Name: ph and poh in water
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 41 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

pH Scale

水溶液中のヒドロニウムイオンの濃度は、通常、負の指数で記述され、1 × 10-14 Mまで小さくすることができます。pHスケールは、1909年に化学者のSoren Sorensonによって、さまざまな溶液の酸性度をすばやく比較するためのより便利な方法として開発されました。
溶液のpHは、そのヒドロニウムイオン濃度の負の対数です。例えば、1 × 10-5 Mのヒドロニウム濃度の溶液のpHは5です。溶液のヒドロニウムイオン濃度が高いほど、そのpHは低くなります。
pHは対数スケールで表されるため、1単位の変化はヒドロニウムイオン濃度の10倍の増加または減少に相当します。pH 3の溶液は、pH 4の溶液の10倍、pH 5の溶液の100倍のヒドロニウムイオンを持ちます。
酸性溶液は、水酸化物イオンよりもヒドロニウムイオンの濃度が高く、pH が 7 未満であるのに対し、塩基性溶液は、水酸化物イオンよりもヒドロニウムイオンの濃度が低く、pH が 7 を超えています。
ヒドロニウムイオンと水酸化物イオンを等濃度に含む中性溶液のpHは7です。
水酸化物イオンの濃度はpOHとして表すこともできます。水酸化物イオン濃度が高いほど、そのpOH値は低くなります。
水溶液のpHまたはpOH値は0〜14の範囲です。これは、水の自己イオン化の平衡定数である K W が 1 × 10-14 に等しいためです。 
方程式の両側の負の対数を取ると、pKWがpHとpOHの合計に等しい方程式が得られます。1 × 10-14の負の対数が14であるため、水溶液のpHとpOHの合計は常に14になります。これは、pOH値がわかっている場合、またはその逆の場合にpH値を決定するために使用できます。
例えば、pOHが10の溶液のpHは4です。

<ol>
	<li>「pHとpOHを1つの本当の酸性度グレード、AGに置き換えてみませんか?」化学教育ジャーナル76、第7号(1999):892。<a href="https://pubs.acs.org/doi/pdf/10.1021/ed076p892">https://pubs.acs.org/doi/pdf/10.1021/ed076p892</a></li>
</ol>

phスケール

ヒドロニウムイオンと水酸化物イオンは、純水にもすべての水溶液にも存在し、その濃度は水のイオン積（Kw）で決まる反比例の関係にあります。水溶液中のこれらのイオンの濃度は、しばしば溶液の特性や他の溶質の化学的挙動を決定する重要な要素となります。2つの異なる溶液のヒドロニウムイオンや水酸化物イオンの濃度は、100万倍、10億倍、あるいは1兆倍の差があります。このような桁違いの量を表現するには、対数目盛を用いるのが一般的です。ここで、[H3O+]は溶液中のヒドロニウムイオンのモル濃度です。
  <img alt="Eq1" src="/files/ftp_upload/11397/11397_Equation_1.png" style="height: 50px;" />
 この式を再整理してヒドロニウムイオンのモル濃度を分離すると、同等の式が得られます。
  <img alt="Eq2" src="/files/ftp_upload/11397/11397_Equation_2.png" style="height: 50px;" />
 同様に、水酸化物イオンのモル度はpOHとして表すことができます。:
  <img alt="Eq3" src="/files/ftp_upload/11397/11397_Equation_3.png" style="height: 50px;" />
 or
  <img alt="Eq4" src="/files/ftp_upload/11397/11397_Equation4.png" style="height: 50px;" />
 最後に、これらの2つのイオン濃度の関係をpHあるいはpOHで表すと、KWの式から簡単に導かれます。
 :
  <img alt="Eq5" src="/files/ftp_upload/11397/11397_Equation_5.png" style="height: 135px;" />
 25 °Cでは、KWの値は1.0 × 10−14となります。
  <img alt="Eq6" src="/files/ftp_upload/11397/11397_Equation_6.png" style="height: 50px;" />
 純水（または中性溶液）中のヒドロニウムイオンのモル比は、25 °Cで1.0 ×10−7 Mです。したがって、この温度での中性溶液のpHとpOHは次のようになります。
  <img alt="Eq7" src="/files/ftp_upload/11397/11397_Equation_7.png" style="height: 95px;" />
 そして、この温度ではヒドロニウムイオンのモル数が1.0 × 10−7Mより大きく、水酸化物イオンのモル数が1.0 × 10−7Mより小さいものが酸性溶液となる（pH値が7.00より小さく、pOH値が7.00より大きいものに相当）。塩基性溶液とは、ヒドロニウムイオンのモル数が1.0 × 10−7 M未満で、水酸化物イオンのモル数が1.0 × 10−7 M以上のもの（pH値が7.00以上、pOH値が7.00未満に相当）を指します。
 自己解離定数KWは温度に依存するため、pH値と酸性/中性/塩基性の相関関係は、25 °C以外の温度では異なります。例えば、80°Cでの純水のヒドロニウムモル度は4.9 × 10−7 Mであり、これは次のpH値とpOH値に対応します。
  <img alt="Eq8" src="/files/ftp_upload/11397/11397_Equation_8.png" style="height: 95px;" />
 この温度では、中性溶液はpH = pOH = 6.31、酸性溶液はpHが6.31未満でpOHが6.31以上、塩基性溶液はpHが6.31以上でpOHが6.31未満となります。この区別は、36 - 40℃前後の恒温動物の酵素反応など、他の温度で起こるプロセスを研究する際に重要となります。特に断りのない限り、pH値は25℃での値であると推定されます。
 上記の文章は以下から引用しました。<a href="https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh"> Openstax, Chemistry 2e, Section 14.2: pH and pOH.</a>

