Chapter 5. Salinity Determination

1.0 Scope and field of application
This procedure describes the method for the determination of seawater salinity. The method is suitable for the assay of oceanic levels (0.005–42). The method is suitable for the assay of oceanic salinity levels of 2-42. This method is a modification of one published by Guildline Instruments (1978).

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:

where: for: 9.0 Quality assurance

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.