Vaginal pH. Doctor performing a vaginal pH test. The cotton swab has been wiped inside a patient's vagina and is now being wiped on a piece of pH paper. The pH paper changes colour depending upon the level of acidity in the vagina. The colour of the paper is compared to a chart (upper left) to determine the pH value. The vagina normally has an acidic pH (around 4.5), which is maintained by the presence of normal vaginal bacteria. A vaginal infection, such as a sexually transmitted disease, or the menopause, can change the vagina's pH.