1326 Journal of Applied Microscopy 



V. Ammonhim Dichromate added to solutions containing Barium precipitates 

 Barium Chromate. 



2BaCl2 + (NH4)2Cr207 + H2O = 2BaCr04 + 2NH4CI + 2HC1. 



Method. — Employ a very dilute solution of the barium salt. Add a little 

 acetic acid, and then a small fragment of the reagent. Do not stir. In adding 

 the reagent avoid scratching the glass slide with the glass rod or platinum wire. 



Barium chromate separates in the form of very 

 ^ ^-^, minute, light yellow, globular masses, and tiny rods 



c^ & ^ with rounded ends. These rods are often arranged 



^^<0 (j^ ^ -^ crosses and Xs (Fig. 59). 



(m ^^ ^ c\ /p O Occasionally rectangular plates are obtained. 



^^ ^ o^ "^ ^^?^ J^^^^'^^^^- — The complete precipitation of all the 



^ ^^ barium present is slow. 



^e>> <SP III 



\ p>v Jo.ov.i s»,. Strontium will not separate in acid solutions, and 



Fig. 59. calcium not even in the presence of ammonium 



hydroxide. 



When strontium is to be tested for as well as barium, the test drop, after the 

 addition of the reagent and examination for barium, is gently heated, then 

 allowed to stand for some time. The supernatant solution is drawn off, and to 

 it a tiny fragment of the reagent is again added, and the preparation warmed ; 

 if no precipitate results, showing that all the barium has been removed, strontium 

 can be tested for by adding ammonium hydroxide. 



In warming the preparation to accelerate the separation of the barium salt, 

 great care must be exercised in order to avoid concentrating the drop to a point 

 where strontium might be precipitated. 



It is often better to allow a drop of ammonium hydroxide to flow slowly into 

 the side of the test drop, rather than add it at once to the center of the prepara- 

 tion. 



Normal potassium chromate produces, with barium salts, a precipitate similar 

 to that obtained with dichromate, but is not to be recommended as a reagent 

 because of its property of also precipitating strontium compounds in acid 

 solution. 



Ordinarily the precipitate of barium chromate is largely amorphous in 

 appearance. Here and there, however, will be found spots where there are 

 recognizable crystals. A high power is always required for the recognition of 

 the form of the crystals, hence the drop to be studied must be spread out quite 

 thin. 



Free mineral acids interfere with the test. 



Exercises for Practice. 



Try reaction on salts of Ba ; Sr ; Ca ; in acid, neutral and ammoniacal solu- 

 tions, both in concentrated and in dilute solutions. 



Try mixtures of Ca and Ba ; Sr and Ba ; use solutions acidified with acetic 

 acid, draw off the clear solution, and to it add ammonium hydroxide. 



