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ChemicalOceanography

Dr. David K. RyanDepartment of ChemistryUniversity of Massachusetts Lowell&Intercampus Marine Sciences Graduate ProgramUniversity of Massachusettshttp://faculty.uml.edu/David_Ryan/84.653

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Cl- has been Described as theUltimate Conservative Tracer

Highest concentration in SWNot biologically depletedNot chemically limitedOne of the longest Residence Times (1 x 108 yr)Generally pretty boringOceanographers have used Cl- concentration to define the concentration of ocean water massesConcept of Chlorinity = Cl- (+ Br-) content of SW

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Chlorinity (Cl)

Amount of Cl-, Br- and I- in grams, contained in 1 kg of seawater assuming Br- and I- replaced by Cl-

The number giving chlorinity in per mille of a seawater sample is by definition identical with the number giving the mass with unit gram of atomic weight silver just necessary to precipitate the halogens in 0.3285234 kg of the seawater sample (Jacobsen & Knudsen, 1940).

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Salinity (S)

Historical Definition - Total amount of solid material, in grams, contained in 1 kg of seawater when all carbonate has been converted to oxide, the bromide and iodine replaced by chlorine, and all organic matter completely oxidizedPractical Salinity Scale – Conductivity of seawater compared to KCl at 32.4356 g/kg (15 oC)

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Practical Salinity Scale(PSS 1978)

RT = C (sample)/C (std seawater)C = conductivity at specified temp. & pressureFormerly used units of parts per thousand (o/oo)Unitless since based on a ratioOften see PSU or practical salinity unitsCalibrate instrumentation with SW standard

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Absolute Salinity (SR)

SCOR/IAPSOScientific Committee on Oceanic ResearchInternational Agency for the Physical Sciences of the Oceans

WG 127 Thermodynamics & Equations of State of SW Density, Enthalpy, Entropy, Potential

temp.,Freezing temp., Dissolved oxygen, Alkalinity, TCO2 , Ca, Silica

SR = (35.16504 / 35) g/kg x S

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Precision in Salinityby Various Methods

1) Composition Studies of major components ± 0.01

2) Evaporation to dryness ± 0.01

3) Chlorinity ± 0.002

4) Sound Speeds ± 0.03

5) Density ± 0.004

6) Conductivity ± 0.001

7) Refractive index ± 0.05

8) Inductive Salinometer

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Relationship betweenSalinity & Chlorinity

S = 1.80655 Cl

See Website for Salinity Handouts 1 - 4

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CTDs

www.seabird.comwww.valeport.co.uk

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Ion-Ion InteractionsMany types – non-specific, bonding, contact, solvent shared, solvent separatedNon-specific i.e., long range interactions and the concepts of ionic strength, activity & activity coefficientSpecific interactions e.g. complexation, ion pairing (strong or weak)Millero cartoons

http://fig.cox.miami.edu/~lfarmer/MSC215/MSC215.HTM

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Non-specificInteractions -electrostatic in nature & limit effectiveness of

the ion

Long Range(Non-Specific)Repulsion

Long Range(Non-Specific)Attraction

δ–

OrientedOutward

δ + OrientedOutward

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Non-specific InteractionElectrostatic in natureLimits effectiveness of ion in solutionUse concept of activity to quantify effect

(accounting for non-ideal behavior in solution)

ai = [i]F γF(i) where ai = activity of ion i[i]F = free ion conc. (m or M)γF(i) = activity coefficient

of ion I ( < 1)a = [i] γIn short

Chemical Equilibria

General representation

a A + b B c C + d D

Where uppercase letters are chemical species and lowercase letters are coefficients (i.e. # of atoms or moles)

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Equilibrium Constant

[C]c [D]d

K = ---------------[A]a [B]b

where [ ] = concentration, usually molar

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True ThermodynamicEquilibrium Constant

o (aC)c (aD)dK = ----------------

(aA)a (aB)b

Fora A + b B c C + d D

Ko Defined for standard conditions of 25 oC,1 atm pressure and I = 0 (infinite dilution)

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Equilibrium Constant

[C]c [D]d

K = ---------------[A]a [B]b

where [ ] = concentration, usually molar

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Many types of K’s(equilibrium constants)

Ka for acid dissociationKb for base hydrolysisKw for water auto ionizationKsp for solubility productKf for a formation constantK1, K2, K3, etc. for stepwise formation constantsβ1, β2, β3, etc. for overall formation constants

