174 METHODS OF ANALYSIS [Chap. 



(b) N / 100 iodin.— The solution must be standardized frequently against (d), con- 

 taining asbestos, and treated as described in 7, omitting the precipitation and boil- 

 ing with hydrochloric acid and potassium chlorate. To obtain exact results the 

 tin solution used for standardization should contain about the same amount of tin 

 as is found in the sample under examination. 



(C) N/100 sodium thiosulphate. 



(d) Standard tin solution. — Dissolve 1 gram of tin in about 500 cc. of concen- 

 trated hydrochloric acid. Make up to 1 liter with water. One cc. contains 1 mg. 

 of tin. 



(e) Sheet aluminium. — Use sheet aluminium, about 30 gauge, free from tin. 



7 DETERMINATION. 



Proceed as directed in 5 to "Digest on the hot plate for an hour and allow to 

 stand 1-2 hours longer". 



Filter the precipitate of tin sulphid upon asbestos in a Gooch crucible with a 

 detachable bottom, using suction. Wash the precipitate a few times and then 

 transfer the detachable bottom, asbestos pad, and tin precipitate to a 300 cc. Er- 

 lenmeycr flask. Remove all traces of the precipitate from the inside of the cru- 

 cible by means of a jet of hot water and a policeman, using a minimum amount of 

 water for washing. 



Add 100 cc. of concentrated hydrochloric acid and 0.5 gram of potassium chlorate 

 to the flask. Boil for about 15 minutes, making about 4 more additions of smaller 

 amounts of potassium chlorate as chlorin is boiled out of the solution. Wash the 

 particles of potassium chlorate down from the neck of the flask with water and 

 finally boil to remove chlorin. Then add about 1 gram of the sheet aluminium to 

 dispel the last traces of chlorin. 



Attach the flasks, in duplicate, as described below, to a large carbon dioxid gener- 

 ator. Pass the carbon dioxid through a scrubber containing water and then divide 

 into 2 streams by means of a Y-tube, each stream of carbon dioxid entering one of 

 the flasks by means of a long rubber tube connected with a bulbed tube, passed 

 through the rubber stopper of the flask and having its lower end near the surface of 

 the liquid in the flask. The carbon dioxid leaves the flask by a second bulbed tube, 

 the opening of which is near the top of the flask. This glass tube is connected by a 

 long rubber tube to a second glass tube about 10 inches long which is immersed in a 

 cylinder containing water. This gives a water-seal to the delivery tube and a pres- 

 sure against which the current of carbon dioxid must work. It also restrains any 

 strong flow of gas when not desired and permits a gas pressure in the Erlenmeyer 

 flask. 



After the flasks are connected, raise the tubes in the water-seal cylinders so that 

 the generator has practically no pressure to overcome. Allow the carbon dioxid 

 to run for a few minutes. Drop the tubes to the bottom of the cylinders, creating 

 pressure in the flasks. Lift the rubber stoppers of the flasks alternately about a 

 dozen times, in order to force out any air remaining in the flasks. Slightly raise the 

 stopper on one of the flasks and quickly drop about 2 grams of sheet aluminium into 

 the flask. The aluminium should be folded into a strip about 1 cm. wide and slightly 

 bent so as to prevent it from striking directly on the bottom of the flask. After 

 the aluminium has entirely dissolved, raise the tubes in the water-seal cylinders so 

 as to allow carbon dioxid to pass through, place the flasks upon hot plates, and heat 

 to boiling. After boiling for a few minutes, remove the flasks from the hot plates 

 and cool in ice water (or cold running water), still maintaining within them an 



