Seawater standards

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. 

To reduce costs, users sometimes replace standard seawater with substandards to calibrate their salinometer. The use of substandards will inevitably result in reduced accuracy, but under certain circumstances may be acceptable. Their usage is described in Section 3.5.7. 

Only lAPSO recognized SSW provides a reliable standard for conductivity ratio measurements. It should be used to standardize each bench salinometer before being used for sample salinity measurements and to detect and eventually trace any drift. The use of so-called substandards for standardization is not recommended, as it will significantly decrease accuracy (see Section 3.5.7 for exceptions if a decrease in accuracy is acceptable). 

Experience shows that about 5 % of SSW vials from the same batch may deviate in salinity from their printed value by more than the claimed accuracy (0.001). After an instrument has been standardized if a vial shows differences larger than twice the instrument s precision (0.001 for both, the AUTOSAL and the RSIO), it should immediately be checked with a follow-up vial to identify a possibly bad vial. 

After a saUnometer has been standardized, SSW measurements are required on a regular basis to detect any drift or malfunction. Small offsets of SSW measurements from its printed salinity value can be used for linear trend corrections. Large offsets usually indicate nonstable room temperatiure or a malfunction of the salinometer. 

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The theoretical yield of the method is less than 100%, as only 80 - 90% of the aqueous phase is removed after back-extraction. The actual yield obtained by 54 Mn counting was 69.5 7.8%, and this can be allowed for in the calculation of results. Environmental Protection Agency standard seawater samples of known manganese content (4370 ng/1) gave good manganese recoveries (4260 ng/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%. 

Ultraviolet photo-oxidation techniques can be used as a method for organic nitrogen. The organic nitrogen compounds are oxidised to nitrate and nitrite then determined by the standard seawater analysis methods [17,18,23]. 

Merks and Vlasbom [114] carried out some comparative experiments on standard seawater with potassium hydrogen phthalate and water from Eastern Scheidt and Western Scheidt. These three types of samples were stored in three different ways and in three types of storage bottles. 

The inductive salinometer has become the measurement technique of choice because of its ease of use, speed, and precision. The reason for this high precision lies in the inductive salinometer s ability to detect very small differences in the ratio of the conductivity of a seawater sample as compared to that of a standard. By international agreement, this standard is prepared by diluting filtered Atlantic ocean water to produce seawater of chlorinity 19 374%o. The International Association for Physical Sciences of the Ocean (lAPSO) has designated Ocean Scientific International Ltd. (OSH) in Wormley, UK, as the sole preparer and distributor of this standard, which is called lAPSO Standard Seawater. ... 

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... 

Fig. 5.7. The stability of water ice and ChRThRO in standard seawater (solid lines). The dashed line separates the analogous equilibrium between C02(g) and C02-6H20. In both cases, the assumption was made that the mole fraction in the gaseous phase (x) is 1.0 [i.e., pure CH4(g) or CC>2(g)]. Reprinted from Marion et al. (2006) with permission...

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. 

Poisson A., Brunet C., and Brun-Coltan J. C. (1980a) Density of standard seawater solutions at atmospheric pressure. Deep-Sea Res. 27, 1013-1028. 

The apparent constants are not those for standard seawater where seawater composition has been significantly altered. Composition changes can be especially important in anoxic environments where extensive sulfate concentration changes are produced. The carbonate ion concentration can be calculated from any two of the four parameters pH, total CO2 (TCO2), the partial pressure of CO2 (pcof) (c-g-> Morse... 

Thus the formation of HSO4" and of MgOH ([CaOH ] and [HF] are nearly negligible) explains the differences in ion products between 0.7 NaCl and standard seawater (25 °C) ... 

This variability for the reported concentrations of mercury in open ocean waters may indicate that there are significant analytical diflBculties associated with the proper sample collection and the accurate measurement of mercury in seawater. These problems tend to override and preclude precise geochemical calculations and marine geochemical interpretations regarding the sources, sinks, and interactions of mercury in the oceans. These observational discrepancies for trace seawater constituents such as mercury can be resolved only through intercalibration programs and the use of standardized seawater samples. Such standards are not presently available for mercury concentrations at 100 ng Hg/1. or less in seawater. 

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. 

The following table is based upon an ocean model which takes into account the equation of state of standard seawater and the dependence on latitude of the acceleration of gravity. The tabulated pressure value is the excess pressure over the ambient atmospheric pressure at the surface. 

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