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

De concentratie van hydronium-ionen in een waterige oplossing wordt meestal geschreven met negatieve exponenten en kan zo klein zijn als 1 10⁻¹⁴ M.De pH-schaal is ontwikkeld door chemicus Søren Sørenson in 1909 als een gemakkelijkere manier om snel de zuurgraad van verschillende oplossingen te vergelijken. De pH van een oplossing is de negatieve logaritme van de hydronium-ionenconcentratie. Een oplossing met 1 10⁻⁵ M hydroniumconcentratie heeft bijvoorbeeld een pH van 5.Hoe hoger de hydronium-ionenconcentratie van een oplossing, hoe lager de pH. Omdat de pH op een logaritmische schaal wordt uitgedrukt, komt een verandering van één eenheid overeen met een 10-voudige toename of afname van de hydronium-ionenconcentratie. Een oplossing met een pH van 3 heeft tien keer meer hydronium-ionen dan een oplossing met een pH van 4 en honderd keer meer dan een oplossing met een pH van 5.Een zure oplossing heeft een hogere concentratie aan hydronium-ionen dan hydroxide-ionen en een pH lager dan 7, terwijl een basische oplossing een lagere concentratie aan hydronium-ionen heeft dan hydroxide-ionen en een pH hoger dan 7. Een neutrale oplossing met een gelijke concentratie aan hydronium-en hydroxide-ionen heeft een pH van 7. De concentratie van hydroxide-ionen kan ook worden uitgedrukt als pOH.pOH is de negatieve logaritme van de hydroxide-ionenconcentratie. Hoe hoger de hydroxide-ionenconcentratie, hoe lager de pOH-waarde. Een pH-of pOH-waarde van een waterige oplossing varieert van 0-14.Dit komt doordat Kw, de evenwichtsconstante voor de auto-ionisatie van water, gelijk is aan 1 10⁻¹⁴. Als we de negatieve log van beide zijden van de vergelijking nemen, resulteert dit in een vergelijking waarin pKw gelijk is aan de som van pH en pOH. Omdat de negatieve log van 1 10⁻¹⁴ 14 is, is de som van de pH en pOH van een waterige oplossing altijd 14.Hiermee kan de pH-waarde worden bepaald als de pOH-waarde bekend is en vice versa. Een oplossing met een pOH van 10 heeft bijvoorbeeld een pH van 4.

ph schaal

Hydronium- en hydroxide-ionen zijn zowel in zuiver water als in alle waterige oplossingen aanwezig, en hun concentraties zijn omgekeerd evenredig, zoals bepaald door het ionenproduct van water (Kw). De concentraties van deze ionen in een oplossing zijn vaak kritische bepalende factoren voor de eigenschappen van de oplossing en het chemische gedrag van de andere opgeloste stoffen. Twee verschillende oplossingen kunnen in hun hydronium- of hydroxide-ionconcentraties een miljoen, miljard of zelfs biljoen keer verschillen. Een gebruikelijke manier om grootheden uit te drukken die vele ordes van grootte kunnen omspannen, is het gebruik van een logaritmische schaal. De pH van een oplossing wordt daarom gedefinieerd zoals hier weergegeven, waarbij [H3O+] de molaire concentratie van hydroniumionen in de oplossing is:  <img alt="Eq1" src="/files/ftp_upload/11397/11397_Equation_1.png" style="height: 50px;"> Het herschikken van deze vergelijking om de molariteit van het hydroniumion te isoleren levert de equivalente uitdrukking op:  <img alt="Eq2" src="/files/ftp_upload/11397/11397_Equation_2.png" style="height: 50px;"> Op dezelfde manier kan de molariteit van het hydroxide-ion worden uitgedrukt als een p-functie of pOH:  <img alt="Eq3" src="/files/ftp_upload/11397/11397_Equation_3.png" style="height: 50px;"> of  <img alt="Eq4" src="/files/ftp_upload/11397/11397_Equation4.png" style="height: 50px;"> Ten slotte kan de relatie tussen deze twee ionenconcentraties, uitgedrukt als p-functies, eenvoudig worden afgeleid uit de KW-uitdrukking:  <img alt="Eq5" src="/files/ftp_upload/11397/11397_Equation_5.png" style="height: 135px;"> Bij 25 °C is de waarde van KW 1,0 × 10−14, en dus:  <img alt="Eq6" src="/files/ftp_upload/11397/11397_Equation_6.png" style="height: 50px;"> De molariteit van hydroniumionen in zuiver water (of een andere neutrale oplossing) is 1,0 × 10−7 M bij 25 ° C. De pH en pOH van een neutrale oplossing bij deze temperatuur zijn daarom:  <img alt="Eq7" src="/files/ftp_upload/11397/11397_Equation_7.png" style="height: 95px;"> En dus zijn bij deze temperatuur zure oplossingen oplossingen met een molariteit van hydroniumionen groter dan 1,0 x 10−7 M en een molariteit van hydroxide-ionen kleiner dan 1,0 x 10−7 M (overeenkomend met pH-waarden van minder dan 7,00 en pOH-waarden groter dan 7,00). Basisoplossingen zijn oplossingen met een molariteit van hydroniumionen kleiner dan 1,0 × 10−7 M en molariteiten van hydroxideionen groter dan 1,0 × 10−7 M (overeenkomend met pH-waarden groter dan 7,00 en pOH-waarden kleiner dan 7,00). Omdat de autoionisatieconstante KW temperatuurafhankelijk is, zullen deze correlaties tussen pH-waarden en de zure/neutrale/basische bijvoeglijke naamwoorden anders zijn bij andere temperaturen dan 25 °C. De hydroniummolariteit van zuiver water bij 80°C is bijvoorbeeld 4,9 × 10−7 M, wat overeenkomt met de pH- en pOH-waarden van:  <img alt="Eq8" src="/files/ftp_upload/11397/11397_Equation_8.png" style="height: 95px;"> Bij deze temperatuur vertonen neutrale oplossingen pH = pOH = 6,31, zure oplossingen vertonen een pH lager dan 6,31 en pOH groter dan 6,31, terwijl basische oplossingen een pH groter dan 6,31 en pOH lager dan 6,31 vertonen. Dit onderscheid kan van belang zijn bij het bestuderen van bepaalde processen die plaatsvinden bij andere temperaturen, zoals enzymreacties in warmbloedige organismen bij een temperatuur rond de 36 – 40 °C. Tenzij anders vermeld, wordt aangenomen dat verwijzingen naar pH-waarden die bij 25 °C zijn.

pH schaal

Hydronium and hydroxide ions are present both in pure water and in all aqueous solutions, and their concentrations are inversely proportional as ...

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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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