Agriculture Reference
In-Depth Information
All nonaqueous solvents may contain traces of water, even when they are
said to be or are sold as anhydrous. If truly anhydrous solvents are needed,
additional drying (see Section 7.3) may be needed.
7.2.1.1
Water
Water is a common soil extracting solvent. Pure water for use in extraction or
as the solvent for extracting solutions would seem to be a simple matter, and
in the past it has been much simpler than today. This is due in part to increas-
ing demands on the purity of the water used and partly on changes in tech-
nology used in water purification. Distilled, doubly distilled, and sometimes
triply distilled water might have been called for in the past or in older litera-
ture and procedures. Today there are many other types of purified water, and
some extractions may call for water purified by a specific procedure.
In some situations it may be desirable to combine one or more purification
procedures in order to obtain water of sufficient purity for the extraction and
analysis to be undertaken. It is common to combine deionization with reverse
osmosis to obtain what is commonly referred to as
deionized water
or
DI water
.
In some cases deionization or filtration may be desirable before distillation is
carried out. Or all three purification methods may be combined to produce
highly purified water.
Highly purified water must be kept under conditions that maintain its purity.
Clean containers are usually constructed of glass or plastic; plastic is the
preferred container material today because highly purified water will dissolve
small amounts of glass. Also once in the container, the water must be protected
from contamination with foreign material and gases. Carbon dioxide, from the
atmosphere, will dissolve in water and form bicarbonate and carbonic acid.
Highly purified water can be protected from carbon dioxide by storing it under
a nitrogen atmosphere or by filling the container completely and having
displacing air pass through an upside-down drying tube filled with a layer
of ascarite between two layers of indicating drieate or for short time periods
sodium hydroxide as illustrated in Figure 7.1. Although this will protect the
water from carbon dioxide, it will not prevent the water from absorbing other
gaseous atmospheric contaminants. If the absorption of other potential con-
taminates is of particular concern, drying tubes containing other absorbents
arranged in tandem can be added. All drying tubes must be changed as soon
as deterioration is seen. Water must always be checked to verify its purity.
A way to avoid contamination problems occurring during storage is to
prepare purified water fresh just before use. However, even in this case care
must be taken to ascertain the purity of the water being produced [19].
7.2.1.2
Organic Solvents
Common laboratory solvents are most frequently used as they come from the
supplier and will be supplied with some information as to their purity. However,
the purity statements provided by the manufacturer or supplier may not provide
