Chemistry of the Marine Environment

Chemical Equilibrium and Speciation in Seawater

This equation is used both in hydrothermal vents but also to describe gases in the swim bladder of fish.

Acid and Bases

The first concept we use is the one of speciation: any element of the periodic table can be present in the seawater as different species.

An example is I: it can be present as I-, IO-, IO^2-. The speciation determines the fate of each compound, the bioavailability (iron is an example) or the potential toxicity (an example is Cr: Cr⁶⁺ is carcinogenic, Cr³⁺ isn't).

Many of the compounds present in the seawater are present as different species with different availability or distribution.

The most important variable for speciation is the pH, the degree of acidity of our water. One of the easiest ways to measure it is to observe the amount of CO₂ in the seawater, which may work as a buffer system.

All the processes involved in the transformation of CO₂ are also involved in the buffer capability of the ocean (this influences the speciation of all the compounds).

other compounds).

Acids and Bases: most of the acid-bases reactions are very fast so chemical equilibrium and the apparent equilibrium constant can be used to describe them. In most of the equations we will use H+ as the concentration of protons in the water even tough every ion is hydrated by water molecules so it is only present as H3O+, or better as H3O+ (coordination with four molecules), so it is a simplification.

According to the Arrhenius definition:

• Acid is a substance that dissociates in water to produce H+ (also called as Brønsted acids):
  HA ⇌ H+ + A-(aq)
• Base is a substance that dissociates in water to produce OH- :
  BOH ⇌ B+ + OH-(aq)

Brønsted-Lowry definition:

• an acid is a substance that gives the H+ to another substance (base)
• A base is a substance that accept H+ ions from another substance (acid)
• The reaction determine the formation of a conjugated base and a conjugated acid and the reaction takes place

or to accept H+. It is not an absolute value but it always acts in relation to another compound, a reference is always needed. The reference species is water: the strength of an acid or a base is measured thanks to its capability to react with water. The parameter used is the apparent dissociation constant (K or K_a). Strong acids are those that are completely dissociated into seawater (an example: oxyacids). To quantify the strength we have to introduce the equilibrium constant: the reference is water.

In water, the molecules can undergo a self-dissociation:

H₂O + H₂O <—> H₃O⁺ + OH^-

The equilibrium constant of the reaction is Kw = [H₃O⁺][OH^-]³ (The value is dependent on temperature, salinity, and pressure. The [H₂O] is not present at the denominator of the equation because it is yet embedded into the values of K). Kw is equal to 1.008*10^-14. In pure water, the concentrations of H₃O⁺ and OH^- are the same: Kw = [H₃O⁺]² = [OH^-]² which is 10^-7.

We can have a table in which we have values

of Kw depending on temperature and salinity. Values are obtained thanks to the algorithm of Millero (non linear equation).

Chemical “p” concept: it refers to the -log(of a quantity). For example pK is the -log (K). Used to handle easiest number (K can be 10^-14, pK instead is 14. Easier to use!).

Same thing for pH: the pH is equal to -log[H O+]³ (pOH can be similarly defined).

Equilibrium constants for acids and bases reactions: the constant for the self dissociation of water is Kw, the constant for a dissociation of an acid is Ka (gives the strength of the acid), the one for the bases is Kb (strength of the basis).

Ka = [H O+][A-]/[HA] Kb = [HA][OH-]/[A-] (water is absent because the concentration is always constant when the values of temperature, salinity and pressure are fixed).

Into the seawater K is the apparent equilibrium constant (describes the activity of species under investigation, they come out from the constant medium reference state).

We can also note them as pK values (pK = -logK).

What happens when the reactant is low in concentration?

When the reactant is so low in concentration that any error in its definition will give a rate of error too high, the quantification is not accurate.

What are the major constituents in this scenario?

All the major constituents are very weak bases or acids conjugated with a strong acids or bases.

What is the explanation for the residence time of some species?

The residence time of some species may increase because they are very weak, making the reactions unlikely to happen.

What can be determined by knowing the system of the ocean?

By knowing the system of the ocean, including molecules in solution and equilibrium constant, calculations can be performed in equilibrium conditions, speciations can be determined in function of the pH, and what happens in a buffer solution can be understood.

What are the steps to determine the pH and species at equilibrium?

1. Establish...

The species in the solution (we have to know all of the species that may be formed during certain reactions and dissociations. The 2 species that are always present are H+ and OH-, coming from the self dissociation of water)

• Describe the equilibrium in solution and use the corresponding equilibrium constant
• Determine the mass balance equation
• Write down the condition of electroneutrality
• Solving the system of n equations for n variables (any chemical species in the system)
• Apply appropriate approximations to simplify the problem (accurate results avoiding some steps)

On the slides there is an example of an exercise (C is the sum of all possible species in which the A element A can be present. For example HCl is present as such and also as Cl-. C is the summatory of the concentrations of HCl and Cl-. This may be important for example when dealing with nutrients, when it is important to evaluate the presence of all of the species containing a certain

1. Mass balance: the total concentration is the summatory of all of the species in which my element is present. It acts in a so called close system: this means that in a box model if the total concentration is constant this means that I am neglecting all of the exchanges (it is a close system).
2. The last equation that we have to resolve in the four system equation is the equation for the conditions of electroneutrality (HA is not influencing the electroneutrality because it doesn’t present charges. Only H+, A- and OH- are considered. The summatory of negative and positive charges should be equal to 0 to be in an electroneutral system).
3. Once we have the 4 equations we have to solve them.
4. At the end the result that we need to have is the [H+] as the value to look for, which will give us the informations regarding pH.
5. In order to simplify the system we can approximate (“approximate solution”). In additive equations tiny terms are negligible (they can be removed!).

When considering an acid, the assumption is that the [H3O+] >> [OH-] so this second term can be neglected. In the electroneutrality equation, by neglecting OH- we see that [H3O+] = [A-]. The same process can be done in all of the other equations: the system can be now resolved removing the OH- (the resolving process can be followed on slides). What we obtain at the end is a second degree equation, easier to resolve.

Simplification can be further performed: the enhanced simplification will consider the fact that HA is very weak (Ka is 7 x 10^-10, pKa is around 9.2, for this reason we consider it as weak. At the equilibrium the highest amount will be as HA, the ratio between [A-] and [HA] will be around 10^-3, since the H+ is around 10^-7. The dominant species at the equilibrium is HA) so only a very small amount of it will be ionized: [HA] >> [A-] at the equilibrium. So the total concentration will be almost equal to the [HA]. Now we have only two variables, the system is even easier.

To resolve. The results obtained do not deviate too much from the first result (if the error is <_ 5% the approximation is justified).

Another system we can try to resolve is the one of H₃PO₄, a triprotic acid (which is also one of the most important nutrients into the water). Expected species in solution are 6. (Resolving process on slide) (P is the total amount of phosphorous into the seawater). Simplifications will be made thanks to the knowledge of the equilibrium constants of the dissociations reactions.

In the electroneutrality equation we have to consider equivalents (Eq = [M] * z)!x x x

Assumptions to simplify:

• since it is an acid, [H₃O⁺] >> [OH^-]
• As K₂ and K₃ are negligible with respect to K₁ is a reliable as