Salinity measurement

Salinity measurements are most often used in oceanography to determine seawater density. The conventional measure used by oceanographers for determining salinity is conductivity. This is feasible because the salt content of seawater is well defined, as is the temperature-related compressibility. As an alternative, the refractive index of water is a good descriptor of density when temperature is known or can be measured. Refractive index provides a high-precision method for determining the density of pure water. As various salts are added, the refractive index is a less exact predictor of density, although relative measurements can still be useful. 

Figure 4. A comparison of measured and diatom-inferred salinity in Devils Lake, North Dakota, for the period of historic record. No salinity measurements were made between 1923 and 1948, hence the dotted line and question mark between these dates. (Reproduced with permission from reference 11. Copyright 1990 American Society of Limnology and Oceanography.)...

Salinity is a measure of the salt content of seawater. Developments in analytical chemistry have led to an historical evolution of the salinity concept. Intrinsically, it would seem to be a relatively straightforward task to measure. This is true for imprecise determinations that can be quickly performed using a hand-held refractometer. The salinity affects seawater density and thus, the impetus for high precision in salinity measurements came from physical oceanographers. 

Not all the major constituents consistently exhibit conservative behaviour in the ocean. The most notable departures occur in deepwaters, where Ca and HCO3 exhibit anomalously high concentrations due to the dissolution of calcite. The concept of relative constant composition does not apply in a number of atypical environments associated with boundary regions. Inter-element ratios for major constituents can be quite different in estuaries and near hydrothermal vents. Obviously, these are not solutions of sea salt with the implication that accuracy of salinity measurements by chemical and conductometric means is limited. 

Calculation of Calcium and Carbonate Total Molalities. Because of the constancy of composition of seawater, the total ion calcium concentration in seawater can be calculated, in "open ocean seawater samples, directly from high precision salinity measurements, using the relationship (14) ... 

Estimation of Uncertainty in The determination of the total ion molal concentration of calcium from salinity measurement is relatively precise with a probable error of less than 0.3% under open ocean conditions. Dickson and Riley (37) have recently discussed the effect of analytical errors on the evaluation of the components of the aquatic carbon-dioxide system for seawater at 25°C and 1 atmosphere total pressure. Their conclusions Indicate that if alkalinity and total carbon dioxide are the measured parameters a probable combined uncertainty in the total carbonate ion molal concentration from 3 to 6 percent results, depending on Fco2 If pH and alkalinity are the measured parameters the uncertainty is approximately 4 percent. In addition to the probable error introduced by analytical precision, the absolute accuracy of the measurements introduces an error which is difficult to evaluate. The results of the GEOSECS intercalibration study (38) were indicative of this problem. A conservative guess is that accuracy introduces at least a one percent further uncertainty. It is also difficult to determine exactly what error is introduced through temperature and pressure corrections to situ conditions. For the deep sea this may introduce a further uncertainty of at least... 

Whereas the composition of dissolved main solid compounds in seawater is rather constant all over the oceans, the freshwater in the Baltic Sea outbalanced by river discharge is dominated by calcium bicarbonate. For this reason, significant anomalies are observed in Baltic waters from the standard composition of seawater (Nehring and Rohde, 1966), in particular in the brackish surface water, with amount increasing toward the eastern and northern margins of the Baltic Sea. Directly measured densities of Baltic water compared with density determined from the seawater equation of state with Baltic water salinity measured by chlorinity titration resulted in a deviation of up to 0.123 kg/m (Millero and Kremling, 1976). This may result in uncertainties in estimating the thermodynamic properties of Baltic water, for example, the vertical stability. 

On the basis of decadal means and maximum values of salinity measured at I/v Gedser Rev, Matthaus (1995) later identified 15 MBls between 1880 and 1896. 

Surface sediment samples were collected from Loch Sunart, a sea loch (fjord) on the NW coast of Scotland, during cruises aboard the R. V. Clupea (April 1999) and the R. V. Envoy (July 2001 and June 2002). Bottom water temperature and salinity measurements were recorded using a STD Plus 646 conductivity, temperature and depth (CTD) probe at each sample site (Table 1). No data were collected on pH and carbonate ion concentration. Two stationary Anderaa RCM-7 current meters complete with temperature and conductivity sensors and a data logger were deployed in the inner basin (56.6842°N, -5.6211°W) between 21 June 2001 and the 18 June 2002, and the main... 

For instance, it is fouitd that for an alkylbenzenesulfonate-alkane-NaCI brine system an increase in ACN of four units is compensated by an increase in salinity measured by an increase of 0.64 of the natural logarithm of the salinity, whatever the other variables, as seen in Fig. 14. 

To derive Eq. (3-1), Knudsen and co-workers mainly used samples from the Baltic and from the Mediterranean and the Red Sea. The offset of 0.03 in Eq. (3-1) reflects that the salt composition especially in the Baltic Sea is not exactly constant (Millero and Kremling, 1976) and thus contradicts the basic assumption of constancy that led to Eq. (3-1). However, the proposed titration method also had advantages it could be performed in reasonable time onboard a ship, and for salinity measurements in open ocean areas where S is close to 35 %o, the error induced by the non-constancy is less than that from titration (0.02 %o in salinity). Therefore, the so-called Mohr-Knudsen titration method (Mohr, 1856 see Chapter 11) and the Knudsen formula (3-1) served oceanographers for more than 60 years to determine salinity from chlorinity. 

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. 

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). 

Samples for salinity measurements are transferred from water samplers to salinity sample bottles and stored in the laboratory before they are analysed with a salinometer. 

Before a major cruise, each salinometer needs all the checks and the maintenance recommended in the manual to be carried out including checks of the electric circuits. As an accurately controlled bath temperature is essential for high accuracy in salinity measurement. 

Computer programmes may not be able to decide at which step of the process the cell has been thoroughly rinsed and then to measure a sample. It therefore must be the operator s and not the programme s decision at which point of the process data for a salinity value are to be read from the interface. On the AUTOSAL, this is executed by pressing the Data hog button. Having read a set of data for a single salinity measurement, the programme then can easily deliver the statistics from which the operator will finally accept the measurement or not. 

The concentration of the 4 major cations Ca++, Mg++, Na++ and K+ and 4 major anions, HCO3-, CO3-, SO42- and Cl- usually constitute the total ionic salinity of water for all practical purposes (Wetzel, 2001). Concentrations of ionized components of other elements such as N, P and Fe and numerous minor elements are of immerse biological importance but are usually minor contributors to total salinity (Wetzel, 2001). The sum of all ionic concentrations is the basis for salinity measurement (Covich, 1993). Total dissolved solids and ionic conductivity of water, are generally used measurement (Covich, 1993). 

Despite recent developments in salinity measurement based on electrical conductivity, density, or the refined measiurement of refractivity, a silver nitrate titration still remains the internationally acceptable referee procedure and it is also still the most convenient method to use when a high precision is required in a laboratory having only limited and sporadic requirements for salinity analyses. 

Figure 6.17. CDOM (grayscale colors) and salinity (black lines) contours across the Mississippi plume in June, 2000. Salinity is represented by the contour lines. Inset map shows the loeation of the survey transect in relation to the Mississippi Delta. Gray lines represent path of towed vehiele and gray boxes represent missing data. Bottom depths were approximately 5 m helow the maximum depths of the fluorescence and salinity measurements. Subsurface CDOM maxima are highlighted with circles. (Redrawn from Chen and Gardner, 2004.) (See Plate 13.)...

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