Geology Reference
In-Depth Information
The first techniques utilised for the determination of salinity, involving
the gravimetric analysis of salt left after evaporating seawater to dryness,
were fraught with difficulties. Variable amounts of water of crystallisation
might be retained. Some salts, such as MgCl
2
, can decompose leaving
residues of uncertain composition. Other constituents, especially organic
material, might be volatilised or oxidised. Overall, such methods led to
considerable inconsistencies and inaccuracies.
The second set of procedures for salinity measurement made use of the
observation from the Challenger expedition of 1872-1876 that sea-salt
composition was apparently invariant. Hence, the total salt content
could be calculated from any individual constituent, such as Cl
that
could be readily determined by titration with Ag
1
. At the turn of the
century, Knudsen defined salinity to be the weight in grams of dissolved
inorganic matter contained in 1 kg of seawater, after bromides and
iodides were replaced by an equivalent amount of chloride and carbon-
ate was converted to oxide. Clearly from the adopted definition, the
analytical technique was not specific to Cl
and so the term chlorinity
was introduced. Chlorinity (Cl%) is the chloride concentration in sea-
water, expressed as g kg
1
, as measured by Ag
1
titration (i.e., ignoring
other halide contributions by assuming Cl
to be the only reactant). The
relationship of interest was that between S% and Cl%, given as
S%
¼
1.805Cl%
þ
0.030
As a calibrant solution for the AgNO
3
titrant, Standard Seawater was
prepared that had certified values for both chlorinity and salinity.
Unfortunately, the above salinity-chlorinity relationship was derived
from only nine seawater samples that were somewhat atypical. It has
since been redefined using a much larger set of samples representative of
oceanic waters to become
S%
¼
1.80655Cl%
The third category of salinity methodologies was based on conductome-
try, as the conductivity of a solution is proportional to the total salt
content. Standard Seawater, now also certified with respect to conduc-
tivity, provides the appropriate calibrant solution. The conductivity of a
sample is measured relative to the standard and converted to salinity in
practical salinity units (psu). Note that although psu has replaced the
outmoded %, usually units are ignored altogether in modern usage. These
techniques continue to be the most widely used methods because con-
ductivity measurements can provide salinity values with a precision of
0.001 psu. Highly precise determinations require temperature control of
samples and standards to within
0.001 1C. Application of a non-specific
technique like conductometry relies upon the assumption that the sea-salt
