Methods of Determination of Salinity

Determination of salinity, as the total measure of inorganic dissolved matter, is by evaporation of the water and weighing of the residue. This is a difficult process, because some carbon dioxide and hydrogen chloride escape during the evaporation process and corrections must be made for this. Furthermore, at sea methods involving weighing can not be used. So the methods to be applied on board a ship have to be indirect ones.
In the past century only two major methods have been used in the oceanography for measurement of the seawater samples salinity: chlorinity titration and conductometry.

 

180px-Mohr_Karl_Friedrich
Karl Mohr

1.Chlorinity Titration

Chemical determination of halide content by titration was for many years the usual routine method for determining the “sea salt” content of sea water samples. This method, which known as Mohr method (Mohr, 1856), consist of titrating a sample of seawater with silver nitrate solution of known concentration to the point where all halides (chloride plus a small amount of bromide ) have been precipitated as silver halide, as detected by suitable indicators or electrode systems. A 15ml Knudsen pipette is used to measure the sea water sample into the titration vessel. This pipette differs from the standard type in that, after filling, the volume of sample is defined by rotation of a 3-way stopcock fitted at the upper end.

KnudsenIngolf
    M.Knudsen during Ingolf Expedition, c.1895

Clu = Cl×  Tu × W\   Ts × Wu

where:
Clu and Cls - chlorinity of unknown and standard,
Wu and Ws - weight of unknown and standard,
Tu and Ts - silver nitrate titre for unknown and standard.

Knudsen burette
Knudsen burette

 

A modification of the Knudsen titration has been suggested by the Grasshoff and Wensk (1972). This uses a Metrohm incremental piston burette in place of Knudsen burette; the absence of greased stopcocks from the system is stated to improve markedly the convience and accuracy of shipboard analysis.
Potentiometric end point determination has been utilized by several workers to enhance the precision of the silver nitrate titration. Reeburgh and Carpenter (1964) used a differential electrochemical system for end point detection.
One of the major drawbacks of manual titration methods lies in the time taken per sample and the operators skill requiered. A semi-automatic method of chlorinity titration has been described (Jarner and Aren,1970) which reduces the time per sample to 5min while retining high precision (0.004‰ in chlorinity).

Chlorinity was then converted to salinity by means of equation [1] or later equation [2] , prior to the introduction of the Practical Salinity Scale 1978. Chlorinity is now regarded as an independent chemical parameter to describe the properties of seawater and has no defined relationship to salinity.

 

2. Electrical Conductivity.

In the past fifty years, the chlorinity titration, which was time-consuming and requied a certain degree of analytical skill, has been largely replaced by the measurement of electrical conductivity as a mean of estimating salinity.

The laboratory method consist of comparing by use salinometers the electrical conductivity of the sample with that of a standard (IAPSO Standard Seawater) of known salinity at the same temperature.

The measured conductivity ratio is than converted to the practical salinity by means of the equation of the PSS-78.

 

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