8 



turbid, make alkaline with potassium hydroxid and then acid with 1 c. c. excess. The 

 solution must neither be alkaline nor too acid, or there will be trouble in filtering and 

 washing. Heat on a steam bath for half an hour, stirring two or three times. Filter 

 onto a 12.5 cm. white ribbon paper. The precipitate will sometimes run through, and 

 must be returned until the filtrate is perfectly clear and brilliant. If the solution to 

 be filtered is stirred vigorously, allowed to stand until the precipitate clots, and then 

 poured onto the filter, refiltering may usually be avoided. The filtrate must be per- 

 fectly clear or some tin will be lost. Wash alternately with distilled water and the 

 ammonium acetate solution previously mentioned until the filtrate obtained from a 

 washing with distilled water is free from chlorids. (Do not mistake the precipitate 

 given by the acetate solution and silver nitrate, which is soluble in water, for silver 

 chlorid.) This requires washing until the volume of the filtrate is 200 c. c. or more. 

 Fill the filter at each washing. Place the moist filter in a porcelain crucible, dry and 

 char on an asbestos gauze, and burn off all carbon over the free flame. Cool in a desic- 

 cator and weigh as stannic oxid. 



Tab^e 8 gives a comparison of the results obtained by this method and those obtained 

 by Schryver's method. 



TABLE 8. Comparison of proposed method with Schryver's sulphuric-acid method. 

 [Grams of tin in 100 grams of sample.] 



1 Duplicates not run, as there was not sufficient material. 



Experiments were made to determine the relative value of potas- 

 sium hydrate and ammonium sulphid as solvents for tin sulphid and 

 it was found that they gave concordant results. In one instance the 

 solution by potassium hydrate gave 0.0313 gram of stannic oxid, and 

 the ammonium sulphid gave 0.0318 gram. The sample of ketchup 

 on which these determinations were made contained a large amount 

 of sand, and the results show that there is no danger of silica con- 

 taminating the stannic oxid when potassium hydroxid is used. 



[Cir. 67] 



