴定することにより測定します。
分析種が複合体からグループを置換する場合、置換されたグループの量は変位滴定を使用して決定されます。

滴定方法: 種類と一般的に使用される戦略

化学における滴定法は、容積法、重量法、電量法の 3 種類に大別されます。容積滴定には、分析物と完全に反応するために必要な既知の濃度の滴定剤の容積を測定することが含まれます。重量滴定では、標準溶液が分析対象物と反応して不溶性の沈殿物を形成し、これを濾過、乾燥させ、重量を量ります。電量滴定では、反応が完了するまで電気化学反応に電流が適用され、この反応に必要な電荷が計算されて、反応物の量または濃度の決定に役立ちます。

一般に、滴定では、分析対象物が消費されるまで、つまり終点に達するまで、既知の濃度の滴定剤が分析対象物に追加されます。この場合、このプロセスは直接滴定と呼ばれます。場合によっては、反応が遅すぎて明確な終点が得られないことがあります。このような場合には、逆滴定が使用されます。逆滴定では、検体を過剰量の標準液と反応させ、この標準液の残りの量を 2 番目の標準液で滴定します。分析物と反応するのに必要な最初の標準溶液の正確な量を計算できます。

場合によっては、直接滴定または間接（逆）滴定のいずれにも適切な指示薬が見つからないことがあります。この場合、置換滴定を使用できます。この場合、検体が通常複合体から試薬を置換し、置換された試薬が滴定によって測定されます。

化学では、滴定法は体積、重量、電量計の3つのタイプに大別されます。体積滴定には...

titrimetric-methods-types-and-commonly-used-strategies

Education

JoVE Core

Analytical Chemistry

分析化学

Unit 1

ユニット1

Chemical Equilibria

化学平衡

KEY TERMS AND CONCEPTS

主要用語と概念

Titrimetric Methods: Types and Commonly Used Strategies

direct, back, and displacement titrations

Titrimetric methods use a standard solution to measure the amount of an analyte in chemistry. The main categories are volumetric, gravimetric, and coulometric methods. Each one reaches the endpoint in a different way.

In volumetric titrations, the key measurement is the volume of a titrant with a known concentration. The titrant is added until it reacts completely with the analyte. In gravimetric titrations, the standard solution forms an insoluble precipitate with the analyte. That solid is then filtered, dried, and weighed.

Coulometric titrations use electric current instead of measuring a liquid volume. Current is applied to drive an electrochemical reaction until it is complete. The charge needed for that reaction is then calculated to determine the amount or concentration of the reactant.

Most titrations stop when the analyte is used up at the endpoint, and this is called direct titration. If the reaction is too slow to give a sharp endpoint, back titration is used. In back titration, the analyte reacts with an excess of a standard solution, and the leftover amount is titrated with a second standard solution.

Sometimes neither direct titration nor back titration works well because no suitable indicator is available. In that case, displacement titration can be used. The analyte displaces a reagent, usually from a complex, and the displaced reagent is then measured by titration.

Direct, Back, and Displacement Titrations

ionic strength: overview

Ionic Strength in Salt and Acid Solutions

Ionic strength describes the total electrolyte concentration in a solution. It is a quantitative measure of how many ions are present and how strongly they contribute to the solution. The concept was introduced in 1921 by Gilbert N. Lewis and Merle Randall while they were describing the activity coefficient of strong electrolytes. To calculate ionic strength, written as I or μ, both cations and anions are included. The concentration of each ion matters, but the charge number z matters even...

ionic strength: effects on chemical equilibria

ionic-strength-effects-on-chemical-equilibria

Salt Effect on Salt Solubility

Salt effect can change the solubility of a sparingly soluble salt in water. When an inert ionic compound is added, the dissolved salt may become more soluble. A common example is potassium nitrate added to a saturated calcium sulfate solution, which increases the amount of calcium sulfate that dissolves. Le Châtelier’s principle does not explain this shift well. Instead, the change is described by the effective concentration of ions in solution. Each ion is surrounded by an ionic atmosphere,...

thermodynamics: chemical potential and activity

thermodynamics-chemical-potential-and-activity

Chemical Potential in Solution Activity

Chemical potential and activity describe how a species behaves in a solution. Activity is the effective concentration of a species. It accounts for the electrolytes near that species and depends on the ionic strength of the solution. Activity is calculated as molar concentration multiplied by the activity coefficient. This makes it a more precise way to describe a species in solution than concentration alone. The thermodynamic equilibrium constant is also defined more accurately using activity...

