Partial negative charge

Water is a dipolar molecule. Partial positive and negative charges reside on opposite ends of the molecule causing the molecules to orient themselves. The polar nature of water makes it a strong solvent for salts, compounds held together by ionic bonds

O

H H+ +

-Partial negative charge

Partial positive charges

H2O

Physical properties of water

O

H

H

+

+-

OH H

+ +

-

O

H

H

+

+-

OH H

+ +

-

O

H

H

+

+-

O

H

H

+

+-

OH

H

+

+

-

OH

H

+

+

-

O

H

H

+

+-

Despite its importance, there is still much uncertainty about the true nature of the molecular arrangement in water and ice – new papers appear all the time!

Water has an “ordered” structure

Unique physical properties of water.

• Unusually high boiling and freezing points (about 90 oC higher than predicted by molecular weight) - caused by the hydrogen bonds in H2O

• Extremely high latent heats of fusion and evaporation - - result from the energy required to break up the “ordered nature” of water molecules caused by hydrogen bonding.

• Very high surface tension - also caused by hydrogen bonding - water molecules want to stick together (think of a meniscus, or capillary action).

Colligative properties of waterCalled colligative because they act together – change in proportion to one another.

Adding solutes to solution:

Decreases the freezing pointDecreases the vapor pressureIncreases the boiling pointIncreases the osmotic pressure

Technically these properties change in response to the ratio of moles of solute to moles of solvent (molal units – mol/kg solvent).

Osmotic Pressure

= cRT

Where = osmotic pressure (in atmospheres)C is the concentration of substance in moles liter-1

R is the gas constant = 0.082 liter atm mol-1 oK-1

And T = temperature in oK

At 0 oC, seawater of 35 ppt has an osmotic pressure of about 25 atmospheres! That is enough to support a column of water 250 m tall, or in other words it is the same pressure experienced at 250 m depth in the ocean! A very powerful force indeed!

Temperature of maximum density for water

• Lakes freeze at surface with warmer (more dense) water beneath.

For fresh water the temperature of maximum density is ~4 oC and it becomes less dense at lower temperatures.

For seawater of salinity 35, the temperature of maximum density is -3.5 oC. This temperature is below the freezing point of 35ppt seawater (-1.92 oC). Therefore seawater continues to get dense as it approaches the freezing point and it sinks.

• The entire water column must be cooled to the freezing point for seawater to freeze.

When seawater freezes….

- Salt is partially excluded from the ice, so water below ice becomes more salty, hence more dense.

- Some salt stays in ice and forms brine pockets, giving sea ice a distinct texture.- Heavier isotopes in water molecules are slightly concentrated in the ice (e.g. 18O)

When sea ice melts …

- Surface water is freshened leading to stratification

Sea ice

Glacial ice

In this microscopic view of a thin section of natural sea ice, note the open spaces within the ice matrix (sea ice is porous, unlike freshwater ice). Ice algae (diatoms) live within these open spaces. These pigmented organisms discolor the bottom layer of ice cores.

(Photo by Christopher Krembs, University of Washington)

http://www.google.com/imgres?imgurl=http://www.whoi.edu/cms/images/oceanus/IceAlgae_71868.jpg&imgrefurl=http://www.whoi.edu/oceanus/viewSlideshow.do%3Fclid%3D27772%26aid%3D43826%26mainid%3D71868%26p%3D71854%26n%3D71856&h=450&w=600&sz=158&tbnid=UAJVVjglSQp1eM:&tbnh=101&tbnw=135&prev=/images%3Fq%3DIce%2Balgae&usg=___ZINH3ftK_Zuk33ScLZX-ly7Q7Y=&ei=iG-ZSpSnF8Ox8Qbd0PzFAQ&sa=X&oi=image_result&resnum=1&ct=image

Oden Station 2 - Chl a

Chl a (g L-1)

0 10 20 30 40 50

De

pth

in c

ore

(cm

)

180

160

140

120

100

80

60

40

20

0

Core 1Core 2

Oden Station 2 - DMSPt

DMSPt (nM)

0 50 100 150 200

Dep

th in

cor

e (c

m)

180

160

140

120

100

80

60

40

20

0

Core 1Core 2

Oden Station 2 - DMSP/Chl a

DMSP/Chl a (nmol g-1)

0 200 400 600

De

pth

in c

ore

(cm

)

180

160

140

120

100

80

60

40

20

0

Core 1Core 2

Antarctic Sea Ice – Ice Core Data

Kiene, unpublished

Salts, ions, chemical charge

Salt - def. A molecule containing one or more cations and anions held together by an ionic bond arranged in a crystal lattice.

