XXVIII] BAKING POWDERS 345 



The treatment with 3% hydrochloric acid, without dissolving the starch, removes 

 effectively the lime, which otherwise would be precipitated as tartrate by the 

 alkaline copper solution. 



19 Modified McGlll Method.— Tentative. 



Digest 1 gram of the powder with 150 cc. of 3% hydrochloric acid for 24 hours 

 at room temperature, with occasional shaking. Filter on a Gooch crucible, wash 

 thoroughly with cold water and then once each, with alcohol and ether. Dry at 

 110°C. (4 hours is usually sufficient), cool and weigh. Burn off the starch, weigh 

 again and determine the starch by difference. 



The results by this method on cream of tartar powders and tartaric acid pow- 

 ders agree closely with those obtained by copper reduction. On phosphate, alum 

 and alum-phosphate powders the results are usually satisfactory, but in some 

 instances they may be over 2% too high. 



ALUM IN THE PRESENCE OF PHOSPHATES*. 



20 Qualitative Test. — Tentative. 



(a) In baking powder. — Burn about 2 grams of the sample to an ash in a porce- 

 lain dish. Extract with boiling water and filter. Add to the filtrate a few drops 

 of ammonium chlorid solution. A flocculent precipitate indicates alum. 



(b) In cream of tartar. — Mix about 1 gram of the sample with an equal quantity 

 of sodium carbonate, burn to an ash and proceed as in (a). 



ASHi". 



21 INSOLUBLE ASH AND PREPARATION OF SOLUTION.-TENTATIVE. 



Char 5 grams of the sample in a platinum dish at a heat below redjxess. Boil the 

 carbonaceous mass with dilute hydrochloric acid, filter into a 500 cc. graduated 

 flask and wash with hot water. Return the residue, together with the paper, to 

 the platinum dish and burn to a white ash. Boil again with hydrochloric acid, 

 filter, wash, unite the 2 filtrates and dilute to 500 cc. 



Incinerate the residue after the last filtration and determine the ash insoluble 

 in acid. 



22 IRON AND ALUMINIUM.-TENTATIVE. 



Draw a 100 cc. aliquot of the solution, prepared as directed in 21, and separate 

 silica, if necessary. Mix the solution with sodium phosphate solution in excess. 

 Add ammonium hydroxid until a permanent precipitate is obtained, then hydro- 

 chloric acid, drop by drop, until the precipitate is dissolved. Heat the solution 

 to about 50°C., mix with a considerable excess of 50% ammonium acetate solution 

 and 4 cc. of 80% acetic acid. 



As soon as the precipitate of aluminium phosphate, mixed with iron phosphate, 

 has settled, collect on a filter, wash with hot water, ignite and weigh. 



Fuse the mixed phosphates with 10 parts of sodium carbonate, dissolve in dilute 

 sulphuric acid, reduce with zinc, and determine the iron by titration with a standard 

 permanganate solution. In the same solution determine the phosphoric acid, as 

 directed under I, 6 or 9. To obtain the weight of alumina (AI2O3) subtract the 

 smn of the weights of ferric oxid (Fe203) and phosphorus pentoxid (P2O6) from the 

 weight of the mixed phosphates. 



