I am reading an old paper and I have came to the passage shown in the picture: k, CA, CB and V0 are all just some constants. I think I know what they did - they differentiated the first equation calculating dpH/dV and then took reciprocals of both sides of the equation. Is it a valid approach? I am not able to get the same result solving the first equation for V and calculating dV/dpH then. Bear in mind I am mathemathically challenged and English is my second language, I can be missing something obvious.