Aldehyde Blocks

Aldehyde blocks do precisely what the term infers, they block the reactions of aldehydes, in this case with reference to the PAS reaction. They may be used at two stages depending on which aldehydes are the target. If applied prior to periodic acid oxidation, pre-existing aldehydes will be blocked. This may be necessary if the tissue was glutaraldehyde fixed, for instance, since this reagent leaves free aldehyde groups attached to the tissues, and these can give a non-specific positive reaction with Schiff's reagent.

They are more commonly applied after periodic acid oxidation to block any aldehydes formed during that step. This is invariably done in conjunction with a second section which is not blocked. Comparison of the two sections shows which materials are positive due to periodic acid engendered aldehydes.

Note: Aldehyde blocks are usually efffective, but Lillie cautions that the times specified are for a ten minute application of Schiff's reagent. The block may be overcome and a positive reaction obtained if the Schiff's reagent is applied for extended periods (hours). This list of aldehyde blocks is not exhaustive.

Blocking Solutions

Sulphite block
Sodium bisulphite 1.041 g
Distilled water 100 mL

Apply for 2-4 hours at 22°C. It is reversed by 2-10 minute application of 3% hydrogen peroxide, 1% ferric chloride, 1% sodium iodate or 1% potassium chlorate.

Aniline acetic block
Aniline 10 mL
Acetic acid, glacial 90 mL

Apply for 20-30 minutes at 22°C.

Aniline chloride block
Aniline 9 mL
Hydrochloric acid, conc 8 mL
Distilled water 100 mL

Shake well when adding the aniline to the acid, then dilute with water. Apply 1-6 hours at 22°C.

Phenylhydrazine block
Phenylhydrazine hydrochloride 5 mL
Acetic acid, glacial 10 mL
Distilled water 35 mL

Apply for 2-3 hours at 60°C.

Hydroxylamine block
Hydroxylamine hydrochloride 10 g
Sodium acetate 20 g
Distilled water 40 mL

Apply for 1-3 hours at 22°C. It is reversed by periodic acid and should not be used to block pre-existing aldehydes.

Semicarbazide block
Semicarbazide hydrochloride 2 g
Sodium acetate 5 g
Distilled water 40 mL

Apply for 2-3 hours at 60°C.


  1. Place into the solution for the time and at the temperature given.
  2. For the sulphite block transfer directly to Schiff's reagent.
  3. For the other blocks wash with running tap water for 5 minutes.
  4. Rinse with distilled water.
  5. Place into Schiff's reagent



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

Pearse, A. G. E., (1968, 1972)
Histochemistry: Theoretical and Applied, Ed. 3
Churchill Livingstone, Edinburgh, London, UK



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