Name: acids, bases, and ph in titration
Uploaded: 2020-03-26T07:22:25.000Z
Duration: 6 min 35 s
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Chemistry

1.2.1 9L.4.2.2.1

Acid and Base Concentrations

アレニウス酸は水に溶けると水素イオンを生成する物質で、塩基は水酸化物イオンを生成する物質です。水素イオンはすぐに水と反応してヒドロニウムイオンを形成しますが、簡単にするために、水素イオンと考え続けます。溶液中の水素イオンまたは水酸化物イオンの量に応じて、酸性または塩基性と見なされます。
酸性度や塩基性度は、水素イオンの濃度の負の対数として算出されるpHを用いて測定します。したがって、7未満のpH値は酸性であり、7を超えるpH値は塩基性です。pH 7は中性です。
酸と塩基は、pHとは異なる強度に基づいても比較されます。酸の強さは、水素イオンが共役塩基と呼ばれる陰イオンからどれだけ簡単に解離するかに関係しています。水酸化物イオンとその共役酸に関する塩基についても同じ考え方が成り立ちます。その強度には、酸解離定数(KA)を使用して値を割り当てることができます。
KAは、非解離酸と解離水素イオンおよび共役塩基の濃度を使用して定義されます。この関係は、単に Ka の負の対数である pKa として表されることがよくあります。pKaが小さいほど、酸は強くなります。
塩酸のような一部の酸はモノプロトン性であり、水素イオンを1つだけ解離できます。ポリプロトン酸は、リン酸と同様に、いくつかの水素イオンを解離させることができます。各解離には独自のpKaがあります。
では、どうすればpKaを決定できるのでしょうか?1つの方法は、酸塩基滴定を行うことです。滴定は、濃度不明の溶液に濃度が不明な溶液をゆっくりと加え、その間の反応を観察しながら行います。この場合、酸は中和反応で塩基と反応して塩と水を形成します。
したがって、酸中の水素イオンの濃度を測定したい場合は、酸が中和されるまで、既知の水酸化物イオン濃度を持つ強塩基で単純に滴定できます。滴定を正確に行うには、塩基を標準化する必要があります—つまり、正確な水酸化物イオン濃度がわかっていることを意味します。これは必ずしも簡単ではありません。
例えば、実験で使用するNaOHは吸湿性が高く、大気中の水分を吸収します。これは、NaOHに固体と溶液の両方で発生します。そのため、NaOH溶液の真の濃度は、予想よりも低い可能性があります。
NaOHの正確な濃度を決定するには、まず酸塩基滴定を行う必要があります。これを行うには、塩基を使用して既知の濃度で酸を滴定する必要があります。フタル酸水素カリウム(KHPは非吸湿性)であるため、その質量からその濃度を正確に計算できます。
滴定が完了した時期、つまり酸が中和された時期は、フェノールフタレインなどのpHインジケーターを使用して確認できます。フェノールフタレインは、約pH0からpH8の間で中性で無色です。
pHが上昇すると、2つの水素イオンが解離します。この陰イオンの形はピンクです。したがって、滴定を開始すると、KHP溶液は酸性で、フェノールフタレインは無色です。NaOHを添加し、水素イオンを中和すると、pHが上昇します。
この反応では、等量の酸と塩基が混合されている場合、溶液は中性です。その後、NaOHを少し加えるとpHが塩基性になり、溶液がピンク色に変わります。これはエンドポイントと呼ばれます。KHPのモル数とそれを中和するために使用されるNaOHの量がわかれば、塩基の正確な濃度を計算できます。
標準化された塩基が得られたら、pHを監視しながら、標準化された塩基で酸の既知の濃度を滴定することにより、酸のpKaを決定できます。pHと添加された塩基の量のプロットは、滴定曲線と呼ばれます。曲線は通常、S字型またはシグモイド形状をたどり、曲線の最も急な部分の変曲点は等価点を示します。
ここでは、水酸化物イオンと解離水素イオンのモルは等しくなっています。pKaと同様に、解離した水素イオンごとに1つの当量点が表示されます。したがって、モノプロトン酸には当量点が1つしかなく、トリプロトン酸には3つの当量点があります。
滴定を実行すると、pHインジケーターが無色からピンクにかろうじて変わるときに当量点を通過したことがわかります。これは滴定エンドポイントと呼ばれます。塩基を標準化する場合と同様に、これは溶液に水酸化物イオンがわずかに過剰にあるため、わずかに塩基性である場合です。
グラフ内の別の変曲点は、等価点の中間で発生します。ここでは、解離した酸と解離していない酸の濃度は等しくなります。したがって、この時点でのpHはpKaに等しくなります。したがって、滴定を行い、当量点の体積を決定すると、この体積の半分のpHとしてpKaを計算できます。
このラボでは、まず塩基を標準化し、次にその標準化された塩基を使用して滴定を行い、ポリプロトン酸の 2 pKa を測定します。

