What is pH?
pH or potential of hydrogen ion is a scale used to determine the hydrogen ion (H+) concentration in a solution. It is a quantitative measure of the acidity or alkalinity of a solution. It equals the negative log of hydrogen ion (H+) concentration.
pH = – log [H+]
What is pOH?
pOH or potential of hydroxide ion is a scale used to determine the hydroxide ion (OH–) concentration in a solution. It is a quantitative measure of the acidity or alkalinity of a solution. It equals the negative log of hydroxide ion (OH–) concentration.
pH = – log [OH–]
Table of Content
- Understanding pH and pOH in detail
- pH to pOH
- pH and pOH value of a few compounds
- Difference between pH and pOH
- Frequently Asked Questions – FAQs
Understanding pH and pOH in detail
pH or potential of hydrogen ion is a scale used to determine the hydrogen ion (H+) concentration in a solution. It is a quantitative measure of the acidity or alkalinity of a solution. It equals the negative log of hydrogen ion (H+) concentration.
pH = – log [H+]
At pH 7 solution is found to be neutral. In contrast, if the pH value is less than 7, the solution will be acidic, and if the pH value is more than 7, the solution will be basic.
In contrast, pOH or potential of hydroxide ion is a scale used to determine the hydroxide ion (OH–) concentration in a solution. It is a quantitative measure of the acidity or alkalinity of a solution. It equals the negative log of hydrogen ion (OH–) concentration.
pH = – log [OH–]
At pOH 7 solution is found to be neutral. In contrast, if the pOH value is less than 7, the solution will be basic, and if the pOH value is more than 7, the solution will be acidic.
Both pH and pOH are related to each other. pH is inversely proportional to pOH, i.e. pH increases with decreasing pOH.
pH ∝ 1 / pOH
pH to pOH
Consider a reaction,
H2O ⇆ H+ + OH–
For the above reaction dissociation constant, Kw would be,
Kw = [H+][OH–]
Here, Kw refers to dissociation constant, and [H+] and [OH–] refer to hydrogen and hydroxide ion concentrations.
We are taking the negative logarithm of both sides.
-log Kw = – log [H+] [OH–]
-log Kw = -(log [H+] + [OH–] )
-log Kw = -log [H+] – log [OH–] . . . 1
We know that,
Kw = 1 X 10 -14 at 298 K
-log [H+] = pH
-log [OH–] = pOH
Putting value in equation 1
-log 1 X 10 -14 = pH + pOH
14 = pH + pOH
pOH = 14 – pH
pH = 14 – pOH
Hence if the pH is known, we can quickly determine the pOH value.
pH and pOH value of a few molecules
| S No. | Name of the Molecule | pH at 1 mM concentration | pOH at 1 mM concentration |
|---|---|---|---|
| 1. | Sodium Hydroxide | 1 | 13 |
| 2. | Potassium Hydroxide | 1.48 | 12.52 |
| 3. | Sulfuric acid | 2.75 | 11.25 |
| 4. | Hydrobromic Acid | 3.01 | 10.99 |
| 5. | Nitric Acid | 3.01 | 10.99 |
| 6. | Hydrochloric Acid | 3.01 | 10.99 |
| 7. | Hydroiodic Acid | 3.01 | 10.99 |
| 8. | Orthophosphoric Acid | 3.06 | 10.94 |
| 9. | Hydrofluoric Acid | 3.27 | 10.73 |
Difference between pOH and pH
| S No. | pH | pOH |
|---|---|---|
| 1. | pH is the potential of hydrogen ions. | pOH is the potential of hydroxide ions. |
| 2. | It is a scale used to determine the solution’s hydrogen ion (H+) concentration. | It is a scale used to determine the solution’s hydroxide ion (OH–) concentration. |
| 3. | It equals the negative log of hydrogen ion (H+) concentration.
pH = – log [H+] |
It equals the negative log of hydroxide ion (OH– ) concentration.
pH = – log [OH–] |
| 4. | If the pH value is less than 7, the solution will be acidic. | If the pOH value is less than 7, the solution will be basic. |
| 5. | If the pH value is more than 7, the solution will be basic. | If the pOH value is more than 7, the solution will be acidic. |
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Acids and Bases
Frequently Asked Questions on pH to pOH
What is pH?
pH or potential of hydrogen ion is a scale used to determine the hydrogen ion (H+) concentration in a solution. It is a quantitative measure of the acidity or alkalinity of a solution. It equals the negative log of hydrogen ion (H+) concentration.
At pH 7 solution is found to be neutral. In contrast, if the pH value is less than 7, the solution will be acidic, and if the pH value is more than 7, the solution will be basic.
What is pOH?
pOH or potential of hydroxide ion is a scale used to determine the hydroxide ion (OH–) concentration in a solution. It is a quantitative measure of the acidity or alkalinity of a solution. It equals the negative log of hydrogen ion (OH–) concentration.
At pOH 7 solution is found to be neutral. In contrast, if the pOH value is less than 7, the solution will be basic, and if the pOH value is more than 7, the solution will be acidic.
Are pH and pOH the same?
No, pH and pOH are not the same. However, they were found to be associated. pH is inversely proportional to pOH, i.e. pH increases with decreasing pOH.
pH ∝ 1 / pOH
What is the difference between pH and pOH?
| S No. | pH | pOH |
|---|---|---|
| 1. | pH or potential of hydrogen ion is a scale used to determine the hydrogen ion (H+) concentration in a solution. | pH or potential of hydrogen ion is a scale used to determine the hydroxide ion (OH–) concentration in a solution. |
| 2. | It equals the negative log of hydrogen ion (H+) concentration.
pH = – log [H+] |
It equals the negative log of hydroxide ion (OH– ) concentration.
pH = – log [OH–] |
Calculate the pOH of a solution having a pH value of 1?
Given,
pH = 1,
We know that,
pH + pOH = 14
pOH = 14 – pH
pOH = 14 – 1
pH = 13
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