F. Hi. Storer—Schenbein’s Test for Nitrates. 177 
as extraordinary and hardly credible. It was neither consistent 
with Schoenbein's statement as to the delicacy of the test nor 
with the reputation which the test had acquired. I have thus 
n led to examine the matter somewhat attentively and to 
subject the test anew to critical study. It appears from this 
examination that the lack of delicacy observed by Carius was 
due to the kind of manipulation employed -him, and_ that 
while his statement is doubtless literally correct it fails to con- 
vey a just idea of the much higher degree of delicacy which is 
readily obtainable by applying the test in a somewhat different 
wa 
lwo methods of using the test were described by Schoen- 
bein,* viz: Ist, To add dilute sulphuric acid and iodo-starch 
paste directly to the nitrate solution and to stir the mixture 
with a zine rod; or, 2d, and better, as we must infer from 
Scheenbein’s statement, to reduce the neutral solution of the 
nitrate in the first place, by means of zine or cadmium, there- 
after to acidulate it with dilute sulphuric acid, and finally to 
add the iodo-starch paste. Both of these modifications have 
come into general use, but the second has been applied perhaps 
even more frequently than the first in cases where very sm 
amounts of nitrates were to be sought for. It is in fact more 
delicate than the first method. Carius, however, in the experi- 
thents above referred to, employed the first modification and 
not the second. 
or cadmium, as if to test it for a nitrate, will react upon iodo- 
Starch precisely as if a trace of some nitrate had been dissolved 
in the water, 
The explanation of this behavior is not far to seek. The 
eee of the iodo-starch is caused by peroxide of hydrogen 
Which has been formed in the water by the action of the metal, 
according to the familiar experiment of Schcenbeint in which 
Peroxide of hydrogen is prepared by shaking zinc-amalgam in 
Water and aint 
. a . 
Pp. ay thoy example, his paper in Fresenius’s Zeitschrift analyt. Chemie, 1862, i, 
Ms, 16 
R peendorit's Annalen, 1861, exii, 288. 
- that tnt has himself shows (ctinant far prakt. Chemie, 1861, Ixxxiv, 206) 
aqueous soluti of hydrogen is formed simultaneously with a nitrite, when the 
the {lution of a nitrate is treated with zinc or cadmium, | to 
looked the the i test, but he seems to have completely over- 
of his “sedi Aroha? hmong of the peroxide yee we preclude ra ain only 
. rt ; cases ion 
® small quantity ite where the solutio: examined contained 
a 
