and Laboratory Methods. l-')3o 



containing a higher percentage of aluminum, separates in the form of tiny rods. 



Double fluorides of aluminum and sodium, potassium, rubidium and cesium, 

 having the general formula AlFg • 3RF, are also known. Or we can indicate the 

 composition by the formula R.^AlFg, calling the compounds fiuoaluminates, a 

 term preferred by some chemists. 



With lithium fluoride the double fluoride formed is less soluble than in the 

 case of the alkali metals ; its crystallizing power is also considerably less. 



Crystalline double fluorides of aluminum with copper, nickel, and zinc have 

 been described, but these are too soluble to appear under the conditions which 

 usually obtain in an analysis. 



In testing for aluminum with ammonium fluoride, salts of lithium, sodium, 

 and iron must be absent. 



The presence of silicon and analogous elements will generally seriously com- 

 plicate matters, and may ruin the test, owing to the formation of fluosilicates. 

 (See ammonium fluosilicate tests, under Sodium and Barium.) Aluminum fluo- 

 silicate is gelatinous, and does not crystallize. 



Testing for aluminum with ammonium fluoride generally yields results a 

 trifle quicker than Method I, but the delicacy of the reaction is but very little 

 greater. Moreover, Method II is subject to many complications and interfer- 

 ences, and there is always danger, in spite of great care, of damaging objectives 

 by the corrosive vapors arising from the test drop. For these reasons, testing 

 with ammonium fluoride will never be considered as being as satisfactory as the 

 cesium method. One of the chief reasons for inserting the test in this series is 

 the fact that crystals of ammonium fluoaluminate may occasionally appear when 

 this reagent is being employed for other purposes, and the presence of aluminum 

 is not yet suspected. 



///. Wif/i Primary Potassium Sulphate, HKSO4. 



This salt, added to sulphate solutions of aluminum, leads to the formation 

 and separation of beautiful, large crystals of potassium alum. This reaction is 

 an elegant and satisfactory one, but is not nearly so good as that with cesium 

 sulphate, for the reasons which have already been stated above, still with due 

 observance of the precautions, etc., there given, testing for aluminum with 

 primary potassium sulphate, in the absence of the cesium salt, can be depended 

 upon to give neat and satisfactory tests. 



IV. Staining the Precipitated Hydroxide. 



Owing to the fact that aluminum hydroxide has the property of uniting with 

 various pigments to form colored compounds, it is possible to detect this element 

 by staining methods. 



Of the various dyes proposed, Congo red and cochineal (Carmine) have 

 been most favorably received, the former being the better. 



An aqueous solution of the dye, added to freshly precipitated aluminum 

 hydroxide, stains the latter a more or less deep red. 



The reaction is subject to many errors, is of very limited application, and is 

 unsatisfactory in the routine work of chemical analysis. 



Cornell University, E. M. ChamOT, 



