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.


Chemical Amount
Methanol 99.17 mL
Hydrochloric acid, conc. 0.83 mL
Note: This is 0.1N hydrochloric acid.


Place dewaxed sections into the solution in a sealed container at the temperature specified for the time given.

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.

Tissue Temp Time State
Glycogen 58°C 3 days complete
Epithelial mucin 58°C 3 days complete
Glycogen 37°C 7 days not complete
Epithelial mucin 37°C 7 days not complete



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



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