METHODS OF ANALYSIS 35 



boils at 180° C. The zinc chloride may be made by adding suc- 

 cessively to 1 pound of pure stick zinc 500 cc. of water and 1250 

 cc. of HC1. After the zinc is in solution, bring to a boil and evap- 

 orate a little to bring the boiling point up to 180° C, this making 

 about 1100 cc. of solution. 



Standard Iodine Solution. — This is best made so that 1 cc. 

 equals .005 grams of As. Dissolve about 40 grams KI in a mini- 

 mum of water, and to this add 17 grams of I. After the iodine is 

 all dissolved make up to 1 litre with distilled water. 



To standardize, dissolve 300 mgs. of C.P. arsenious acid in a 

 little caustic soda or potash, dilute to about 200 cc, acidify slightly 

 with HC1, add about 2 grams NaHC0 3 some starch solution and 

 titrate to a permanent blue. The arsenious acid contains 75.76$ 

 arsenic. 



MODIFICATION. (THOMSON.) 



Treat the ore with HN0 3 , and, when violent action ceases, add 

 5 to 10 cc. HC1 and evaporate to dryness. (If the sample contains 

 organic matter, as in the case of flue dusts, treat with HN0 3 + 

 KC10 3 ). Take up with water and HC1, filter off the residue, neu- 

 tralize with ammonia, make acid with HC1 to redissolve the ppt.; 

 then add some sodium sulphide solution, and heat to drive off 

 excess of SO,. Ppt. with H 2 S, and proceed as in above method. 



