What is pH?
The pH or power of hydrogen ions is a quantitative measure of the acidity or alkalinity of a solution. It is equivalent to the negative logarithm of hydrogen ion concentration.
pH = – log[H+]
What is pKa?
pKa is a standard used to specify the nature of the solution. It is generally used to estimate the acidity of a solution. If the pKa of a solution is less, the proton would be less tightly bound; hence the solution would be more acidic.
Table of Content
- Understanding pH and pKa in detail
- pH to pKa
- pH and pKa value of few acids
- Frequently Asked Questions – FAQs
Understanding pH and pKa in detail
The pH or power of hydrogen ions is a quantitative measure of the acidity or alkalinity of a solution. It is equivalent to the negative logarithm of hydrogen ion concentration.
pH = – log[H+]
If the pH of a solution is less than 7, the solution is acidic, or if the pH of a solution is more than 7, the solution is alkaline, or if the pH is equivalent to 7, then the solution is neutral.
In contrast, pKa is a standard used to specify the nature of the solution. It is generally used to estimate the acidity of a solution. If the pKa of a solution is less, the proton would be less tightly bound; hence the solution would be more acidic.
The pKa value varies from -2 to 12 for weak acid, while a strong acid has a pKa value less than -2.
Though pH and pKa are associated, pKa is more explicit. pKa signifies how a molecule will behave at a certain pH. Primarily, pKa determines at what pH a solution will effectively lose or gain proton.
Henderson-Hasselbalch has given a relation between pH and pKa.
pH to pKa
Henderson-Hasselbalch has given a relation between pH and pKa. He said pH is equal to pKa + log {[salt] / [Acid]}.
pH = pKa + log([salt]/[Acid])
If ([salt]/[Acid]) is equal to 10.
pH = pKa + log10
pH = pKa + 1
If ([salt]/[Acid]) is equal to 1/10.
pH = pKa + log1/10
pH = pKa + log1 – log10
pH = pKa – 1
Thus by using the Henderson-Hasselbalch equation, we can quickly convert pH to pKa.
pH and pKa value of a few acids
| S No. | Name of the Acid | pH at 1 mM concentration | pKa value |
|---|---|---|---|
| 1. | Sulfuric acid | 2.75 | -3 |
| 2. | Nitric Acid | 3.01 | -1.4 |
| 3. | Hydroiodic Acid | 3.01 | -9.3 |
| 4. | Hydrobromic Acid | 3.01 | -8.7 |
| 5. | Hydrochloric Acid | 3.01 | -6.3 |
| 6. | Hydrofluoric Acid | 3.27 | 3.17 |
| 7. | Orthophosphoric Acid | 3.06 | 2.16 |
| 8. | Arsenious Acid | 6.07 | 9.29 |
| 9. | Boric Acid | 6.12 | 9.24 |
| 10. | Hydrocyanic Acid | 6.11 | 9.21 |
Frequently Asked Questions on pH to pKa
What is pH?
The pH or power of hydrogen ions is a quantitative measure of the acidity or alkalinity of a solution. It is equivalent to the negative logarithm of hydrogen ion concentration.
pH = – log[H+]
If the pH of a solution is less than 7, the solution is acidic, or if the pH of a solution is more than 7, the solution is alkaline, or if pH is equivalent to 7, then the solution is neutral.
What is pKa?
pKa is a standard used to specificity the nature of the solution. It is generally used to estimate the acidity of a solution. If the pKa of a solution is less, the proton would be less tightly bound; hence the solution would be more acidic.
The pKa value varies from -2 to 12 for weak acid, while a strong acid has a pKa value less than -2.
Are pH and pKa the same?
Though pH and pKa are associated, pKa is are more explicit. pKa signifies how a molecule will behave at a certain pH. Primarily, pKa determines at what pH a solution will effectively lose or gain proton.
Henderson-Hasselbalch has given a relation between pH and pKa. He said pH is equal to pKa + log {[salt] / [Acid]}.
pH = pKa + log([salt]/[Acid])
What is the pH and pKa of pure water at 25℃?
The pH of pure water is equal to 7 at 25℃. In contrast, the pKa value of pure water at 25℃ is equal to 14.
How are pH and pKa related?
Henderson-Hasselbalch has given a relation between pH and pKa. He said pH is equal to pKa + log {[salt] / [Acid]}.
pH = pKa + log([salt]/[Acid])
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