PRIMARY fixatives: NON-COAGULANTS 127 



There is a striking difference between the hydronium-ion 

 concentration of the two solutions. A 2-5% solution of potassium 

 dichromate has a pH of 4-05 ; in a solution of chromium trioxide 

 containing the same amount of chromium as the dichromate solu- 

 tion the pH is 0-85.^1^ 



If a solution of potassium dichromate be acidified to the same 

 pH as a solution of chromium trioxide containing the same 

 weight of chromium, the ions present in the two solutions will be 

 the same, except that the former will contain potassium ions and 

 the anions of the added acid. If hydrochloric acid be used, one has 

 a fluid almost identical with a solution of chromium trioxide to 

 which some potassium chloride has been added. Since potassium 

 and chloride ions are inactive in fixation, it follows that an acidified 

 potassium dichromate solution will act like a solution of chromium 

 trioxide. 



Casselman does not give the pH of a 1*5% solution of potassium 

 dichromate, but a 1% solution (pH 4-10) difi'ers only slightly from 

 the 2-5% solution. Lassek ^^^ gives pH 4-0 for Miiller's fluid, 

 which is 2 or 2-5% potassium dichromate with 1% sodium sul- 

 phate (see below). 



Oxidation-potential. The oxidation-potential of a 3% solution 

 of potassium dichromate is 0-79 volt.^^^ A 2-5% solution has 

 almost exactly the same oxidation-potential.^^^ 



Manufacture. Chromium occurs naturally as chromite, 

 FeO.Cr^O.^. It is treated with sodium carbonate to produce 

 sodium chromate, and this with sulphuric acid to give sodium 

 dichromate. Potassium chloride is added to a strong, warm 

 solution of the latter, and large crystals of potassium dichromate 

 separate out on slow cooling, or small ones if the tank is 

 shaken. 



Introduction as fixative. Potassium dichromate was introduced 

 into microtechnique in i860 by H. Miiller,^^^ who used it in 

 studies of the human eye. His first fluid consisted of this salt and 

 sodium sulphate (presumably Glauber's salt, crystallized with 10 

 molecules of water), both at about 1*5%, with etwas chromium 

 trioxide. Later in the same year ^^^ he mentioned another fluid, 

 from which the chromium trioxide was omitted ; the concentration 

 of the other components was not stated, but was presumably the 

 same as before. The fluid called Miiller'sche AugenflUssigkeit con- 

 sists of the same two salts, at 2-2^^% and 1% respectively.^ ^^ It 

 forms the basis of Zenker's ^^* and Helly's ^^* fluids. 



