2.0 Definition
The method determines the practical salinity
(S) of seawater samples which is based on electrical conductivity measurements.
The Practical Salinity Scale 1978 (PSS 78) defines the practical salinity
of a sample of seawater in terms of the conductivity ratio (K 15
) 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 1 kg of solution.
3.0 Principle
A salinometer is used to measure the conductivity
ratio of a sample of seawater at a controlled temperature. The sample is
continuously pushed through an internal conductivity cell where electrodes
initiate signals that are proportional to the conductivity of the sample.
Using an internal preset electrical reference, these signals are converted
to a conductivity ratio value. The number displayed by the salinometer
is twice the conductivity ratio. The internal reference is standardized
against the recognized IAPSO standard seawater.
4.0 Apparatus
Guildline model 8400A Autosal Salinometer.
The Autosal has a 4 electrode cell which measures the conductivity ratio
of a sample seawater in less than one minute. The salinity range of the
instrument is about 0.005–42 and has a stated accuracy of ± 0.003
by the manufacturer. In practice, accuracies of 0.001 are possible with
careful analysis.
5.0 Reagents
IAPSO Standard Seawater. Standard seawater
for instrument calibration.
6.0 Sampling
Salinity samples are collected from Niskin
bottles at all depths. These samples are collected after the oxygen and
CO2 samples have been drawn. The bottles used are 125 and 250
ml borosilicate glass bottles with plastic screw caps. A plastic insert
is used in the cap to form a better seal. The remaining sample from the
previous use is left in the bottles between uses to prevent salt crystal
buildup from evaporation and to maintain an equilibrium with the glass.
When taking a new sample, the old water is discarded and the
bottle is rinsed three times with water
from the new sample. It is then filled to the bottle shoulder with sample.
The neck of the bottle and inside of the cap are dried with a Kimwipe.
The cap is then replaced and firmly tightened. These samples are stored
in a temperature controlled laboratory for later analysis (1-5 days after
collection). Every six months the bottles are acid washed (1 M HCl), rinsed
with deionized and Milli-Q water.
After this cleaning they are rinsed five
times with copious amounts of sample before filling.
7.0 Procedures
The samples are analyzed on a Guildline
AutoSal 8400A laboratory salinometer using the manufacturer’s recommended
techniques. The salinometer is calibrated with IAPSO standard seawater.
Two standards are run prior to running the samples. If those two standards
agree, the samples are run. At the end of the run, two new standards are
run to check for instrument drift. The drifts are generally found to be
zero. Using this procedure, the instrument can give a salinity precision
of ± 0.001- 0.002.
8.0 Calculation and expression of results
The calculation of salinity is based on
the 1978 definition of practical salinity (UNESCO, 1978). The following
gives the necessary computation to calculate a salinity (S) given a conductivity
ratio determined by the calinometer:
9.1 Quality control: The bottle salinities are compared with the downcast CTD profiles to search for possible outliers. The bottle salinities are plotted against potential tem-perature and overlaid with the CTD data. Historical envelopes from the time-series station are further overlaid to check for calibration problems or anomalous behavior.
9.2 Quality assessment: Deep water samples (>3000 m) are duplicated. These replicate samples are found to agree in salinity of ±0.001.
9.3 Regular intercalibration exercises should be preformed with other laboratories.
10.0 References
Guildline Instruments. (1981). Technical
Manual for ‘Autosal’ Laboratory Salinometer
Model 8400.
UNESCO. (1978). Technical Papers in Marine
Science, 28, 35pp.