NON-COAGULANT PRIMARY FIXATIVES 49 



The fixation is good for methacrylate embedding, but tliere is 

 very poor resistance to the distortion caused by embedding in 

 paraffin. Cell-aggregates are shrunken, so that wide artificial 

 spaces appear; the tissue tends to crack; the cytoplasm contracts 

 round the nuclei; chromosomes (especially in the first meiotic 

 prophase) are poorly fixed. 



Acid dyes, as we have seen (p. 47), scarcely act on tissues fixed 

 bv osmium tetroxide. Basic dyes are taken up by both nuclear sap 

 and cytoplasm, and differential dyeing of chromatin is therefore 

 not obtained. 



Compatibility with other fixatives. Osmium tetroxide reacts with 

 ethanol and formaldehyde, but is compatible with mercuric 

 chloride, chromium trioxide, and potassium dichromate. These 

 three substances prevent its reduction by daylight. 



POTASSIUM DICHROMATE 



Standard concentration. \-S% w/v aqueous solution. 



Formula. K2Cr20;. 



Description. Orange-red crystals, soluble at about 10 °o w/v in 

 water. 



Ionization. The ions produced by the solution of potassium 

 dichromate are the same as those produced by the solution of 

 chromium trioxide, but the proportions are somewhat different. 

 In particular, the amount of hydronium ion is much less, the pH 

 of the standard solution being about 4-1. The chief ion containing 

 chromium is the dichromate, [CraO;]^, with some hydrogen 

 chromate, [HCrOJ". 



Oxidation-potential. This is a strong oxidizer (o.p. 0-79 volt), 

 but not nearly so strong as chromium trioxide. 



Reactions with proteins. Potassium dichromate is a non-coagu- 

 lant of proteins, including nucleoproteins. It very gradually 

 renders egg-white more viscous and eventually transforms it into 

 a weak gel. It gels hist ones more powerfully. ^^® With these excep- 

 tions, it seems doubtful whether potassium dichromate can be 

 regarded as a fixative for proteins, in the circumstances of its 

 ordinary use in microtechnique; but it must be remarked that 



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