Seawater Chemistry The ions of sodium and chloride in NaCl (table salt) are held together by ionic bonds, electrostatic attraction that exists between ions that have opposite charge. Sodium and chloride are the two most abundant ions in sea water.
Summary of Important Concepts Water is a polar molecule with the ability to dissolve more substances than any other natural solvent. Salinity is the measure of dissolved inorganic solids in water. Salinity can be expressed as a percent (%), but is more commonly expressed as parts per thousand (ppt). Average ocean water salinity is about 3.5 % which equals 35 ppt. The most abundant ions dissolved in seawater are chloride, sodium and sulfate. The proportions of dissolved ions in seawater is nearly the same everywhere. The most abundant dissolved gases in sea water are nitrogen, oxygen, and carbon dioxide. Oxygen and carbon dioxide levels are controlled mostly by the photosynthesis and/or respiration by marine organisms. The ocean is in chemical equilibrium, meaning equal proportions and amounts of dissolved materials both enter and leave.
The Dissolving Power of Water What are solutions? A solution is made of two components, with uniform molecular properties throughout: The solvent, which is usually a liquid, and is the more abundant component. The solute, often a solid or gas, is the less abundant component, and is dissolved in the solvent. Seawater is a solution of water molecules (the solvent) and dissolved solutes consisting of both gases and solids (mostly ions).
Water is a polar molecule, having a positive and a negative side. This polarity means water easily dissolves many ionic compounds. Note in the figure how the water molecules pull apart the crystal of sodium chloride (table salt) by surrounding each sodium and chloride ion. The Dissolving Power of Water
Salinity Salinity is the total quantity of dissolved inorganic solids in water. You can see here that the most abundant dissolved solids are chloride, sodium, and sulfate.
Salinity Salinity can expressed as a percentage (%). For example, if you had 100 kilgorams of sea water consisting of 96.5 kilograms of water molecules and 3.5 kilograms of dissolved salts, the salinity would be 3.5% However OCEANOGRAPHERS more commonly measure salinity as parts per thousand (ppt). In this case, if you had 1000 kilograms of sea water consisting of 965 kilograms of water molecules and 35 kilograms of dissolved salts, the salinity would be 35 ppt. Note that 35ppt is equal to 3.5% Average ocean water salinity is about 3.5 % or 35 ppt. The salinity of seawater varies in different areas of the ocean. But the proportions of the major salts in seawater stays about the same everywhere. This is known as the principle of constant proportions (also called Forchhammer s principle).
Measuring Salinity How do oceanographers measure the salinity of seawater? Salinity can be determined in several ways. These include: 1. Chlorinity: The amount of chloride ions in the water is proportional to the total salinity. 2. Electrical conductivity: The saltier the water, the more easily it conducts electricity. 3. Refraction: The saltier the water the more it bends (refracts) light. One of the most common and simple ways of determining salinity is to measure the chlorinity. Chlorinity refers to the abundance of chloride ions in the water. Since chlorinity is easy to measure, and the principle of constant proportions applies to all seawater, scientists can use the chlorinity to measure the total salinity using this formula: Salinity (in parts per thousand) = 1.80655 Chlorinity (ppt)
Chemical Equilibrium Is the salinity of the ocean changing over time? No, the ocean appears to be in chemical equilibrium. The proportion and amounts of dissolved solids remain constant over time. This concept is known as the steady state ocean. Ions are added to and removed from the ocean at about the same rate, so salinity stays about the same.
Residence Time Residence time is the average length of time an element spends in the ocean. Residence time can be calculated by the equation: Residence Time = Amount of element in the ocean The rate at which the element is added to (or removed from) the ocean The residence time of different elements in the oceans varies over a great range, as shown by the table on the next slide...
Residence Time This table summarizes the residence times of the different types of dissolved materials in sea water.
Conservative & Nonconservative Constituents Conservative constituents of seawater are those substances that occur in constant proportions. Conservative elements have long residence times and are the most abundant dissolved material in the ocean. Nonconservative constituents are substances which have short residence times, and are usually associated with seasonal, biological or short geological cycles.
Dissolved Gases in Seawater All types gas dissolve more readily in cold water than in warm water. Nitrogen (N 2 ) oxygen (O 2 ) and carbon dioxide (CO 2 ) are the most common dissolved gases in seawater. Oxygen (O 2 ) and carbon dioxide (CO 2 ) levels are controlled mostly by the activity of living things. Photosynthesis by plants and phytoplankton produces oxygen and uses up carbon dioxide. Respiration by animals, bacteria, and other organisms produces carbon dioxide and uses up oxygen. The effects of these processes are shown on the next slide..
Dissolved Gases in Seawater The figure shows how O 2 and CO 2 levels vary with depth. Photosynthesis by plants at the ocean surface (where there is sunlight) causes high levels of O 2 and low levels of CO 2. Below the surface where there is not enough light for plants, respiration by animals causes lower levels of O 2 and higher levels of CO 2.
Acid-Base Balance An acid is a substance that releases a hydrogen ion (H+) in a solution. Acidity is caused by an excess of H+ ions. A base is a substance that combines with a hydrogen ion (H+) in solution. Alkalinity is caused by an excess of OH- (hydroxide) ions. The acidity or alkalinity of a solution is measured by the ph scale. On the ph scale, 7.0 is neutral (meaning equal amounts of H+ ions and OH- ions). Numbers smaller than 7.0 mean greater amounts of H+ ions (acidic solutions). Numbers larger than 7.0 mean greater amounts of OHions (alkaline solutions).
The ph Scale
ph of Seawater The ocean has an average ph of 8.0, meaning it is slightly alkaline. Note that the ph of seawater varies slightly with depth.
Sea water samples are often collected using a Nansen bottle. The bottle is lowered to the depth the scientist wants to collect a water sample. Then a small weight is sent down the line. The weight hits a trigger, which causes the bottle to flip upside down and also close off at both ends. The sealed bottle is then hauled up to the surface. T Sampling Seawater
{And We have made from Water every living thing. Will they not then believe?} (Al-Ambiya: 21, Verse: 30) A solution is made of two components: solvent and solute. Seawater is a solution of water molecules (the solvent) and dissolved solutes consisting of both gases and solids (mostly ions). Water is a polar molecule, having a positive and a negative side. This polarity means water easily dissolves many ionic compounds. Salinity is the total quantity of dissolved inorganic solids in water. Average ocean water salinity is about 3.5 % or 35 ppt. The proportions of the major salts in seawater stays about the same everywhere. This is known as the principle of constant proportions (also called Forchhammer s principle). Salinity can be determined in several ways. These include: Chlorinity, Electrical conductivity, and Refraction. Salinity (in parts per thousand) = 1.80655 Chlorinity (ppt)
{And We have made from Water every living thing. Will they not then believe?} (Al-Ambiya: 21, Verse: 30) The ocean appears to be in chemical equilibrium. This concept is known as the steady state ocean. Residence time is the average length of time an element spends in the ocean. Residence Time = Amount of element in the ocean The rate at which the element is added to (or removed from) the ocean. Conservative & Nonconservative Constituents. Photosynthesis by plants and phytoplankton produces oxygen and uses up carbon dioxide. Respiration by animals, bacteria, and other organisms produces carbon dioxide and uses up oxygen. Acidity is caused by an excess of H+ ions. Alkalinity is caused by an excess of OH- (hydroxide) ions. The acidity or alkalinity of a solution is measured by the ph scale. On the ph scale, 7.0 is neutral. Numbers smaller than 7.0 mean greater amounts of H+ ions (acidic solutions). Numbers larger than 7.0 mean greater amounts of OH- ions (alkaline solutions).