140 McDonnell and Smith — Lead-Chlor Arsenate. 



It was found that a chlor compound was also formed (slowly 

 in the cold, and very rapidly on warming) when a mass of 

 dilead arsenate was digested with ammonium, sodium or potas- 

 sium chloride. Sodium chloride seemed to he particularly 

 effectiye in forming a product of nearly theoretical composi- 

 tion. Two samples resulting from the digestion of dilead 

 arsenate for 5 minutes at hoiling temperature with (a) 2 per 

 cent and (b) 10 per cent sodium cliloride* solutions analyzed as 

 follows : 



n, b 



Lead oxide, PhO 74-75$ 74-56$ 



Arsenic pentoxide, As Q O ri 23-25 23-39 



Chlorine, CI ... 1 2-39 2-39 '. 



100-39 100-34 



O, equivalent to CI 0-54 0*54 



9980 



These transformations are accompanied by the solution of 40 

 per cent of the total arsenic in the dilead arsenate,f the super- 

 natant liquid showing a distinct acid reaction toward methyl 

 orange. The reaction may be expressed by the following equa- 

 tion : 



5PbHAs0 4 + NaCl =Pb 4 (PbCl)(As0 4 ) 3 + NaH,As0 4 + H 3 Ab0 4 



A particularly interesting reaction is that between dilead 

 arsenate and a solution of lead chloride, which substances were 

 found to react readily on warming, giving a solution contain- 

 ing free hydrochloric acid and a residue corresponding closely 

 to theory for mimetite. This may be expressed by the follow- 

 ing reaction : 



3PbHAs0 4 + 2PbCl 2 ^z> Pb 4 (PbCl)(As0 4 ) a + 3HC1. 



In all of these reactions we have the rather strange phenome- 

 non of the transformation of one lead arsenate into a more 

 basic one with liberation of a strong acicl. 



* Dilead arsenate boiled for about 5 bours witb solutions of potassium 

 bromide, potassium iodide or sodium fluoride (35 grams to 1 1.) gave prod- 

 ucts similar in composition. Tbe bromine compound contained 5'79 per 

 cent Br (theory 5 -22) and 21-92 per cent As 2 O s (theory 22'51) ; the iodine 

 compound 23-57 per cent As 2 5 (theory 21-84) ; and the fluorine compound 

 23-71 per cent As 2 6 (theory 23 - 44). The reaction with KI apparently did 

 not proceed as rapidly as the others, and did not go to completion during 

 the time of the digestion. 



f It was noted by Headden (Colorado Agricultural Experiment Station 

 Bulletins, 131, 22, 1908; 157, 30, 1910), and by Haywood and McDonnell 

 (Bur. Chem. Bull. No. 131, 46, 1910), that water containing sodium chloride 

 extracts a greater amount of arsenic from commercial lead arsenates than 

 does pure water. This also explains why greater injury to foliage occurs 

 when lead arsenate is applied with water containing chlorides in solution, 

 as has been shown by the latter authors (ibid., p. 49). 



