INORGANIC PLANT CONSTITUENTS. 23 



5. Sulphuric Acid, Sodium, and Potassium. Official. 



Heat to boiling an aliquot of Solution A, corresponding to from 0.5 to 1 gram 

 of ash, add barium chlorid solution in small quantities until no further pre- 

 cipitation is produced, and proceed as under soils (5 (g) and (7i) p. 17). 



6. Chlorin. Official. 



Determine as silver chlorid, either gravimetrically or by one of the standard 

 volumetric processes (as the Volhard method given below), in a nitric acid or 

 aqueous solution of the ash. 



VOLHARD METHOD OF DETERMINING CHLOBIN. 

 (a) SOLUTIONS REQUIRED. 



(1) Silver nitrate. Prepare a decinormal solution in the usual way. 



(2) Ammonium or potassium thiocyanate. Standardize a decinormal solution 

 by means of the standard silver solution. 



(3) Ferric indicator. Prepare a saturated solution of iron alum. 



(//) Pure nitric acid. This reagent must not contain any of the lower oxids 

 of nitrogen. 



(b) DETERMINATION. 



Dissolve the material (in this case ash) in nitric acid and filter. Add a 

 known volume of the tenth-normal silver nitrate to the filtrate until an excess 

 is present. Stir well, filter, and wash the silver chlorid precipitate thoroughly. 

 To the filtrate and washings add o cc of the ferric indicator and a few cubic 

 centimeters of nitric acid. Titrate the excess of silver with the tenth-normal 

 thiocyanate solution until a permanent light-brown color appears. From the 

 amount of tenth-normal silver nitrate originally used subtract the excess as 

 shown by the thiocyanate solution to obtain the amount used by the chlorin. 



7. Potassium in Plants. Official. 



Determine potash as directed under fertilizers for potash in organic com- 

 pounds ((2) (&), p. 11), using sufficient plant material to get from 0.5 to 

 1 gram of ash in the aliquot of the solution used for the potash determination. 



8. Sulphur in Plants Peroxid Method. Provisional. 



Place from 1.5 to 2.5 grams of material in a nickel crucible of about 100 cc 

 capacity and moisten with approximately 2 cc of water. Mix thoroughly, using 

 n nickel or platinum rod. Add 5 grams of pure anhydrous sodium carbonate 

 and mix. Add pure sodium peroxid, small amounts (approximately 0.50 gram) 

 at a time, thoroughly mixing the charge, after each addition. Continue adding 

 The peroxid until trie mixture becomes nearly dry and quite granular, requiring 

 usually about 5 grams of peroxid. Place the crucible over a low alcohol flame 

 (or other flame free from sulphur) and carefully heat with occasional stirring 

 until contents are fused. (Should the material ignite, the determination is 

 worthless.) After fusion remove the crucible, allow to cool somewhat, and 

 cover the hardened mass with peroxid to a depth of about 0.5 cm. Heat 

 gradually, and finally with full flame until complete fusion takes place, rotating 



