2o8 THE CONNECTICUT POMOLOGICAL SOCIETY. 



The Chemistry of Sulphur-Arsenical Injury and Its 

 Prevention. 

 The so-called practical facts have largely been stated 

 above. The chemical basis for some of the preceding 

 statements is very instructive, however, and not particu- 

 larly difficult to comprehend. Hence, we have worked it 

 out for presentation here. The chemistry of the injury^ 

 or of the production of arsenic acid from either pyro or 

 acid arsenates, is shown in the two following reactions : 



Pyro-arsenate+^^'ater=Acid Arsenate 

 (1) FbaAssO^ + H„0 = 2 PbHAsO.i 



„ Alkalin Ortho-arsenate+Arsenic acid 



(2) ^PbHAsU4+Soi^^tiQj,g= pb3(As04)o + H3ASO4 



As noted above, this arsenic acid is soluble and will 

 injure vegetation severely unless immediately re-precipi- 

 tated into harmless forms by satisfactory bases. Of these, 

 the most satisfactory is doubtless lead itself, and if an ex- 

 cess of it is maintained in the solution, any arsenic acid 

 that may be formed would at once be converted into the 

 desirable ortho-arsenate, so long as alkalin conditions were 

 maintained. This reaction, using lead acetate (or sugar 

 of lead) as the carrier of soluble lead, is as follows: 



Arsenic acid -{- Lead acetate^ortho-arsenate-|-acetic acid 

 (3) 2H3As04+3Pb(CoH,jOo),= Vh,{hsO,)._-^6nC^Yi^0.2 



The acetic acid, in turn, attacks the lime-sulphur in 

 solution, forming hydrogen sulphide, free sulphur and cal- 

 cium acetate, as indicated by the following reaction : 



(4) 2 HC2H302+CaS4=:H,S-f 3 S+CaCC.HgOJo- 



The practical bearing of all this appears in the fact 

 that the danger of injury from mixed commercial lead 

 arsenates in alkalin spray solutions can at least be greatly 

 reduced by the simple addition of a small amount of solu- 

 ble lead, preferably in the form of lead acetate, before add- 

 ing them to the fungicidal solutions. 