Inorganic salt: Na+ + Cl- <=> NaCl (sodium chloride)

Organic salt: Na+ + CH3COO- <=> CH3COONa (sodium

acetate)

• Cations - positively charged ions• Anions - negatively charged ions

• neutral solutes - no formal charge on molecule, e.g. glucose, benzene etc. Some neutral solutes are composed of charged species that join together in solution to form a neutral species in solution e.g. Ca2+ + SO4

2- <=> CaSO4o(aq) A neutral ion-

pair

Dissolution of NaCl by hydrationSalt crystals dissolve through their interaction with the solvent. The polar water molecules neutralize the electrostatic forces holding salts together causing the ionic bonds to break. Water molecules “hydrate” the resulting ions.

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

NaNa++

ClCl--ClCl--

ClCl--

ClCl--

ClCl--

ClCl--

ClCl--

ClCl--

ClCl--

ClCl--

ClCl-- ClCl--

ClCl--

ClCl--

ClCl--

Crystals are more, or less, soluble depending on lattice energy and degree of order

NaCl(s) <=> Na+(aq) + Cl-

(aq) aq = aqueous = hydrated = solvated

What are the major salt ions in seawater?

Six ions constitute > 99.8% of all dissolved solids in seawater

Cl-chloride

SO42-

sulfate

Na+

sodium

Ca2+

calcium

Mg2+

magnesium

K+

potasium

Cations

Anions

Seawater contains 33-37 grams of dissolved solids per kg of seawater.

Why are major ions so abundant?

Because they do not undergo appreciable removal reactions relative to the water replacement time.

Slow removal results in major ion elements having long residence times in the ocean.

The residence time of Cl- in the ocean is over 100 million years!

For the most part - the elements with the longest residence times have the highest concentrations.

Residence Time (y)

Con

cent

rati

on (

M)

C

Mg

C

SCa

K

P

108102 106

1

10-4

10-8

10-12

Na+ and Cl- are VERY unreactive chemicals in aqueous solution and are only slowly removed from the ocean!

Residence time vs. concentration of different elements in seawater.

Residence time (τ, tau) = (reservoir size)/(input flux) or pool/flux. It assumes steady state, that is, pool size does not change with time.

Ocean(mass of Na+)

Riverine input of Na+

Loss of Na in sedimentary pore waters

Loss of Na in evaporites

Atmospheric dust input of Na+

At steady state:

Inputs = outputs (sources = sinks)

influx

pool

(Mass/time)

(Mass/time) (Mass/time)

(Mass/time)Example of Na+ residence time in ocean

In this example we are considering the amount (mass) of sodium (Na+) in the entire ocean divided by the sum of all the inputs (which balance the exports because steady state is assumed). At steady state, the amount of the element in the ocean (i.e. the pool size) does not change appreciably over time.

(Pool size)

Important!

Steady State is a relative concept – it depends on the time frame of reference. The oceans are in steady state from year to year, but on billion year time scales they are not.

Time

Pro

pert

y

Apparent steady state - over short time interval

Time

Pro

pert

y

Apparent non-steady state condition

Seawater Charge balance - The major ions of seawater include 4 cations (Na+, K+,Ca2+, Mg2+), and two anions (Cl-, SO4

2-), which together make up > 99.8% of salinity. Chloride is by far the major anion, with a concentration of ~540 mM compared to sulfate 28 mM. Seawater is about 86% NaCl in terms of mass.

Seawater, like all solutions, is electronically neutral. The negative charge of anions is balanced by the positive charges of cations. This must be satisfied everywhere.

Absolute salinity, SA is defined as the ratio of the mass of dissolved material in seawater to the mass of solution indicated as g/kg or ppt or o/oo.

In practice, absolute salinity cannot be measured directly on seawater samples, so a Practical Salinity Scale was developed in line with conductivity methods for determining salinity. The symbol S, is used for reporting salinity.

Salinity

The practical salinity scale defines a 35.000 salinity point when a seawater sample has an electrical conductivity equal to that of a KCl solution (@15 oC and 1 atm) containing 32.4356 g KCl in a mass of 1 kg of solution (equivalent to 19.37 o/oo chlorinity).

