Definition of salinity

As with solids, colloids, and solutes, marine scientists have developed practical definitions of salinity that are based on measurement technique. Those in current use are listed in Table 3.2 with their typical measurement precision. Each method has its advantages and disadvantages. For example, refractometers are small, lightweight, and low cost, but have low precision, so they are used for field work in which only a rough... 

An element that is relatively conservative through water-rock reaction is chlorine in the form of the anion chloride. Chloride is key in hydrothermal fluids, because with the precipitation and/or reduction of SO4 and the titration of HC03"/C03, chloride becomes the overwhelming and almost only anion (Br is usually present in the seawater proportion to chloride). Chloride becomes a key component, therefore, because almost all of the cations in hydrothermal fluids are present as chloro-complexes thus, the levels of chloride in a fluid efiectively determine the total concentration of cationic species that can be present. A fundamental aspect of seawater is that the major ions are present in relatively constant ratios—this forms the basis of the definition of salinity (see Volume Editor s Introduction). Because these constant proportions are not maintained in vent fluids and because chloride is the predominant anion, discussions of vent fluids are best discussed in terms of their chlorinity, not their salinity. 

Salinity is presently determined by measuring the conductance of seawater by using a salinometer. The modem definition of salinity uses the practical salinity scale, which replaces the chlorinity-salinity relationship with a definition based on a conductivity ratio (Millero, 1996). A seawater sample of salinity S= 35 has a conductivity equal to that of a KCl solution containing a mass of 32.435 6g KCl in 1 kg of solution at 15 °C and 1 atm pressure. No units are necessary on the practical salinity scale however, in practice, one often sees parts per thousand, ppt, or the abbreviation psu. New salinometers using this method are capable of extremely high precision so that the salinity ratio can be determined to 1 part in 40 000. At a typical salinity near 35 this procedure enables salinities to be determined to an accuracy of 35.000 0.001. This is much better than most chemical titrations, which, at best, achieve routine accuracy of 0.5 parts per thousand. 

In this chapter, a brief overview of the early definitions of salinity based on the chlorinity concept is given. It is followed by a description of the definition of the Practical Salinity Scale 1978 (PSS78), which is based on the measurement of electrical conductivity. Finally, methods are described that are used to derive salinity (and thus density) with modern instrumentation, both from bottle samples on a bench and in situ. 

The definitions of 1902 and 1969 give identical results at a salinity of 35 %o and do not differ significantly for most applications. The definition of salinity was reviewed again when techniques to determine salinity from measurements of conductivity, temperature, and pressure were developed. The Practical Salinity Scale defined in 1978 is a complex function related to the ratio (K) of the electrical conductivity of a seawater sample to that of a potassium chloride (KCl) solution with a mass fraction in KCl of 0.0324356, at the same temperature and pressure. 

The offered method can be recommended for changing of more laborious and prolonged standard procedures of definition of total water salinity, which are used now. 

Although the GLP revisions of 1987 excluded animal feed and water from the definition of control article, it would appear that such common vehicles as saline solutions and carboxymethylcellulose solutions still fall within the definition. Such a strict definition of the term for such innocuous vehicles as saline solutions is quite burdensome when one considers the requirements for control articles that are found in other sections of the GLPs characterization [ 58.105(a)], stability testing [ 58.105(b)], sample retention [ 58.105(d)], and inventory [ 58.107(d)]. It does not appear that this comprehensive definition is enforced by FDA field investigators in the course of GLP inspections. 

It may be expected that this disquisition should be commenced by giving a rigid definition of the term salt, or saline substance. But the complex ideas of natural substances are not subject to very definite descriptions, nature in her several productions proceeds by imperceptible gradations, seldom having any decisive marks, by which we can invariably discriminate them into sorts. ... 

Particulates in estuarine systems are composed of both seston (discrete biological particles) and inorganic lithogenic components. An operational cutoff definition of 0.45 pm is used in this section to discriminate particulates—more details on the role of colloidal particles are discussed in chapters 8, 14, and 15. The highly dynamic character of estuarine systems (e.g., tides, wind, resuspension) can result in considerable variability in particle concentration over diurnal time intervals (Fain et al., 2001). Moreover, the reactivity of these particles can change over short spatial intervals due to rapid changes in salinity, pH, and redox conditions (Herman and Heip, 1999 Turner and Millward, 2002). 

The definition of the sea boundary of the mouth area is related to the term mouth-mixing zone. Water salinity within this zone increases from the salinity inherent in river water (usually 0.2-0.5%o) to the salinity of seawater (usually 10-40%o in different seas). The salt composition of water radically changes within the mixing zone river water of hydrocarbonate class and calcium group transforms into seawater of chloride class and sodium group. 

It should be noted that the complexity and ill definition of carbonate salts in highly saline solutions is documented in literature by the complete absence of single salt osmotic and activity coefficient data. This situation requires the use of this less than satisfying technique to define heavy metal chemistry in brines. 

Brine. Water of salinity higher than that of average seawater, i.e., more than 3.5 X 10" mgL TDS. The majority of oil-field waters are brines according to this definition, whereas only a small fraction could be classified as brines based on the definitions of Davis (1964) and Carpenter et al. 

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). 

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