1650 Journal of Applied Microscopy 



silver chromate is isomorphous with normal silver sulphate, normal silver selenate, 

 and anhydrous sodium sulphate, all are to be referred to the orthorhombic system. 

 Because of this isomorphism of the sulphate and chromate very interesting and 

 instructive preparations can be obtained. Such preparations are best under- 

 stood by performing the experiments outlined in the last paragraph of the 

 Exercises for Practice given under this method (//). Silver sulphate separates 

 from solution generally in the form highly refractive, transparent, colorless, 

 rhombic octahedra, but in the presence of silver chromate these colorless 

 octahedra increase in size, turn first yellow, and finally a more or less intense 

 brownish red. 



Mercurous salts yield with ammonium dichromate in solutions acidified with 

 nitric acid a number of different compounds (see Mercury) varying in composi- 

 tion and appearance according to the conditions which obtain. There is, how- 

 ever, little danger of confusing these salts with the silver dichromate, though 

 they are sometimes apt to interfere with the test for silver. 



Normal potassium chromate added to neutral solutions of silver causes the 

 precipitation of normal silver chromate; but when the test drop is first acidified 

 with nitric acid the crystals separating probably consist of both the chromate 

 and dichromate. When recrystallized from hot nitric acid the precipitlte will 

 usually consist of the dichromate alone. When ammonium hydroxide is the 

 solvent employed to recrystallize the silver chromate, the compound separating 

 is thought to have the formula Agj Cr04 • 4NH3* 



When working with test drops acidified with nitric acid there is little danger 

 of any interference by members of the calcium group. 



In all analytical work it is safe to work on the assumption that the presence 

 of any elements which are precipitated as chromate or dichromate in acid solution 

 will interfere with the reaction for silver, particularly when such elements are in 

 excess of the latter. 



Large amounts of the salts of the alkalies seem to have an injurious effect 

 when but little silver is present. 



White alloys believed to contain silver can be tested for this element by 

 drawing across them a streak of a solution of ammonium dichromate in nitric 

 acid. The color of the streak is generally sufficient to indicate the presence or 

 absence of silver; but if the streak of the reagent be examined under the micro- 

 scope (best with an illuminating objective or some form of vertical illuminator) 

 in the presence of silver the characteristic dark red crystals of the silver dichro- 

 mate will be easily distinguished. 



Exercises for Practice. 



To a moderately concentrated neutral test drop add a fragment of 

 (NH4)2Cr20,. 



Acidify a test drop with HNO3, then add the reagent. 



Draw off the mother liquor from a precipitated test drop and recrystallize the 

 silver salt by heating with HjO. Try another preparation by heating with dilute 

 HNO3. Recrystallize a third portion of the silver compound, using NH4OH. 



* Ladenburg, Ilandworterbuch, 10: 713. 



