Name: finding ph in acid mixtures
Uploaded: 2025-05-22T09:06:25.000Z
Duration: 1 min 19 s
Channel: JoVE

K12 Standards

Alabama Course of Study Science

9-12

Chemistry

Matter and Its Interactions

9-12.C.1.6.e

Mixtures of Acids

Dans un mélange d’acide fort et d’acide faible, l’acide fort se dissocie complètement, augmentant considérablement la concentration d’ionsH3O+, tandis que l’acide faible ne se dissocie que partiellement.
Le principe de Le Châtelier explique que plus l'acide est fort, plus il supprime la dissociation de l'acide le plus faible en déplaçant l'équilibre vers les réactifs.
Considérons un mélange de 0,15 M HCl et 0,30 M HCN. L’acide fort donne une concentration de H3O+ de 0,15 M, ce qui rend négligeable la concentration de H3O+ de l’acide faible.
Ainsi, le pH d’un mélange d’acides est principalement dicté par la concentration de l’acide fort.
De même, lorsqu’un mélange de deux acides faibles de quantités égales est présent, l’acide relativement plus fort est le principal déterminant du pH.
Par exemple, dans un mélange de HF et de HCN, HF sera le principal déterminant du pH du mélange, car il a un Ka de 3,5 × 10−4, ce qui est près d'un million de fois plus élevé que le Ka de l'acide cyanhydrique.

Aucun conflit d’intérêts n’a été déclaré.

mélange d'acides

Le pH d'une solution contenant un acide peut être déterminé en utilisant sa constante de dissociation acide et sa concentration initiale. Si une solution contient deux acides différents, son pH peut être déterminé à l'aide de plusieurs méthodes en fonction de la force relative des acides et de leurs constantes de dissociation.

Dans un mélange d’acides forts et faibles, l’acide fort se dissocie complètement et devient une source de presque tous les ions hydronium présents dans la solution. En revanche, l’acide faible présente une dissociation partielle et produit une concentration négligeable d’ions hydronium. La forte concentration d'ions hydronium produite par l'acide fort réduit encore la dissociation de l'acide faible. Selon le principe de Le Chatelier, cela se produit : « Lorsqu’un système chimique en équilibre est perturbé, le système se déplace dans une direction qui minimise la perturbation. » Les ions hydronium en excès produits par l’acide fort perturbent l’équilibre et la réaction se déroulera donc dans le sens inverse jusqu’à ce que l’équilibre soit établi. Cela conduit à une diminution de la dissociation de l'acide faible. En raison de cette diminution, le pH d’un mélange d’acides forts et faibles peut être calculé uniquement à partir de la concentration de l’acide fort. Par exemple, le pH d’un mélange contenant une concentration égale d’acide chlorhydrique (HCl), un acide fort, et d’acide formique (HCHO2), un acide faible, peut être déterminé à partir de la concentration de HCl uniquement. Si la concentration de HCl dans le mélange est de 0,0020 M, son pH peut être calculé comme suit.

pH = -log(0,002) = 2,7

Ici, la concentration d'ions hydronium produits par HCHO2 et l'autoionisation de l'eau sont négligeables et peuvent donc être ignorées.

Un mélange de deux acides faibles avec des constantes de dissociation différentes

Dans un mélange de deux acides faibles, le pH d’un mélange sera déterminé par l’acide le plus fort si sa constante de dissociation est nettement supérieure à celle de l’acide le plus faible. Par exemple, dans un mélange avec une concentration égale d’acide nitreux (HNO2) et d’acide hypochloreux (HClO), le HNO2 sera le principal déterminant du pH du mélange puisque son Ka (4,6 × 10−4) est environ 10 000 fois supérieur au Ka (2,9 × 10−8) de HClO. Selon le principe de Le Chatelier, HClO présente une diminution de la dissociation en présence de HNO2.

Mélange d'acides

Dans un mélange d’acide fort et d’acide faible, l’acide fort se dissocie complètement, augmentant considérablement la concentration d’ions H3O+, tandis que l’acide faible ...

Education

Enseignement

JoVE Core

Analytical Chemistry

Chimie analytique

Unit 1

Unité 1

Acid–Base Titration

Titrages acido-basiques

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

finding ph in acid mixtures

Finding pH in acid mixtures depends on the acids present and their dissociation constants. When a solution contains two acids, the pH can be found by choosing the method that matches their relative strengths. The key idea is that the stronger acid often controls the hydronium ion level, which is the amount of H3O+ in solution.

