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

La concentrazione di ioni idronio in una soluzione acquosa è solitamente scritta con esponenti negativi e può essere piccola come 1 10⁻¹⁴ M.La scala del pH fu sviluppata dal chimico Soren Sorenson nel 1909 come modo più conveniente per confrontare rapidamente l'acidità di soluzioni diverse. Il pH di una soluzione è il log negativo della sua concentrazione di ioni idronio. Per esempio, una soluzione con una concentrazione di idronio 1 10⁻⁵ M ha un pH di 5.

Maggiore è la concentrazione di ioni idronio di una soluzione, minore è il suo pH. Poiché il pH è espresso su una scala logaritmica, una variazione di una singola unità corrisponde all'aumento o diminuzione di 10 volte della concentrazione di ioni idronio. Una soluzione con un pH di 3 avrà dieci volte più ioni idronio di una soluzione con un pH 4, e cento volte più ioni idronio di una soluzione con un pH di 5.

Una soluzione acida ha una maggiore concentrazione di ioni idronio rispetto agli ioni idrossido e un pH inferiore a 7, mentre una soluzione basica ha una concentrazione inferiore di ioni idronio rispetto agli ioni idrossido, e un pH maggiore di 7. Una soluzione neutra con una concentrazione uguale di idronio e ioni idrossido ha un pH di 7. La concentrazione di ioni idrossido può anche essere espressa come pOH.

pOH è il log negativo della concentrazione di ioni idrossido. Maggiore è la concentrazione di ioni idrossido, minore è il suo valore di pOH. Un valore di pH o pOH di una soluzione acquosa è compreso fra 0 e 14.

Questo perché Kw, la costante di equilibrio per l'autoionizzazione dell'acqua è pari a 1 10⁻¹⁴. Prendendo il log negativo di entrambi i lati dell'equazione si ottiene un'equazione in cui pKw è uguale alla somma di pH e pOH. Poiché il log negativo di 1 10⁻¹⁴ è 14, la somma del pH e del pOH di una soluzione acquosa sarà sempre 14.

Ciò può essere usato per determinare il valore di pH quando il valore di pOH è noto, e viceversa. Per esempio, una soluzione con un pOH di 10 ha un pH di 4.

<ol>
	<li>van Lubeck, Henk. "Perché non sostituire il pH e il pOH con un solo vero grado di acidità, l'AG?. " Giornale di educazione chimica 76, n. 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>

scala del ph

Gli ioni idronio e idrossido sono presenti sia nell'acqua pura che in tutte le soluzioni acquose e le loro concentrazioni sono inversamente proporzionali come determinato dal prodotto ionico dell'acqua (K_w). Le concentrazioni di questi ioni in una soluzione sono spesso determinanti critici delle proprietà della soluzione e dei comportamenti chimici degli altri suoi soluti. Due diverse soluzioni possono differire nella concentrazione degli ioni idronio o idrossido di un milione, un miliardo o addirittura un trilione di volte. Un mezzo comune per esprimere quantità che possono estendersi su molti ordini di grandezza è utilizzare una scala logaritmica. Il pH di una soluzione è quindi definito come mostrato qui, dove [H_3O^+] è la concentrazione molare dello ione idronio nella soluzione:

 <img alt="Eq1" src="/files/ftp_upload/11397/11397_Equation_1.png" style="height: 50px;" />

Riorganizzando questa equazione per isolare la molarità degli ioni idronio si ottiene l'espressione equivalente:

 <img alt="Eq2" src="/files/ftp_upload/11397/11397_Equation_2.png" style="height: 50px;" />

Allo stesso modo, la molarità degli ioni idrossido può essere espressa come funzione p o pOH:

 <img alt="Eq3" src="/files/ftp_upload/11397/11397_Equation_3.png" style="height: 50px;" />

O

 <img alt="Eq4" src="/files/ftp_upload/11397/11397_Equation4.png" style="height: 50px;" />

Infine, la relazione tra queste due concentrazioni di ioni espressa come funzioni p è facilmente derivabile dall'espressione K_W:

 <img alt="Eq5" src="/files/ftp_upload/11397/11397_Equation_5.png" style="height: 135px;" />

A 25 °C il valore di K_W è 1,0 × 10^−14, quindi:

 <img alt="Eq6" src="/files/ftp_upload/11397/11397_Equation_6.png" style="height: 50px;" />

La molarità degli ioni idronio nell'acqua pura (o in qualsiasi soluzione neutra) è 1,0 × 10^−7 M a 25 °C. Il pH e il pOH di una soluzione neutra a questa temperatura sono quindi:

 <img alt="Eq7" src="/files/ftp_upload/11397/11397_Equation_7.png" style="height: 95px;" />

E quindi, a questa temperatura, le soluzioni acide sono quelle con molarità degli ioni idronio maggiori di 1,0 × 10^−7 M e molarità degli ioni idrossido inferiori a 1,0 × 10^−7 M (corrispondenti a valori di pH inferiori a 7,00 e valori di pOH superiori a 7,00). Le soluzioni basiche sono quelle con molarità degli ioni idronio inferiori a 1,0 × 10^−7 M e molarità degli ioni idrossido superiori a 1,0 × 10^−7 M (corrispondenti a valori di pH superiori a 7,00 e valori di pOH inferiori a 7,00).

Poiché la costante di autoionizzazione K_W dipende dalla temperatura, queste correlazioni tra i valori di pH e gli aggettivi acido/neutro/basico saranno diverse a temperature diverse da 25 °C. Ad esempio, la molarità dell'idronio dell'acqua pura a 80°C è 4,9 × 10^−7 M, che corrisponde a valori di pH e pOH di:

 <img alt="Eq8" src="/files/ftp_upload/11397/11397_Equation_8.png" style="height: 95px;" />

A questa temperatura, le soluzioni neutre presentano pH = pOH = 6,31, le soluzioni acide presentano pH inferiore a 6,31 e pOH maggiore di 6,31, mentre le soluzioni basiche presentano pH maggiore di 6,31 e pOH inferiore a 6,31. Questa distinzione può essere importante quando si studiano determinati processi che avvengono ad altre temperature, come le reazioni enzimatiche negli organismi a sangue caldo a una temperatura intorno ai 36 – 40 °C. Salvo diversa indicazione, i riferimenti ai valori di pH si presumono quelli a 25 °C.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh"> Openstax, Chemistry 2e, Section 14.2: pH and pOH.</a>

Scala del pH

Gli ioni idronio e idrossido sono presenti sia nell'acqua pura che in tutte le soluzioni acquose, e le loro concentrazioni sono inversamente proporzionali ...

Education

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KEY TERMS AND CONCEPTS

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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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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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pH and pOH in Water

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