Electrolysis of Solutions

So far I've only encountered formulas to solve for solutions of single molecules while learning about electrolysis, i.e. H2O or FeCl3. This generally has involved determining the resultants, identifying whether they are attracted to the anode or cathode, and performing some multiplications.


What approach is used for analyzing complex solutions? For example, an everyday soda solution may contain water (H20), fructose (C6H12O6), phosphoric acid (H3PO4), and carbon dioxide (CO2). I'm not sure how to approach the resultants. It would seem the decomposition potential should for each molecule should first be considered to identify which molecules are likely to decompose, but given so many elements how can we determine what resultants will remain? It would seem C6H12O6 alone could result in 6(C) + 6(H2O) or in 6(C) + 6(H2) + 3(O2), for example.





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> I'd like to learn how to approach more complex problems. Is there a
> standard set of steps to apply, perhaps iteratively?
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Every electrolyte has its own decomposition potential and a stability window. One can carefully design mixtures and study their electrolytic behaviour, for example, what happens if we electrolyze an aqueos solution of transition metal salts. There are no standard set of steps or iterations, although for simple ionic cases, people look up the electrode potential table to predict to products of electrolysis. For example, in higher classes one might study the electrolysis of KCl and KI in water. What products do we expect under standard conditions? These can be answered systematically. Electrolysis products of random mixtures like soda are difficult to predict, because once you have organic compounds, they start reacting at the electrodes via radical reactions.


For the conceptualization of electrolysis, take binary mixtures which represent soda such as a very dilute phosphoric acid and citric acid solution. If you electrolyze this mixture at the decomposition potential of water, you will the see expected products, hydrogen and oxygen.


Let us say you have water and tons of fructose in it (this is why sodas are bad!), it will almost be a non-conductor of electricity. Simple underivatized sugars are not good electrolytes. On the other hand, carefully controlled (pH adjusted) solutions of sugars can be oxidized. Search electrolysis of carbohydrates.


Again think about the electrolysis carbon dioxide saturated water; this solution will also be a poor conductor of electricity. Still since we will have plenty of water molecules, and using the right decomposition potential, hydrogen and oxygen will be the main electrolysis products.


If you are still curious, search voltammetry of mixtures.


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Fructose and $\ce{CO2}$ are covalent solute molecules. They are not modified by electrolysis when dissolved in a solution of phosphoric acid. Fructose, for example, does not produce ions in solution : it will never be decomposed (by electrolysis) partially into fragments, or totally into $\ce{C, H2}$ and $\ce{O2}$. Water is a solvent, made of covalent molecules. It can be electrolyzed.


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