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

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North Carolina K-12 Science Standard Course of Study

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Chemistry

Matter and its Interactions

PS.Chm.6.3

pH Scale

A concentração de íons hidrônicos numa solução aquosa é geralmente escrita com expoentes negativos e pode ser tão pequena quanto 1 10-¹⁴ mols. A escala de pH foi desenvolvida pelo químico Soren Sorenson em 1909 como uma forma mais conveniente para comparar rapidamente a acidez de diferentes soluções. O pH de uma solução é o logaritmo negativo da sua concentração de íons hidrônicos.Por exemplo, uma solução com uma concentração hidrônico de 1 10-⁵ M tem um pH de 5. Quanto maior a concentração de íons hidrônicos de uma solução, mais baixo o seu pH. Como o pH é expresso numa escala logarítmica, corresponde a uma alteração de uma unidade para um aumento de 10 vezes ou diminuição da concentração de íons hidrônicos.Uma solução com um pH de 3 irá ter dez vezes mais íons hidrônicos do que uma solução com um pH 4 e cem vezes mais do que uma solução com um pH de 5. Uma solução ácida tem uma maior concentração de íons hidrônicos do que íons hidróxidos e um pH inferior a 7, enquanto que uma solução básica tem uma menor concentração de íons hidrônicos do que íons hidróxidos e um pH superior a 7. Uma solução neutra com uma concentração igual de íons hidrônicos e hidróxidos têm um pH de 7.A concentração de íons hidróxidos também pode ser expressa como pOH. O pOH é o logaritmo negativo da concentração de íons hidróxidos. Quanto mais alto a concentração de íons hidróxidos, mais baixo o seu valor pOH.Um valor de pH ou pOH de uma solução aquosa é de 0-14. Isto é porque Kw, a constante de equilíbrio para a autoionização da água, é igual a 1 10-¹⁴. Tomando o registo negativo de ambos os lados da equação resulta numa equação onde pKw é igual à soma de pH e pOH.Uma vez que o registo negativo de 1 10-¹⁴ é 14, a soma do pH e pOH de uma solução aquosa será sempre 14. Isto pode ser utilizado para determinar o valor de pH quando o valor de pOH é conhecido e vice-versa. Por exemplo, uma solução com um pOH de 10 tem um pH de 4.

<ol>
	<li>van Lubeck, Henk. "Por que não substituir o pH e o pOH por apenas um grau de acidez real, AG?. " Jornal de Educação Química 76, nº 7 (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>

