lo THE ULTIMATE CONSTITUENTS OF PLANTS 



or all of the magnesium, and sulphate, and is examined 

 for these substances as usual : — 



Calcium. — NH^Cl and (NHJgCOg precipitates the 

 calcium as carbonate, confirmed by dissolving the 

 latter in dilute acetic acid and treating with ammonium 

 oxalate (above). 



Magnesium. — To the filtrate from the precipitation 

 of calcium carbonate, ammonium phosphate is added, 

 whereby the magnesium is thrown down as phosphate. 



Potassium and Sodium. — The filtrate from the 

 magnesium precipitate is evaporated to dryness and 

 ignited, in order to expel ammonium salts. The 

 presence of sodium and potassium is confirmed by the 

 usual flame colorations, while the presence of the latter 

 element is also shown by precipitation with platinic 

 chloride or sodium cobaltinitrite. 



Sulphates and Chlorides. — The presence of sulphate 

 and chloride can be most easily demonstrated in a 

 portion of the original ash by dissolving in hydro- 

 chloric acid and precipitating with BaClg for the 

 sulphate, and by dissolving in nitric acid and precipi- 

 tating with AgNOg for the chloride. 



7. Quantitative Estimation of Nitrogen by 



KjeldahVs Process. 



This process depends upon the fact that, excepting in the case 

 of a few special compounds, all the nitrogen present in the plant 

 product is converted into ammonium sulphate by digesting the 

 substance with concentrated sulphuric acid at as high a tempera- 

 ture as possible. The ammonia is expelled by boiling the 

 resultant solution with caustic soda and is estimated by absorption 

 in a known volume of standard acid. 



From I to 4 grams of the substance (the amount 

 taken depends upon the probable nitrogen content) 

 are placed in a Kjeldahl flask (Fig. i), care being 



