126 FIXATION 



to darken certain cytoplasmic inclusions. This process of 'postos- 

 mication' was introduced by the Polish cytologist Weigl.^^^ It is 

 useful for directing attention to a particular part of a cell, but it 

 should be used with caution. Ringkorner are often seen in post- 

 osmicated preparations. There is a tendency for a black material 

 (presumably osmium dioxide) to be deposited on the surfaces of 

 granules or other cytoplasmic inclusions, and especially to fill up 

 the spaces between crowded granules. The appearances given can 

 be very misleading morphologically, and should not be trusted 

 unless they can be confirmed by study of the living cell. 



POTASSIUM BICHROMATE 



Standard concentration for fixation. 1-5% w/v aqueous solution. 



Formula and formula- weight. K2Cr207. 294-2. 



Description. Potassium dichromate crystallizes readily in large, 

 orange-red prisms or tables. These melt with decomposition at 

 396° C. They are soluble at about 10% w/w in water at room- 

 temperature (18% at 30° C), but insoluble in absolute ethanol. 



Potassium dichromate is more expensive than the sodium salt, 

 but the fact that it is anhydrous and not deliquescent gives it an 

 advantage for certain industrial purposes. It is used in making 

 matches and fireworks and in the chrome tanning of leather ; dis- 

 solved with sulphuric acid it acts as a bleaching agent for tallow 

 and palm oil. 



It is wrong to call this substance potassium bichromate, because 

 the name would only be applicable to potassium hydrogen chro- 

 mate, which does not exist. 



Ionization. The ionization of potassium dichromate has been 

 carefully considered by Casselman.^^^ The ions are the same as 

 those produced by chromium trioxide, but in different proportions. 

 The ions in solutions of the two substances containing the same 

 weight of chromium are compared in fig. 14 (p. 105). In both 

 solutions by far the greater part of the chromium is in the form of 

 dichromate [Cr207]" and hydrogen chromate [HCr04]~, the 

 former predominating in both cases, especially in the solution of 

 potassium dichromate. The chromate ion [CrOJ" is present in 

 minute quantities in both. Undissociated chromic acid, H2Cr04, 

 is present in considerable amount in the solution of chromium 

 trioxide, but there is scarcely any of it in the solution of potassium 

 dichromate. 