酸および塩基濃度

<h4>酸と塩基</h4>
アレニウス酸は、水に溶解すると水素イオンを生成します。
HA + H2O → H+(aq) + A-(aq)
ここで、HAは非解離酸、H+は水素カチオン、A-は共和性陰イオン(共役塩基と呼ばれます)です。アレニウス塩基は、水に溶解すると水酸化物イオンを生成します。
BOH + H2O → B+(aq) + OH-(aq)
ここで、BOHは非解離塩基、OH-は水酸化物イオン、B+は共和型カチオン(共役酸と呼ばれます)です。共役塩基は、酸が水素イオンを失い、水素を得る可能性があるときに形成されます。共役酸についても同じことが言え、塩基がヒドロキシル基を失い、それを取り戻す可能性があるときに形成されます。すべての酸には共役塩基があり、すべての塩基には共役酸があります。
<h4>pH</h4>の
pHは溶液の酸性度であり、溶液中の水素イオンの量の尺度です。pHスケールは対数で、0から14まで実行されます。pH 7未満の水溶液は酸性と説明され、pH 7を超える水溶液はアルカリ性または塩基性と呼ばれます。pH 7の溶液は中性と見なされます。
溶液のpHは、溶液中の水素イオン濃度の負の対数塩基10に等しくなります。
<img src="/CDNSource/lm/labs/44/44_Concepts_3.jpg">
水は、その強い正電荷が周囲の水分子の負極を引き付けるため、水素イオンと強く相互作用します。実際、それらは非常に強く相互作用するため、共有結合とヒドロニウムと呼ばれるH3O +カチオンを形成します。これを反映するために、上記の式を書き換えます。
<img src="/CDNSource/lm/labs/44/44_Concepts_4.jpg">
簡単にするために、pHについて説明するときには、ヒドロニウムイオンの代わりに水素イオンの濃度を参照します。溶液のpH値が低いほど、存在する水素イオンが多くなり、ひいては溶液の酸性度が高くなります。たとえば、硫酸の 1 mM の pH は 2.75 ですが、塩酸 1 mM の pH は 3.01 です。硫酸溶液中の水素イオンの濃度は1×10-2.75と計算されますが、塩酸溶液中の水素イオンの濃度は1×10-3.01です。したがって、硫酸にはより多くの水素イオンが存在し、より酸性です。2つの溶液のpHがpH値の半分程度しか変化しない場合でも、pHスケールの対数的な性質により、水素の量は大きく異なることを忘れないでください。
<h4>酸と塩基の強度</h4>
酸の強度は、共役塩基の電気陰性度と酸性水素の極性に影響されます。したがって、強度とは、水素カチオン(H+)が陰イオンからどれだけ容易に解離するかを指します。強酸と強塩基は水溶液中で完全に解離しますが、弱酸と弱塩基は部分的にのみ結合イオンに解離します。
解離定数 Ka は酸の強さを表します。KAは、非解離酸HAの濃度と、水素カチオンと共役塩基A-の濃度を用いて計算されます。Ka値が高いほど強い酸を表し、Ka値が小さいほど弱い酸を表します。
<img src="/CDNSource/lm/labs/44/44_Concepts_5.jpg">
Ka は数値的に非常に小さく、Ka の負の対数基数 10 である pKa の形で報告されることがよくあります。pKaの値が低いほど強い酸に対応し、pKaの値が高いほど弱い酸に対応します。
<img src="/CDNSource/lm/labs/44/44_Concepts_6.jpg">
