Exploring Our Fluid Earth

Concentration of Mixtures

Seawater is a mixture of many substances, the most abundant of which is water. Water, H₂O, is the solvent in the mixture of seawater. The properties of water determine many of the properties of seawater. The solutes in seawater, including dissolved salts, gases, and other substances, also contribute to the properties of seawater. Concentration is the amount of solute or solutes dissolved in a mixture. The concentration of dissolved substances in seawater affects the properties of seawater.

Concentration of Elements in Seawater

Concentration is a measure of the relative amount of dissolved substances in a solution. There are many elements dissolved in seawater and different amounts of each element, which means that each element has a different concentration. Concentration can be expressed in many different ways. Sometimes concentrations are expressed in terms of a particular compound. Other times, concentrations are expressed in terms of individual elements, regardless of the compounds in which they are found. In aquatic science, concentration is most often expressed in terms of the number of grams (g) of an element dissolved in 1,000 g of seawater. The unit for this measure of expressing concentration is parts per thousand, or ppt. For example, salinity is the measure of the number of grams of salts per kilogram (kg) of seawater. The average salinity of seawater is about 35 g/kg of seawater, or 35 ppt.

Other common units of concentration in aquatic science are parts per million (ppm), the number of grams of an element dissolved in 1,000,000 g of seawater and parts per billion (ppb), the number of grams of an element dissolved in 1,000,000,000 g of seawater.

Elements with concentrations measured in ppm or ppb are called trace elements. For example, if 40 g of a trace element were found in 1,000,000 g of water, the trace element’s concentration would be 40 ppm.

This means there are 40 parts of trace element for every million parts of the total sample of water.

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Fig. 2.18. Placement of the decimal point to show concentrations of major, minor, and trace elements in 1,000 g of seawater.

Image copyright and source

Image by Byron Inuoye

Fig. 2.18 shows how to read numbers in ppm and ppb. A common use of ppm is in measuring carbon dioxide in the atmosphere. In the ocean, ppb is often used to express the concentration of trace metals, like mercury or lead.

The concentration of elements in seawater is extremely important and can make a difference to the physical and biological properties of the water. For example, fish and other organisms require a minimum concentration of oxygen to live. If the concentration of oxygen goes below this threshold, hundreds of fish can die at one time, an event called a fish kill (Fig. 2.19).

Fig. 2.19. (A) A fish kill in Narragansett Bay, Rhode Island, due to extremely low oxygen concentrations (August 2003). Each silver spot is a fish.

Image courtesy of Chris Deacutis

Fig. 2.19. (B) A close-up of fish in the fish kill.

Image courtesy of Chris Deacutis

Concentration Categories of Elements

There are four different classifications of elements in seawater based on their concentrations.

1. Hydrogen and oxygen make up most of seawater. These elements make up the water molecule and parts of other compounds and are so abundant they are considered to be in their own category.
   
    
2. Major elements in seawater occur in concentrations greater than or equal to one-tenth of a gram per thousand grams of seawater (expressed as >0.1 g/1,000 g of seawater or >0.1 ppt).
   
    
3. Minor elements in seawater have a concentration between one-tenth of a gram per thousand grams of seawater and one-thousandth of a gram per thousand grams of seawater (0.1 to 0.001 g per 1,000 g of seawater, or 0.1 to 0.001 ppt).
   
    
4. Trace elements in seawater have a concentration of less than one-thousandth gram per thousand grams of seawater (<0.001 g/1,000 g of seawater, or <0.001 ppt).
   
    

Elemental Abundance in Different Systems

The concentration of elements in different systems is related to the sources of elements in those systems. For example, hydrogen and helium are abundant in the sun and other stars as they are common elements formed after the Big Bang. Oxygen and silicon are common in the earth’s crust as they are very stable elements that form the minerals that make up rocks. In seawater, oxygen and hydrogen are abundant as they make up the water molecule. Sodium and chlorine make up sodium chloride, a common salt. The source of many of the dissolved substances in the ocean is the Earth’s crust. Elements in the Earth’s crust that can easily dissolve in rain and river water are carried to the ocean by rivers and runoff. The most abundant elements in the human body, oxygen, carbon, hydrogen, and nitrogen, reflect the composition of the human body, which is made up of water and organic compounds. The concentrations of elements in seawater are affected by human activity, as opposed to the concentration of the elements in the sun or in the earth’s crust.