336 ELEMENTARY CHEMICAL MICROSCOPY 



mate, the compound separating is thought to have the formula 

 Ag2Cr0 4 - 4 NHg. 1 



Normal potassium chromate produces in neutral or slightly acid 

 solutions of manganous salts sheaves and bundles of a cinnamon 

 brown manganous chromate soluble in excess of acid. Bichro- 

 mates cause no precipitates in solutions of manganous salts. 



Precautions. 



The test drop must be moderately concentrated with respect 

 to silver. 



When working with test drops acidified with nitric acid there 

 is little danger of any interference by members of the calcium 

 group. 



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

 injurious effect when but little silver is present. 



In all analytical work it is safe to assume that the presence of 

 any elements which are precipitated as chromate or bichromate 

 in acid solution will interfere with the reaction for silver, particu- 

 larly when such elements are in excess of the latter. 



White alloys believed to contain silver can be tested for this 

 element by drawing across them a streak of a solution of ammo- 

 nium bichromate in nitric acid. The color of the streak is gener- 

 ally sufficient to indicate the presence or absence of silver, but 

 if the streak of the reagent be examined under the microscope 

 (best with an illuminating objective or some form of vertical 

 illuminator) in the presence of silver the characteristic dark red 

 crystals of silver bichromate will be easily distinguished. 



EXPERIMENTS. 



a. To a moderately concentrated neutral test drop add a fragment of 

 (NH 4 )2Cr 2 O7. Then try K 2 CrO 4 . 



b. Acidify test drops with HNO 3 , then add the above reagents in turn. 



c. Decant the mother liquor from a precipitated test drop and recrystallize the 

 Ag salt by heating with H^O. Try another preparation by heating with dilute 

 HNO 3 . Recrystallize a third portion of the Ag compound, using NH 4 OH. 



d. Make a mixture of AgNO 3 and PbNO 3 , acidify with HNO 3 , then add a drop 

 or two of dilute H 2 SO 4 and finally a fragment of (NH4) 2 Cr 2 O 7 . 



1 Ladenburg, Handworterbuch, 10, 713. 