一部の酸は1つの水素イオンのみを解離するため、1pKあたり1つの値を持ちます。これらの酸はモノプロティックと呼ばれます。ただし、一部の酸は複数の水素イオンを解離させる可能性があり、ポリプロティックと呼ばれます。これらの酸は、水素イオンの解離ごとにpKa値を持っています。
pKaは、ヘンダーソン・ハッセルバルヒの式に示すように、酸塩基反応の平衡pHを計算するためにも使用できます。
<img src="/CDNSource/lm/labs/44/44_Concepts_7.jpg">
ヘンダーソン・ハッセルバルヒ方程式は、共役塩基と弱酸の濃度がわかっている場合にpHを計算するために、またはpHと濃度がわかっている場合にpKaを計算するために使用されます。
<h4>滴 定</h4>
酸塩基反応は、滴定を使用して定量的に研究されます。滴定実験では、標準溶液と呼ばれる既知の濃度の溶液を使用して、別の溶液の濃度を決定します。酸塩基滴定では、標準化された塩基溶液を未知の濃度の酸にゆっくりと添加します(または酸を塩基に添加します)。酸塩基反応は中和反応であり、塩と水を形成します。酸中の水素イオンのモルが塩基から添加されたヒドロキシルイオンのモルと等しい場合、溶液は中性のpHに達します。
酸塩基滴定を行うには、標準化された塩基をビュレットを使用して未知の酸の攪拌フラスコにゆっくりと添加し、体積の測定と塩基の滴下添加を可能にします。溶液のpHは、酸に添加されたpH指示薬を使用して、滴定全体を通して綿密に監視されます。通常、フェノールフタレインは、溶液が塩基性になり、淡いピンク色に変わるまで無色のままであるため、使用されます。
滴定が当量点(水素イオンのモルが添加されたヒドロキシルイオンのモルと等しくなる)に近づくと、ヒドロキシルイオンが過剰になるため、pHインジケーターの色が一時的に変化します。フラスコを回転させると、pHインジケーターの酸性色に戻ります。滴定は完了し、わずかな過剰のヒドロキシルイオンがインジケーターを恒久的に基本色に変化させたときに終了点に達します。
滴定曲線は、溶液のpHと標準化された塩基の添加量をプロットしたものです。等価点は曲線の変曲点に位置し、滴定曲線の 2 次導関数として計算されます。
酸がポリプロティックの場合、水素イオンの解離ごとに1つずつ、複数の当量点があります。モノプロトン酸の場合は当量点までの中間点、またはポリプロトン酸の場合は当量点間のpHは、酸のpKaに等しくなります。
<h4>参照</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012).化学および化学反応性。カリフォルニア州ベルモント:ブルックス/コール、Cengage Learning。</li>
	<li style="font-size:17px">Silberberg, M.S. (2009).化学:物質と変化の分子的性質。マサチューセッツ州ボストン:マグロウヒル。</li>
	<li style="font-size:17px">ハリス、ワシントンD.C.(2015年)。定量的化学分析。ニューヨーク州ニューヨーク:WHフリーマンアンドカンパニー。</li>
</ol>

酸と塩基
アレニウス酸は、水に溶解すると水素イオンを生成します。
HA + H2O → H+(aq) + A-(aq)
ここで、HAは非解離...