pHスケール

ヒドロニウムイオンと水酸化物イオンは、純水とすべての水溶液の両方に存在し、それらの濃度は次のように反比例します...

Education

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化学

Acids and Bases

酸と塩基

KEY TERMS AND CONCEPTS

主要用語と概念

ph and poh in water

pH and pOH show how hydronium and hydroxide ions behave in water and other aqueous solutions. These ion concentrations are linked by the ion product of water, K w , so when one goes up, the other goes down.

Because ion concentrations can differ by huge amounts, a logarithmic scale is used to describe them. pH is defined from the hydronium ion concentration, [H3O+], and pOH is the matching p-function for hydroxide ion concentration. These two values are also connected through the K w relationship.

At 25 °C, K w is 1.0 × 10−14. In pure water, or any neutral solution at this temperature, the hydronium ion concentration is 1.0 × 10−7 M. That gives pH = 7.00 and pOH = 7.00.

At 25 °C, acidic solutions have hydronium ion concentrations greater than 1.0 × 10−7 M and hydroxide ion concentrations less than 1.0 × 10−7 M. This means acidic solutions have pH values below 7.00 and pOH values above 7.00. Basic solutions show the opposite pattern, with pH above 7.00 and pOH below 7.00.

The link between pH and acidity changes with temperature because K w changes with temperature. For example, pure water at 80 °C has a hydronium ion concentration of 4.9 × 10−7 M, which gives pH and pOH values of 6.31. At this temperature, neutral solutions have pH = pOH = 6.31, acidic solutions have pH below 6.31, and basic solutions have pH above 6.31.

This temperature effect matters in processes that happen away from 25 °C, including enzyme reactions in warm-blooded organisms at about 36–40 °C. Unless another temperature is stated, pH values are usually assumed to refer to 25 °C.

pH and pOH in Water

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ph scale

pH and pOH show how hydronium and hydroxide ions behave in water and other aqueous solutions. These ion concentrations are linked by the ion product of water, K w , so when one goes up, the other goes down. Because ion concentrations can differ by huge amounts, a logarithmic scale is used to describe them. pH is defined from the hydronium ion concentration, [H3O+], and pOH is the matching p-function for hydroxide ion concentration. These two values are also connected through the K w...

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determining the ph of salt solutions

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acid/base strengths and dissociation constants

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lewis acids and bases

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key terms and concepts

Learn how hydronium and hydroxide ion levels define pH and pOH, why neutral water changes with temperature, and how acidic or basic solutions are identified.

pH and pOH in Water | JoVE K12

chemistry

JoVE Core 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

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Ohio’s Learning Standards and Model Curriculum

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CHEM.12.E

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Florida Department of Education

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SC6.

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Acids and Bases: Percent Dissociation and pH

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pH and pOH in Water

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