The pH of a liquid determines whether it’s acidic or basic. Mixing liquids requires technical know-how to predict the final pH. A simple formula helps calculate the pH when combining acids, bases, or their mixtures in any ratio.
pH Solution Mixer
Professional grade pH calculation system
These can also be used for:
- Mixing acid and bases (up to 10 different liquids)
- Desired pH of liquid with dilution
Now, let’s see how is the calculation done:
Example 1: Finding the pH of a Mixed Solution of 400ml of solution A of pH 6 with 600ml of solution B of pH 4.
Given Data
We have two solutions:
Solution A:
- Volume = 400 mL
- pH = 6
Solution B:
- Volume = 600 mL
- pH = 4
We need to determine the final pH after mixing both solutions.
Step 1: Convert pH to [H⁺] Concentration
The relationship between pH and hydrogen ion concentration is:
[H+]=10−pH
For Solution A (pH = 6):
[H+]A=10−6 moles/litre
For Solution B (pH = 4):
[H+]B=10−4 moles/litre
Step 2: Calculate Moles of [H⁺] in Each Solution
Since concentration = moles/volume, we find the total moles of [H+] in each solution.
For Solution A (400 mL = 0.400 L):
Moles of H+ = (10−6 mol/L) × (0.400 L)
=4.0×10−7 moles
For Solution B (600 mL = 0.600 L):
Moles of H+ = (10−4 mol/L) × (0.600 L)
=6.0×10−5 moles
Step 3: Find Total [H⁺] in the Mixture
To determine the total moles of hydrogen ions [H+] in the mixed solution, we add the moles from Solution A and Solution B
Total moles of H+ :
[H+]mix=[H+]A+[H+]B
=(4.0×10−7) + (6.0×10−5)
=6.04×10−5 moles
Total volume of the mixture:
400 mL+600 mL=1000 mL
Final [H+] concentration:
[H+]mix= 6.04×10−5 moles / 1.0 L
= 6.04×10−5 moles
Step 4: Calculate the Final pH
pH = −log(6.04×10−5)
≈ 4.22
The final pH of the mixed solution is 4.22.
Example 2: Mix 6.0 mL of a solution with pH = 5.0 and 6.0 mL of a solution with pH = 9.0. What is the final pH?
The final pH is 8.65
Note: In the above example, it seems like an equal volume of pH 5 and pH 9 will neutralize each other and give a pH value of 7. However, we have seen that the pH is different as the concentration of hydrogen ions is different. So, the calculation of the pH value is important.
Important Note
The pH equation used for mixing solutions is accurate for strong acids and bases because they fully dissociate in water, allowing direct calculation of hydrogen and hydroxide ion concentrations.
However, for weak acids and bases, partial dissociation and buffer effects complicate pH predictions. In such cases, the Henderson-Hasselbalch equation provides a more accurate approach, accounting for the equilibrium between the weak acid and its conjugate base or the weak base and its conjugate acid.
To determine the final pH when mixing weak acids and bases, it is essential to consider buffer capacity, equilibrium constants, and the extent of dissociation rather than relying solely on direct concentration calculations.