COAGULANT PRIMARY FIXATIVES 37 



thr: cells tend to separate from one another; chromosomes are not 

 well fixed ; mitochondria and lipid droplets are usually not present. 



Proteins are more readily coloured by basic dyes than they are 

 after the action of any other fixative. The -COOH side-groups of 

 their acidic amino-acids are presumably intact in the usual cir- 

 cumstances of fixation. The special affinity for basic dyes must be 

 attributed in part to the coagulation of protein chains in a form 

 that permits the dye ions to penetrate easily between them to 

 reach their sites of action. There is some affinity for acid dyes, 

 and it is therefore clear that the amino-groups of the proteins are 

 not completely blocked. 



Compatibility with other fixatives. Compatible with any of the 

 selected primary fixatives. 



CHROMIUM TRIOXIDE 



Standard concentration. 0-5% w/v aqueous solution. 



Formula. CrOg. 



Description. Brownish-red deliquescent crystals, extremely 

 soluble in water (a saturated solution is about 62% w/W). 



Ionization. In water it forms chromic acid, HoCr04, which 

 cannot be isolated. A small amount of this remains undissociated 

 in water, while the greater part of it ionizes as dichromate, 

 [CroOv]^, and the rest mostly as hydrogen chromate, [HCrOJ", 

 with sufficient hydronium ions to give a 1 ^o solution a pH of 



1.2.51,52 



Oxidation-potential. This is much the strongest oxidizer of all 

 fixatives (o.p. about IT volt), the dichromate ion being readily 

 reduced to the chromic ion, Cr+++, or to chromic oxide, CrgOg. 



Reactions with proteins. A powerful coagulant of albumin and 

 many other proteins, including nucleoprotein. Chromium trioxide 

 does not fix gelatine gels. It is probably an additive fixative, but 

 the reactions involved are not known. The affinity of proteins for 

 basic dyes is rather low after fixation by chromium trioxide, and 

 this suggests blocking of the -COOH groups of the acidic amino- 

 acids. It has been suggested, however, that acidic proteins are not 

 fixed but simply dissolve away.^^^ 