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Solubility Equilibria

Ba2+(aq) + SO4

2-(aq) BaSO4(s)

or by convention

BaSO4(s) Ba2+(aq) + SO4

2-(aq)

We can write an equilibrium constant for rxn19

Solubility Product(equilibrium constant)

[Ba2+] [SO42-]

Ksp = ------------------ = [Ba2+] [SO42-]

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aBa aSO4

Ksp = ------------------ = aBa aSO4 1

activity of solid is defined as = 120

Solubility Calculated

Solubility (S) is the concentration of individual ions generated from an insoluble compound

BaSO4(s) Ba2+(aq) + SO4

2-(aq)

S = [Ba2+] = [SO42-]

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Solubility Calculation(continued)

Given KSP = [Ba2+][SO42-] = 2.0 x 10-10

Then S = √ KSP = √ 2.0 x 10-10 = 1.4 x 10-5 M

So S = [Ba2+] = [SO42-] = 1.4 x 10-5 M

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Activity Correction

a Ba aSO4KSP = --------------- = aBa aSO41Since

aBa = γBa [Ba2+] & aSO4 = γSO4[SO42-]

Substituting

KSP = aBaaSO4 = γBa [Ba2+]γSO4[SO42-]

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Solubility Calculation(completed)

Since

KSP = γBa [Ba2+]γSO4[SO42-] & γBa = γSO4

Then KSP S = -------√ γ2

To determine solubility of BaSO4 in a solution containing other ions (as in SW), you must calculate the activity coefficient (γ) 24

Two ways to correctfor activity

1) Correct each ion as discussed

KSP = aBaaSO4 = γBa [Ba2+]γSO4[SO42-]

2) Correct the equilibrium constant K

KSPK´ = --------- = [Ba2+] [SO42-]

γ225

Common Ion Effect

In seawater the total concentration of sulfate

is 2.86 x 10-2 moles/kg must use here ↓

KSP = aBaaSO4 = γBa [Ba2+]γSO4[SO42-]

KSPK´ = --------- = [Ba2+] [SO42-]

γ226

Water Hydrolysis(very important)

H2O H+ + OH-

Applying same rules for K expressions

aH+ aOH-Kw = -------------- = aH+ aOH-1

Where H2O (the solvent) is assigned activity = 1

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Remember pHpH is defined as the negativelogarithm of the hydrogen ion activity

pH = -log aH+

Given the numerical value Kw = 1 x 10-14

& Kw = aH+ aOH- then we can always calculate OH- from the pH

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pH Examples

At neutral pH aH+ = aOH- and

aH+ = √Kw = 1 x 10-7 = pH 7.00At seawater pH (e.g., 8.2)

aH+ = 1 x 10-8.2 = 6.31 x 10-9 M

Kw 1 x 10-14

aOH- = -------- = ------------ = 1.58 x 10-6 MaH+ 6.31 x 10-9

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Hydronium Ion

Water actually hydrolyses to form a hydronium ion (H3O+) rather than the lone proton (H+)

(Once again an ion-water interaction akin to those discussed previously)

For the sake of simplicity, we will refer to this species as H+ which is common practice

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A Note on Strong& Weak Electrolytes

Salts, Acids & Bases are all ionic compounds that dissociate (i.e., form ions) in water either partially or completelyComplete dissociation = a strong electrolyte

NaCl H2O Na+ + Cl- no equilibriumPartial dissociation = a weak electrolyteH2CO3 H+ + HCO3

- Ka1

HCO3- H+ + CO3

2- Ka2

Two step equilibrium = forward & back reactions31

Acid-Base EquilibriaFictitious Weak Acid (HA)

HA H+ + A-

[H+] [A-] aH+ aA-Ka = -------------- or --------------

[HA] aHA

The smaller the Ka the weaker the acid

Strong acids have no Ka it approaches infinity32

Acid-Base EquilibriaFictitious Weak Base (B)

Capable of accepting a proton (H+)

B + H2O BH+ + OH-

[BH+] [OH-] aBH+ aOH-Kb = ----------------- or ------------------

[B] aB

The smaller the Kb the weaker the base

Strong bases have no Kb it approaches infinity33

Ion Pair or ComplexFormation EquilibriaDozens of Ion Pairs form in SW & even

more complexes – deal with them the same way

Mg2+(aq) + SO4

2-(aq) MgSO4(aq)

aMgSO4Kf = ----------------aMg aSO4

Larger Kf = stronger formation – reaction 34