32 METHODS OF ANALYSIS [Chap. 



on the filter with ammonium hydroxid, etc." or as under I, 9 (a), beginning with 

 "Wash with cold water until the filtrate from 2 fillings of the filter yields a pink 

 color, etc." 



12 SULPHURIC ACID, SODIUM, AND POTASSIUM.— OFFICIAL. 



Boil an aliquot of A, under 4, corresponding to 0.5-1 gram of ash, add barium 

 chlorid solution in small quantities until no further precipitate is formed, and pro- 

 ceed as directed under II, 20 and 21 . 



■|3 CHLORIN.— OFFICIAL. 



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

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

 aqueous solution of the ash. 



Volhard Methods 



14 REAGENTS. 



(a) N/10 silver nitrate. 



(b) N/iO ammonium, or potassium sulphocyanate. 



(C) Ferric indicator. — Saturated solution of ferric alum. 



(d) Nitric acid. — Free from lower oxids of nitrogen, secured by diluting the 

 usual pure acid with about 1/4 part of water, and boiling till perfectly colorless. 



15 DETERMINATION. 



Dissolve a weighed portion of the ash, prepared under 2, in nitric acid (1 to 10). 

 filter and wash with water. Add a known volume of the N/10 silver nitrate in slight 

 excess to the combined filtrate and washings. Stir well, filter, and wash the silver 

 chlorid precipitate thoroughly. To the filtrate and washings add 5 cc. of the ferric 

 indicator and a few cc. of the nitric acid. Titrate the excess of silver with the 

 N/10 sulphocyanate until a permanent light brown color appears. Calculate the 

 amount of chlorin. 



16 POTASSIUM IN PLANTS.-OFFICIAL. 



Determine potassium as directed under I, 42, using sufficient plant material to 

 yield 0.5-1 gram of ash in the aliquot of the solution used for the potassium determi- 

 nation. 



SULPHUR IN PLANTS. 



17 Peroxid Method. — Official. 



Place 1.5-2.5 grams of material in a nickel crucible of about 100 cc. capacity and 

 add 5 grams of pure anhydrous sodium carbonate. Mix thoroughly, using a nickel or 

 platinum rod, and moisten with approximately 2 cc. of water. Add pure sodium 

 peroxid, approximately 0.5 gram at a time, thoroughly mixing the charge after each 

 addition. Continue adding the peroxid until the mixture becomes nearly dry and 

 quite granular, requiring usually about 5 grams of peroxid. Place the crucible over 

 a low alcohol or other sulphur-free flame and heat carefully with occasional stirring 

 until the 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 fusion again takes place, rotating the crucible from time to 

 time in order to bring any particles adhpring to the sides into contact with the 

 oxidizing material. Continue the heating for 10 minutes after fusion is complete. 



