454 Permeability 



As for the metal ions of sea water, sodium could easily be dem- 

 onstrated within the cell by allowing some of the sap to evapo- 

 rate on a watch-glass when crystals of sodium chloride were seen 

 to be formed. The presence of calcium was easily demon- 

 strated. When a saturated solution of ammonium oxalate was 

 added to the cell sap a white precipitate was produced which 

 was soluble in hydrochloric acid but insoluble in acetic acid. 

 This precipitate was washed in acetic acid and water and when 

 dissolved in a small amount of hydrochloric acid gave an unmis- 

 takable flame test for calcium. 



A similar condition, however, was not found to hold for mag- 

 nesium. In sea water this can easily be demonstrated by precipi- 

 tating with any of the fixed alkali hydroxides (e.g., NaOH) 

 forming Mg(OH) 2 , which is white, gelatinous, and insoluble in 

 excess of the reagent but readily soluble in the presence of am- 

 monium salts, and from such solution can readily be precipitated 

 by the alkali phosphates (e.g., Na 2 HP0 4 ) in the form of mag- 

 nesium ammonium phosphate. When this method was tried on 

 the cell sap, however, only the merest trace of a precipitate was 

 obtained by the addition of NaOH and, though this disappeared 

 upon the addition of ammonium sulfate, the addition of diso- 

 dium phosphate to the solution would not cause the precipita- 

 tion of any magnesium ammonium phosphate. This shows that, 

 though magnesium is probably present within the cell, it does 

 not exist in nearly as high a concentration as in sea water. 



Exactly the opposite obtains in the case of potassium. In sea 

 water the concentration of potassium is so low that no precipi- 

 tate was produced by the addition of picric acid, and the dried 

 residue gave only a doubtful flame test. With the Valonia sap, 

 however, the dried residue showed an abundance of potassium 

 by the flame test; when picric acid was added to the sap long 

 acicular crystals settled out, and when these were dried and 

 touched with a hot needle they detonated, giving an unmistak- 

 able potassium-colored flash, showing that these crystals, formed 

 by the addition of picric acid, were potassium picrate. From 

 this it is plain that there is a considerably higher concentration 

 of potassium within the cell than without. 



The Valonia sap gives an abundant precipitate with chloro- 

 platinic acid. The solution was heated to complete the reaction 



