If you would refer to PH of bogs and look at fredsbog answer to What would be the best PH reading for bogs you will learn the difference. It sure opened my eyes and established that I had a Fen instead of a bog. I might add the Fen is more fun because of a larger variety of plants and the ease of providing water for it.

In reality the difference is not only in PH but in the type of water that supplies the respective habitats.

A bog is fed strictly by rain water, it receives ALL it's nutrients from rain and what rain picks up as it travels through the atmosphere (whether that's good or bad these days!)

A fen receives it's water from the both rain and the ground, often (in the case of the local fens) from percolating through limestone bedrock. Hence it is more pure than say river water because of the rock substrate but it has more mineral and nutrient content than just rain water so the plant material that grows in fens ins much more diverse. Many bog plants will grow in a fen but a large number of fen plants will not grow in a bog. An example is I cannot grow Sarracenia Purpurea in my fen garden because the PH is too high. there reported to be a variety of this plant that grows on limestone along lake Huron...I'd like to visit these areas!

There was quite a debate about this difference a few years ago on this forum and I think most came to the agreement on the above...It was eye opening for me, I'd assumed it was all about PH and only PH.

Fred