48 METHODS OF ANALYSIS [Chap. 



sodium hydroxid to the combined alcoholic filtrates and evaporate to dryness. Dis- 

 solve this last residue in 2-3 cc. of water and repeat as above described the precipi- 

 tation with alcohol, heating, and filtering. Add a drop of the sodium hydroxid to 

 this alcoholic filtrate and evaporate to dryness. Dissolve this residue in a little 

 water, acidify with the sulphuric acid, using 3 or 4 drops in excess, and transfer to 

 a small flask. Add 4 drops of the potassium nitrite and about 5 cc. of the carbon 

 disulphid. Shake until all the iodin is extracted, filter off the acid solution from the 

 carbon disulphid, retaining the latter in the flask. Wash the flask, filter and con- 

 tents with cold water and transfer the carbon disulphid (containing the iodin in 

 solution) to a Nessler tube, using approximately 5 cc. of the carbon disulphid. In 

 washing the filter make the contents of the tube up to definite volume, usually 12-15 

 cc, and compare the color with that of other* tubes containing known amounts of 

 iodin dissolved in carbon disulphid. Prepare these standard tubes by treating 

 measured quantities of a solution of known potassium iodid content as described 

 above. Transfer the sample and standards, from which the iodin has been removed, 

 severally to small flasks. To the standards add definite measured quantities of a 

 bromid solution of known strength, and to each of the flasks containing sample and 

 standards add 5 cc. of purified carbon disulphid. Add the saturated chlorin water, 

 1 cc. at a time, shaking after each addition until all the bromin is set free. (Avoid 

 a large excess of chlorin, since a bromo-chlorid may be formed which spoils the color 

 reaction.) Filter off the water solution from the carbon disulphid through a mois- 

 tened filter, wash the contents of the filter 2 or 3 times with water, and then transfer 

 to a Nessler tube by means of about 1 cc. of carbon disulphid. Repeat this extrac- 

 tion of the filtrate twice, using 3 cc. of carbon disulphid each time. The combined 

 carbon disulphid extracts usually amount to 11.5-12 cc. Add enough carbon di- 

 sulphid to the tubes to bring them to a definite volume, usually 12-15 cc, and com- 

 pare the sample with the standards. In some cases when using this method near its 

 upper limit the amounts of carbon disulphid recommended do not extract all the 

 bromin. In these cases, make 1 or 2 extra e.xti'actions with carbon disulphid, trans- 

 fer the extracts to another tube, and compare the color with some of the lower 

 standards and add the readings thus obtained to the others. 



Results closely approximating the true values for iodin and bromin can be ob- 

 tained on most samples by omitting the extractions with alcohol given above and 

 by comparing the color of the carbon disulphid solutions directly in the extraction 

 flasks, thus shortening the method. 



ARSENIC-OFFICIAL. 

 57 REAGENTS. 



(a) Zinc, arsenic-free. 



(b) Sulphuric acid (1 to 5), arsenic-free. 



(C) Standard arsenious oxid solution. — Dissolve 0.0132 gram of pure arsenious 

 oxid in 100 cc. of water containing about 50 mg. of sodium carbonate. One cc. of 

 this solution is equivalent to 0.1 mg. of As. 



DETERMINATION. 



58 



Evaporate to dryness an aliquot of the alkaline filtrate under 52. Acidify with 

 the sulphuric acid and subject to the action of the zinc and the sulphuric acid in a 

 Marsh-Berzclius apparatus. Compare the mirror obtained with a mirror prepared 

 from an arsenious oxid solution of known strength. Calculate to the arsenic acid 



