Sodium cacodylate trihydrate is the most favoured buffering vehicle used with fixatives in specimen preparation for electron microscopy. Also called cacodylate or cacodylic acid, this chemical avoids the microprecipitation on thin sections that can occur with phosphate buffers if the specimen is not well rinsed between pre- and post-fixation. The chemical does not form a precipitate in sea water. Ultrastructural preservation is excellent.
After some years the chemical breaks down at room temperature, so storage in the refrigerator is recommended. Made up solutions should also be stored refrigerated where they last for many months before some growths occur.
Note: This compound contains arsenic. Arsenic is poisonous and carcinogenic. It is absorbed through the skin. Avoid contact with the powder and solutions. Use gloves and fume-hood. Wash hands after use. Electron microscopists use only small quantities of this chemical. Clean working habits and simple precautions can make its use safe.
C2H6AsNaO2.3H2O F.W. 214.02 CAS #124-65-2
Certificate of Analysis:
|Aqueous solution (2g+10 ml)||Colourless|
|Loss on drying %||25.39|
|Inorganic Arsenic %||0.0030|
For animal tissue, buffers are normally used at 0.1 molar and for plant tissue at 0.01 molar. To prepare the latter, the same stock solution of 0.2M may be used and is diluted 1 9 parts of water before use.
To make up a 0.2 molar stock solution:
- Into a 100mL volumetric flask, with a magnetic stirring bar, weigh 4.3g of sodium cacodylate.
- Add double distilled water to 100mL mark.
- Stir to dissolve.
- Pour solution and stirring bar into a suitable container and adjust the pH to the desired reading (plant tissue most commonly pH 6.8, animal tissue pH 7.2) using 0.2N HCl. The natural pH of the solution is about 7.8.
Most commonly the solution is mixed 1:1 to make:
- With water to make single strength buffer for storage and rinsing.
- With 5% glutaraldehyde to make 2.5% glutaraldehyde in single strength buffer.
- With 2% osmium tetroxide to make 1% OsO4 in single strength buffer.