```
pH = -log10[H3O+]
```
where [H3O+] is the concentration of hydrogen ions in the solution, in moles per liter (M).
The pH difference between two solutions can be calculated by subtracting the pH of the second solution from the pH of the first solution:
```
pH difference = pH1 - pH2
```
For example, if the pH of a solution is 4 and the pH of another solution is 6, the pH difference is 6 - 4 = 2. This means that the second solution is 2 units more basic than the first solution.
The pH difference can also be used to calculate the concentration of hydrogen ions in a solution, using the following equation:
```
[H3O+] = 10^-pH
```
For example, if the pH of a solution is 4, the concentration of hydrogen ions in the solution is:
```
[H3O+] = 10^-4 = 1 × 10^-4 M = 100 μM
```
