pH is a scale that indicates acidity and alkalinity. The range goes from 0, being the most acidic, to 14, being the most alkaline. A healthy vaginal pH value is somewhere between 3.8 and 4.5, well into the acidic range.
The acidic nature of the vagina is very important. It cleans itself by secreting natural fluids and maintaining a healthy pH which encourages the growth of good bacteria that is responsible for keeping the vagina clean and healthy. Also, it inhibits the growth of harmful bacteria which could lead to infection and disease.
The pH of semen is commonly measured between 7.2 and 8.0 (slightly basic or alkaline in nature). Semen also has a high buffering capacity; it is capable of rendering the slightly acidic environment of the vagina neutral, shortly after ejaculation.