what is the PH to be maintained for soap?

[Zany_in_CO]

I have seen many soap making videos in which it is told to maintain the PH of the soap to be between 8 and 9.But one of my friend who is a dermatologist says that the ph of skin is 5.5 and any top grade soap needs to have a PH of somewhere 7.

That's true, but he's speaking of commercial soaps. Many of us, including me, here on SMF started making soap because commercial bars (including "top grade") irritated our skin or the skin of a loved one. Please be sure to give your dermatologist friend a bar of your handmade soap to try as soon as you can. Just don't mention the pH and see what he thinks. LOL

If we cure the soap for a longer time will the PH get lowered or the oils we use must be chosen to maintain lower PH.

I spent a good deal of time researching pH when I was where you are right now. I spent some time adding citric acid (20% solution) to lower the pH. When I got close to pH 7.5 the batch would separate. Also, while citric acid in small amounts did help with soap scum, it was not noticeably more gentle to my skin and scalp.

I've been making soap since 2003 and I just don't worry about the pH any more.