.. _example-phreeqc-manual-04:

04 - Redox
==========

Rainwater from Central Oklahoma, evaporated to a twentieth of its volume, and
what the redox chemistry does while that happens.

Two things are being shown. Evaporation as a *reaction* -- removing water
rather than adding a reactant -- and what happens to a solution that contains
both an oxidised and a reduced form of nitrogen when it is asked to come to
redox equilibrium.


Evaporation by removing water
-----------------------------

.. raw:: html
   :file: study_04 - Redox.html

There is no "evaporate" keyword. Evaporation is written as a reaction with a
negative coefficient:

.. code-block:: text

   REACTION 1
           H2O     -1.0
           52.73 moles

-- 52.73 moles of water taken out, which is the amount that concentrates the
solution twentyfold. The second simulation then uses ``MIX`` with a factor of
20 to scale the result back to a kilogram of water, so that the concentrations
are reported in the usual units.

This is worth seeing because it is how PHREEQC handles most processes that
are not obviously a reaction. Anything expressible as adding or removing a
quantity of something is a ``REACTION``, and the sign is the whole
difference.


The redox part
--------------

The rainwater analysis carries nitrogen in two oxidation states at once --
``N(-3)`` as ammonium, 0.208 mg/l, and ``N(5)`` as nitrate, 0.237 mg/l. In
the real rainwater they coexist; thermodynamically they should not, because
ammonium and nitrate are not at equilibrium with each other in an oxidising
solution.

When PHREEQC is asked for an equilibrium distribution it resolves this: the
two are brought to the same redox potential, which in practice means the
reduced form is largely oxidised. The nitrogen total is conserved, and how it
is divided between the states changes.

This is the general caution about redox in geochemical models. A measured
analysis records what was there; an equilibrium model reports what would be
there if the couples had come to equilibrium, which in natural waters they
very often have not. The difference is not an error in either -- it is what
the two are for. :ref:`example-phreeqc-manual-01` shows the other side of it,
where particular elements are told which couple governs them.


The result
----------

.. csv-table:: table_ex4
   :file: table_ex4.csv
   :header-rows: 1
   :delim: ;

The table gives the evaporated composition and the redox-equilibrated
distribution.


Source
------

* Parkhurst, D. L. and Appelo, C. A. J. (2013). *Description of input and
  examples for PHREEQC version 3.* U.S. Geological Survey Techniques and
  Methods, book 6, chapter A43. This is Example 4 of that manual, using a
  precipitation analysis from Central Oklahoma.