METHODS OF ANALYSIS 21 



which has settled by this time, is now filtered out and the filtrate 

 at once placed on the hot plate to evaporate down for the deter- 

 mination of the magnesia. 



The calcium oxalate is washed carefully (the washings being 

 added to the Mg filtrate), and the lime determined by titration with 

 potassium permanganate. 



The filtrate containing the Mg is evaporated down to 25 cc, 

 cooled, 10 cc. sodium phosphate solution added, then 10 cc. am- 

 monia, is well stirred and allowed to stand (preferably over night), 

 filtered, washed with dilute ammonia, ignited, and weighed as 

 Mg 2 P 2 7 . 



ANTIMONY AND ARSENIC. 



Antimony and Arsenic. — These determinations are often called 

 for, especially in an analysis of dross or speiss. 



Take 1 gram of material, add dilute HC1 (50$) and evaporate 

 almost to dryness, take up in 5 to 10 cc. HC1 and boil. Add an ex- 

 cess of sodium sulphide and boil well. (In the presence of Cu avoid 

 ammonium sulphide.) Filter and re-treat the black sulphides. 

 Now combine the filtrates, acidify with HC1, boil and filter (avoid 

 an excess of HC1). Wash the sulphides off the filter with water, 

 add KC10 3 , and boil until free chlorine ceases to come off. Cool, 

 make strongly alkaline with ammonia, add magnesium chloride (in 

 case very little arsenic is present, let stand 5 to 6 hours, agitating), 

 filter and wash with strong ammonia. Dissolve the magnesium 

 arsenate in 50$ HC1, add KI and let stand, because the action is 

 slow. Titrate with hyposulphite solution as in copper. 



Acidify the filtrate from the magnesium arsenate with HC1, 

 warm and filter, wash the antimony into a flask, washing the filter 

 paper with HC1 containing a little potassium chlorate. Now add 

 in the flask a little more chlorate and boil until no chlorine is ap- 

 parent on iodide starch paper (be careful at this stage not to vol- 

 atilize the antimony), cool, add KI and 2 cc. of carbon bisulphide 

 and titrate with hyposulphite solution.* 



Tin. — When tin is present, run down with 50$ HC1, but avoid 

 dryness. Add yellow potassium sulphide, or sodium sulphide, and 

 boil well for an hour. Filter (Sb and Sn are in the filtrate). -j- Acid- 



*This method does not work when tin is present. 



t Volumetrically tin does not interfere with As and Sb. 