thermodynamics: activity coefficient

Activity Coefficient in Ionic Solutions

Activity coefficient in ionic solutions describes how much a species behaves like its measured concentration. Activity is the effective concentration of a solute in solution. It equals the molar concentration multiplied by the activity coefficient, which is a unitless number. The activity coefficient depends on the total ionic strength of the solution. It shows how far a solution is from ideal behavior. When ionic strength is very low, the activity coefficient of an ionic species is close to...

chemical equilibria: redefining equilibrium constant

chemical-equilibria-redefining-equilibrium-constant

Salt Effect in Equilibrium Constant Calculations

Salt effect changes how a sparingly soluble salt dissolves when an inert salt is added. The size of this effect depends on the ionic strength of the solution. Ionic strength reflects how strongly the ions in the solution interact. In solution, the activity of a species is the product of its concentration and its activity coefficient. A redefined equilibrium constant must account for these activity effects when the ionic strength is moderately high. This form is also called the thermodynamic...

factors affecting activity coefficient

How Ionic Strength Changes Activity Coefficient

The activity coefficient of an ion in water depends on ionic strength, ion charge, and ion size. The extended Debye-Hückel equation describes these effects for aqueous solutions at 25°C. These three factors are partly linked, so changes in one can influence the others. When a solution has almost zero ionic strength, the activity coefficient is close to one. That means the solution behaves almost ideally. As ionic strength rises from 0 to 0.1 mol/L, the activity coefficient usually drops.

chemical equilibria: systematic approach to equilibrium calculations

chemical-equilibria-systematic-approach-to-equilibrium-calculations

Equilibrium Calculations with Mass and Charge

Chemical equilibrium calculations become easier when they are organized around mass balance and charge balance. This approach is useful for systems with multiple equilibria, where several reactions must be considered together. It is especially helpful when finding the solubility of a sparingly soluble salt in water with a common ion present. The first step is to identify every chemical reaction in the system. Next, write the equilibrium constant expression for each reaction. These expressions...

acid&#8211;base equilibria: activity-based definition of ph

acidbase-equilibria-activity-based-definition-of-ph

How Ionic Strength Changes pH

pH depends on hydrogen ion activity, not just hydrogen ion concentration. In an ideal solution, pH is the negative logarithm of the hydrogen ion concentration. In a non-ideal solution, a more accurate pH uses hydrogen ion activity, which combines concentration with an activity coefficient. This difference matters most in very low ionic strength solutions, such as pure water. At first, the hydrogen ion activity coefficient is close to one. When an electrolyte that does not donate or accept a...

ladder diagrams: acid&#8211;base equilibria

Using Ladder Diagrams to Predict pH

Ladder diagrams help show how acid-base equilibria change with pH. They are graphical tools that make the chemistry of a system easier to read during analysis. In a ladder diagram, the vertical axis shows pH. Horizontal bars, or steps, place the pKa values in the system.

ladder diagrams: redox equilibria

Redox Potential Shifts in Ladder Diagrams

Ladder diagrams show redox equilibrium using electrochemical potential, E, as the vertical scale. They help students see how changes in concentration affect which species is most stable. The areas of predominance are set with the Nernst equation. A useful example is the Fe3+/Fe2+ half-reaction, which has a standard-state potential of +0.771 V. At potentials more positive than +0.771 V, Fe3+ predominates. At potentials more negative than +0.771 V, Fe2+ predominates. Ladder diagrams also show...

ladder diagrams: complexation equilibria

Reading Metal-Ligand Ladder Diagrams

Ladder diagrams help show equilibria in metal-ligand complexes. In these diagrams, the vertical scale shows pL, or the concentration of free ligand that has not reacted. The horizontal lines mark the log of formation constants and show which species is dominant in each region. For the Cd and ammonia system, the formation constant K1 for Cd(NH3)2+ is 3.55 × 10^2. Its log value is 2.55, so pNH3 = 2.55 is the dividing line between Cd2+ and Cd(NH3)2+ dominance. Above 2.55, Cd2+ is the predominant...

