Salinity standard seawater

A commonly used procedure for the determination of phosphate in seawater and estuarine waters involves the formation of the molybdenum blue complex at 35-40 °C in an autoanalyser, and spectrophotometric evaluation of the colour produced [3]. Unfortunately, when applied to seawater samples, depending on the chloride content of the sample, peak distortion or even negative peaks occur which make it impossible to obtain reliable phosphate values. This effect can be overcome by the replacement of the distilled water used in such methods by a solution of sodium chloride of appropriate concentration related to the chloride concentration of the sample. The chloride content of the wash solution need not be exactly equal to that of the sample. For chloride contents in a sample up to 18 000 mg/1, (i.e., seawater), the chloride concentration in the wash should b e within 15% of that in the sample. The use of saline standards is optional but the use of saline control solutions is mandatory. Using good equipment, down to 0.02 mg/1 phosphate can be determined by such procedures. For chloride contents above 18 000 mg/1, the chloride content of the wash should be within 5% of that in the sample. See also Sect. 3.6.1. 

Jagner and Kerstein [654,655] used computer-controlled high-precision complexiometric titration for the determination of the total alkaline earth metal concentration in seawater. Total alkaline earths were determined by photometric titration using EDTA with eriochrome Black as indicator. The method yielded 63.32 (xmolekg-1 for the total alkaline earth concentration in standard seawater of 3.5% salinity. The precision was about 0.01%. 

Early scientists recognized that standards were needed to determine reliable values of the chlorinity and salinity of seawater. The IAPSO Standard Sea Water Service (originally based in Copenhagen) collected and distributed seawater from the North Atlantic with a known, measured chlorinity. This sample was supplied to oceanographers to standardize the AgNOg solutions used to determine chlorinity in various laboratories. 

The concept of salinity was introduced by Georg Forchhammer in 1865. From extensive analyses of seawater samples, he was able to demonstrate the validity of Marcet s principle for the most abundant of the salt ions chloride, sodium, calcium, potassium, magnesium, and sulfete. Thus, he recognized that the salinity of seawater could be inferred from the easily measurable chloride concentration or chlorinity. The details of this relationship were worked out by Martin Knudsen, Carl Forch, and S. E L. Sorenson between 1899 and 1902. With the international acceptance of their equation relating salinity to chlorinity (S%o = 1.805 Cl%o + 0.030), the standardization necessary for hydro-graphic research was provided. A slight revision in this equation (S%o = 1.80655 Cl%o) was made in 1962 by international agreement. 

Because the major ions are present in nearly constant proportions, the salinity of seawater can be inferred from any of their individual concentrations. The easiest concentration to measure is that of the chloride ion, which is also the most abundant. In practice, this concentration is determined by titrating a sample of seawater with a standardized solution of silver nitrate. The reactions that take place are ... 

This standard seawater has since proven problematic because it is based on real seawater, whose conductivity is influenced by concentration variations in the nonconservative ions and subtle fluctuations in the ratios of the major ions. To eliminate these issues, a practical salinity scale (PSS-78) was adopted by international agreement in 1978. As a... 

However, salinity values are easily obtained with a salinometer (which measures electrical conductivity and is appropriately calibrated with standard solutions and adjusted to account for T effects). The salinity of seawater increases if the loss of H2O (evaporation, formation of ice) exceeds the atmospheric input (rain plus rivers), and diminishes near deltas and lagoons. Salinity and temperature concur antithetically to define the density of seawater. The surface temperature of the sea reflects primarily the latitude and season of sampling. The vertical thermal profile defines three zones surface (10-100 m), where T is practically constant thermoclinal (100-1000 m), where T diminishes regularly with depth and abyssal... 

Therefore, a standard seawater sample with a salinity (5) of 35 (with no %o units needed) has a conductivity ratio of 1 at 15°C and 1 atmosphere, using a standard KC1 solution of 32.4356 g in a 1 kg mass of solution. Finally, recent applications of microwave remote sensing have been used to determine surface water of gradients of salinity in coastal regions, particularly in river plume regions (Goodberlet et al., 1997). 

