The scientific definition of this value reads: "The pH value is the negative decadic logarithm of the hydrogen ion concentration" and thus a measure of the acid concentration in aqueous solutions. The pH therefore indicates how acidic or alkaline a solution is. The pH scale ranges from 0 to 14.
Acids have a pH of less than 7 and bases a greater than 7, the mean of pH 7 is called neutral. You can measure the pH with indicator solutions, indicator papers or with a pH meter.
The skin can also be assigned a ph value, since the hydrolipid film contains water on the skin surface. Already one hundred years ago it was known that the skin is slightly acidic. With modern measurement methods, it was found that the average ph value of the skin is between 5.4 and 5.9.
Horn scales, sebum, sweat and microparticles together form an acidic environment. One speaks also of the so-called acid protective coat of the skin. This protective coating helps to ward off harmful microorganisms as well as negative environmental influences and thus protect the skin from infections, irritations, allergies and dehydration.
With every cleaning with soap, this protective coating is lost and the physiological balance of the skin is disturbed. The ph value of the skin is raised to about 9 for a short time. However, healthy skin will be able to lower the level to normal levels within 30 to 200 minutes.