In a mixture of a strong acid and a weak acid, the strong acid dissociates completely. It becomes the main source of hydronium ions in the solution. The weak acid dissociates only partly, so it adds a very small amount of hydronium ions. The high hydronium ion concentration from the strong acid also suppresses the weak acid.

This effect follows Le Chatelier’s principle. When an equilibrium system is disturbed, it shifts to reduce the disturbance. In this case, the extra hydronium ions from the strong acid push the weak acid equilibrium in the reverse direction. As a result, the weak acid dissociates less. Because of this, the pH of a strong and weak acid mixture can be calculated from the strong acid concentration only.

For example, a mixture with equal concentrations of hydrochloric acid, HCl, and formic acid, HCHO2, can be treated using the HCl concentration alone. If the HCl concentration is 0.0020 M, then pH = −log(0.002) = 2.7. The hydronium ions from HCHO2 and the autoionization of water are negligible here and can be ignored.

A mixture of two weak acids works differently. If one weak acid has a much larger dissociation constant, Ka, than the other, it will usually determine the pH. For example, in a solution with equal concentrations of nitrous acid, HNO2, and hypochlorous acid, HClO, HNO2 is the main factor because its Ka value, 4.6 × 10−4, is about 10,000 times higher than the Ka of HClO, 2.9 × 10−8. Le Chatelier’s principle also explains why HClO dissociates less in the presence of HNO2.

Finding pH in Acid Mixtures

acid&#8211;base titration: overview

Titration Curves and pH Changes

Acid-base titration uses a known acid or base to find the concentration of an unknown solution. A base can titrate an acid, or an acid can titrate a base. As titrant is added little by little, the pH changes in a predictable way. The pH change is shown on a titration curve. A titration curve plots how the pH of the solution changes as more titrant is added. Scientists use this curve to follow the progress of the titration and to identify the endpoint, which is the point used to stop the...

titration of a strong acid with a strong base

titration-of-a-strong-acid-with-a-strong-base

pH Changes in Strong Acid-Base Titration

pH changes in a strong acid-strong base titration depend on the amount of leftover hydronium or hydroxide ions. A strong acid such as hydrochloric acid fully dissociates in water. That gives the solution hydronium ions and chloride ions, so the starting pH is low. When sodium hydroxide is added, it neutralizes hydronium ions. As more base is added, the concentration of hydronium ions falls and the pH rises. At the equivalence point, the hydronium ions have been neutralized and the solution is...

titration of a weak acid with a strong base

titration-of-a-weak-acid-with-a-strong-base

Weak Acid Titration Curve and pH Stages

Weak acid titration with a strong base changes pH in clear stages. The acid in this example is acetic acid, and the base is sodium hydroxide. Each stage uses a different pH calculation method. At the start, the pH of the weak acid is found with its dissociation constant, Ka, and an ICE table. An ICE table tracks the initial, change, and equilibrium amounts of the acid. This step describes the acid before much base is added. As sodium hydroxide is added, a buffer forms. A buffer is a solution...

titration of a weak base with a strong acid

titration-of-a-weak-base-with-a-strong-acid

Ammonia Titration Curve with HCl

Ammonia titration with hydrochloric acid shows how a weak base behaves as acid is added. The curve is the mirror image of a weak acid titrated with a strong base. In this case, ammonia acts as the weak base, and hydrochloric acid acts as the strong acid. The starting pH of 50 mL of 0.1 M ammonia is found with an ICE table and the Kb value. That calculation gives an initial pH of 11.11. As hydrochloric acid is added, the pH begins to fall. After 25 mL of 0.1 M hydrochloric acid is added, the...

titration of a weak acid with a weak base

titration-of-a-weak-acid-with-a-weak-base

Finding the Endpoint in Weak Acid–Base Titration

Weak acid and weak base titration is used to follow how pH changes as base is added. In this kind of titration, both the acid and the base dissociate only partly. The titration curve shows a slow, gradual pH change instead of a sharp jump. A common example is the titration of acetic acid with ammonia. During the reaction, these two species form ammonium acetate and water. Because the pH changes so gradually, there is no simple indicator that shifts color sharply at the equivalence point. One...

solution composition during acid/base titrations

solution-composition-during-acidbase-titrations

Acetic Acid Buffer Changes in Titration

Acetic acid buffer changes during a weak acid–strong base titration. When acetic acid is titrated with sodium hydroxide, the reaction forms water and the conjugate base of the acid, sodium acetate. The mixture of acetic acid and sodium acetate acts as a buffer, which helps show how solution makeup shifts during the titration. The relative amounts of acetic acid and acetate ions are tracked with α values. Here, α0 represents the equilibrium concentration of acetic acid, and α1 represents the...

