F, H. Storer-—Scheenbein’s Test for Nitrates, 183 
00005 grm. N,O, in 50 c. c. water, gave an immediate colora- 
tion on being mixed with iodo-zine-starch plus acid, after hav- 
ing been boiled for five minutes upon cadmium; with a solution 
containing 0°0002 grm. N,O, the blue coloration appeared 
about 5 minutes after the addition of the iodo-starch, and with 
a solution containing 0:0001 grm. N,O, the color began to ap- 
pear in about 8 minutes. The last named quantity indicates 
very nearly the limit of applicability of the test, since the de- 
gree of coloration derivable from an amount of the nitrate any 
smaller than this, could hardly be distinguished from that due 
to the peroxide of hydrogen that is obtained on boiling pure 
water upon cadmium or zinc. It is true that the coloration 
caused by the products of the reduction of a nitrate generally 
appears rather more speedily than the coloration produced by 
peroxide of hydrogen, but since the reaction of the peroxide 
often begins to show ten or fifteen minutes after the addition 
of the iodo-zine-starch and acid, and sometimes éven sooner, no 
dependence can be placed upon mere rapidity in the appearance 
of the coloration, as a means of distinguishing the nitrate from 
the peroxide. In case the mixtures are left to stand over night, 
or for a number of hours, after the iodo-starch has been added, 
this seeming advantage in favor of the nitrate solutions disap- 
= for after long standing the coloration due to peroxide of 
ydrogen is often as deep as that obtained from 0:00005 grm. 
N,0,, and the difference between this tint and that obtained 
from 0-001 is by no means large enough to permit of distin- 
Suishing the one from the other with any certainty. 
ults very different from the foregoing were obtained 
When 50 cc. of the pure nitrate solution, mixed directly with 
two drops of the dilute acid and 2 c.c. of the iodo-zine-starch 
Solution, were left to stand in contact with a rod of zine, accord- 
he ‘o the method employed by Carius* On proceeding in 
this way, a solution containing 0-01 grm. of N,O, (=0°01872 
tely when the zine was added; and a solution containing 
grm. 
blue coloration at the lower end of the zinc rod in a few 
doe While in a solution containing 0-002 grm. N20, 
=000874 orm, KNO,) no coloration could be perceived 
even after the lapse of two hours, though the liquid was 
con at frequent intervals. On repeating this trial with 
whi N rely short time. Trials similar to the above, in 
mMalgamated zinc was used instead of simple zine, gave 
* Annalen der Chemie, 1874, clxxiy, 14.. 
