Salinity Analysis
been carried out at the Laboratory of the Government
Chemist by a silver nitrate titration method. Since the early
1980s, Guildline salinometers have been used, starting
with the Autosal, and latterly the Portasal (Fig. 1). The
Portasal as its name suggests, is smaller and easily
transportable. Designed for use on research vessels, it
maintains a high level of accuracy.
Why take salinity samples?
The majority of the salinity samples collected by FRS are
used to calibrate conductivity, temperature and depth
profiling instruments (CTDs), and thermosalinograph
instruments used to measure sea surface temperature
and salinity (Fig. 2). Other salinity samples are collected
at shallow water positions, such as sea lochs and at the
FRS Coastal Long Term Monitoring sites. Salinity data are
used, (usually in conjunction with temperature values)
to characterise the marine environment for studies of
marine biology, water circulation and climate change.
Figure 1. Portasal salinometer
What is salinity?
The salinity of sea water is the measure of its saltiness,
Figure 2.
CTD conductivity calibration graph. The results of this linear
calibration are applied to the data collected by the CTD
instruments.
usually determined by measuring the electrical
38
‘The Practical Salinity Scale 1978 (PSS 78) defines the
practical salinity of a sample of seawater in terms of the
conductivity ratio (K15) of the conductivity of the sample
at a temperature of 15°C and pressure of one standard
atmosphere to that of a potassium chloride (KCl) solution
containing 32.4356 g of KCl in a mass of 1kg of solution’.
The measure of salinity is a ratio and has no units, but is
usually labelled as practical salinity units (PSU) for clarity.
Sample conductivity
conductivity of a water sample. Expressed scientifically,
36
CTD Conductivity Calibration 911+
y = 1.000210x - 0.004419 (y = mx + c)
34
32
30
28
28
30
32
34
36
38
CTD Conductivity
History of salinity analysis at FRS
The Oceanographic Services Group at FRS presently
Salinity analysis at Fisheries Research Services (FRS)
analyse approximately 2,500 salinities per year; almost
began in the early 1960s with the introduction of the
half of these are analysed at sea and the remainder at
Autolab electronic salinometer. Prior to this, analysis had
FRS’ Marine Laboratory.
Automatic sampling carousel with salinity sample bottle.
Sample stations
Sample collection
FRS has an extensive water sampling programme in
support of fisheries and environmental research.
On-going programmes include the recently established
Long Term Coastal Monitoring project, long-term data
collection east of Orkney, and in the Faroe-Shetland
Channel. The latter has been in operation for over 100
Sea water is collected at specified depths by a set of
specially designed bottles called Niskens. Twelve Niskens
are attached to a carousel, which also contains a CTD.
The salinity sample is drawn from the Niskin into a
200 ml glass bottle which is filled to the neck (see picture
above). Samples are stored in a temperature controlled
room until analysis.
years and has yielded a unique and valuable data-set.
Figure 3.
Long-term coastal monitoring sites (white circles), Faroe-Shetland
Channel (Fair Isle – Munken and Nolso – Flugga Lines) and east
of Orkney (Jonsis Line) sampling stations.
Nolso – Flugga Line
Scalloway
Fair Isle – Munken Line
Fair Isle
Scapa
Loch Ewe
Sample analysis
Once collected, the sample is pumped into the measurement cell four times to ensure adequate flushing out of
the previous sample before the conductivity is measured
and the result displayed. The result can be displayed as
either conductivity ratio or Practical Salinity Units (PSU).
The range of conductivity measurement is wide, from
0.004 milli Siemens/centimetre (mS/cm)-76 mS/cm with
a resolution of 0.0003 mS/cm. This means salinity can
be measured to an accuracy of better than 0.003 Practical
Salinity Units.
Jonsis Line
Calibration
Cromarty
Loch Maddy
Mallaig
Millport
Ythan
Findon
Stonehaven
The salinometer is regularly calibrated in-house
to a reference sample of precisely known salinity. The
reference is IAPSO Standard Sea Water*. The salinometer
is also returned to the supplier for an electronic alignment
and linearity check every second year.
For further information contact oceanography staff at the
address below.
* ‘The IAPSO Standard Sea Water Service is the only internationally recognised standard for the calibration of salinity measurement devices. Its widespread
use over 100 years of IAPSO history has been of great importance to the quality and comparability of salinity data worldwide’.
FM23|05|07