LEAD. 
§§ 828, 829.] 
burnt sugar. Then, by the method of trial and error (well known in 
water analysis as “ Nesslerising ”), is determined the quantity of the 
dilute solution of lead PbT which must be added to the auxiliary solution 
in order that there may be equal colorations produced upon the addition 
of 2 drops of the solution of sodium sulphide PbT to both the primary 
and the auxiliary solution, after dilution to the 50-millilitre mark. In 
these circumstances each millilitre of the dilute solution of lead PbT 
required corresponds to 1 part per million of lead in the substance 
examined. The colorations may be viewed by light reflected from a 
horizontal white tile through the Nessler glasses inclined at an angle to 
the observer. 
§ 828. Tetra-methyl-diamino-phenyl-methane as a Test for Lead 
Peroxide. —The salt is the leuco-derivative of malachite green. It may be 
made by the reduction of malachite green and by various processes ; but 
the most convenient method of preparation is to boil 30 grms. of dimethyl- 
aniline with 25 c.c. of a 40 per cent, solution of formaldehyde, the 
solution being made up to 200 c.c. with water. The excess of dimethyl- 
aniline is got rid of by blowing steam through. On cooling, the tetra- 
methyl compound crystallises out, and should be dissolved in hot alcohol 
and recrystallised from that solvent. 
The test solution is made by dissolving 5 grms. of the crystals in 
100 c.c. of water, with the addition of 10 c.c. of acetic acid. The 
solution is very sensitive to light and air. Paper dipped in the solution 
and dried in the dark will readily “ print if exposed behind a negative, 
the unshaded parts of the paper becoming green. 
A fragment of lead binoxide or manganese binoxide strikes with the 
reagent a deep blue or blue-green colour—chlorine, chromic acid, and 
oxidising substances generally also reproduce, in more or less purity, 
malachite green (which, by the way, is not a true green, but a blue- 
green). Hence, if utilised as a test for lead binoxide or peroxide, the 
test must be used in such a way as to exclude possibility of confusion 
with other oxidising substances. This is fairly easy when the test is 
applied to the results of an electrolytic operation ; under such circum¬ 
stances the test is of great delicacy, a just visible speck of lead binoxide, 
or a just visible stain of the same substance deposited by the galvanic 
current on a platinum wire, giving at once a blue colour. 
§ 829. Electrolytic Method of separating Lead as Dioxide. —Lead 
can be separated, estimated, and identified in the most minute quantity 
by an electrolytic method under exact conditions of acidity, strength of 
current, and temperature. According to Paul Vortmann ( Annalen , 
1907), the results are good in presence of Cu, but incomplete if As, 
Mn, Se, Bi, or phosphoric acid should be present. 
The lead should be converted into nitrate ; the proportions of lead, 
acid, and water in 100 c.c. should be not more than 0-2 grm. lead as 
