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This procedure is used to block acid groups, particularly carboxyl groups. As a consequence basophilic staining materials cease to be basophilic. Extended treatment destroys PAS positivity of some carbohydrates. This may be used as a means of identifying them, particularly if done in conjunction with other blocking procedures.


Hydrochloric acid, conc.0.83mL

Note: This is 0.1N hydrochloric acid.


  1. Place dewaxed sections into the solution in a sealed container at the temperature specified for the time given.
Glycogen58°C3 daysComplete
Epithelial mucin58°C3 daysComplete
Glycogen37°C7 daysNot complete
Epithelial mucin37°C7 daysNot complete

It should be noted that methylation is more commonly used to abolish metachromasia or cytoplasmic basophilia. Treatment of sections at 37°C with 1% hydrochloric acid in methanol for 2 days will usually accomplish this, while not affecting PAS reactivity.


  1. Lillie, R.D., (1954)
    Histopathologic technique and practical histochemistry Ed.2
    Blakiston, New York, USA