XXVni] BAKING POWDERS 349 



cm. folded filter into a liter Erlenmeyer flask, rinse the 3 liter flask 2-3 times with 

 small portions of water and pour the rinsings through the filter. Reserve the fil- 

 trate for the recovery of dissolved lead salts. Open the filter, containing the pre- 

 cipitate, over a 600 cc. beaker and wash the precipitate into it. Then transfer the 

 contents of the beaker to a 2 liter Erlenmeyer flask together with whatever pre- 

 cipitate remains in the 3 liter flask. Dilute the contents of the flask so as to nearly 

 fill the latter, stir thoroughly to dissolve the calcium sulphate, add 20 cc. of strong 

 acetic acid and saturate the liquid thoroughly with hydrogen sulphid. Stopper 

 the flask and set aside until the precipitate settles. Siphon off the supernatant 

 liquid. When much calcium sulphate is present, one such treatment is not suffi- 

 cient to dissolve all of it. In this case refill the flask with water, again acidify, 

 saturate with hydrogen sulphid and allow to stand till the calcium sulphate is 

 practically all dissolved and the residue of sulphids is dark colored. Solution may 

 be hastened by the addition of lead-free sodium acetate to the water (50-75 grams 

 to each 2 liters). 



Treat the liquid containing the soluble sulphates separately to recover the trace 

 of lead which it may contain. Partially neutralize with ammonium hydroxid solu- 

 tion just short of the point of producing a permanent precipitate of aluminium 

 phosphate, saturate with hydrogen sulphid and allow the precipitate to settle. 

 Some iron sulphid also will usually be precipitated. The sulphid precipitations 

 should be made in very slightly acid solutions, otherwise lead sulphid will not be 

 precipitated completely. Siphon off the supernatant liquid, transfer the precipi- 

 tate to an 11 cm. filter and wash with hydrogen sulphid water. Transfer the first 

 precipitate remaining in the 2 liter flask to a second 11 cm. filter and treat in the 

 same way. Place the 2 filters with their contents in a 200 cc. Erlenmeyer flask, 

 add 10 cc. of concentrated nitric acid and 5 cc. of concentrated sulphuric acid, 

 insert a stemless funnel in the neck and heat to completely oxidize the material. 

 When the nitric acid has all been expelled and the residue darkens, add more nitric 

 acid until no such darkening occurs. Finally heat the residue till fumes of sulphur 

 trioxid are given off, cool and add 15 cc. of water. Filter through a 7 cm. filter, 

 rinse, then wash the filter twice with small portions of dilute sulphuric acid and 

 finally with a little water. Place a clean 150 cc. beaker under the filter, dissolve 

 the precipitate in 15-25 cc. of ammonium acetate solution [1 part of 99% acetic 

 acid, 1 of water and 1 of ammonium hydroxid (sp. gr. 0.90); made neutral to lit- 

 mus paper] and wash thoroughly with water. 



Acidify the filtrate and washings with acetic acid, add an excess of potassium 

 dichromate solution, heat on a steam bath and allow to cool and settle. Filter on 

 a tared Gooch prepared with a thick layer of asbestos which has been previously 

 dried at 125°C., wash with water, dry at about 125°C. and weigh as lead chromate. 



33 Method IV.— Tentative. 



(Applicable in the absence of alum and phosphates.) 



Weigh 100 grams of the sample into a liter beaker and add an excess of hydro- 

 chloric 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 hydrolj'zed and the 

 solution is quite limpid. Cool, add ammonium hj'droxid solution until distinctly 

 alkaline, dilute to about 800-900 cc. and saturate with hydrogen sulphid. Allow 

 the mixture to stand for 3-4 hours or until the precipitate has settled, filter on a 

 12.5 cm. close-textured paper and wash the precipitate several times with hydrogen 

 sulphid water. Place the filter paper and precipitate in a 100 cc. Erlenmeyer flask, 

 add 5 cc. of concentrated sulphuric acid and 5 cc. of concentrated nitric acid and 



