Chlorinity, salinity and
density are the basic physico-chemical properties of seawater, depending primarily upon the
concentration of the following essential constituents of seawater:
Version 2012-III
Chlorinity, Salinity and Density of Natural Sea Water
Chlorinity, salinity and density are the basic physico-chemical properties of seawater, depending primarily
upon the concentration of the following essential constituents of seawater:
Cl-
Na+
Mg2+
SO42-
Ca2+
K+
These ions form 99,8% of the total mass of all seawater solutes (sodium
chloride alone 86%). All stated quantities are interrelated and can be empirically
derived one from another at a given temperature (and pressure). The
issue in reefkeeping is what quantity should be measured as primary parameter,
why, how to determine it and what can be derived from the results.
Salinity
The most common measure of seawater “saltiness” is defined as follows:
S(‰) = (total mass of dissolved inorganic
ions (g) / I kg of seawater ) * 1000
The salinity of 99% of seawater in world oceans is in the range of 33-37‰, on average 35‰, which corresponds to a 3,5% solution. The salinity of seawater can be determined most accurately ( +- 0,001‰) by using of
high-quality induction salinometer, which measures the seawater conductivity.
Reefkeeping Specifics
Measuring salinity using salinometer assumes that the sample of the
seawater contains only negligible amounts of those substances, which
affect the water conductivity:
Trace elements and other mineral nutrients
Dissolved organic matter (DOM)
This assumption may not be met in many cases of water coming from reef
tanks.
Density
The density is defined as follows:
ρ (g/cm3) = mass of 1 cm3 of seawater ( = mass
(kg) of one liter of seawater)
The density of seawater dependents significantly on temperature and pressure
and thus is not suitable as a primary parameter – it is mostly derived
from salinity or chlorinity, which are easier to measure with higher accuracy.
Chlorinity
Chlorinity is defined as follows:
The total mass (g) of chloride (Cl-), bromide (Br-)
and iodide (I-) ions converted to chlorides,
which can be precipitated
from 1 kg of seawater by the silver cation (Ag+.)
According to this definition chlorinity is expressed in g/kg, yet can
also be expressed as g/l at a given temperature (and pressure).
Why to measure “saltiness” of seawater
The basic reason is the control of correct concentration of the seawater
“fundament” – sodium chloride (NaCl). Even though this concentration is
constant in natural seawater, the situation is different in reef tanks
and the NaCl concentration can be significantly influenced by:
Water exchange – concentration of NaCl in marine salt used for its
make-up
certain risk of deviation in the NaCl concentration in both
directions
Application of Balling Method
certain risk of deviation
in the NaCl concentration in both directions
Application of chloride-based
additives (mainly calcium and magnesium)
definite increase in NaCl
concentration in the long-run
Selection of primary parameter for reefkeeping
Disadvantages
Advantages
Salinity
Requires investment in a costly apparatus
Maximum accuracy
Risk of erroneous results at high concentrations of trace
elements, additional mineral nutrients and dissolved organic matter (DOM)
in the sample
Density
Significant dependence on temperature (and pressure)
Approximate density can be determined very quickly
Maximum accuracy of measurements in common reefkeeping
practice is not adequate.
This parameter is “most remote” from the means of determination
of correct composition of seawater - required density can be in principle
mimicked by infinite number of combinations of ions' concentration
Chlorinity
Requires a careful performance of the titration
test
High accuracy
Measurement results depend primarily on concentration of chloride
ion, e.i. NaCl
How to measure chlorinity and make use of the results
Chlorinity can be determined with high accuracy by titration of
seawater sample with the solution of silver nitrate (AgNO3) after masking
of interfering substances, which may be present in the sample. As already been stated above, it is possible to correlate empirically
values of salinity, density and chlorinity. In reefkeeping practice the
most important relationship is between chlorinity and density, see table
for temperature 25°C:
Salinity
Density
Chlorinity
‰
kg/l
g/l
42,50
1,029
22,86
41,00
1,028
22,08
40,00
1,027
21,56
38,50
1,026
20,77
37,00
1,025
19,98
36,00
1,024
19,46
34,50
1,023
18,67
33,00
1,022
17,88
32,00
1,021
17,35
30,50
1,020
16,55
In case of seawater, whose composition corresponds to natural seawater
(especially with respect to NaCl), its density at 25°C can be directly
found in the table from the determined value of chlorinity - chlorinity
determination serves in this case for direct reading of density (salinity).
In case of control measurement, this is the way how to proceed:
Measure as accurately as possible the density of the sample
(preferably
using a high-quality refractometer) at 25°C.
The result gives the
real density of the sample
Determine the chlorinity of the sample by
a test at 25°C.
The theoretical density of the sample is then found
in the table.
In principle, 2 situations may occur, indicating some
problems:
The real and theoretical density of the sample differ by
10% (2 lines of the table) and more
The real and theoretical density
of the sample do not differ, but they are considerably higher or
lower (again by +/- 10%) than the expected figure, corresponding to e.g.
dosing of marine salt in the seawater make-up or to parameters
of the application of Balling Method
CS Reefkeeping Concept
CS TEST CHLORINITY
Chlorides Profi Plus
Marine Minerals
Chlorinity = >>
Chlorinity = <<
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