in] PLANT CONSTITUENTS 31 



acid are given off. Cool, transfer to a flask, add water, and digest till the solution is 

 clear. Reduce with zinc, cool, titrate with N/50 potassium permanganate and cal- 

 culate to ferric oxid. 



If it is desired to use a larger amount of the sample for the iron determination, 

 evaporate a suitable aliquot of A, under 4, with sulphuric acid, reduce with zinc, 

 and titrate as above. 



MANGANESE, CALCIUM AND MAGNESIUM. 



(Applicable for plant materials other than seeds.) 



7 Method I. —Official. 



To an aliquot of A, under 4, corresponding to 0.5-2 grams of ash, add a quantity 

 of pure ferric chlorid solution, more than sufficient to combine with the phosphoric 

 acid which may be present, and neutralize with ammonium hydroxid. Dissolve the 

 precipitate in a very slight excess of hydrochloric acid and add 1-2 grams of sodium 

 acetate. Boil for 1-2 minutes, filter at once, and wash with boiling water. Dissolve 

 the precipitate in hydrochloric acid and reprecipitate as above. Concentrate the 

 combined filtrates and washings to about 50 cc. and determine manganese, calcium 

 and magnesium as directed under II, 14, 15, and 16. 



8 Method 11.— Tentative. 



Calcium. — Make alkaline with ammonium hydroxid the combined filtrates and 

 washings, C, under 6, and proceed as directed under 11,15. If the ignited calcium 

 oxid has a brown coloration, due to manganese, dissolve in dilute hydrochloric acid 

 and determine the manganese as directed below. Deduct the weight of mangano- 

 manganic oxid thus obtained from the weight of the impure calcium oxid. 



Manganese. — Acidify the combined filtrates and washings from the calcium de- 

 termination and evaporate to dryness in a porcelain casserole. Expel the ammonium 

 salts by carefully heating the casserole from above, treat with a few cc. of hydro- 

 chloric acid and water, filter off molybdic acid, and wash the precipitate until it is 

 free frc orin. Bring the filtrate to a volume of 100 cc, add 1-2 drops of bromin, 



make a with a'^^monium h/drSx'"^'"; ..1' let stand for several minutes without 



agitation. Filter off the precipitated manganese, wash, dry, ignite, and weigh the 

 precipitate as manganomanganic oxid. To this add the weight of the mangano- 

 manganic oxid found as an impurity in the calcium determination. 



Magnesium. — Concentrate the alkaline filtrate from the manganese determination 

 to 75 cc. and determine magnesium as directed under II, 16. 



PHOSPHORIC ACID. 



9 Method I.— Official. 



Determine phosphoric acid in an aliquot of A, under 4, corresponding to 0.2-1 

 gram of ash, as directed under I, 6 or 9. 



10 Method II. —Official. 



Determine phosphoric acid in the plant substance as directed under 1, 6, using suf- 

 ficient material to give 0.2-1 gram of ash in the aliquot of the solution employed. 



11 Method III. -Official. 



The phosphomolybdate precipitate obtained in 6 is used for the determination 

 of phosphoric acid as directed under I, 6, beginning with "Dissolve the precipitate 



