_Procedure._—Place in a standard Nessler tube 50 cc. of the sample,
decolorized if necessary with nitrite-free aluminium hydroxide (see p.
42) or a smaller amount diluted to 50 cc. At the same time prepare in
Nessler tubes a set of standards, by diluting to 50 cc. with
nitrite-free water, various amounts of the standard nitrite solution.
The following amounts of standard solution are suggested: 0.0, 0.1, 0.2,
0.4, 0.7, 1.0, 1.4, 1.7, 2.0, and 2.5 cc. Add 1 cc. of the sulfanilic
acid solution and 1 cc. of the α-naphthylamine acetate or hydrochloride
solution to the sample and to each standard. Mix thoroughly and allow to
stand 10 minutes; then compare the sample with the standards. Do not
allow the sample to stand more than one-half hour before making the
comparison. If the color of the sample is deeper than that of the
highest standard repeat the test on a diluted sample. If 50 cc. of the
sample is used 0.01 times the number of cc. of the standard matched
equals parts per million of nitrite nitrogen. Satisfactory results can
be obtained by using either hydrochloric or acetic acid in preparing the
test solutions, but the speed of the reaction is more rapid if acetic
acid is used.[112]
Permanent standards may be prepared by matching the nitrite standards
with dilutions of the fuchsine solution. Fuchsine standards have been
found to be sufficiently accurate for waters high in nitrite and for
sewage. The standards should be checked once a month and kept out of
bright sunlight.