Ask A Scientist Chemistry Archive

 
name         Bob
status       educator
grade        6-8
location     IL

Question -   We are using sodium sulfate in a distilled water solution
as the electrolyte in a electrolysis procedure. By the end of the
process, the solution has become discolored. Could you identify the
product that accounts for that color change? On a larger scale, could you
identify a balanced chemical equation for the reactants and products in
this reaction. Thank-you for your time and expertise.
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What is the applied voltage you are using? I would expect the reaction
to be the hydrolysis of water: 2 H2O = O2(gas) + 2 H2(gas).
None of the components you mention should produce a colored product,
not the color of the product produced, which could be a lead.  However, you
do not mention the materials of construction of the electrodes. Side
oxidation reactions involving Fe, Cu at the electrodes could produce a
colored product.

Vince Calder
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I have to guess at some of the stuff you did not include in your 
description, Bob.
But I think this is the story.

Water (H2O) and sulfuric acid (H2SO4)  can be changed by electrolysis,
    at the anode to  oxidation products:
         oxygen O2,   hydrogen peroxide H2O2, peroxy-di-sulfuric 
 acid  (HSO4)2,  Caro's acid H2SO5,
    at the cathode to reduction products:
         hydrogen H2, sulfite-acid H2SO3,
but all of these products are colorless.

The metal of your anode (positive electrode) must be oxidizing and 
dissolving in the solution.
Metal ions are often colored.   Copper ions are blue or green or similar,
and iron(+3)  is yellow/orange/brown-purple, in order of increasing 
concentration.

Cu    -->   2e- + Cu(2+)     (The "2e-" electrons are pulled out of the 
cell, down the wire, to the positive end of your battery.)
Fe  -->  2e-  + Fe(2+)
Fe    -->   3e- + Fe(3+)
Fe(2+)  <-->  Fe(3+)  + e-

If your solution gets dark enough with iron, or if much of the sulfuric 
acid gets reduced at the other electrode,
you might see some powder forming due to precipitation with the metal ions 
you've made.
    2 Fe(3+)  + 3 SO4(2-)  <-->    Fe2(SO4)3   yellow/orange-ish
     Cu(2+)  +   SO3(2-)    <-->     CuSO3  reddish
However,  that kind of thing usually takes more amp-hours per milliliter 
than people give these experiments.

So look at what metal you used for your anode.
See if the color in the solution is one of the colors of one of the ions 
of that metal.
It is difficult to find a metal that will not electrolytically dissolve in 
acid solutions.
That is why people go to the expense of using Platinum anodes for 
electrolysis.
If you used graphite rather than a metal,  perhaps it had some metal 
dispersed in it.
The best grades of motor-commutator graphite have copper or silver in them.
I think I have seen some graphite break into fine black powder and make 
the solution start looking dark,
but that might settle to the bottom in a while.

Jim Swenson

PS- Palladium solutions can be dark ruby-red. (pinkish-clear in small 
concentrations).
So if you are already using jewelry-platinum for your anode and a color is 
forming,
maybe it is not 100% platinum, and palladium is part of the alloy.
White gold might have palladium in it too.
Gold, silver, platinum, and nickel do not usually have much color.
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