Education

JoVE Lab Manual

化学

acids, bases, and ph in titration

Acids, bases, and pH explain how acidic or basic a solution is. An Arrhenius acid releases hydrogen ions when it dissolves in water, while an Arrhenius base releases hydroxide ions. These reactions also create conjugate partners: an acid forms a conjugate base after losing hydrogen, and a base forms a conjugate acid after losing hydroxyl. Every acid has a conjugate base, and every base has a conjugate acid.

pH measures the amount of hydrogen ions in a solution. The pH scale runs from 0 to 14 and is logarithmic, so small changes in pH can mean large changes in hydrogen ion concentration. Solutions below pH 7 are acidic, solutions above pH 7 are basic or alkaline, and solutions at pH 7 are neutral. Water strongly attracts hydrogen ions and forms hydronium, H3O+, but pH is often discussed using hydrogen ion concentration for simplicity.

Lower pH means more hydrogen ions and greater acidity. For example, 1 mM sulfuric acid has a pH of 2.75, while 1 mM hydrochloric acid has a pH of 3.01. Because the pH scale is logarithmic, even a difference of about half a pH unit can reflect a large difference in hydrogen ion amount. In this example, sulfuric acid is more acidic because it contains more hydrogen ions.

Acid and base strength describes how readily hydrogen ions separate from the anion. Strong acids and bases dissociate completely in water, while weak acids and bases dissociate only partly into their conjugate ions. Acid strength is measured with the dissociation constant, Ka, which uses the concentrations of the non-dissociated acid HA, hydrogen cations, and conjugate base A-. Larger Ka values indicate stronger acids, and smaller values indicate weaker acids. Ka is often written as pKa, the negative base-10 log of Ka, so lower pKa means a stronger acid and higher pKa means a weaker acid.

Some acids release only one hydrogen ion, so they have one pKa and are called monoprotic. Others release more than one hydrogen ion and are polyprotic, with a pKa value for each step. The Henderson-Hasselbalch equation uses pKa to find the equilibrium pH of an acid-base reaction when the weak acid and conjugate base concentrations are known, or to find pKa when pH and concentrations are known.

Acid-base reactions can also be studied by titration. In a titration, a standard solution with a known concentration is used to find the concentration of another solution. For acid-base titrations, a standardized base is added slowly to an unknown acid, or the acid is added to the base. The reaction is a neutralization reaction that produces salt and water. When the moles of hydrogen ions match the moles of hydroxyl ions added, the solution reaches neutral pH.

During the experiment, the standardized base is added drop by drop from a burette to a stirring flask of acid. The pH is monitored with an indicator such as phenolphthalein, which stays colorless until the solution turns basic and becomes light pink. Near the equivalence point, the indicator may change color because of a temporary excess of hydroxyl ions, but swirling can return its acidic color. The endpoint is reached when a tiny extra amount of hydroxyl ions makes the basic color stay permanent.

A titration curve plots pH against the volume of standardized base added. The equivalence point appears at the inflection point of the curve and is found from the second derivative. Polyprotic acids have multiple equivalence points, one for each hydrogen ion dissociation. For monoprotic acids, the pH halfway to the equivalence point, or between equivalence points for polyprotic acids, is equal to the pKa.

Acids, Bases, and pH in Titration

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boiling points

Boiling Point and Vapor Pressure

Boiling point and vapor pressure are key physical properties of a liquid. For a pure compound, the boiling point can help identify the substance. In experiments, the boiling point is usually recorded as a range because exact values are hard to measure and may differ by a few degrees from the literature value. Vapor pressure explains why a liquid boils. In a sealed container, some molecules at the liquid surface gain enough energy to escape into the gas phase. At the same time, some gas...

recrystallization

Purifying Crystals by Solubility

Purifying crystals by solubility is a key organic chemistry technique. Recrystallization uses differences in solubility to separate a desired solid product from starting materials, solvent, and impurities. It is one of the three standard purification methods, along with distillation and extraction. Solubility is the maximum amount of a substance that can dissolve in a fixed volume of solvent at a given temperature. Different solutes dissolve in different solvents, and their behavior can vary...