As a calibrant solution for the AgNOs titrant, Standard Seawater was prepared that had certified values for both chlorinity and salinity. Unfortunately, the above salinity-chlorinity relationship was derived from only nine seawater samples that were somewhat atypical. It has since been redefined using a much larger set of samples representative of oceanic waters to become... 

The third category of salinity methodologies was based on conductometry, as the conductivity of a solution is proportional to the total salt content. Standard Seawater, now also certified with respect to conductivity, provides the appropriate calibrant solution. The conductivity of a sample is measured relative to the standard and converted to salinity in practical salinity units (psu). Note that although psu has replaced the outmoded %o, usually units are ignored altogether in modern usage. These techniques continue to be the most widely used methods because conductivity measurements can provide salinity values with a precision of 0.001 psu. Highly precise determinations require temperature control of samples and standards to within 0.001 °C. Application of a non-specific technique like conductometry relies upon the assumption that the sea-salt... 

There are two types of conductometric procedures commonly used. Firstly, a Wheatstone Bridge circuit can be set up, whereby the ratio of the resistance of unknown seawater to standard seawater balances the ratio of a fixed resistor to a variable resistor. The system uses alternating current to minimise electrode fouling. Alternatively, the conductivity can be measured by magnetic induction, in which case the sensor consists of a plastic tube containing sample seawater that links two transformers. An oscillator establishes a current in one transformer that induces current flow within the tube, the magnitude of which depends upon the salinity of the sample. This in turn induces a current in the second transformer, which can then be measured. This design has been exploited for in situ conductivity measurements. 

Culkin F. and Smith N. D. (1980) Determination of the concentration of potassium chloride solution having the same electrical conductivity at 15 °C and infinite frequency as standard seawater of salinity 35.000% (Chlorinity 19.37394). lEEEJ. Ocean. Eng. OE-5, 22-23. 

Poisson A. (1980) Conductivity/salinity/temperature relationship of diluted and concentrated standard seawater. IEEE J. Oceanic Eng. OE-5, 41-50. 

The accuracy of the method was indicated by the value of (F )g, which was 1.05 ppm, and which would correspond to a value of 1.3 ppm for the sample of salinity, S = 35%o. For comparison, the calculated value for salinity of 35%o would be 1.28, based upon the reported value for standard seawater (13). The precision was estimated for samples 6S and 6B for which the mean and standard deviations were 34.2 0.8 and 35.0 0.3, respectively the corresponding relative deviations were 2.3 and 0.9%. There was no significant variation in fluoride values during the 24 hr after being stored in a plastic container and refrigerated at < 4°C. In addition, fluoride was also determined for one unique sample colorimetrically, using an lanthanum-alizarin complexone reagent (14). Data are compared in Table I. 

Up to 1974, hydrographic data were collected with reversing water bottles and thermometers, and titration of chlorinity for salinity, using certified standard seawater as the reference. From 1974, temperature and salinity profiles were measured with CTD, but up to 1988, nutrient samples were still collected with reversing water bottles. From 1988, CTD-Rosette samplers have been used. An overview of the NERI devices, methods, and uncertainties is given in Table 11.8. 

Millero, F. J., Feistel, R., Wright, D. G., McDougall, T. J., 2008. The Composition of Standard Seawater and the Definition of the Reference-Composition Salinity Scale. Deep-Sea Research, I, 55, 50-72. 

TABLE 20.1 The Definition of Reference Composition, Providing the Cnrrently Best Estimate of the Absolute Salinity of Standard Seawater, Equation 20.1. X-Mole Fractions, Z-Valences, W-Mass Fractions (Millero et al., 2008). 

Another item of the forthcoming recommendation by the WG127 concerns a new salinity definition called Reference-Composition Salinity, bringing this fundamental oceanographic measure back into the interdisciplinary scientific and engineering family of the SI system of units (BIPM, 2006 Millero et al., 2008). The chemical Reference Composition is given in Table 20.1. On the basis of this definition, the recommended conversion formula between Absolute Salinity, S, and Practical Sahnity, S, of standard seawater is given by... 

Standard seawater from Ocean Scientific International Ltd, Southampton. Salinity 34.998. Batch 35NI. No further information about longitude and latitude of locahties. 

