Potassium dichromate

Please read this explanation about safe working with chromium compounds and cleanup of spills.

Wikipedia has an article about potassium dichromate

PropertyData
ChemicalPotassium dichromate
Synonyms
FormulaK2Cr2O7
StateOrange crystals & solution
Concentration1 - 3%
Fixation timeHours to weeks
AdditiveYes
CoagulantNo
HardensModerately
Acid dyesImproves
Basic dyesReduced
AftertreatmentWash well

Potassium dichromate fixes differently than does chromic acid, although both contain chromium. This is due to the differing pH at which they are used. A simple 1% potassium dichromate solution has a pH of just over 4. Chromic acid is much lower than that.

The pH difference results in different fixation characteristics. It should be noted that if potassium dichromate is used in solutions with a pH lower than 3.4, it will have the same characteristics as chromic acid, since the ions present will be the same.

To be explicit, if used at a pH below 3.4 potassium dichromate fixes the same as chromic acid. If used at a pH above 3.8 it fixes as described below.

How it fixes
Proteins are converted into a gel rather than precipitated. DNA is dissolved, but the histones of the nucleus are preserved and nuclear structure may be demonstrated with appropriate techniques.

Carbohydrates
There is little to no effect on carbohydrates.

Lipids
Potassium dichromate is a very valuable fixing agent where lipids are concerned. This is best observed with the post chroming procedure, which makes many unsaturated lipids insoluble. During this procedure the chromium attaches to the lipid and may be used as a mordant for some dyes, including hematoxylin. It has to be stressed that this is a slow process, and post chroming may need to be applied for weeks to be fully effective. For faster demonstration of lipids in paraffin sections, post osmication should be used.

Morphology
Morphological preservation is reasonable when combined with other fixing agents, although it is rarely used alone as the primary fixative.
 
Fixatives containing potassium dichromate may also contain an indifferent salt. This is the term given to a compound included in a fixative which appears to have no function. The presence of an indifferent salt in a simple aqueous solution of potassium dichromate improves the quality of the morphological preservation.

Time
It fixes relatively slowly, but mixtures containing it are usually effective overnight. Post chroming may take up to six weeks or so in some cases, although seven to ten days is more common.

Simple solution
It is not usually used alone as a primary fixative, but applied as a 1 - 3% aqueous solution for post chroming.

Compatability
Potassium dichromate is compatible with mercuric chloride and osmium tetroxide. It is incompatible with ethanol as it causes chromic oxide to be precipitated into the tissue. That is the reason for extended washing after fixation.
 
Formaldehyde is a reducing agent, while potassium dichromate is an oxidising agent. The reaction between them is relatively slow and mixtures containing both are not unusual, i.e. Helly's fluid. These solutions deteriorate so they should be prepared immediately before use and fixation in them should not be extended.
 
Too high an acid content in a mixture containing potassium dichromate may reduce the pH to below 3.4 and change the characteristics of the preservation. Acetic acid, picric acid and chromium trioxide can all reduce the pH to below 3.4 if used in other than very small amounts.

Aftertreatment
Wash the tissues very well with running tap water, preferably overnight. Transferring to ethanol without this being done is likely to cause precipitation of chromium compounds, which appear as dark deposits in the sections.

 

Reference
Baker, John R., (1958)
Principles of biological microtechnique
Methuen, London, UK.


 

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