XXVni] BAKING POWDERS 347 



until carbon dioxid is no longer evolved. Dilute a little of this solution with 10 

 volumes of water as needed in the determination. 



(b) 10% -potassium cyanid solution. 



(C) Standard lead solution. — Dissolve 1.6 grams of crystallized lead nitrate, pre- 

 viously dried over sulphuric acid, in a liter of water containing a few drops of dilute 

 nitric acid. One cc. of this solution is equivalent to 1 mg. of metallic lead. Dilute 

 1 cc. of this solution to 100 cc. immediately before use in making up the color 

 standards. 



(d) Lead-free tartrate solution. — Dissolve 200 grams of tartaric acid in about 500 

 cc. of hot water, cool, add 40 cc. of the sodium bisulphite solution, heat to incipient 

 boiling and test a few drops of the solution with potassium sulphocyanate solution 

 to ascertain if all the iron is reduced to the ferrous state, repeating the treatment 

 with about 10 cc. of the sodium bisulphite solution in case ferric iron is still present. 

 Cool, add 20 cc. of the 10% potassium cyanid solution, and then strong ammonium 

 hydroxid solution until the solution is distinctly alkaline to litmus paper. Boil 

 until the solution is clear, cool, add 2 cc. of freshly prepared, colorless ammonium 

 sulphid solution, dilute to 1 liter and allow to stand overnight. Filter to remove 

 the precipitated sulphids, boil the filtrate until hydrogen sulphid is removed, cool 

 and dilute to 1 liter with water. 



PREPARATION OF SOLUTION. 



29 



(a) Baking powder. — Weigh 20 grams of the sample into a 250 cc. casserole, add 

 water a little at a time with stirring until foaming ceases, then hydrochloric acid 

 (1 to 1) a little at a time until all the carbonate is decomposed and finally 5 cc. 

 excess of the hydrochloric acid. Cover with a watch glass and digest on a steam 

 bath until all the starch is hydrolyzed as shown by testing 1 or 2 drops of the mix- 

 ture with iodin. Filter through a folded filter and wash the filter several times with 

 small portions of hot water. Treat the residue on the filter with several small por- 

 tions of hot nitric acid (sp. gr. 1.2), collect the acid solution in a separate, small 

 porcelain dish, evaporate this solution to dryness on a water bath and expel nitric 

 acid by several treatments and evaporations with a few drops of concentrated 

 hydrochloric acid. Rinse the contents of the dish through a small filter into the 

 main solution and make up to 100 cc. 



(b) Tartaric acid and cream of tartar. — Dissolve 100 grams of the sample in hot 

 water, add 50 cc. of hydrochloric acid (1 to 1), filter into a liter graduated flask, 

 wash the filter several times with water, and then treat the residue on the filter 

 with several small portions of hot nitric acid (sp. gr. 1.2), collect the acid solution 

 in a separate, small porcelain dish, evaporate this solution to dryness on a water 

 bath and expel nitric acid by several treatments and evaporations with a few drops 

 of concentrated hydrochloric acid. Rinse the contents of the dish through a 

 small filter into the main solution, finally diluting the combined filtrates and wash- 

 ings to a liter. 



30 DETERMINATION. 



Introduce 50 cc. of the solution, prepared as directed in 29, into a beaker, add 

 2 cc. of the sodium bisulphite solution, heat to incipient boiling, and test a few 

 drops of the solution with potassium sulphocyanate to determine if all the iron is 

 reduced to the ferrous state, repeating the treatment with the sodium bisulphite 

 solution if ferric iron is still present. Cool, add 1 cc. of the 10% potassium cyanid 

 solution and neutralize to litmus with strong ammonium hydroxid solution; finally 

 add an excess of 1 cc. of. the last reagent. Boil gently until clear and colorless, cool 



