The salinity of the seawater in the South China Sea is 3.4%. What is the resistivity of such seawater?

This post was last edited by Wang Genrong on 2009-8-6 at 16:42. The following is a repost; I hope it will be useful to the original poster. Salinity, measurement of sea salinity: Over billions of years, a large amount of chemical substances from land have dissolved and accumulated in the oceans. If all the oceans were to evaporate, the salt that remained would cover the entire Earth with a layer 70 meters thick. According to measurements, there are 11 chemical substances that are most abundant in seawater: namely five cations such as sodium, magnesium, calcium, potassium, and strontium ； Five anions including chlorine, sulfate, bicarbonate (including carbonate), bromide, and fluoride, as well as boric acid molecules. The top three among them are sodium, chlorine, and magnesium. To indicate the amount of chemical substances in seawater, seawater salinity is commonly used as a measure. The salinity of seawater is a quantitative measure of the salt content in seawater, and it is one of the most important physicochemical properties of seawater. It is closely related to coastal runoff, precipitation, and sea surface evaporation. Changes in salinity distribution are also important factors that influence and constrain the distribution and changes of other hydrological elements; therefore, the measurement of seawater salinity is an essential part of marine hydrological observations. Definition and evolution of salinity: Absolute salinity refers to the ratio of the mass of dissolved substances in seawater to the mass of the seawater itself. Since absolute salinity cannot be measured directly, corresponding definitions of salinity have been introduced in practical applications as measurement methods for salinity have evolved and improved. Knudsen’s salinity formula: At the beginning of this century, Knudsen and others established a definition for salinity. According to that definition, salinity refers to the total amount of solid substances present in 1000 grams of seawater, after all carbonates have been converted into oxides, bromine and iodine have been replaced by chlorine, and all organic substances have been oxidized. The measurement method involves taking a certain amount of seawater, adding hydrochloric acid and chlorine water, evaporating it to dryness, then drying it at constant temperatures of 380°C and 480°C for 48 hours, and finally weighing the weight of the remaining solid substance. Measuring seawater salinity using the aforementioned weighing method is extremely complicated; it takes several days to analyze a single sample, which makes it unsuitable for marine surveys. Therefore, in practice, the chlorinity of seawater is measured, and salinity is calculated indirectly based on the constancy of seawater composition. The relationship between chlorinity and salinity (the Knudsen salinity formula) is as follows: S‰ = 0.030 + 1.8050 × Cl‰. The Knudsen salinity formula proves to be highly effective when combined with a standardized silver nitrate titration method and commonly used marine tables, and it has been in use for 70 years. However, long-term use has also shown that Knudsen’s salinity formula is merely an approximate relationship with poor representativeness ； Titration is also inconvenient to perform on board a ship. So people sought more precise and faster methods. We know that when there is a potential difference across the ends of a conductor, an electric current flows through it. The current I flowing in a conductor is proportional to the potential difference () across its ends; this is the famous Ohm’s law, namely I = G(), where G is the proportionality constant, also known as conductance. Let G=; then the constant R is called the resistance of this conductor, and it depends on the properties and geometric shape of the conductor. The unit of resistance is the ohm (W). Experiments show that for a conductor with uniform thickness, when the material of the conductor and its temperature are constant, the resistance of the conductor is proportional to its length L and inversely proportional to its cross-sectional area S. The proportionality constant r in this equation is called the resistivity, whose unit is W · m; it depends on the properties of the material, and different materials have different resistivities. The reciprocal of resistivity is called conductivity, with the unit of S/m. Given that the conductivity of seawater depends on its temperature and salinity, the salinity of seawater can be determined by measuring its conductivity and temperature. The definition of conductivity salinity in 1969: In the early 1960s, Cox and others from the National Oceanographic Institution in the UK collected 135 seawater samples from depths up to 200 meters in various oceans as well as the Baltic Sea, Black Sea, Mediterranean Sea, and Red Sea. They first used standard seawater to accurately determine the chloride content of these samples, and then measured the conductivity ratio of samples with different salinities compared to standard seawater with a salinity of 35.000‰ and a temperature of 15°C, under one standard atmosphere. This allowed them to establish a relationship between salinity and relative conductivity; this is what is known as the definition of conductivity salinity from 1969: S‰ = 1.80655 × Cl‰. The method of determining salinity using conductivity offers high accuracy, fast speed, and simple operation, making it suitable for field measurements at sea. However, in practical applications, there are still some issues: first, the two salinity formulas used in the definition of electrical conductivity are still based on the assumption of constant seawater composition, which is an approximation. In conductivity-based salinity measurement, the chlorinity value assigned to the standard seawater used in salinometers is corrected; when certain changes occur in the standard seawater, the chlorinity value may remain unchanged, but the conductivity value will change. Secondly, the water samples used in the definition of conductivity salinity are all from the surface layer (up to 200 m depth), and thus cannot reflect the changes in conductivity values in the deeper ocean due to variations in seawater composition. Finally, the temperature range in international marine tables is 10–31°C; when the temperature is below 10°C, the conductivity value must be corrected using other methods, which leads to errors and confusion in the data. To overcome the problem of saltiness standards being affected by seawater composition, the Practical Salinity Scale of 1978 (PSS78) was established. The Practical Salinity Scale of 1978: The Practical Salinity Scale still determines the salinity of seawater using the conductivity method. Unlike the definition of conductivity salinity from 1969, it overcomes the problem whereby the standards for seawater salinity are affected by changes in the composition of seawater. In practical salinity measurements, high-purity KCl is used; it is prepared into a solution of a specific concentration (32.4357‰) using standard weighing methods, and this solution serves as an accurate reference standard for salinity. It is independent of the chlorinity level of the seawater sample. Salinity is defined as the ratio of the conductivity of a seawater sample to that of the standard KCl solution at 15°C and 1 standard atmosphere: where C represents the conductivity value. In this case, the practical salinity value of the sample is exactly 35. If, then the expression for practical salinity is given by (1-1), where S denotes practical salinity and is a dimensionless quantity. For example, if the salinity of seawater is 35‰, its practical salinity is also 35; in equation (1-1), this value can be substituted in place of . It represents the ratio of the conductivity of a seawater sample to that of standard seawater with a salinity of 35‰ at an atmospheric pressure of 15°C. In the formula, it is valid when . The salinity expression for the specific conductivity of seawater samples at any temperature is given by equation (1-2). The second term in this equation represents the temperature correction factor; the coefficient a is the same as that in equation (1-1), with values of K=0.0162, for temperatures ranging from -2℃ to 35℃