1532 Journal of Applied Microscopy 



Cesium alum is one of a group of double sulphates known as " alums," hav- 

 ing the general formula M^(SO^)^ • 7V^2^04 • 24H2O, where -Jlf- can be Al, Cr, 

 Mn, Fe, In, Cxa, Tl ; and -A- Na, K, Rb, Cs, NH^, Ag, or Tl. All alums are 

 isomorphous, and are to be referred to the isometric system. Theoretically, 

 therefore, one would be led to expect that the presence of elements capable of 

 taking the place of aluminum in alums would be liable to interfere with the test 

 for aluminum. But in addition to their property of being able to replace 

 aluminum in these double sulphates, we must consider the crystallizing power of 

 the compounds formed. It is herein that lies the explanation of the value of 

 cesium sulphate over and above that of any other of the sulphates we might be 

 inclined to select. Of the above listed alum forming elements, aluminum is the 

 only one which unites with cesium or rubidium sulphates to form easily crystal- 

 lizable alums. The other elements unite with these two sulphates only with 

 difficulty, and the alums formed can be regarded, from a micro-chemical stand- 

 point, as practically uncrystallizable. Sodium, potassium, and ammonium sul- 

 phates readily unite to form more or less crystallizable alums with the other alum 

 forming elements as well as with aluminum. 



Exercises for Practice. 



To a test drop consisting of a solution of aluminum sulphate add a fragment 

 of the reagent. 



Precipitate another drop with ammonium hydroxide, draw off, wash the 

 precipitate, dissolve in the least possible amount of sulphuric acid, and test. 



Try rubidium sulphate as reagent ; then potassium sulphate ; sodium sulphate ; 

 ammonium sulphate. Try cesium chloride. 



Test for Al in the presence of free hydrochloric acid ; free nitric acid. 



Test preparations containing Al and Fe ; Al and Cr ; Al and Mn ; Al, Fe, 

 Cr ; Al and Mg; Al and Gl; Al in the presence of phosphates. 



Prepare slides of chrome alum, iron alum, etc., then mixtures of these various 

 alums ; note isomorphism. 



//. Ammouiian Fluoride in excess leads to the separatiofi of a Double Fluoride 

 of Aluminufn and Ammonium. 



Al2(SOj3 4- 12 NH4F = 2(A1F3. 3NH4F) + 3(NH4)2S04. 



Method. — Place on a celluloid slip a drop of a moderately dilute neutral solu- 

 tion of the substance to be tested, and to it add several small 

 ^ ^ S fragments of ammonium fluoride. Very minute crystals im- 

 ' ^ ^ c> mediately separate. The preparation is set aside for a few 



^ ^ m ^ seconds, and is then examined near the circumference of the 

 I I I I I drop. Small but clear cut octahedral crystals of the double 



\p\V.»O.Ol»x\»y». 



Fie 74 fluoride of ammonium and aluminum will be seen (Fig. 74). 



/Remarks. — The solution must contain no appreciable amount 

 of free mineral acid. The best results seem to be obtained when the test drop 

 is neutral. 



Unless the reagent is present in excess, a compound of different composition, 