escala de ph

Os iões hidrónio e hidróxido estão presentes tanto em água pura como em todas as soluções aquosas, sendo as suas concentrações inversamente proporcionais, conforme determinado pelo produto iónico da água (Kw). As concentrações desses iões em uma solução são muitas vezes determinantes críticos das propriedades da solução e dos comportamentos químicos dos seus outros solutos. Duas soluções diferentes podem diferir nas suas concentrações de iões hidrónio ou hidróxido em um milhão, mil milhões, ou mesmo um bilião de vezes. Um meio comum de expressar quantidades que podem abranger muitas ordens de grandeza é usar uma escala logarítmica. O pH de uma solução é, portanto, definido como mostrado aqui, onde [H3O+] é a concentração molar de iões hidrónio na solução:
 <img alt="Eq1" src="/files/ftp_upload/11397/11397_Equation_1.png" style="height: 50px;" />
Reorganizando esta equação para isolar a molaridade do ião hidrónio produz a expressão equivalente:
 <img alt="Eq2" src="/files/ftp_upload/11397/11397_Equation_2.png" style="height: 50px;" />
Da mesma forma, a molaridade do ião hidróxido pode ser expressa como uma função p ou pOH: 
 <img alt="Eq3" src="/files/ftp_upload/11397/11397_Equation_3.png" style="height: 50px;" />
ou
 <img alt="Eq4" src="/files/ftp_upload/11397/11397_Equation4.png" style="height: 50px;" />
Por fim, a relação entre estas duas concentrações de iões expressas como funções p é facilmente derivada da expressão KW:
 <img alt="Eq5" src="/files/ftp_upload/11397/11397_Equation_5.png" style="height: 135px;" />
A 25 °C, o valor de KW é 1,0 × 10−14, pelo que:
 <img alt="Eq6" src="/files/ftp_upload/11397/11397_Equation_6.png" style="height: 50px;" />
A molaridade de iões hidrónio em água pura (ou qualquer solução neutra) é de 1,0 × 10−7 M a 25 °C. O pH e o pOH de uma solução neutra a esta temperatura são, por conseguinte:
 <img alt="Eq7" src="/files/ftp_upload/11397/11397_Equation_7.png" style="height: 95px;" />
E assim, a essa temperatura, soluções ácidas são aquelas com molaridades de iões hidrónio maiores que 1,0 × 10−7 M e molaridades de iões hidróxido menores que 1,0 × 10−7 M (correspondendo a valores de pH menores que 7,00 e valores de pOH maiores que 7,00). Soluções básicas são aquelas com molaridades de iões hidrónio inferiores a 1,0 × 10−7 M e molaridades de iões hidróxido superiores a 1,0 × 10−7 M (correspondendo a valores de pH superiores a 7,00 e valores de pOH inferiores a 7,00).
Como a constante de autoionização KW é dependente da temperatura, essas correlações entre os valores de pH e os adjetivos ácido/neutro/básico serão diferentes a temperaturas diferentes de 25 °C. Por exemplo, a molaridade de hidrónio da água pura a 80°C é de 4,9 × 10−7 M, o que corresponde a valores de pH e pOH de:
 <img alt="Eq8" src="/files/ftp_upload/11397/11397_Equation_8.png" style="height: 95px;" />
A essa temperatura, as soluções neutras apresentam pH = pOH = 6,31, as soluções ácidas apresentam pH menor que 6,31 e pOH maior que 6,31, enquanto que as soluções básicas apresentam pH maior que 6,31 e pOH menor que 6,31. Esta distinção pode ser importante ao estudar certos processos que ocorrem a outras temperaturas, tais como reações enzimáticas em organismos de sangue quente a uma temperatura de cerca de 36 – 40 °C. Salvo indicação em contrário, presume-se que as referências aos valores de pH são as de 25 °C.
Este texto é adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh"> Openstax, Chemistry 2e, Section 14.2: pH and pOH.</a>

Escala de pH

O hidrônio e os íons hidróxido estão presentes tanto na água pura quanto em todas as soluções aquosas, e suas concentrações são inversamente proporcionais à medida que ...

Education

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Química

Acids and Bases

Ácidos e Bases

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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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Proton Transfer in Acids and Bases

Acids and bases can be understood through proton transfer. Early scientists described acids by their sour taste, their effect on dyes, and their reaction with limestone, which releases carbon dioxide. Robert Boyle also noted that acid solutions can lose these properties after contact with alkali, or base, solutions. The modern view developed step by step. Humphry Davy showed that hydrogen is the essential part of acids. Joseph Louis Gay-Lussac then described acids as substances that neutralize...

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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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Brønsted-Lowry acid-base chemistry is based on proton transfer. That idea helps show how the strength of an acid is linked to the strength of its conjugate base, and vice versa. Acid and base strength is measured with ionization constants, Ka and Kb, which tell how far an ionization reaction goes. For the conjugate pair HA and A−, the acid ionizes in water and the base reaction can be written for the partner species. When the two reactions are added, they give the autoionization of water. The...

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determining the ph of salt solutions

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acid/base strengths and dissociation constants

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Acid and Base Strength in Water

Acid and base strength in water depends on how much a substance ionizes after it dissolves. If ionization is nearly complete, the substance is called strong. If only a small amount ionizes, it is called weak. Weak acids and weak bases are much more common than strong ones. The most common strong acids are HClO4, HCl, HBr, HI, HNO3, and H2SO4. The most common strong bases are LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2. These strong acids and bases ionize essentially completely in water. <br...

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lewis acids and bases

Electron-Pair Reactions in Acid-Base Chemistry

Electron-pair reactions explain Lewis acid-base chemistry. In 1923, G. N. Lewis described acids and bases by how they accept or donate a pair of electrons. When that electron pair forms a coordinate covalent bond, one atom supplies both bonding electrons. This bond is also called a dative bond. A simple example is the reaction of water with a hydrogen ion to make a hydronium ion. Ammonia can also combine with a hydrogen ion to form an ammonium ion. In both cases, the reaction creates a Lewis...

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

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Acids and Bases: Percent Dissociation and pH

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pH and pOH in Water

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