Chemical Speciation

OCN 623 – Chemical Oceanography

30 January 2014

Reading

Libes, Chapter 5

© 2014 Frank Sansone

Outline

Up until now, we have assumed that ionic solutes dissolve in

their solvent (water) as simple, single ions. We now look at

actual ion speciation in a complex solvent (seawater).

This lecture will cover:

• Overview of speciation

• Activity and its calculation; ionic interactions

• Speciation control of seawater distributions

Speciation defines the chemical reactivity of elements in

the ocean:

• Affects residence time (e.g., reactivity of ions vs. neutral species)

• Cation = positive ion

• Anion = negative ion

• Affects biological uptake (e.g., free Cu2+ is the bioactive species of Cu)

• Important in redox reactions (e.g., NH4 → NO3 under oxidizing conditions)

• Affects other properties of seawater – e.g.:

• MgSO40 (ion pair) is main determinant of sw sound absorption

• MgCO30 is a major competitor for CO3

2-, affecting calcification

Overview

Consider the dissolution of NaCl in water:

NaCl (s) → Na+(aq) + Cl- (aq)

If the reaction goes to completion, get 1 mole Na+ and 1 mole Cl- (i.e.,

2 moles of ions total)

Colligative properties of solutions (e.g., lowering of vapor pressure, elevation of boiling point, depression of freezing point) depend on number of ions in solution:

E.g., the depression of freezing point of water in dilute soln = ∆t = -nkfm

n = # of ions per molecule of solute (e.g., 2 for NaCl, 3 for BaCl2)kf = constant = 1.86˚C kgwater molsolute

-1

m = molality of solute (molsolute kgwater-1)

Activity

For NaCl solution with m = 2 mol kg-1:

• Expect depression of -3.72˚C, but in fact get -3.01˚C

• NaCl is acting as though there were less than 2 moles of ions

• Thus, solution is non-ideal

Ionic interactions cause this (and other) non-ideality

• As ionic strength increases, interactions and non-ideality increase [What might these be?]

To understand these effects, we need to know the effective concentration of ions ≡ ACTIVITY

Activity of an ion i is defined: ai ≡ {i} ≡ γi mi

where: γi ≡ activity coefficient for the ion “i”

mi ≡ ion molality (measured concentration, mol kg-1)

Activity coefficient:

• Can be calculated by equations

• Depends on ionic strength, temp, pressure

• May or may not be considered dimensionless:– Activity is dimensionless, and γ has units of kg mol-1 (most texts)

-- OR –

– Activity has units of mol kg-1, and γ is dimensionless (Libes)

Calculating Activity

In dilute aqueous solutions:

• Ions behave independently of one another

• γ = 1

• Activity = measured molality

As concentrations of ions in solution increase:

• Electrostatic and covalent interactions increase between ions

• Activities of ions decrease from measured (analytical) concentrations

Debye-Huckel equation

log γi = -A zi2 √I

where A = a constant for the ion i

zi = ion charge

I = solution ionic strength

I = ½ ∑ mi zi2 (the sum of

total charge from ions):

As I → 0, γ →1

Ionic strength of actual seawater is ≈ 0.7

4

(very roughly seawater)

As ionic strength gets larger, formula must get more complex:

• Interactions modeled are mostly electrostatic

• These equations only work for individual ions (not for ion complexes)

Note:

• Uncharged species not affected much by ionic strength

• Activity coefficients for uncharged species can be >1

Stumm and Morgan (1981)

γ

Note:

• Affect of increasing charge

• Ionic strength of seawater ≈ 0.7!

Use these values with the

concentrations of each

species (e.g., [MgHCO3+]),

not with the total elemental

concentration [Mg]

This table is valid for seawater (not freshwater)

Consider Dissolved Inorganic Phosphorus (DIP), which has pH-dependent speciation of free Orthophosphate:

• H3PO4

• H2PO4-

• HPO42- (most important at sw pH)

• PO43-

Libes Fig. 5.19

Chemical Speciation

Libes Fig. 5.20

But actual seawater speciation is far more complex,

because you have to consider complexes and ion pairs:

Interaction types

Non-specific: • Interaction between the ion

and the solvent• “Ordered shell” -- drops off

considerably with distance• Well developed when no other

interactions -- free ions (e.g., Na+,Cl-)

Specific interactions:• Continuum of effects• Weak ion pairs, sharing

hydration shells• Complex ions that are sharing

electrons

Ion Interactions

Open University

Ionic bonding, spheres of hydration largely intact

Covalent bonding, merged spheres of hydration

Coordination complexes:

• A metal ion (M+) is coordinated with (associated with) an electron-donating ligand (L-)

• Ligands are also called chelants, chelators, chelating agents, or sequestering agents (discussed later)

• Strongest bonds are favored -- greatest decrease in energy

• Ligands may include water, which can share its non-bonded electron pair with a cation:

• The higher the charge, the more likely it is to form a strong and abundant complex• Trivalent (and higher valent) cations associate with OH- at neutral pH (i.e., they react with water)

• Major cations are mainly present as free ions• Significant ion pairing of SO4

2-

• Cl- is assumed to be unpaired

• Because Cl- is very abundant, other anions are present in lower concentrations than are cations – other anions are thus are relatively more complexed

• Special-interest ion pair: MgSO40 absorbs sound in the kilohertz

range -- affects sound propagation

General Trends in Ion Speciation

Increase pressure, (mostly) decrease pairing

[ WHY??? ]

Decrease temp,(mostly) increase pairing

Temperature and Pressure Effects

Speciation of Major Ions in Seawater

Open University

Compare with Libes Fig. 5.18 �

Availability Reflects Single-ion Activity

Coefficient and Ion Pairing!

x =

Speciation Largely Controls Trace

Element Distributions in Seawater

• Not well understood, lack of thermodynamic data

• More likely to be complexed than major ions (effect of low concentrations)

• Functional groups on dissolved org matter can act as ligands

(e.g., R-COOH; R-OH; R2-NH; R-NH2; R-SH)

Speciation of Trace Metals in Seawater

Complications:• Major ions compete for sites

• Some biomolecules are highly specific for certain trace metals (e.g.,siderophores, hydroxamate for Fe, etc.)

The net result:• Some metal species in upper

waters may be predominantly complexed

• Complexation increases solubility in seawater

• Complexation affects bioavailability (toxicity, uptake)