Introduction to Soil Chemistry

122 titrimetric measurement
6.6. NITRITE AND NITRATE
Both nitrite and nitrate are highly mobile in soil and easy to extract. However,
it is also possible to reduce each individually to ammonia and subsequently
steam-distill the ammonia, capturing and titrating it as described above for
ammonia. If this procedure is to be followed, naturally occurring ammonia in
soil must first be determined as described above. After this step, a reducing
agent is placed in the flask and nitrite and nitrate reduced to ammonia. The
soil is then rendered basic again and the ammonia steam-distilled and titrated.
If both nitrate and nitrite are reduced at the same time, the combined amount
of both is obtained. At this point selective reduction of either nitrate or nitrite
with subsequent distillation and titration will allow for calculation of the
amounts of all three forms of nitrogen in the soil sample.
The determination of nitrite and nitrite in soil can be accomplished by
reactions carried out before digestion or steam distillation to oxidize nitrite to
nitrate and to reduce nitrate to ammonia. Alternately, nitrite can be reacted
with an organic compound, typically salicylic acid and subsequently digested
in a typical kjeldahl procedure. This procedure starts with an organic molecule
that reacts with nitrite to form a nitro compound that is subsequently reduced
to an amine. The amine is then subject to digestion just as with organic matter
in soil. In this way the total nitrogen content in soil can be determined. Also,
when this procedure is combined with those described above, the amounts of
the various forms of nitrogen in soil—organic, ammonia, nitrite, and nitrate—
can be determined. This is thus an extremely powerful method for elucidating
the nitrogen status of soils. Figure 6.7 gives a flow diagram for determining
nitrite and nitrate in soil using a kjeldahl apparatus.
The procedures described above determine soil nitrogen that is often taken
to be the total nitrogen in soil. Because very few other organic or inorganic
nitrogen compounds are commonly found in soil, there is little call for additional
analytical procedures. However, there are some exceptions to this. It
may be necessary to determine the gaseous oxides of nitrogen, formed during
denitrification. This is typically accomplished using gas chromatography (see
Chapter 9). Also, using simple steam distillation ammonia trapped in clay
structures will not be determined. Such determination requires complete
destruction of soil minerals and the subsequently released ammonia
determined.
Although kjeldahl procedures are capable of providing information about
all the common nitrogen components in soil, it is a time-, labor-, equipment-,
and reagent-intensive procedure. Typically kjeldahl equipment, namely, a
digestion–steam distillation apparatus, can accommodate 6, 12, or more
digestion tubes or flasks and distillation, and titration can be done quickly;
however, it will take hours to do any significant number of samples. For 15 N
work, additional steps, time, and money are required to convert ammonia to
nitrogen gas and determination of 15 N by mass spectrometry (see Chapter 8)
[6].