Sampling and sample storage
Much of salinity data collected at sea are derived from a variety of in situ probes. The quality of such data is dependent on proper calibration and correct operation of the probes used. Most in situ measurements are corrected and verified by separate bottle samples collected in the same area. It is not the remit of this course to cover all aspects of oceanographic water sampling, but many of the ground rules which apply to bottling and storage of samples for salinity analysis also apply to the sampling devices. In particular, attention must be paid to ensuring that sampling devices are clean, free from salts prior to operation and that all efforts are applied to reduce contamination or evaporation of the samples. Sampling devices, ideally, should be adequately flushed with the sample water before filling and then sealed tightly before retrieval. This operation should be relatively automatic when using oceanographic water bottles on a wire. When taking surface samples manually, the neck of the container must be thoroughly dried before fixing the cap.
A number of surveys have been carried out on the suitability of a variety of sample bottles:
- Polythene and, in general, other plastic bottles are not suitable. Many plastics "breathe", other absorb dissolved components over relatively short time periods.
- Single plastic screw caps and rubber bungs do not provide a sufficiently reliable seal.
- There should be no metal contact with the water sample during storage.
The most widely tested and acceptable bottle is that of glass, (Type III is satisfactory), with a plastic cap insert (disposable) and a strong screw cap (bakelite). These bottles are relatively cheap and have a good working life-spin of about 10 years, before glass deteriorates from salt water contact.
For cleaning the bottles the following schedule is suggested:
- Detergent soak
- Multiple tap water rinses (until detergent remo
- Multiple distilled water rinses
- Oven dry (130°C)
Ideally, dry bottles, prepared in this way, should be used when sampling. However, if this is not possible, the bottles can be stored containing seawater (preferably of the salinity of the samples).
Filling schedule:
- Rinse bottle and cap three times with sample water.
- Fill bottle to base of neck (shoulder). Avoid overfilling as this can lead to sample seeping at the neck and subsequent salt crystal formation.
- Dry cap and outside of bottle neck thoroughly with clean tissue, insert plastic plug and screw cap on firmly. This is essential to prevent formation of salt crystals in cap.
- The delay between filling and capping the bottle should be kept to a minimum.
Store bottles upright in well protected crates. Avoid large temperature changes. Do not allow the samples to freeze. Shake the sample 2-3 times prior to analysis to ensure homogeneity.
Devise an organized system for identifying bottles so that quick and orderly sampling can take place. It can be safer to label bottles and screw caps.
Permanent numbering system is more useful as bottles are re-used many times.
Methods of the salinity measurements
In the past century only two major methods have been used in the oceanography for measurement of the seawater samples salinity:
1.Chlorinity Titration.
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.
Usually most of the silver nitrate is added as a strong solution, to just short of the end-point , and then the titration is completed with a more dilute solution of silver nitrate.
The silver nitrate solutions are calibrated against IAPSO Standard Seawater certified in chlorinity (nowadays only in practical salinity and K15) .
The chlorinity of the unknown seawater is calculated as follows:
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.
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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