mixtures of acids

Finding pH in acid mixtures depends on the acids present and their dissociation constants. When a solution contains two acids, the pH can be found by choosing the method that matches their relative strengths. The key idea is that the stronger acid often controls the hydronium ion level, which is the amount of H3O+ in solution. In a mixture of a strong acid and a weak acid, the strong acid dissociates completely. It becomes the main source of hydronium ions in the solution. The weak acid...

titration in nonaqueous solvents

Nonaqueous Solvents for Sharper Titration Endpoints

Nonaqueous solvents can make acid-base titrations easier to read by giving sharper endpoints. Most acid-base titrations are done in water, but water can compete with weak acids or bases for proton donation or acceptance. That competition can blur the titration curve and make the endpoint less clear. Water also changes the partial ionization of weak acids and bases. For example, water can accept a proton from acetic acid to form hydronium and acetate ions. The hydronium ion is a stronger acid...

titration of polyprotic base with a strong acid

titration-of-polyprotic-base-with-a-strong-acid

Carbonate Buffering in Acid Titration

The titration of sodium carbonate with hydrochloric acid shows how a polyprotic base reacts in stages. A polyprotic base can accept more than one proton, so its titration curve has two equivalence points. These points mark where one form has been fully changed into the next. At the first equivalence point, carbonate ions are completely converted to bicarbonate ions. The region before this point is the carbonate/bicarbonate buffer. In this buffer region, both forms are present in the solution. <br...

titration of polyprotic acids with a strong base

titration-of-polyprotic-acids-with-a-strong-base

Polyprotic Acid Titration Curve Features

Polyprotic acid titration curves show how acids with more than one ionizable proton react with a strong base. Each proton is released in a separate step. Each step has its own dissociation constant, or Ka, and each later Ka is weaker than the one before it. When a polyprotic acid such as sulfurous acid is titrated with sodium hydroxide, the base first neutralizes the first ionizable proton. This forms an intermediate species, such as hydrogen sulfite ions. The first part of the curve looks...

composition of polyprotic acid solutions as a function of ph

composition-of-polyprotic-acid-solutions-as-a-function-of-ph

Polyprotic Acid Titration Fractions at Each pH

Polyprotic acid titration shows how the main species in solution change as pH changes. In a compound of the form H2M, there are two ionizable protons. That means the titration with a base produces two equivalence points on the curve. As base is added, three species can be present in the solution: H2M, HM−, and M2−. Their amounts are shown as fractions called α0, α1, and α2. These alpha values track how much of each form is present at different stages of the titration. At the start of the...

buffers: overview

Buffers and pH Stability

Buffers help keep the pH of a solution stable when small amounts of acid or base are added. They are made from a weak acid and its conjugate base, or from a weak base and its conjugate acid. This balance lets the solution resist large pH changes until the buffer capacity is exceeded. One common buffer uses acetic acid and sodium acetate. Here, CH3COOH acts as the weak acid and CH3COONa provides its salt. Another buffer uses ammonia and ammonium chloride, written as NH3 and NH4Cl. These...

buffers: buffer capacity

Buffer Capacity and pH Resistance

Buffer capacity describes how well a buffer resists changes in pH. It is a quantitative measure, so it tells how much acid or base a one-liter buffer solution can take before its pH shifts by 1 unit. In the equation, beta is the buffer capacity, while Ca and Cb represent the moles of acid and base added. The term dpH means the change in pH. A graph of pH versus the moles of base added to a weak acid with a pKa of 5 shows how buffering works as more base is added. Taking the derivative of that...

leveling effect

How Solvents Limit Acid and Base Strength

The leveling effect explains how a solvent can limit acid and base strength in solution. In acid-base chemistry, the solvent sets a ceiling on how strong an acid or base can act. Stronger acids or bases may react with the solvent itself instead of with the intended compound. When a base is stronger than the conjugate base of the solvent, it deprotonates the solvent first. A conjugate base is the species left after an acid loses a proton. This reaction keeps going until the stronger base is...

key terms and concepts

Learn how pH is determined in acid mixtures, from strong acid and weak acid combinations to two weak acids with different Ka values.

Finding pH in Acid Mixtures | JoVE K12

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Finding pH in Acid Mixtures

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