Salinity refers to the concentration of dissolved salts in water

Salinity refers to the concentration of dissolved salts in water. At the suggestion of Forch, Sorensen and Knudsen the salinity has been defined as the total amount of solid material in programmes contained in 1 kg of sea-water.

When all the bromine and iodine have been replaced by the equivalent amount of chlorine, all the carbonate converted to oxide and all organic matter has been completely oxidized.

It is noteworthy that such a definition of salinity facilitates its determination with greater accuracy. This technical definition can also serve as a basis for the investigation of the relationship between any single component and the total salinity.

However, with this definition, it appears that salinity must be a very complex property and very difficult to measure.

The degree of salinity is generally expressed in terms of the number of parts of salt per thousand parts of sea-water; if in 1000 grammes of water there are 35 grammes of dissolved salts, the salinity is said to be 35%o.

Considering the average salinity of the world ocean, 34.7%o by weight, a 1000 gramme sample of the sea-water would contain 34.7 grammes of dissolved solids. Even though the average salinity of the ocean-water is almost constant, numerous salinity differences are recorded in horizontal as well as vertical direction.

However, it is interesting to observe that if samples of water from different parts of the oceans and seas are chemically analysed, the proportions of different ions (major salts) remain virtually constant, even though the total salinity varies substantially.

In other words, the major component ions are distributed in ocean water in relatively constant proportions to one another, but the proportion of water to dissolved salts may vary form place to place within the ocean.

Water makes up, on the average 96.5% of the ocean’s mass, so this substance (water) determines most of the physical properties observed in ocean water.

When techniques are developed for making such measurements, almost all of the known elements will be found to be dissolved in ocean water. However, 99% of the dissolved solids found in ocean water comprise six elements. These major constituents are chlorine, sodium, sulfur (S0₄), calcium and potassium.

Since water in the ocean is in constant motion, it is so very well mixed and the proportion of the major constituents is virtually constant, this condition makes the measurement of salinity relatively simple.

Under these conditions, in order to determine the salinity of a given water sample, we have to measure the concentration of only one of the major salts. It is the chlorine (CO that occurs in the greatest abundance.

Chlorinity is the proportion of the weight of a given sample of water that is the direct result of the presence of chloride ion. It is usually expressed in terms of grammes per kilogram of ocean water (g/kg) or parts per thousand (%o).

It is certain that the chloride ion accounts for 55% of the dissolved solids in any sample of ocean water. Thus, by measuring its concentration, the total salinity in parts per thousand can be determined by the following equation:

Salinity (%o) = 1.80655 x Chlorinity (%o)

Average salinity of the oceans 34.7%o = 1.80655 x Average chlorinity of the oceans 19.2%

From 1902 to 1975, the arduous task of determining the salinity of sea – water samples was entrusted to the Hydrographic Laboratory atCopenhagen, Denmark. But in 1975 this duty was handed over to the Institute of Oceanographic Services located in Wormly, England.

Standard sea water comprises ocean water analyzed for chloride ion content to the nearest ten-thousand of a part per thousand. It is sealed in ampoules and labelled. Then it is sent to various laboratories throughout the world.

Now, the chlorinity of water samples measured in other parts of the world can be compared, if the titration solution or instrumentation is calibrated with the standard water sample. However, the chemical determination of the chloride content is very time-consuming and must be done with utmost care.

Now, with the rapid strides made in the sphere of instrumentation in the field of oceanography, salinity is now most commonly determined by measuring the electrical conductivity of ocean water with the salinometer. It is a modern conductance-measuring instrument. Now, it has been made possible to determine salinity to 0.01 parts per thousand.

A large-scale oceanographic research project under way is the Geochemical Ocean Sections (GEOSECS) programme. This project has undertaken a huge systematic programme of understanding the circulation patterns and mixing processes in the oceans. Ocean-water samples have been taken from different depths of all the major ocean basins.

The present sampling system makes use of electronic sensors that telemeter temperature, salinity, pressure etc. This programme not only provides a much deeper understanding of the physical processes, but will supply a lot of information regarding the biological cycles in the ocean.