solubility equilibria: overview

How Solubility Reaches Equilibrium

Solubility equilibrium describes what happens when a sparingly soluble salt dissolves in water and then reaches a balance with the solid left over. In a saturated solution, dissolved ions and the undissolved solid are in dynamic equilibrium. The process is reversible, so this topic is also called precipitation equilibrium. This balance matters in real life. Sugar-rich foods can produce organic acids that dissolve hydroxyapatite, the mineral in tooth enamel, which can lead to tooth decay. A...

solubility equilibria: ionic product of water

solubility-equilibria-ionic-product-of-water

Water Ionization and Kw in Equilibrium

Water ionization and the ionic product of water describe how pure water reaches equilibrium. Water is a weak electrolyte, so only a small amount breaks into hydrogen ions and hydroxide ions. At a fixed temperature, the amount of undissociated water stays almost constant. Because of this, the ionic product of water is the product of the hydrogen ion concentration and the hydroxide ion concentration. It is written as K w . The square root of K w gives the concentration of each ion in pure water.

complexation equilibria: overview

Stepwise Metal-Ligand Complexation

Stepwise metal-ligand complexation forms coordination compounds when metal ions and ligands share electrons through dative, or coordinate covalent, bonds. In these reactions, the metal ion binds to one or more ligands until its coordination number is reached. This number tells how many bonds form between the metal ion and the ligands. In aqueous solution, many metal ions are already surrounded by water molecules. These species are called aqua complexes. Complexation reactions are described by...

complexation equilibria: the chelate effect

complexation-equilibria-the-chelate-effect

Chelating Ligands and Metal Complex Stability

Metal complexes form when metal atoms or cations interact with ligands in a complexation reaction. These donor-acceptor adducts are central to understanding how metal ions bind in chemistry. Ligands can bind in different ways. A monodentate ligand uses one donor site. A bidentate ligand uses two donor sites, and a polydentate ligand uses more than two. Ethylene diamine is a bidentate ligand that binds through two nitrogen donor atoms and forms a five-membered ring. Polydentate ligands can...

complexation equilibria: factors influencing stability of complexes

complexation-equilibria-factors-influencing-stability-of-complexes

Complex Ions: Metal and Ligand Stability

Complex ions form when a metal cation accepts electron pairs from ligands, which are the electron pair donors. The stability of these metal complexes depends mainly on the metal ion and on the ligands attached to it. The metal ion’s size and charge play a major role in stability. As the size of the metal ion increases, complex stability usually decreases when the valency of the metal ion and the ligands stays the same. As the charge on the metal ion increases, complex stability increases. <br...

redox equilibria: overview

Redox Electron Transfer and Half-Reactions

Redox reactions involve the transfer of electrons from one species to another. In this type of reaction, electrons are not shared between atoms. They move from the species that loses them to the species that gains them. The species that loses electrons is the reducing agent, also called the reductant. It is oxidized during the reaction. The species that gains electrons is the oxidizing agent, or oxidant, and it is reduced. Redox reactions are often written as two half-reactions. One...

titrimetric methods: types and commonly used strategies

Titrimetric methods use a standard solution to measure the amount of an analyte in chemistry. The main categories are volumetric, gravimetric, and coulometric methods. Each one reaches the endpoint in a different way. In volumetric titrations, the key measurement is the volume of a titrant with a known concentration. The titrant is added until it reacts completely with the analyte. In gravimetric titrations, the standard solution forms an insoluble precipitate with the analyte. That solid is...

classification of titrimetric analysis based on reaction types

classification-of-titrimetric-analysis-based-on-reaction-types

Four Reaction Types in Titrimetric Analysis

Titrimetric analysis measures solution volume and is also called volumetric analysis. A standard solution with a known concentration is placed in the burette and is called the titrant. The solution with an unknown concentration in the flask is the analyte, or titrand. Titrimetric analysis is grouped into four types based on the reaction between the titrant and the analyte. Acid-base titrations involve a neutralization reaction between an acid and a base. This reaction forms water molecules.

key terms and concepts

Learn how volumetric, gravimetric, and coulometric titrations work, plus direct, back, and displacement titration methods used to find concentration.

Direct, Back, and Displacement Titrations | JoVE K12

JoVE Core 分析化学 は、複雑な化学原理をアニメーションで視覚化することで、SI 単位から NMR 分光法を含むトピックをよりわかりやすくします。実験を撮影した実写動画では、これらの概念が世界中の研究所でどのように応用されているかを取り上げています。

analytical chemistry

Direct, Back, and Displacement Titrations

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