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Aldehyde Blocks

for the Periodic Acid-Schiff Reaction

Aldehyde blocks do precisely what the term infers, they block the reactions of aldehydes, in this case with reference to the Periodic Acid Schiff (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 effective, 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.


The production and demonstration of aldehydes occupies an important place in histological practice. The demonstration of carbohydrates would hardly be possible without the ability to produce aldehydes by the oxidation of 1-2 glycols and their subsequent condensation with Schiff’s reagent in the PAS reaction, for instance. Several other techniques depend on similar procedures as well, both with Schiff’s reagent and with other means of visualizing their presence.

There are, however, occasions when it may be necessary to show that the aldehydes being visualized did come from the specific treatment given and were not there prior to that, from the fixative, for instance. Similarly, it may be necessary to prove that the colouration finally obtained is due to an aldehyde and not something else, since very few techniques are inherently so specific that they will react with a single tissue component. This includes Schiff’s reagent and silver reductions, although anomalous results are more likely with silver reduction than with Schiff’s reagent.

Applying treatment which blocks aldehydes from taking part in reactions can overcome those difficulties.

  • Applying an aldehyde block before applying reagents to produce aldehydes can prove that the reagents were responsible for producing the aldehydes responsible for a positive result.
  • If one of two adjacent sections has an aldehyde block applied after generating an aldehyde but before the visualizing step,
    • If the unblocked section is colored and the blocked section is not colored, it proves that an aldehyde is responsible for the color.
    • If the unblocked section is colored and the blocked section is also colored to the same degree, it proves that some other constituent is responsible for the color in those areas, but that it is not an aldehyde.
    • If the unblocked section is colored and the blocked section is also colored but to a greater degree and in different areas, it proves that both an aldehyde and some other constituent are both responsible for the color.

Blocking Solutions

Sulphite Block

Sodium bisulfite1.041g
Distilled water100mL

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

Acetic acid, glacial90mL

Apply for 20-30 minutes at 22°C.

Aniline Chloride Block

Hydrochloric acid, conc8mL
Distilled water100mL

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

Phenylhydrazine Block

Phenylhydrazine hydrochloride5mL
Acetic acid, glacial10mL
Distilled water35mL

Apply for 2-3 hours at 60°C.

Hydroxylamine Block

Hydroxylamine hydrochloride10g
Sodium acetate20g
Distilled water40mL

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 hydrochloride2g
Sodium acetate5g
Distilled water40mL

Apply for 2-3 hours at 60°C.

General Procedure

  1. Place into the solution for the time and at the temperature given.
  2. For the sulfite 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


  1. Lillie, R.D., (1954)
    Histopathologic technique and practical histochemistry Ed.2
    Blakiston, New York, USA.
  2. Pearse, A. G. E., (1968, 1972)
    Histochemistry: Theoretical and Applied, Ed. 3
    Churchill Livingstone, Edinburgh, London, UK