Name: buffer ph and the common ion effect
Uploaded: 2020-03-26T07:22:25.000Z
Duration: 4 min 35 s
Channel: JoVE

バッファーは、強酸や強塩基の添加に関係なく、システム内のpHを一定に保つために機能する溶液です。緩衝液がない場合、溶液に強酸または塩基を添加すると、pHが大幅に変化します。バッファーは日常生活で非常に重要です。たとえば、血液は緩衝液として機能し、7.8を超えるまたは6.8未満のpHは死に至る可能性があるため、7.35から7.45の間のpHを維持することができます。
では、バッファはどのように機能するのでしょうか?バッファーは、基本的に弱酸または弱塩基とその共役塩基または酸が互いに平衡状態にあるものです。たとえば、酢酸と酢酸ナトリウムで作られたバッファーを考えてみましょう。緩衝液には弱酸とその共役塩が含まれており、弱酸が解離して共役塩基アセテートを形成します。酢酸のKAが共役塩基のKBよりも高いため、溶液は酸性です。
解離定数KAまたはKBBは、水中での化合物の解離の平衡定数であるため、それぞれ酸または塩基の強度を定義することを忘れないでください。解離定数は、pKa、またはKaの負の対数として表すこともでき、pKaが小さいほど酸が強くなります。同じことがpKbにも当てはまり、pKbが小さいほど、ベースは強くなります。
アセテートバッファーに戻ると、水酸化ナトリウムのような強塩基を追加すると、水酸化物イオンは溶液中に存在するヒドロニウムイオンと反応します。これにより、より多くの酢酸が水と反応して平衡状態に戻り、より多くのヒドロニウムイオンが形成されます。塩酸のような強酸を加えると、溶液中により多くのヒドロニウムイオンが形成されます。これらのイオンは酢酸イオンと反応して、より多くの酢酸を形成します。どちらの場合も、溶液のpHにはほとんど変化がありません。
各バッファーには、溶液をバッファーできる特定のpH範囲があることに注意してください。ここでは、酢酸バッファーの滴定曲線を示します。バッファリング範囲が強調表示されます。この領域はヘンダーソン・ハッセルバルヒの式によって決定され、緩衝液のpHは弱酸のKAと共役塩基と弱酸の比によって決定されることがわかります。
このpHから遠く離れると、バッファーの弱酸または弱塩基が枯渇し、pHをバッファーできなくなります。したがって、アプリケーション用のバッファーを選択するときは、pKaが目的のpHに近いものを選択してください。
最後に、すべてのバッファーには緩衝能力、つまりpHが大幅に変化する前にバッファー溶液に添加できる酸または塩基の量に制限があります。たとえば、酢酸と酢酸ナトリウムの緩衝液に酸を加えすぎると、すべての酢酸イオンがプロトン化され、ヒドロニウムイオンが大量に蓄積されるため、pHが低下します。
同様に、塩基を添加しすぎると、すべての酢酸が脱プロトン化され、水酸化物イオンが大量に蓄積されるため、pHが上昇します。したがって、緩衝能力は、弱酸塩と共役塩の濃度に影響されます。したがって、1モルのアセテートバッファーは、0.1モルのアセテートバッファーよりも高い緩衝能力を持っています。
このラボでは、さまざまなpH範囲のバッファーを調製し、検査します。次に、バッファーを使用して、ニュートラルレッドと呼ばれるpHインジケーターのpKaを測定し、リボフラビン結合タンパク質が導入されたときのニュートラルレッドのpKaの変化を調査します。

