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

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.6.3

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

Titration of a Weak Acid with a Weak Base

酢酸のような弱酸とアンモニア水のような弱塩基を滴定すると、中性の酢酸アンモニウム塩と水が生成されます。
弱酸と弱塩基の滴定曲線は、滴定のさまざまなポイントでpHを測定することによってプロットされます。
100 mLの0.1 M酢酸溶液と100 mLの0.1 Mアンモニア水溶液の滴定では、初期pHは2.87です。
弱酸と弱塩基が同一のKA値とKBB値を有する場合、当量点でのpHは7であり、中性溶液を示唆しています。KaがKbより大きい場合、pHは当量点で7未満です。あるいは、KaがKbより小さい場合、pHは7より大きい。
滴定曲線全体にわたるpH変化は緩やかで、鋭い終点は観察されません。ただし、ニュートラルレッドとメチレンブルーのインジケーターを組み合わせて、エンドポイントを概算できます。

利益相反は宣言されていません。

弱塩基による弱酸の滴定

弱酸と弱塩基は完全には解離しないため、これら 2 つの間の滴定はほとんど研究されません。たとえば、弱塩基による弱酸の滴定などの研究を行う場合、滴定曲線は、加えた塩基の量の関数として pH の変化をプロットします。たとえば、アンモニアによる酢酸の滴定を考えてみましょう。滴定中、これら 2 つの種は酢酸アンモニウムと水を形成しますが、pH の変化はゆっくりと緩やかです。


その結果、当量点を正確に特定するのに役立つ、色を急激に変える簡単なインジケーターはありません。これにアプローチする比較的簡単な方法は、2 つの指示薬 (この場合はニュートラルレッドとメチレンブルー) を混合し、混合物が当量点で鋭い色の変化 (ここでは紫青から緑) を示すようにすることです。
 

同じ Ka 値と Kb 値を持つ弱酸と弱塩基の場合、当量点で計算される pH は 7 です。Ka が Kb より大きい場合、pH は 7 未満となり、終点溶液は酸性になります。ただし、Ka が Kb より小さい場合、計算された pH は 7 より大きくなり、終点では塩基性溶液になります。

酢酸のような弱酸とアンモニア水のような弱塩基を滴定すると、中性の酢酸アンモニウム塩と水が生成されます。
滴定...

Education

JoVE Core

Analytical Chemistry

分析化学

Unit 1

ユニット1

Acid–Base Titration

酸塩基滴定

KEY TERMS AND CONCEPTS

主要用語と概念

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

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

Alabama Course of Study Science

Matter and Its Interactions

9-12.C.1.6.e

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New York State Science Learning Standards

High School

Physical Science

HS. Chemical Reactions

HS-PS1-11.

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Finding the Endpoint in Weak Acid–Base Titration

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