Since the major ions are present in seawater at constant ratios to one another, it is normally not necessary to measure the concentrations of all the ions since the concentration of one will allow the prediction of the others. Thus, chloride has traditionally been measured using a silver nitrate titration, and from this the salinity (i.e., total dissolved salt concentrations) can be derived. Now, however, conductivity is the routinely measured parameter and this is converted to salinity by a relationship agreed internationally with interlaboratory agreement ensured by the distribution of standard seawater samples for instrumental calibration. The use of modern inductively coupled conductivity measurements with careful temperature controls allows salinities to be determined with accuracy and precisions of the order of +0.01% or better. 

To achieve the accuracy of 0.02 %o in salinity for a single determination with the above method, and also to make salinity determinations from different institutions comparable, Knudsen required frequent comparison with an internationally accepted standard of known chlorinity. On behalf of the International Council for the Exploration of the Sea (ICES), standard seawater (SSW), later often called Copenhagen Water as it was prepared in Copenhagen for a long time, served this purpose. Each batch was numbered and distributed to oceanographers in sealed ampoules with the chlorinity indicated. For a detailed description of the history of standard seawater see Culkin and Smed (1979). 

In the early 1960s, bench salinometers were developed that allowed measurement of the electrical conductivity of a seawater sample relative to that of a standard with high precision. Cox et al. (1967) had related chlorinity and conductivity ratios of seawater to standard seawater at temperatures higher than 10 °C and tabulated their results (UNESCO, 1966). Following their work, the responsible international oceanographic organizations adopted a redefinition of salinity (Wooster et al, 1969). Firstly, it was assumed that salinity was proportional to chlorinity, to be consistent with the assumed constancy of the ionic composition. The constant was chosen so that for 5=35 %o, both the Knudsen formula (3-1) and the new relationship... 

To preserve continuity with the previous 1%9 definition, the reference KQ solution has a mass fraction of 32.4356 x 10 of KQ, to have the same conductivity as a standard seawater with S=35 (Cl=19.3739%o in the 1969 definition, see Eq. (3-2)). Equation (3-4) is valid for a practical salinity S from 2 to 42. For 15= , the practical salinity is 5=35. As the coefficients in Eq. (3-4) were determined by comparison of diluted and evaporated fractions of standard seawater with the KO solution, a standard sea water with known K15 value may replace the KCl solution as a reference. This is the general practice for preparing standard seawater. 

On the 1978 scale, practical salinity S no longer depends on the ionic composition of seawater. Any improvement in the knowledge of the ionic composition of seawater will change the coefficients o and b in Eq. (3-3), but not S. For standard seawater and its dilutions, a = 0 and b has a value close to 1 depending on the ionic composition of the batch. 

High accuracy salinity measurements (0.002) require knowledge of the interpretation of standard seawater measurements (Section 3.5.2), and careful sampling, storage and logging (Sections 3.5.3 and 3.5.6). Along with Section 3.5.4 on the operation of the AUTOSAL, these sections describe procedures as recommended for the WOCE (see Stalcup, 1991) with some supplementary instructions and remarks added. 

Figure 1 The solubility of the prineipal atmospherie gases in seawater, as a funetion of temperature. Units are millilitres of gas eontained in a litre of seawater of salinity 35 psu, assuming an overlying atmosphere purely of eaeh gas. Note that salinity is defined in terms of a eonduetivity ratio of seawater to a standard KCl solution and so is dimensionless. The term praetieal salinity unit , or psu, is often used to define salinity values, however. It is numerieally praetieally identieal to the old style unit of parts per thousand by weight...

The saltiness of the ocean is defined in terms of salinity. In theory, this term is meant to represent the total number of grams of dissolved inorganic ions present in a kilogram of seawater. In practice, salinity is determined by measuring the conductivity of a sample and by calibration through empirical relationships to the International Association of Physical Sciences of the Ocean (IAPSO) Standard Sea Water. With this approach, salinity can be measured with a precision of at least 0.001 parts per thousand. This is fortunate, considering that 75% of all of the water in the ocean falls neatly between a salinity of 34 and 35. Obviously, these high-precision measurements are required to observe the small salinity variations in the ocean. 

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