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

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pH Scale

Stężenie jonów hydroniowych w roztworze wodnym jest zwykle zapisywane za pomocą ujemnych wykładników i może wynosić nawet 1 × 10−14 M. Skala pH została opracowana przez chemika Sorena Sorensona w 1909 roku jako wygodniejszy sposób szybkiego porównywania kwasowości różnych roztworów.
pH roztworu jest ujemnym logarytmem stężenia jonów hydroniowych. Na przykład roztwór o stężeniu hydroniowym 1 × 10−5 M ma pH 5. Im większe stężenie jonów hydroniowych w roztworze, tym niższe jest jego pH.
Ponieważ pH jest wyrażane w skali logarytmicznej, zmiana o jedną jednostkę odpowiada 10-krotnemu wzrostowi lub spadkowi stężenia jonów hydroniowych. Roztwór o pH 3 będzie miał dziesięć razy więcej jonów hydroniowych niż roztwór o pH 4 i sto razy więcej niż roztwór o pH 5.
Roztwór kwaśny ma wyższe stężenie jonów hydroniowych niż jonów wodorotlenkowych i pH mniejsze niż 7, podczas gdy roztwór zasadowy ma niższe stężenie jonów hydroniowych niż jony wodorotlenkowe i pH większe niż 7.
Obojętny roztwór o równym stężeniu jonów hydroniowych i wodorotlenkowych ma pH 7.
Stężenie jonów wodorotlenkowych można również wyrazić jako pOH. pOH jest ujemnym logarytmem stężenia jonów wodorotlenkowych. Im wyższe stężenie jonów wodorotlenkowych, tym niższa jest jego wartość pOH.
Wartość pH lub pOH roztworu wodnego mieści się w zakresie od 0 do 14. Dzieje się tak, ponieważ KW, stała równowagi dla autojonizacji wody, jest równa 1 × 10−14. 
Wzięcie ujemnego logarytmu obu stron równania daje równanie, w którym pKW jest równe sumie pH i pOH. Ponieważ logarytm ujemny 1 × 10−14 wynosi 14, suma pH i pOH roztworu wodnego zawsze będzie wynosić 14. Można to wykorzystać do określenia wartości pH, gdy znana jest wartość pOH i odwrotnie.
Na przykład roztwór o pH 10 ma pH 4.

<ol>
	<li>van Lubeck, Henk. "Dlaczego nie zastąpić pH i pOH tylko jednym prawdziwym stopniem kwasowości, AG?. " Journal of Chemical Education 76, nr 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>

skala ph

Jony wodorotlenkowe i wodorotlenkowe występują zarówno w czystej wodzie, jak i we wszystkich roztworach wodnych, a ich stężenia są odwrotnie proporcjonalne. Określa to iloczyn jonowy wody (Kw). Stężenia tych jonów w roztworze są często krytycznymi wyznacznikami właściwości roztworu i zachowania chemicznego innych substancji rozpuszczonych. Dwa różne roztwory mogą różnić się stężeniem jonów hydroniowych lub wodorotlenkowych milion, miliard, a nawet bilion razy. Powszechnym sposobem wyrażania wielkości, które mogą obejmować wiele rzędów wielkości, jest użycie skali logarytmicznej. Dlatego pH roztworu definiuje się w sposób pokazany poniżej, gdzie [H3O+] oznacza stężenie molowe jonu hydroniowego w roztworze:

 <img alt="Eq1" src="/files/ftp_upload/11397/11397_Equation_1.png" style="height: 50px;" />

Przekształcenie tego równania w celu wyizolowania molarności jonu hydroniowego daje równoważne wyrażenie:

 <img alt="Eq2" src="/files/ftp_upload/11397/11397_Equation_2.png" style="height: 50px;" />

Podobnie molarność jonów wodorotlenkowych można wyrazić jako funkcję p lub pOH:

 <img alt="Eq3" src="/files/ftp_upload/11397/11397_Equation_3.png" style="height: 50px;" />

Lub

 <img alt="Eq4" src="/files/ftp_upload/11397/11397_Equation4.png" style="height: 50px;" />

Wreszcie związek między tymi dwoma stężeniami jonów wyrażonymi jako funkcje p można łatwo wyprowadzić z wyrażenia KW:

 <img alt="Eq5" src="/files/ftp_upload/11397/11397_Equation_5.png" style="height: 135px;" />

W temperaturze 25°C wartość KW wynosi 1,0 × 10−14 i tak:

 <img alt="Eq6" src="/files/ftp_upload/11397/11397_Equation_6.png" style="height: 50px;" />

Molarność jonu hydroniowego w czystej wodzie (lub dowolnym roztworze obojętnym) wynosi 1,0 × 10-7 M w temperaturze 25 ° C. Zatem pH i pOH obojętnego roztworu w tej temperaturze wynoszą:

 <img alt="Eq7" src="/files/ftp_upload/11397/11397_Equation_7.png" style="height: 95px;" />

Tak oto w tej temperaturze roztworami kwasowymi są roztwory o molarności jonów hydroniowych większych niż 1,0 × 10-7 M i molarności jonów wodorotlenkowych mniejszych niż 1,0 × 10-7 M (odpowiadające wartościom pH mniejszym niż 7,00 i wartościom pOH większym niż 7,00). Roztwory zasadowe składają się z jonów hydroniowych molarności mniejszej niż 1,0 × 10-7 M i molarności jonów wodorotlenkowych większych niż 1,0 × 10-7 M (odpowiadające wartościom pH większym niż 7,00 i wartościom pOH mniejszym niż 7,00).

Ponieważ stała autojonizacji KW zależy od temperatury, te korelacje między wartościami pH a przymiotnikami kwasowy/neutralny/zasadowy będą inne w temperaturach innych niż 25°C. Na przykład molarność hydroniowa czystej wody w temperaturze 80°C wynosi 4,9 × 10-7 M, co odpowiada wartościom pH i pOH:

 <img alt="Eq8" src="/files/ftp_upload/11397/11397_Equation_8.png" style="height: 95px;" />

W tej temperaturze roztwory obojętne wykazują pH = pOH = 6,31, roztwory kwaśne wykazują pH mniejsze niż 6,31 i pOH większe niż 6,31, natomiast roztwory zasadowe wykazują pH większe niż 6,31 i pOH mniejsze niż 6,31. To rozróżnienie może być ważne przy badaniu pewnych procesów zachodzących w innych temperaturach, takich jak reakcje enzymatyczne w organizmach stałocieplnych w temperaturze około 36–40 °C. O ile nie wskazano inaczej, przyjmuje się, że odniesienia do wartości pH dotyczą temperatury 25 °C.

Skala pH

Jony hydroniowe i wodorotlenkowe są obecne zarówno w czystej wodzie, jak iwe wszystkich roztworach wodnych, a ich stężenia są odwrotnie proporcjonalne do ...

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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.

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pH and pOH in Water

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