The ratio of conductivities (R) between that of seawater and the 35.000 o/oo reference solution is used to calculate practical salinity

Calculation of practical salinity from conductivity ratios

A crude, but simple way of determining salinity is to use a refractometer which measures the amount of dissolved mass in a given volume of water - Refractive index is a function of total dissolved solids.

Neutral solutes (i.e. sugars) also will contribute to “salinity” determined with a refractometer.

Refractometers work well for seawater (+/- 0.5 ppt) since neutral species are insignificant compared with the major ions.

Physical factors that can alter the salinity of seawater

• evaporation

• precipitation i.e. rain or snow

• freezing

• thawing

• diffusion of ions between water masses of different salinity

• turbulent mixing of water masses

The Constancy of Seawater Composition

The relative ratios of major ions in seawater are constant anywhere in the ocean (Marcet's Principle). The concentration of each ion is conservative with salinity (changing only with addition or removal of water).

Therefore, measurement of one (usually Cl-) is usually all that is necessary to estimate salinity with great precision.

Salinity = Chlorinity * 1.80655 UNESCO (1962) (now rarely used)

Constancy also extends to trace elements in seawater B, Br, Fl, U, and Cs.

The major ion Ca2+ is essentially constant, but slight depletions (<0.5%) in surface waters result from biological precipitation of CaCO3.

Taken from Pilson, Introduction to the chemistry of the sea. Millero has similar figure in 2.23, presented as normalized Ca (its normalized to salinity)

The major ion Ca2+ is mostly conservative with salinity, except for ~0.5% depletion in surface waters due to biological uptake.

[Ca2+] normalized to salinity

• The Ocean mixing time is approximately 1000 y (103 y; about the time for one cycle of the Ocean Conveyor). Elements with residence times longer than several mixing cycles (say 104 y) will be homogenized with respect to the other elements - yielding a constant ratio amongst these elements.

• Some biologically active elements do not fit this pattern - N & P for example have long residence times (due to biological recycling), but are heterogeneously distributed due to biological processes.

Why are major ions in constant proportion?

Deviations from constancy rule are found in:

• Anoxic basins• pore waters• ice formation areas• hydrothermal vents• evaporite basins

Why?

End

Partial Molal Volume

PTmj

jin

VV ,,)(

The change in volume of solute j (Vj) per mole of solute j (nj) to a solution where the total molality of the solution (mi), the temperature (T) and the pressure (P) is held constant (the addition of solute must be small relative to the total volume of solution so that total molality can remain essentially constant i.e an ideal solution.

Example: 1 mole of H2 gas at 1 atm pressure and 25 oC is about 25000 cm3. The V for H2 in water at the same T & P is only 26 cm3!

1 mole of solid NaCl is 27 cm3. In aqueous solution, the VNaCl is 16.4 cm3.

Isotopic composition of water - SMOW - Standard Mean Ocean Water - now known as VSMOW (Vienna SMOW)

This water is the reference material for isotopic analyses of D (del-deuterium) and 18O.

Water containing 18O instead of 16O is two mass units heavier per molecule and thus is more dense, and a tiny bit slower to evaporate or react in a chemical reaction. A similar thing can be said of water containing one atom of D but in this case the mass difference is smaller (1), so we would expect less fractionation.

When water evaporates, the heavy water is preferentially left behind yielding isotopically heavier (more positive 18O) ocean water and isotopically lighter H2O vapor.

• Different ocean water masses have different isotope signatures that behave as conservative tracers, aiding in distinction of mixing patterns in the ocean.

As water vapor moves through the atmosphere, precipitation removes the heavier isotope preferentially (same principle as in the evaporation) and the vapor becomes lighter still.

+9.47+1.23Glacial ice maxium

-8.97-1.15Melt all present continental ice

δDδ18O

Change in oceanic values

• Since water vapor transport is generally from tropics to high latitude, snow deposited at high latitudes has a lighter 18O isotopic composition than precipitation at lower latitudes. This shifting of the isotope signatures of natural waters can be used to trace processes such as ice sheet buildup during glacial periods, paleotemperatures and ocean temperatures.

Natural waters differ in their isotopic composition

Because there are three naturally occurring isotopes of both hydrogen (1H, 2H, 3H) and oxygen (16O, 17O, 18O) there are many combinations that H2O can take:

1H216O is the most common form and is generally

written as H2O.

2H is called deuterium and is designated as D. 3H is called tritium and is designated as T. You can have HDO, D2O, HTO, DTO, H2

17O, H218O, D2

18O, etc. Generally the light isotopes are more abundant so the combinations with heavier isotopes are relatively rare.