88 
Journal of Agricultural Research 
Vol XXIV. No. » 
FORMUIyAS OF LEAD ARSENATES 
The composition and naming of lead arsenates is a matter that was at 
one time in great confusion. It has been held by most chemical workers 
in this field, however, that the lead arsenates on the market fall inta 
three classes: 
1. Triplumbic ortho arsenate (Pb3(As04)2), called by manufacturers 
“ortho, triplumbic, “normal,, ” or “neutral ’’ lead arsenate. 
2. Diplumbic ortho arsenate (PbHAsOJ, called “monoplumbic,’' 
diplumbic, “ acid arsenate, ” or simply lead arsenate. ’* 
3. Mixtures of these two compounds. 
Tartar and Robinson (4, 7) believe that the substances here designated 
as class i are really of much more complicated structure and suggest the 
formula 
2iPb3(AsO,)2.Pb(OH)2.ioH20. 
They also state that the diplumbic arsenate, PbHAs04, can be readily 
transformed into this basic compound by treating with ammonia and that 
by this treatment, a definite quantity of arsenic is dissolved and may be 
recovered in the ammonia solution. It thus is an accurate method for 
the determination of the quantity of diplumbic arsenate in class 3. 
Their formula for this basic compound was refuted in two papers ap¬ 
pearing at the same time by McDonnell and Smith {2) and by G. Ennis 
Smith (5), both suggesting the same formula, Pb50H(As04)3, and G. 
Ennis Smith suggesting the name lead hydroxy arsenate. By this treat¬ 
ment, with ammonia, 40 per cent of the arsenic present in the diplumbic 
goes into solution and the rest remains insoluble as the basic compound. 
All investigators agree that the diplumbic arsenate is the more active 
compound of the two and that where conditions are such as to cause any 
apprehension of damage from burning the normal compound should be 
preferred. On the other hand, the acid arsenate contains a greater per¬ 
centage of arsenic; hence the consumer can secure the required quantity 
of arsenic with a smaller number of pounds to pay freight and profits 
upon. The acid arsenate is sold in much greater quantity in many 
markets. 
G. Ennis Smith, in the paper already referred to (5), states that soap 
acts upon diplumbic arsenate in the way that Tartar and Robinson 
reported for ammonia (4, 7)—^transforms it into the more basic lead 
arsenate and dissolves some of its arsenic but sets free an equivalent 
quantity of the fatty acid, which is insoluble. Thus assuming that the 
end product is triplumbic arsenate, the reaction is: 
3PbHAs04+ 2NaCi7H35CO,= Pb 3 (As 04 ) 2 + 2 Ci 7 H 35 COOH-f- Na 2 HAs 04 . 
(insoluble) (insoluble) (soluble) 
In this case 33^^ per cent of the arsenic of the diplumbic arsenate is 
made soluble. In case we consider the end product to be lead hydroxy 
arsenate, the reaction is: 
5PbHAs04+4Na(;,H33C03+ H2O- Pb30H(As04)3+4C17H33COOH+ 
(insoluble) (insoluble) 
2 NajHAs 04 . 
(soluble) 
In this case 40 per cent of the arsenic of the diplumbic arsenate goes 
into solution. Under this theory there should be no reaction possible 
