Choice of Buffer.

The properties required of a buffer for a fixative are :-

• Ability to maintain a constant pH during fixation.
• Suitable osmolarity (when mixed with the fixative) so that cells and organelles neither swell nor shrink during fixation.
• Suitable ionic constitution so that materials are neither extracted or precipitated during fixation.

Phosphate buffers.

These are more physiological than any other buffer and are non-toxic. Stabilises the pH of fixative more efficiently than cacodylate. May cause swelling of intracellular organelles, also more likely to produce precipitates. Stable for several weeks at 4 oC if sucrose or glucose have not been added.

Cacodylate buffers.

These contain arsenic and also have an unpleasant smell. On addition of an acid produces arsenic gas. Always handle in a fume cupboard. Effective in the pH range 6.4 - 7.4. Stable for several months at 4 oC and resistant to bacterial contamination during specimen storage.

Most animal tissues	pH 7.2 - 7.4
Highly hydrated tissues e.g.. invertebrates, embryos	pH 8.0 - 8.4
Gastric mucosa	pH 8.5
Plant cells	pH 8.0
Neurosecretory granules	pH 5.0 - 6.0

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