seawater chemistry - kaurfarawati.kau.edu.sa/files/0002674/files/27868_chemistry of... · •...

© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

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)


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.

Chemical Equilibrium


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


Residence Time

This table summarizes

the residence times of

the different types of

dissolved materials in

sea water.


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.

Conservative & Nonconservative Constituents


• All types gas dissolve more readily in cold water than in warm

water.

• Nitrogen (N2) oxygen (O2) and carbon dioxide (CO2) are the most

common dissolved gases in seawater.

• Oxygen (O2) and carbon dioxide (CO2) 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


Dissolved Gases in Seawater

The figure shows how O2

and CO2 levels vary with

depth.

Photosynthesis by plants

at the ocean surface

(where there is sunlight)

causes high levels of O2

and low levels of CO2.

Below the surface where

there is not enough light

for plants, respiration by

animals causes lower

levels of O2 and higher

levels of CO2.


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 OH-

ions (alkaline solutions).

Acid-Base Balance


The pH Scale


The ocean has an average pH of 8.0,

meaning it is slightly alkaline.

Note that the pH of seawater varies

slightly with depth.

pH of Seawater


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)

http://www.searchtruth.com/chapter_display.php?chapter=21&translator=5


{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).



seawater chemistry - kaurfarawati.kau.edu.sa/files/0002674/files/27868_chemistry of... · •...

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