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Chlorinity, Salinity and Density of Natural Sea Water
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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:

  1. Trace elements and other mineral nutrients
  2. 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
 

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