METHODS OF ANALYSIS 49 



the stopper, shake the bottle well, and let it stand for some time, 

 so that the flocculent ppt. of cyanide of zinc may settle, and 

 obtain a clear supernatant liquid. If possible, let it stand over 

 night. When the ppt. has settled sufficiently, the liquid is siphoned 

 off. Use a two-holed rubber stopper, with a siphon passing through 

 one hole, and a short, bent tube through the other, and start the 

 action by blowing through this bent tube, as one would use a wash 

 bottle. The end of the siphon in the liquid should have a small 

 bag of lint tied over it, to prevent the carrying away of any small 

 particles of ppt. In this way, fill the two 293 cc. separatory tubes 

 and put them aside for the present. Draw off the same quantity 

 — 293 cc. — into a beaker, add 1 cc. H 2 S0 4 (half acid and half water), 

 and 1 cc. of iodide of potassium and starch. From a burette, add 

 carefully, drop by drop, dilute bromine water (1 bromine water to 

 2 of water) till a blue color is obtained, and note the number of 

 drops. 



Take the tube with the solution to be tested, add 1 cc. NaN0 2 

 and KI solution and 1.0 cc. H 2 S0 4 (half acid, half water), and the 

 number of drops of bromine water required, put in the stopper 

 and turn over the tube several times. Iodine is at once set free in 

 proportion to the oxygen in the solution, and is ready to be deter- 

 mined by titration with hyposulphite, according to Thresh's method. 



The wide-mouthed glass bottle, having a rubber stopper, 

 pierced with 4 holes, is here used, and coal gas, or CO,, must be 

 passed through it during the experiment. The C0 2 must be puri- 

 fied by passing through a solution of potassium iodide and freed 

 from oxygen. The tube containing the solution is inserted through 

 the third hole and the hyposulphite burette through the fourth. 

 Coal gas (or C0 2 ) is passed through the bottle for 15 minutes, and 

 then the KCN solution is allowed to flow into the bottle, and also 

 a few drops of starch solution, which becomes blue at once. The 

 stopcock is turned off, and the free iodine is determined by drop- 

 ping in hyposulphite solution, slowly, until the blue color dis- 

 appears; 7.75 grams of hyposulphite, in 1 litre of water, gives a 

 solution 1 cc. of which corresponds to .25 milligrams of O. A cor- 

 rection must be made for the O in the reagents used. 



Nitrite of potash (or soda) is oxidized by the addition of 

 bromine water, and liberates iodine. To determine the correction 

 necessary for this iodine, take a 350 cc. extra strong flask, and 

 pour into it the same amount of solution as taken for the analysis. 



