Err.... I think it might just be ordinary NaCl (or maybe KCl), since human physiology is most readily equipped to deal with that.
Sperm cells have a lot of Sodium and Potassium ions in them (it's to do with the molecular 'motor' powering their tails), and as a result (so semen itself is not self-destructively acidic), chloride ions are present in the seminal fluid that the sperm live in. It's those chlorine ions that would cause the basic state of semen. When chloride ions come into contact with common bodily acids, which often have sodium or potassium as components, the result is often ordinary salt (NaCl), or potassium chloride (KCl).
None of this really matters, though: The vagina is coated in a layer of mucus that acts like an acidic buffer, so any salt made by sperm would just be caught in the mucus, and the pH not really affected by it anyway.
This is just an estimated guess, mind you. And why the hell do you want to know anyway?