バッファー

<h4>バッファー</h4>
アレニウス酸(HA)を水に加えると、共役塩基(A-)と水素カチオン(H+)に解離します。
HA + H2O → H+(aq) + A-(aq)
溶液中に存在する水素イオンの量によって溶液の酸性度が決まり、水素イオンが多いほどpHが低いか酸性であることを示します。同様に、強アレニウス塩基(BOH)を水に加えると、共役酸(B +)と水酸化物イオン(OH-)に解離します。
BOH + H2O → B+(aq) + OH-(aq)
一般に、溶液に強酸または強塩基を添加すると、酸または塩基が溶液中の水分子と反応し、水素イオンまたは水酸化物イオンの濃度が増加するため、pHが劇的に変化します。ただし、このpHの変化は、緩衝液を使用して軽減できます。バッファーは、強酸や強塩基の添加に関係なく、システム内のpHを一定に保つために機能する溶液です。
ほとんどの場合、バッファーの成分は、弱酸または弱塩基の共役酸-塩基対です。このため、水中で完全に解離する強酸または強塩基は非常に貧弱な緩衝液を作り、部分的に解離する弱酸または塩基はより優れた緩衝液を作ります。緩衝液が存在する場合、強酸または強塩基は溶液中に存在する水分子と反応せず、代わりに弱酸/共役塩基と反応します。これにより、溶液のpHはほとんどまたはまったく変化しません。
<h4>一般的なイオン効果</h4>
バッファーは、共通イオン効果と呼ばれる現象を通じて機能します。一般的なイオン効果は、特定のイオンがすでに含まれている平衡状態の混合物に特定のイオンが追加されたときに発生します。これが起こると、平衡はそのイオンをより多く形成することから離れます。
例えば、酢酸(CH3COOH)は水中でわずかに解離し、酢酸イオン(CH3COO - )と水素イオンを形成します。
CH3COOH(aq) H2O ⇔ H+(aq) + CH3COO-(aq)
可溶性酢酸ナトリウムから酢酸イオンを多く添加すると、平衡位置が左にシフトして非解離酢酸が多く形成され、水素イオンの濃度が低下します。ここでは、一般的なイオンである酢酸が酢酸の解離を抑制します。
溶液を緩衝するには、バッファーには酸性(HA)成分と塩基性(A-)成分の両方を高濃度で含んでいる必要があります。バッファーに添加される水素イオンまたは水酸化物イオンの量が少ない場合、少量のバッファー成分が他のバッファー成分に変換されます。添加されたイオンの濃度が、バッファーにすでに存在するHAおよびA-の濃度よりもはるかに小さい限り、添加されたイオンは緩衝液成分の1つによって消費されるため、pHにほとんど影響を与えません。水素または水酸化物の濃度が酸とその共役塩基の濃度を超えると、緩衝効果が失われ、pHが変化します。
<h4>Henderson-Hasselbalch 方程式</h4>
弱酸の解離定数Kaは、非解離酸HAの濃度と、水素イオンと共役塩基A-の濃度を用いて計算される。
<img src="/CDNSource/lm/labs/45/45_Concepts_4.jpg" alt="酸解離定数方程式Ka=[H+][A-]/[HA]、化学式。" data-altupdatedat="1754907201000">
Ka値が高いほど酸が強いことを表し、Ka値が小さいほど酸が弱いことを表します。水素イオンの濃度を決定するために、方程式が再配置されます。この形式では、酸性種と塩基種の比率が水素イオンの濃度、ひいてはpHを決定する上で重要であることは明らかです。
<img src="/CDNSource/lm/labs/45/45_Concepts_5.jpg" alt="酸解離方程式、[H +] = Ka × [HA]/[A-]、化学平衡の概念。" data-altupdatedat="1754907201000">
両側の負の共通対数を取ると、ヘンダーソン・ハッセルバルフ方程式が得られます。
<img src="/CDNSource/lm/labs/45/45_Concepts_6.jpg" alt="ヘンダーソン・ハッセルバルチ方程式;pH、pKa、緩衝液、酸塩基平衡、化学式。" data-altupdatedat="1754907201000">
ヘンダーソン・ハッセルバルヒ方程式は、水素イオンの濃度を最初に計算しなくても、バッファーのpHを直接計算することを可能にします。
例えば、0.02モルの強塩基を添加した後の1 L緩衝液のpHを測定するために使用できます。強塩基は完全に解離するため、添加されるヒドロキシルイオンの濃度は0.02 Mです。これにより、酸の濃度が0.02減少します。酸(HA)成分と塩基(A-)成分の元の濃度がそれぞれ0.5 Mであると仮定すると、塩基の新しい濃度は0.02 Mから0.52 M増加し、酸の濃度は0.02 Mから0.48 M減少します。
これは、緩衝液の容量、つまりpHに大きな影響を与えることなく、強酸または強塩基を緩衝液にどれだけ添加できるかを決定するのに役立ちます。
<h4>バッファ容量</h4>
バッファー容量は、pH変化に抵抗するバッファーの能力の尺度です。この能力は、緩衝成分、つまり酸とその共役塩基の濃度に依存します。バッファー濃度が高いほど、バッファー容量が大きくなります。これは、平衡を乱し、バッファーのpHを変化させるために、より多くの水素イオンまたはより強い酸を追加する必要があることを意味します。
バッファ容量は、バッファ成分の相対濃度によっても影響を受けます。バッファーは、成分の濃度が類似している場合により効果的です。緩衝液の成分比が類似している場合、酸や塩基を添加しても成分の濃度比は大きく変化しません。比率を相殺し、pHを変えるために、大量の酸または塩基を添加する必要があります。
緩衝液のpHは、その緩衝容量とは異なります。pH範囲は、バッファーが有効である範囲です。通常、緩衝液の使用可能な範囲は、緩衝液の酸成分のpKaの1 pH単位以内です。
<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>
</ol>

