348 METHODS OF ANALYSIS [Chap. 



and make up to 100 cc. Treat with 2 drops of freshly prepared, colorless ammo- 

 nium sulphid solution, mix and compare in a colorimeter with standard solutions, 

 prepared by adding measured amounts of the standard lead solution to 50 cc. of 

 the lead-free tartrate solution, diluting to 100 cc. and treating with 2 drops of 

 freshly prepared colorless ammoniima sulphid solution. 



The final comparison should be made with a standard containing approximately 

 the same amount of lead, and the addition of ammonium sulphid solution should be 

 made to the standards and the solution of the sample at the same time, as the colors 

 change on standing. 



31 Method II. — Tentative. 



(Applicable to alum or phosphate baking powders or their ingredients.) 



Weigh 100 grams of the sample into a 1..3 liter beaker and add an excess of hy- 

 drochloric acid (1 to 3) in small portions, keeping down excessive frothing with a 

 little ether. Heat the mixture on a steam bath until the starch is hydrolyzed and 

 the solution is quite limpid. Cool and add 200 cc. of 50% lead-free ammonium 

 citrate solution. Place the beaker in a bath of cold water and add carefully ammo- 

 nium hydroxid solution, in small portions with constant stirring, until the mixture 

 is alkaline. If a precipitate forms, add sufficient ammonium citrate solution to 

 dissolve it. Then add 15 cc. of saturated mercuric chlorid solution, dilute the 

 mixture to about 1200 cc, saturate with hydrogen sulphid and allow to stand 

 until the precipitate has settled (15-20 minutes). Filter and wash the precipitate 

 with hydrogen sulphid water. Place the paper and precipitate in a small casserole, 

 add 10 cc. of concentrated nitric acid and 2 cc. of concentrated sulphuric acid and 

 heat on a hot plate until the mixed acids have been slowly driven off. Heat the 

 residue in a muffle at low redness until the mercury salts have volatilized. Cool 

 the casserole and leach the residue several times with 25% ammonium acetate solu- 

 tion, made slightly alkaline with ammonium hydroxid, pass the teachings through 

 a small filter into a beaker and finally wash the residue and filter paper with a 

 little hot water. Acidify the combined filtrate and washings with acetic acid, add 

 an excess of potassium dichromate solution and allow to stand overnight. Filter 

 on a tared Gooch, wash with water, dry for 30 minutes at 125°-150°C., cool and 

 weigh as lead chromate. Calculate the weight of metallic lead. Conduct a blank 

 determination upon all the reagents and correct the result accordingly. 



32 Method III.— Tentative. 



(Applicable to alum or phosphate baking powders or their ingredients.) 



Transfer 200 grams of the sample to a 3 liter Jena flask, add 300 cc. of concen- 

 trated nitric acid in small portions, shake thoroughly after each addition and 

 heat the mixture slowly, shaking repeatedly. When brown fumes begin to appear 

 at the mouth of the flask, discontinue heating and insert a stemless funnel in the 

 neck of the flask. As soon as the action has moderated, place the flask on an 

 asbestos gauze over a small Bunsen flame. When the action becomes weak, add 

 slowly 90 cc. of concentrated sulphuric acid and continue heating until the fumes 

 disappear. Then add 25 cc. of concentrated nitric acid from time to time with con- 

 tinued heating until all the starch is completely oxidized. Usually 3-4 additions 

 of 25 cc. portions of nitric acid suffice. Finally expel the nitric acid as completely 

 as possible. Cool, add 400 cc. of water, shake and allow to settle. The soluble sul- 

 phates of sodium, potassium, aluminium, iron, etc., go into solution, while calcium 

 sulphate and most of the lead sulphate will be precipitated. Filter through an 18 



