The testing of the pickle to check the penetration of the acid through the pelt is typically done with Bromocresol Green (BCG) or Methyl Orange indicators. The liquor pH is best checked by a pH meter. Pickle pH and penetration should be as consistent as possible going into chrome to avoid variation in wet-blue.
When pickling for preservation, the pickle acid can be checked by titration to control the amount of acid in the pelts. But if the pickle liquors are recycled buffer systems caused by build up of soluble protein and neutral salts, they can give misleading results.
When pickling prior to chrome tannage for most productions and certainly for small skins, the penetration should be 100% throughout. But heavy hides will usually be tested in the thickest part, i.e. the neck and also in the tighter fibre parts, i.e. the butt. Providing the penetration is OK in most of the hide, the neck areas will be passed if the penetration is at least 80%.