1 1 6 BO TANICAL MICRO TECHNIQ UE. 



y. Potassium Bichromate and Chromic Acid. 



201. Most tannins form with potass turn bichromate a volu- 

 minous precipitate which is bright brown or blackish brown 

 according to the quantity of tannin present, and which is 

 insoluble in water or in an excess of the salt, and, according 

 to Moeller (I, LXIX), probably consists of purpurogallin. 



Potassium bichromate is most conveniently used by placing 

 large pieces of the tissue to be examined for one or several 

 days in a concentrated aqueous solution of this salt, and 

 then preparing sections after washing out the bichromate. 

 These sections generally show the precipitate in the places- 

 where the tannin was formerly present. They may be pre- 

 served unchanged in glycerine or glycerine-gelatine. Thicker 

 sections or larger fragments may also be transferred to Can- 

 ada balsam. But of course they must first be dehydrated 

 with alcohol and cleared with clove-oil or the like (cf. 14-22), 



The precipitate produced by potassium bichromate does- 

 not change its color with iron salts, or, as Overton (II, 5} 

 has shown, with sulphurous acid. Even hydrogen peroxide 

 does not attack it. 



To obtain a rapid reaction J. af Klercker (I, 8) recom- 

 mends for many cases that the objects be plunged in a boil- 

 ing solution of potassium bichromate. In fact an immediate 

 reaction may thus be obtained with algae and sections of 

 higher plants. 



According to Moeller (I, LXX), the diffusion of the bichro- 

 mate is much hastened by the addition of a few drops of 

 acetic acid. 



202. Dilute chromic acid of about \% appears to give a 

 reaction similar to that of potassium bicarbonate, and may 

 be used, as well as mixtures of chromic and osmic acids, for 

 the recognition of tannins. These have the advantage of 

 fixing the cell-contents well at the same time. 



But it should be noted that, according to Nickel (I, 74), 

 various compounds not related to the tannins give similar 

 precipitates with potassium bichromate. 



