Name: finding the endpoint in weak acid–base titration
Uploaded: 2024-04-04T10:51:19.000Z
Duration: 1 min 8 s
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PS.Chm.6.3

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

Titration of a Weak Acid with a Weak Base

A titulação de um ácido fraco como o ácido acético com uma base fraca como a amônia aquosa produz um sal de acetato de amônio neutro e água.
A curva de titulação do ácido fraco versus base fraca é plotada medindo o pH durante os vários pontos da titulação.
Na titulação de 100 mL de solução de ácido acético 0,1 M com 100 ml de amônia aquosa 0,1 M, o pH inicial é 2,87.
Se o ácido e a base fracos possuírem valores idênticos de Ka e Kb, o pH no ponto de equivalência é 7, sugerindo uma solução neutra. Se Ka for maior que Kb, o pH é menor que 7 no ponto de equivalência. Alternativamente, se Ka for menor que Kb, o pH é maior que 7.
A mudança de pH durante toda a curva de titulação é gradual, sem ponto final agudo observado. No entanto, uma combinação de indicadores neutros vermelho e azul de metileno pode ser usada para aproximar o ponto final.

Não há conflitos de interesse declarados.

titulação de um ácido fraco com uma base fraca

Ácidos e bases fracos não sofrem dissociação completa e as titulações entre estes dois raramente é estudadas. Quando tais estudos é realizados, digamos, para a titulação de um ácido fraco com uma base fraca, a curva de titulação representa graficamente a mudança no pH em função do volume de base adicionado. Leve em consideração a titulação do ácido acético com amônia, por exemplo. Durante a titulação, essas duas espécies formam acetato de amônio e água, mas a mudança de pH é lenta e gradual.

Como resultado, não existe um indicador simples que mude de cor bruscamente para ajudar a identificar com precisão o ponto de equivalência. Uma maneira relativamente simples de abordar isto é misturar dois indicadores – Nesse caso, vermelho neutro e azul de metileno – de modo que a mistura exiba uma mudança acentuada de cor no ponto de equivalência – Nesse caso, de azul-violeta para verde.

Para um ácido fraco e uma base fraca que possuem valores idênticos de Ka e Kb, o pH calculado no ponto de equivalência é 7. Se Ka for maior que Kb, o pH será menor que 7, tornando a solução final ácida. Contudo, se Ka for menor que Kb, o pH calculado é maior que 7, resultando em uma solução básica no ponto final.

Titulação de um ácido fraco com uma base fraca

A titulação de um ácido fraco como o ácido acético com uma base fraca como a amônia aquosa produz um sal de acetato de amônio neutro e água.
A titulação ...

Education

JoVE Core

Analytical Chemistry

Química Analítica

Unit 1

Unidade 1

Acid–Base Titration

Titulação Ácido-Base

KEY TERMS AND CONCEPTS

TERMOS E CONCEITOS-CHAVE

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 practical solution is to mix two indicators, neutral red and methylene blue. The mixed indicator gives a clearer color change at the endpoint, from violet-blue to green. For a weak acid and weak base with the same Ka and Kb values, the pH at the equivalence point is 7. If Ka is greater than Kb, the pH is less than 7 and the endpoint is acidic. If Ka is less than Kb, the pH is greater than 7 and the endpoint is basic.

Finding the Endpoint in Weak Acid–Base Titration

finding-the-endpoint-in-weak-acid-base-titration

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

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

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

Explore how a weak acid and weak base titration changes pH, why acetic acid and ammonia are a challenge, and how mixed indicators help locate the endpoint.

Finding the Endpoint in Weak Acid–Base Titration | JoVE K12

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Finding the Endpoint in Weak Acid–Base Titration

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