Note: Hair is known to have a slightly acidic pH but there is great variance among individuals. Scientific studies have reported values as low as 3 and others as high as 6.6.
Q1: After using soap, does the pH of hair change?
The most likely answer is yes. I could not find an answer directly about hair but I did find one on skin. Even washing with water produces a change in pH, however soap does produce the most change (Dermatology pp258-262, 1997)
pH change to skin
|Synthetic detergent 1||5.5||+0.294|
|Synthetic detergent 2||5.5||+0.291|
Q2: How long does it take skin/hair to 'recover' from alkaline pH
Anywhere from 45 minutes to 3 hours (British Journal of Dermatology, Volume 76, Issue 3, pp 122-125, 1949)
Q3: Does pH affect anything else (hint: bacteria and fungi)
Q4: Are castile soap or baking soda with an alkaline pH irritating?
Let me start by saying the normal test to look for sensitive skin is to use SLS, a superb surfactant. Regardless of pH, (which it can be made to pH 4-7) it can be very irritating. Do remember irritance comes from removal of the oil layer causing dryness and itchiness.
There is however a strong relationship between pH and irritancy. Generally between ph 4 and 9, there isn't much irritation (meaning it may still be there but will not be disturbing). Outside this range, irritation can increase markedly (International Journal of Dermatology, pp494–499, 2002)
Depending on the pH of your baking soda mix or castile soap dilution, you may experience irritation.
I will save the juicy questions for last - Tune in tommorow for Q&A: Should you really use castile soap and or baking soda ( including silicone removal and whether pH is sufficient to relax hair).