132 



PROCEEDINGS OF SECTION B. 



Gyory ("J.C.S.," Abstr., II., 554). This volumetric estimation of 

 aisenious acid in connection with digestion was finally adopted as our 

 standard method, which is carried out as follows: — 10 cc. of the 

 dipping fluid are measured into a Kjeldahl flask, 15 cc. pure sulphuric 

 acid and a pinch of starch aie added, the whole heated to boiling- 

 over a naked flame, and after a few minutes digestion 10 grammes 

 of potassium sulphate are added and the digestion continued until the 

 liquid is colourless. After cooling the residue is diluted with 50 cc. 

 of distilled water, and boiled for about five minutes, or until all odour 

 of SO2 has disappeared. The hot liquid is titrated with N/50 

 potassium bromate solution, using two drops of methyl-orange 

 solution as an indicator. Towards the end of the titration, when the 

 liquid is nearly colourless another drop of the indicator is added, a 

 few more drops of the N/50 standard solution should discolour the 

 liquid. The reaction is exceedingly shai"p, and veiy much better than 

 with indigo-carmine as indicator. Contraiy to the experience of others 

 we found that the N/50 bromate solution keeps very well for any 

 length of time. 



A few of the numerous analyses according to different methods 

 are recorded on Table II. 



All our dipping fluids coming in to be analysed are at first tested 

 qualitatively for the presence of arseniate, by adding a few drops of 

 silver nitrate to a few cc. of the dipping fluid, previously clarified and 

 discoloured by tVie addition of animal charcoal. The slightest trace 

 of arseniate gives an orange tinge to the pure yellow precipitate of 

 silver arsenite. 



Another good test, not generally known, is lead acetate giving 

 white precipitates with arsenite and arseniate, the latter being in- 

 soluble in acetic acid. If arseniates are present; the amount is deter- 

 mined gravimetricaliy, by treating 200 cc. of the dipping fluid with 

 20 cc. strong HCl, warming slightly to coagulate the impurities, and 

 filtering ; 55 cc. of the filtrate, corresponding to 50 cc. dip, are used 

 for the analysis, ammonium chloride solution, ammonia, and magnesia 

 mixture are added in excess, and the arsenic acid determined as 

 magnesium pyro-arseniate. 



I have to acknowledge my indebtedness to my assistants, Messrs. 

 E. H. Gumey and N. C. Christensea, who carried out the experimental 

 and analytical work. 



TABLE I. 



Oxidation of Arsenite into Arseniate by Blowing Air through Solutions. 



Pure arsenite golntion, 1% 

 Same, rpsin in solution added... 

 Same, 1% catholic acid addfd... 

 Same, 1% Stockholm tar in sol. added 



Blowin.t; air through 

 for 3 days. 



AS5O3 changed 

 into As,0,. 



•7 

 14-7 



36-8 



Blowing air through 

 for 2 days more. 



% AS2O3 changed 

 into AS4O5. 



•3 



4-6 



1-6 



.53-9 



