Chemical Speciation. OCN 623 Chemical Oceanography. 30 January Reading Libes, Chapter Frank Sansone

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1 Chemical Speciation OCN 623 Chemical Oceanography 30 January 2014 Reading Libes, Chapter Frank Sansone

2 Outline Up until now, we have assumed that ionic solutesdissolve 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

3 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 Overview Affects biological uptake (e.g., freecu 2+ is the bioactive species of Cu) Important in redox reactions (e.g., NH 4 NO 3 under oxidizing conditions) Affects other properties of seawater e.g.: MgSO 40 (ion pair) is main determinant of swsound absorption MgCO 30 is a major competitor for CO 3 2-, affecting calcification

4 Activity 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 propertiesof 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 = -nk f m n = # of ions per molecule of solute (e.g.,2 for NaCl, 3 for BaCl 2 ) k f = constant = 1.86 C kg water mol solute -1 m = molalityof solute (mol solute kg water -1 )

5 For NaCl solution with m = 2 mol kg -1 : Expect depression of C, but in fact get 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 nonideality increase [What might these be?] To understand these effects, we need to know the effective concentration of ions ACTIVITY

6 Calculating Activity Activity of an ion iis defined: a i {i} γ i m i where: γ i activity coefficientfor the ion i m i ion molality (measured concentration, molkg -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 γhas units of kg mol -1 (most texts) --OR Activity has units of molkg -1, and γis dimensionless (Libes)

7 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

8 Debye-Huckel equation log γ i = -A z i2 I where A = a constant for the ion i z i = ion charge I = solution ionic strength I = ½ m i z i 2 ( the sum of total charge from ions): (very roughly seawater) As I 0, γ 1 Ionic strength of actual seawater is 0.7 4

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

10 Note: Affect of increasing charge Ionic strength of seawater 0.7!

11 This table is valid for seawater (not freshwater) Use these values with the concentrations of each species(e.g., [MgHCO 3+ ]), not with the total elemental concentration [Mg]

12 Consider Dissolved Inorganic Phosphorus (DIP), which has phdependent speciation of free Orthophosphate: H 3 PO 4 H 2 PO 4 - HPO 4 2- (most important at sw ph) PO 4 3- Chemical Speciation Libes Fig. 5.19

13 But actual seawater speciation is far more complex, because you have to consider complexes and ion pairs: Libes Fig. 5.20

14 Ion Interactions 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 ionsthat are sharing electrons

15 Ionic bonding, spheres of hydration largely intact Covalent bonding, merged spheres of hydration Open University

16 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 nonbonded electron pair with a cation:

17 General Trends in Ion Speciation 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 SO 4 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: MgSO 40 absorbs sound in the kilohertz range -- affects sound propagation

18 Temperature and Pressure Effects Decrease temp, (mostly) increase pairing Increase pressure, (mostly) decrease pairing [ WHY??? ]

19 Speciation of Major Ions in Seawater Open University Compare with Libes Fig. 5.18

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21 Availability Reflects Single-ion Activity Coefficient and Ion Pairing! x =

22 Speciation Largely Controls Trace Element Distributions in Seawater

23 Speciation of Trace Metals 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; R 2 -NH; R-NH 2 ; R-SH)

24 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)