extraction

Separating Mixtures with Solvent Extraction

Solvent extraction is a common way to separate and isolate a target compound in organic chemistry. In this process, a solute moves from one phase to another so it can be separated from unreacted starting materials or impurities. Extraction is also useful when a product is dissolved in a reaction solvent that is hard to remove by evaporation, such as a solvent with a high boiling point. There are three main types of extraction. In solid-liquid extraction, the solute moves from a solid phase...

simple distillation

Separating Liquids by Boiling Point

Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

thin-layer chromatography

TLC Separation by Polarity

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column. The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a...

column chromatography

Separating Mixtures by Polarity

Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

hydrolysis of an ester

Soap Formation from Triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

入門化学ラボコースの指導者と学生のための、包括的でカリキュラムに焦点を当てたビデオ。

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Learn how acids, bases, pH, Ka, and pKa connect to titration, neutralization, and titration curves in high school chemistry.

Acids, Bases, and pH in Titration | JoVE K12

new-york-state-science-learning-standards

New York State Science Learning Standards

High School

Physical Science

HS. Chemical Reactions

HS-PS1-11.

Georgia Standards of Excellence

SC3.

Indicators of Chemical Reactions

SC6.

Neutralization Reactions

酸および塩基濃度 を教える準備を、この 5 分間の教員向けガイドで行いましょう。JoVE Lab Manual の Chemistry ラボの主要な準備手順を解説します。

Chemistry の 酸および塩基濃度 教員準備 - 6.2K+ 回の視聴 | JoVE Lab Manual

この 15 分間の学生プロトコル動画で、酸および塩基濃度 の手順をステップごとに学びましょう。JoVE Lab Manual の Chemistry コレクションの一部です。

Chemistry の 酸および塩基濃度 学生プロトコル - 31K+ 回の視聴 | JoVE Lab Manual

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Matter and Its Interactions: Plan and conduct an investigation of acid-base reactions to test ideas about the concentrations of the hydronium ion in an aqueous solution (pH). (P:3, CC: 3, CI: PS1)

9C.1.2.1.2

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

Acids and Bases
An Arrhenius acid produces hydrogen ions when it is dissolved in water:
HA + H2O &#8594; H+(aq) + A-(aq)
Here, HA is the non-dissociated ...

Strong Acid and Base Solutions

A strong acid is a compound that dissociates completely in an aqueous solution and produces a concentration of hydronium ions equal to the initial ...

Polyprotic Acids

Acids are classified by the number of protons per molecule that they can give up in a reaction. Acids such as HCl, HNO3, and HCN that contain one ...

Water: A Bronsted-Lowry Acid and Base

The reaction between a Br&#248;nsted-Lowry acid and water is called acid ionization. For example, when hydrogen fluoride dissolves in water and ionizes, ...

Weak Base Solutions

Some compounds produce hydroxide ions when dissolved by chemically reacting with water molecules. In all cases, these compounds react only partially and ...

Weak Acid Solutions

Few compounds act as strong acids. A far greater number of compounds behave as weak acids and only partially react with water, leaving a large majority of ...

The potential of hydrogen (pH) is a measure of the acidity or basicity of a water-based solution determined by the concentration of hydronium ions (H3O+). ...

water-quality-analysis-via-indicator-organisms

Water Quality Analysis via Indicator Organisms

Source: Laboratories of Dr. Ian Pepper and Dr. Charles Gerba -The University of Arizona
Demonstrating Author: Luisa Ikner
Water quality analysis monitors ...

Matter and Its Interactions: Obtain and communicate information about how Minnesota American Indian Tribes and communities and other cultures construct solutions to mitigate threats to biodiversity.* (P: 8, CC: 7, CI: LS2, ETS1)

1 Exploring phenomena or engineering problems
 1.2 Planning and carrying out investigations
  1.2.1 Students will be able to design and conduct investigations in the classroom, laboratory, and/or field to test students’ ideas and questions and will organize and collect data to provide evidence to support claims the students make about phenomena.

Acids, Bases, and pH in Titration

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