IV] WATERS 43 



Dilute to approximately 25 cc, boil, and again precipitate with ammonium hydroxid; 

 filter, wash thoroughly with hot water, dry, ignite, and weigh as iron and aluminium 

 oxids. (In the presence of phosphoric acid, the weight of this residue must be cor- 

 rected for the phosphorus pentoxid equivalent to the phosphoric acid found in 51 , 

 making due allowance for the difference in the volumes of the water used for these 

 determinations.) Designate the filtrate as B. 



IRON. 



42 Colorimelric Method. 



(If the amount of iron is less than 1 mg.) 



Fuse in a platinum crucible the ignited precipitate of iron and aluminium 

 oxids with fused potassium hydrogen sulphate, dissolve in water, and precipitate the 

 iron and aluminium with ammonium hydroxid. Dissolve the precipitate on the 

 filter paper in hydrochloric and nitric acids, dilute the solution, add ammonium 

 sulphocyanate solution (1 to 20) and compare the color developed with that of 

 calibrated color disks, or standards containing known amounts of iron. 



43 Volumetric Method. 



Fuse in a platinum crucible the residue of iron and aluminium oxids with fused 

 potassium hydrogen sulphate. This fusion takes but a few minutes and must not 

 be continued beyond the time actually needed. When completed, the crucible 

 is set aside and allowed to cool. Add dilute sulphuric acid and heat the crucible 

 until the fused mass is dissolved. Evaporate on the water bath as far as possible; 

 then heat gradually until copious fumes of sulphuric acid are given off. Dissolve 

 in water and allow to stand on the water bath. Cool, transfer to an Erlenmeyer 

 flask, and make up to such a volume that the solution does not contain more than 

 2.5% of free sulphuric acid. Pass hydrogen sulphid through the solution to reduce 

 the iron and precipitate any platinum contaminating the residue from the fusion. 

 (Zinc may be used instead of hydrogen sulphid for reducing the iron.) Filter, wash 

 and again pass hydrogen sulphid through the solution to be certain that all the iron 

 is reduced. Expel the hydrogen sulphid by boiling, at the same time passing a 

 current of carbon dioxid through the solution; test the escaping gas with lead acetate 

 paper to ascertain the complete removal of hydrogen sulphid. When hydrogen 

 sulphid has been removed discontinue boiling and let the flask cool somewhat with- 

 out discontinuing the current of carbon dioxid. Titrate the reduced iron with a 

 standard permanganate solution (1 cc. equivalent to 1 mg. of Fe) and calculate as 



44 ' ALUMINIUM.— OFFICIAL. 



In the absence of phosphates, subtract from the weight of iron and aluminium 

 oxids, under 41 , the iron, under 42 or 43, calculated to oxid, to obtain the weight of 

 aluminium o\id. Calculate to aluminium. 



45 CALCIUM.— OFFICIAL. 



Concentrate B, under 41 , to 150-200 cc. and to this solution, containing not more 

 than 0.6 gram of calcium, calculated as calcium oxid, or 1 gram of magnesium, cal- 

 culated as magnesium pyrophosphate, add 1-2 grams of oxalic acid and sufficient 

 hydrochloric acid to clear the solution. Heat to boiling and neutralize with ammo- 

 nium hydroxid, stirring constantly. Add ammonium hydroxid in slight excess and 

 alloi«' to stand 3 hours in a warm place. Filter off the supernatant liquid and wash 



