TESTS FOR LEAD. 
91 
coloured yellow by the iodide of potassium; but on shaking 
the liquor, the iodide of lead entirely dissolved, forming a 
colourless solution. A similar effect occurred in solutions 
of acetate of soda, and tartrate of potassa containing lead, 
varying to some extent with their degree of concentration, 
and it is probable it occurs with most of the salts of these 
alkalis. When to the liquor containing the precipitated 
iodide of lead, sulphate of soda or carbonate of soda or 
potassa is added, the yellow colour disappears, leaving a 
white precipitate ; the iodide of lead being replaced by sul- 
phate or carbonate of lead. It is evident therefore that we 
cannot rely upon iodide of potassium as a test for the pre- 
sence of lead in a salt of potassa or soda. We do not find 
that their presence effects the action of sulphuretted hydro- 
gen if the proper precautions are observed, and so far as 
we have examined, this gas is the most delicate test for 
this metal. 
From economical reasons the acetates of potassa and 
soda, are frequently made by mixing solutions of acetate 
of lead and carbonate of potassa or soda, carbonate of 
lead precepitating and acetate of the alkali remaining in 
solution ; but this solution constantly retains a trace of lead, 
even if the alkaline salt be in excess. The lead may be 
entirely separated by passing sulphuretted hydrogen through 
the solution, until no further change is produced, and after- 
wards boiling it for some time in order to drive off the ex- 
cess of the gas, and facilitate the precipitation of the sul- 
phuret of lead. It will be necessary after boiling to add 
sufficient acetic acid to neutralize the solution if, as is gen- 
erally the case, it has become alkaline. 