バッファー
アレニウス酸(HA)を水に加えると、共役塩基(A-)と水素カチオン(H+)に解離します。
HA + H2O → H+(水) + ...

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Chemistry

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Buffers

buffer ph and the common ion effect

Buffer solutions help keep pH steady when acids or bases are added. In water, an Arrhenius acid (HA) dissociates into hydrogen ions, H+, and a conjugate base, A-. More hydrogen ions make a solution more acidic and lower its pH. An Arrhenius base (BOH) dissociates into a conjugate acid, B+, and hydroxide ions, OH-. Strong acids and strong bases usually change pH a lot because they react completely in water and raise the amount of H+ or OH-.

Buffers reduce those pH changes. A buffer usually contains a weak acid and its conjugate base, or a weak base and its conjugate acid. Weak acids and weak bases are better buffers because they dissociate only partially in water. When a buffer is present, added strong acid or base reacts with the buffer components instead of causing a large shift in pH.

The way buffers work is called the common ion effect. A common ion is an ion already present in an equilibrium mixture. When more of that ion is added, the equilibrium shifts away from making more of it. A good example is acetic acid, CH3COOH, which dissociates slightly in water to form H+ and acetate, CH3COO-. If sodium acetate adds more acetate ion, the equilibrium shifts left and less hydrogen ion remains in solution.

A buffer needs high amounts of both the acidic form, HA, and the basic form, A-. Small amounts of added H+ or OH- are used up by one buffer component and converted into the other. If the added acid or base is much smaller than the amounts already in the buffer, the pH changes very little. Once the concentration of added H+ or OH- becomes larger than the buffer components, the buffer effect is lost and the pH shifts.

The Henderson-Hasselbalch equation helps calculate buffer pH. It comes from the acid dissociation constant, Ka, which uses the concentrations of HA, H+, and A-. A larger Ka means a stronger acid, while a smaller Ka means a weaker acid. After rearranging and taking the negative log, the equation shows that the ratio of acid to base controls pH, so buffer pH can be found without first calculating hydrogen ion concentration.

For example, a 1 L buffer with 0.5 M HA and 0.5 M A- can change after 0.02 moles of strong base are added. The strong base dissociates completely, so 0.02 M OH- is present. That amount lowers the acid concentration to 0.48 M and raises the base concentration to 0.52 M. The new values can be placed into the Henderson-Hasselbalch equation with the acid pKa to find the pH. This also helps estimate buffer capacity, or how much strong acid or strong base a buffer can absorb without a large pH change.

Buffer capacity depends on concentration. A more concentrated buffer has a greater ability to resist pH change. It also depends on how similar the acid and base concentrations are. Buffers work best when those concentrations are close, because the ratio changes only a little when acid or base is added. Buffer pH is not the same as buffer capacity. The effective pH range is usually within 1 pH unit of the acid component's pKa.

Buffer pH and the Common Ion Effect

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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

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Learn how buffers resist pH change through the common ion effect, the Henderson-Hasselbalch equation, and buffer capacity in high school chemistry.

Buffer pH and the Common Ion Effect | JoVE K12

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Buffer pH and the Common Ion Effect

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