Here is something written by a show breeder of Coton de Tulears. The whole thing about grooming is interesting, but this part addresses the pH issue.
"Some people use “people” shampoo for their dogs, and I've even heard of some who use dish washing liquid. Both are much too harsh for your dog's skin and can cause skin and coat problems in the future. While there are some products made for people that are very mild (such as baby shampoos), there is still a difference in the "pH" of the products. The pH is a measure of the acidity or alkalinity of the products. The pH of dog shampoo is two points different than people shampoo, which is a huge difference. Using products made for people can really dry out a dog's coat. In the long-run, it's cheaper to buy a high quality dog shampoo and dilute it according to the label instructions. I recommend the Vellus Show Shampoo and/or the Vellus Clarifying Shampoo, and the Vellus Show Conditioning Concentrate. Alternately I use the All Systems Super Cleaning and Conditioning Shampoo or the Les Poochs Pooch Bright Shampoo. It is best to rotate shampoos occasionally to prevent product buildup. If you are sensitive to fragrance, Les Poochs makes a wonderful fragrance-free shampoo, Botanique Hypoallergenic Shampoo. There are many excellent shampoos/conditioners on the market that are suitable for the Coton coat. I have mentioned just a few of my favorites. I suggest avoiding any product that contains lanolin."
Bathing Grooming Coton de Tulear