Instrumento de medición 192

The previous paragraphs highlighted the main physical properties of water, but at this point it is pertinent to show a phase diagram (Figure 2.6) representing the different physical states of water depending on pressure and temperature.

This diagram allows us to define the domains in which water exists in its liquid, solid and gaseous state, as well as the limits for transition between the different phases that involve an exchange of energy. For example, a change from the liquid to the gaseous state requires an amount of energy called the latent heat of evaporation Ov which depends on the temperature of the liquid. Likewise, the change from the solid to the liquid state requires that the solid be subjected to a sufficient quantity of thermal energy called the latent heat of fusionOf. Finally, to make the transition from a solid to a gaseous state, the solid requires a quantity of heat called the latent heat of subli-mationOs. It is important to note that change between the different phases occurs at a constant temperature: when ice melts, its temperature remains at 0 ^°C. However, after a certain point, called the critical point, distinguishing between the gas and liquid phases is no longer possible, which explains the interruption in the evaporation/

vaporization curve.

The different phases of water leads us to the study of ice, the solid state of water.

We explained earlier why ice has a lower density than water, so in this section, we will

Isotope Proportion

1H 98.9885%

2H 0.0115%

Total 100%

16O 99.757%

17O 0.038%

18O 0.205%

Total 100%

focus on the crystalline structure of this solid. The basic diagram that is usually used to represent ice is a regular tetrahedron, where the center and vertexes are occupied by the oxygen atoms of the water molecule (Figure 2.7). Repetition of this basic structure produces the structure of ice, which displays a hexagonal shape (Figure 2.8). However, there are in fact a number of different crystalline phases, which are a function of temperature and pressure. These phases, called allotropic phases, are six in number and cover a fairly broad range of pressure and temperature conditions because ice can exists at temperatures above 100^°C if the pressure is extremely high.

When ice melts, there is progressive rupturing of the hydrogen bonds and conse-quently a rupture of the basic tetrahedral structure of the ice crystal. However, the hydrogen bonds persist until high temperatures, which means, as shown in Figure 2.4,

sublimationcurve

Fig. 2.6 : Phase diagram of water (from Musy and Soutter, 1991).

O

2-Fig. 2.7 : Ice crystal of water (based on CNRS, 2000).

that water in the liquid state consists not only of water molecules but also of dimers and trimers, as well as the basic structures of ice.

Figure 2.9 shows the proportion of water molecules that are linked to x neighbors where x varies from 0 to 4. At a temperature of 25^°C, two thirds of the water molecules are still linked by hydrogen bonds to four neighboring molecules. At a temperature of 100°C the ratio is 1/2, which indicates that when water boils, half of its molecules are still connected to four nearby molecules. So we can see that even at high temperatures, water presents as a liquid made of various different structures.

Fig. 2.8 : Structure of ice (based on CNRS, 2000).

0%

10%

20%

30%

40%

50%

60%

70%

80%

0 1 2 3 4

Number of hydrogen bonds per water molecule.

rate of hydrogen bonds

0 °C 25 °C 60 °C 100 °C

Fig. 2.9 : Proportion of hydrogen bonds as a function of the proximity of water molecules (based on Javet et al., 1987).

2.2.4 Seawater

Since 97% of the water on earth is seawater, it is interesting to look at some of its particular physico-chemical properties. Most people think of seawater as containing salt, but in fact it contains a mixture of ions; to date, 60 of the 92 basic elements have been identified in seawater. Table 2.3 gives the average composition of seawater containing 25 g of salt per kilogram, which corresponds to “average” seawater. Bear in mind that if the average concentration of salt is 25 grams per kilogram and we know that the total volume of the oceans is 1350 million km³, then the total volume of salt is approximately 50 million billion tons!

Table 2.3 : Average composition of sea water containing 25 g of salt per kilogram.

In general, the differences in the physical and chemical properties of seawater compared to fresh water are related to its salinity. Average seawater containing 25 grams of salt per kilogram of water has a freezing temperature of – 1.9°C. At this temperature, crystals of fresh water start to form, resulting in ice crystals immersed in a liquid medium of increasing salinity. The temperature must continue to fall to - 23°C before sodium chloride NaCl crystals start to form.

At this point, it is useful to discuss the concept of salinity in greater detail. Salinity is defined as the total quantity of solid residues after all organic matter as well as carbonates have been oxidized and when the bromine and iodine have been replaced by chlorine. In reality, it is difficult to measure the salinity of water directly by the methods of drying and weighing the residue, because a certain number of solids will evaporate during the process. However, the relative proportions between the various ions are almost constant, which means that it suffices to determine the concentration of a single element of the water sample to deduce the concentrations of the other elements. In general, to determine the salinity of a water sample, its chlorine, bromine or iodine is titrated. Then salinity is calculated based on a set relationship between the elements measured, for example between the quantity of chlorite and the salinity. In this context, in 1969 UNESCO proposed a relationship of chlorine content to absolute salinity S as a ‰ of water as follows:

Element Proportion (g/Kg) Chlorine Cl^ 18.9777

Sulfate 2.6486

Bicarbonate 0.1397

Bromine Br^ 0.0646

Fluorine F^ 0.0013

Sodium Na^ 10.5561

Magnesium Mg^ 1.2720 Calcium Ca^ 0.4001 Potassium K^ 0.3800 Strontium Sr^ 0.0135

SO₄– HCO₃

with Cl [^°/_°°] the rate of chlorine (2.1) More recently, the concept of the salinity of water was redefined in relation to its electric properties, in particular its conductivity.

It is important to note that the salinity of seawater is not homogeneous in all waters around the globe. Essentially, like density and temperature, salinity varies as a function of depth and location. However, the variations in the physical and chemical properties of seawater are clearly more important on the vertical scale than on the horizontal scale. The waters of the oceans are highly stratified; and in addition to these spatial variations, there are temporal variations that can be daily or seasonal.