PRIMARY fixatives: COAGULANTS 



105 



[HCr04-] 



[Cr20,=] 



hCrOj 



[HCro/] 



[Crp,-] 



is about 198" C. They are extremely soluble in water, a saturated 

 solution having a concentration of 62-4% w/v. 



Ionization. For a careful study of this subject, see Casselman.^^^ 



When chromium trioxide 

 is placed in water, it forms 

 chromic acid; this may be 

 regarded as H2Cr04, but no 

 such substance can be iso- 

 lated. The acid ionizes to 

 form hydronium ions and 

 three different chromic 

 anions. These are the orange- 

 red dichromate [CrgO^]" 

 and hydrogen chromate 

 [HCr04]^ ions, with traces 

 of the yellow chromate 

 [CrOJ" ion: a certain 

 amount of chromic acid re- 

 mains undissociated. The 

 relative proportions of the 

 four substances are shown in 

 fig. 14 (left-hand column). 



The hydronium ions re- 

 leased by the ionization of 

 chromic acid make the solu- 

 tion strongly acid. A 1% 

 solution of analytical-grade 

 chromium trioxide has a pH 

 of 1-20.^^^ (Lassek ^^^ gives 

 I -12.) If chromium trioxide 

 of 'laboratory reagent grade' 

 is used, the pH of solutions 

 is significantly higher. ^^^ 



There is a very small rise 

 in pH (about o-i unit) dur- 

 ing fixation, almost entirely 

 in the first 3 or 4 hours. ^^^ 



Oxidation-potential. The dichromate ion can readily be reduced 

 to give the chromic ion Cr^~^. The oxidation-potential of a 1% 

 solution of chromium trioxide is i-o8 volt.^^^ Data do not exist for a 

 0-5% solution, but extrapolation from available figures ^^'^ makes 



o 



Otd 

 •_• — 



E o 

 .5 E 



{^ 2 



H2Cr04] and 

 t [Cr04l 



FIG. 14. Graphical representation 

 of the ions present in a 2-5% 

 aqueous solution of potassium 

 dichromate and in a solution of 

 chromium trioxide containing the 

 same weight of chromium. 



(From Casselman/*- by kind permis- 

 sion of himself and of the Company of 

 Biologists, Ltd.) 



