398 Gooch and Phinney — Quantitative Determination 



potassium up to about 0*0013 mg., but that the increase of the 

 potassium to 0*0034 mg. results in diminution of brightness. 

 In other words, it seems that a single molecule of potassium 

 chloride has approximately the same dissociating effect upon 

 the molecule of rubidium chloride in the flame as that brought 

 about bv a greater number, that the presence of potassium in 

 the proportion of five parts to one of the rubidium begins to 

 influence visibility unfavorably, that when this proportion 

 rises to thirty to one of the elements (or twenty to one of the 

 chlorides) the lines appear about as distinctly as if no potas- 

 sium were present, and that an increase of proportion to fifty 

 to one may bring about a sufficient glare of light to reduce the 

 rubidium lines to invisibility. The degree of increase in bril- 

 liance due to the action of potassium when that element is 

 present in proportions suitable to induce the maximum effect 

 is shown in the following record. 



Test solution. 



Standard. Eubidium in a coil-full Line of test 



Rubidium in a with its own weight compared with 



coil-full (tto cm 3 ). of potassium. standard. 



0-00066 mg. 0-00066 mg. Brighter. 



" " 0-00064 " Brighter. 



" " 0-00057 " Brighter. 



" " 0-00049 " Brighter. 



" " 0-00044 " Equally bright. 



" " 0-00040 " Fainter. 



It appears that the presence of 0-00044 mg. of potassium is 

 capable of increasing the brilliance of the lines yielded by 

 '00044 mg. of rubidium to an equality with the lines given 

 by 0*00066 mg. of the pure salt ; or, that the maximum 

 increase of brightness amounts to fifty per cent. 



It is evident, therefore, that means must be found to effect 

 the separation of the rubidium from sodium and potassium, or 

 of bringing test and standard to the same condition as regards 

 the presence of these elements, before any reasonable degree 

 of accuracy can be expected in the spectroscopic determination 

 of rubidium as it ordinarily occurs in nature. The separation 

 from sodium is easily accomplished by the conversion of the 

 salts to the form of chloroplatinates ; but for the quantitative 

 separation of rubidium from potassium there is no good 

 method known. The question as to the practical value of the 

 spectroscopic reaction of rubidium for purposes of approxi- 

 mative quantitative analysis resolves itself into the problem as 

 to whether by matching potassium lines as well as the ru- 

 bidium lines (following the method outlined in the determina- 

 tion of potassium in presence of sodium), and so bringing the 

 lines of test and standard equally under the influence of potas- 



