Name: salt effect on salt solubility
Uploaded: 2024-04-04T10:51:15.000Z
Duration: 1 min 19 s
Channel: JoVE

L’ajout de 50 mmol de nitrate de potassium à un litre de solution aqueuse saturée de sulfate de calcium entraîne une augmentation d’environ 30 % des concentrations d’ions calcium et sulfate dissous.
Avec les contre-ions déjà présents, les ions de charge opposée du sel inerte ajouté – le nitrate de potassium – contribuent aux atmosphères ioniques autour du cation primaire et de l’anion.
En conséquence, les charges nettes des ions calcium et sulfate diminuent, ce qui entraîne moins d’attraction entre eux. Ainsi, l’équilibre de solubilité se déplace vers les ions dissociés.
Cette tendance réduite des cations et des anions à s’associer se reflète dans leur concentration accrue dans la solution, améliorant ainsi la solubilité du sulfate de calcium peu soluble.
Plus la force ionique d’une solution est élevée, plus la charge sur l’atmosphère ionique est importante et plus la dissociation du sel en ions est importante.
Ce phénomène est connu sous le nom d’effet de sel, d’électrolyte ou d’ions divers.
Cet effet est plus important pour les sels peu solubles ayant une charge plus importante sur les ions constitutifs.

force ionique : effets sur les équilibres chimiques

L'ajout d'un composé ionique inerte augmente la solubilité d'un sel peu soluble. Par exemple, l’ajout de nitrate de potassium à une solution saturée de sulfate de calcium améliore considérablement la solubilité du sulfate de calcium. Le principe de Le Châtelier ne peut pas prédire ce changement d’équilibre. Au lieu de cela, cela pourrait s’expliquer par des changements dans la concentration efficace des ions en solution en présence de sel inerte ajouté.

Dans cette solution, le cation primaire – l’ion calcium – est entouré par l’anion primaire – l’ion sulfate – qui à son tour est entouré d’ions calcium, formant des atmosphères ioniques autour de chaque ion. De plus, le cation primaire et l'anion sont entourés par les ions de charges opposées du sel inerte ajouté. Les ions du composé inerte dans l’atmosphère ionique font diminuer la charge nette des ions primaires – calcium et sulfate, dans ce cas –, réduisant ainsi la fréquence des précipitations. Cela déplace l'équilibre vers l'ion dissocié, augmentant la solubilité du sel peu soluble. Ce phénomène est appelé effet sel, effet électrolyte ou effet divers ions.

L'effet salin dépend fortement de la force ionique de la solution. Avec une augmentation de la force ionique de la solution, davantage d'ions diffusent dans l'atmosphère ionique, ce qui entraîne une charge nette encore plus faible sur l'ion primaire, ce qui facilite ainsi une plus grande dissociation du sel. De plus, la charge des ions constituant le sel peu soluble affecte l’étendue de l’effet du sel. Par exemple, la solubilité des ions doublement chargés, tels que ceux constituant le sulfate de baryum, est plus influencée que la solubilité des ions chargés une seule fois, tels que ceux constituant le chlorure d'argent, par la même concentration de nitrate de potassium.

Force ionique : effets sur les équilibres chimiques

L’ajout de 50 mmol de nitrate de potassium à un litre de solution aqueuse saturée de sulfate de calcium conduit aux concentrations de calcium et de sulfate dissous ...

ionic-strength-effects-on-chemical-equilibria

Education

Enseignement

JoVE Core

Analytical Chemistry

Chimie analytique

Unit 1

Unité 1

Chemical Equilibria

Équilibres chimiques

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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, or a cloud of nearby oppositely charged ions. In the calcium sulfate example, calcium ions and sulfate ions also interact with the ions from the added inert salt.

These added ions lower the net charge around the primary ions. That reduces the chance of precipitation and favors the dissociated ions in solution. As a result, the equilibrium shifts toward more dissolved salt. This effect is also called the electrolyte effect or diverse ion effect.

The size of the salt effect depends on ionic strength, which is a measure of how many ions are present in the solution. As ionic strength increases, more ions enter the ionic atmosphere, and the net charge on the primary ion drops further. This makes dissociation easier and can raise the solubility even more. The charge of the ions in the sparingly soluble salt also matters. Doubly charged ions, such as those in barium sulfate, are affected more strongly by the same potassium nitrate concentration than singly charged ions, such as those in silver chloride.

Salt Effect on Salt Solubility

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

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-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...

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

See how added inert salts can increase the solubility of sparingly soluble salts, why ionic strength matters, and why barium sulfate is affected more than silver chloride.

Salt Effect on Salt Solubility | JoVE K12

analytical chemistry

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Salt Effect on Salt Solubility

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