Physical Properties
(Salinity)
Introduction
• Sea water contains majority of known elements and is tasted as salty.
• Salts and other mineral elements have been washed away from the land by rain over billions of years.
• The total amount of salts in the oceans is estimated to be about 5 x 1019 kg.
Dissolved constituents of sea water
Definitions of Salinity
• In 1899, the International Council for the Exploration of the Sea appointed a Commission, headed by Prof. M. Knudsen, to investigate the problems of determining salinity.
• According to the commission, salinity is defined as "the total amount of solid material in grammes contained in one kilogramme of sea water, when all the carbonate has been converted to oxide, all the bromine and iodine replaced by chlorine and all the organic material oxidized".
• The unit of salinity was PPT (o/oo). • Approximately, salinity of open ocean water is 35.
Practical Salinity Unit (PSU)*
• In 1978, conductivity was used to estimate salinity.
• Salinity is estimated from the ratio of conductivity of sea water at standard temperature (15oC) and pressure (1atm) with that of KCl. The unit for salinity is PSU.
Factors Affecting Salinity
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The factors responsible for increase of salinity are: (1)Evaporation
(2)Ice formation
(3)Advection of more saline water
(4)Mixing with more saline water
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The factors that are responsible for decrease of salinity are: (1)Precipitation
(2)Melting of ice
(3)Advection (of less saline water)
(4)Mixing (with less saline water) (5)Inflow of fresh water from land.
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Over large areas in the oceans, changes in surface salinity depends mainly upon changes in the difference between evaporation and precipitation.
Measurement of Salinity
Methods of estimation
(1)Chemical titration method
(2)Electrical conductivity method
(3)Density method and
(4)Refractive index method
(1) Chemical Titration Method
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Prof. Martin Knudsen developed a volumetric method for the estimation of chloride content. He suggested that in view of the constancy of composition of sea water, salinity can be estimated in very good approximation from the chloride content. In this method, sea water is taken in a beaker and is titrated with silver nitrate solution taken in the burette. A little quantity of potassium chromates solution is added to it. During the initial stage of titration, a white deposit of silver chloride precipitate forms. when all the chloride ions that are present in the sea water sample have precipitated as silver chloride, a red colour deposit of silver chromate is formed.
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From the amount of silver nitrate run down until the change of colour has taken place, and from the amount of sea water used, the chloride content of sea water sample is calculated.
The salinity of sea water sample is then determined using the empirical relationship (Knudsen, 1901) as:
Salinity = 0.03 + 1.805 x Chlorinity
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Later (since 1960) the relationship was modified as
Salinity = 1.80655 x Chlorinity
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The main disadvantage with the titration method is that it is not very convenient to use on board the ship and it requires more time for titration.
(2) Electrical Conductivity Method
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In 1902 itself, Knudsen realized that electrical conductivity would provide an alternative method of measuring salinity.
Accurate measurement of conductivity had problems because of the fact that conductivity is a function of not only salinity but also temperature.
Hence it is necessary to remove the effect of temperature on conductivity.
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The first conductivity salinometer for sea water was developed in 1930.
The conductivity measuring technique was capable of an accuracy equal to the Knudsen's titration method, and that it could be used on a ship at sea.
of electrical conductivity is made by means of galvanic method. In this method the resistance between two electrodes is estimated by a Wheatstone bridge.
Wheatstone bridge circuit.
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The resistance
between two
electrodes is
estimated by a
Wheatstone bridge.
Sea water
resistance is
estimated as
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The potentiometer resistance rp is adjusted in such a way that the current passing through the galvanometer (G) becomes zero.
Salinometers (Autosal)*
CTD
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CTD is an acronym for Conductivity, Temperature and Depth, an instrument used to measure subsurface temperature and salinity.
This device actually measures pressure (not depth) and then calculate the depth.
Oceanographers use the Conductivity Measurement, in conjunction with temperature and Pressure to calculate the Salinity of the ocean. This device is one of the most common instruments used by oceanographers.
History:
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1948: electronic profiling system for temperature and salinity was described by Jacobsen.
1950s: A system called the STD (salinity­temp­
erature­depth) profiler was developed by Hamon and Brown in the mid­1950s. 1974: Conductivity measurement led to the introduction of the conductivity­temperature­depth (CTD) profiling system (Brown, 1974).
Measurement:
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CTD is used as a profiler. As it descends (or ascends), the sensors attached records water properties, allowing researchers to understand the vertical structure of water. CTD can be fixed at a particular location/depth to record changes with time.
CTD may be pulled through the water to identify horizontal variations.
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Resistance thermometers are widely used on continuous profilers designed to replace the earlier hydrographic profiles collected using a series of sampling bottles.
Since density also depends on salinity, electronic sensors were developed to measure salinity in situ and were incorporated into the profiling system.
(2) Density Method
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The salinity can also be determined by measuring the density of a sea water sample. The density of a sea water sample can be measured by means of an hydrometer. Since this density determination will be made at room temperature, it will be different from the density of the sea water sample at which it was collected.
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From the density thus determined, the density of the sample is computed from the chart provided with the hydrometer. From this density value, the salinity of the water sample can be found using the Knudsen's Hydrographical Tables.
(3) Refractive Index Method
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To determine salinity through measurement of optical refractive index, the immersion refractometer designed by Pulfrich is used in a thermostatized sea water sample.
Salinity Distribution in the Ocean
General Aspects
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In the open ocean, surface salinity varies generally between 33
and 37.
Lower salinity values are found in the coastal areas where large
rivers enter the oceans.
Higher values of salinity are observed in the ocean regions where
evaporation is high.
Among the oceans, Atlantic is the most saline and Pacific is the
least saline.
The average surface salinity of the oceans as a whole is more in
the southern hemisphere than in the northern hemisphere
High salinities are found in areas such as Persian Gulf, Red sea,
mediterranian sea etc.
Latitudinal variation of surface salinity
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Salinity has subtropical maxima at 20-25° N and
south due to the excess evaporation over
precipitation.
A salinity minimum occurs just north of the
equator at 5°N due to excess precipitation.
Salinity decreases towards poles due to less
evaporation, higher precipitation and melting of
ice.
Annual variation of surface salinity
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The annual variation of surface salinity follows
closely the annual variation in net evaporation
over the oceans.
In general, the annual variation of surface salinity
in the open ocean is usually small, always less
than 0.5.
In Bay of Bengal, the large annual variation of
precipitation and the resultant inflow of fresh
water through large rivers is responsible for the
large annual variation of surface salinity.
Seasonal variations of surface salinity
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Depends mainly upon the changes of
evaporation and precipitation from season to
season over the oceans.
The seasonal variations of surface salinity in
the open ocean are generally weak, not more
than 0.5.
The seasonal variations of surface salinity are
more in coastal waters where there is lot of
fresh water inflow through rivers.
Vertical distribution of salinity
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The vertical distribution of salinity in the ocean is
complex compared to the vertical variation of sea
water temperature.
This is because the density which determines the
stability of the water body depends more on
temperature than on salinity in the open ocean,
except in the polar seas.
Hence, it is possible to have either high or low
salinity in the top warmer layers.
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There is a uniform
layer of salinity from
the surface up to a
few tens of metres
depth due to mixing.
Below this layer,
salinity increases with
depth, called as
'halocline'
Salinity in enclosed seas*
Dead Sea
Black Sea