Can water electrolysis detect the water impurities?

A salesman came to my house to demonstrate his water purifier machine which is a RO Purifier. To show the difference between his purifier machine and the machine I own, he took some water from my machine in a glass. Then he pour water from a bottle he bought in another glass claiming that this was a purified water from his machine (little suspicious). Then he put a small electrolysis machine (that one we used for experiments in school) on both glass and started it. After a minute, there was lot of uhh... particles looking like mud floating in my water, while in his glass, there were very few in dissolved state.

So he said that those particles were salts and bad impurities which my purifier can not filter out, and his purifier can, thus having few particles in his glass. So my question is, can this method detect the impurities in water, or is it just some dirty sales tactic? Because as I remember, i was taught in school that electrolysis does not has to do anything with water, it just transfer cathode to anode or something like that (sorry for my bad chemistry knowledge :P)

• Which kind of "impurities" do You filter out with the "purifier" You have already? What is wrong with the water coming into Your house? Feb 22 '15 at 13:01

[…] can this method detect the impurities in water […]

In principle, yes. The electrolysis can, as an example oxidize iron ions ($\ce{Fe^2+ -> Fe^3+}$) The latter forms a brownish, voluminous oxide hydroxide, $\ce{FeO(OH)·n H2O}$.

The WHO suggests an upper limit of 0,2 mg/l iron in drinking water.

Am I right that your drinking water comes from a well? Did you perform any analyses on the the water quality or consult local authorities concerning possible health risks and their prevention?

[…] or is it just some dirty sales tactic […]

I can't comment on the dirty part, but salesmen need to sell. We don't know whether the reference sample was actually produced by reverse osmosis or just a sample of distilled water (which you would not want to drink over a longer time).

Even if reverse osmosis can reduce the mineral content in your drinking water, you might want to check whether the additional costs for operation are really worth it.

Again, try to obtain some information from local authorities.

• Depending on the electrodes used and the voltage applied, the cloudy material may well be caused by degradation of the electrodes themselves. Here's an interesting related video. Notice that after becoming more conductive, the water allows more current through, which speeds up corrosion of the copper wire. So naturally, after a certain amount of time at a given voltage, regular tap water with salts inside will become cloudier than distilled/deionized/RO water. Feb 22 '15 at 11:59
• @Klaus Warzecha Thanks for the suggestions! I live in a city, and the main water source is a dam. Government purifies it before supplying, I also use water purifier, but still there was impurity in my water, hence I was very surprised. Feb 22 '15 at 15:28
• It is entirely normal and indeed desirable for your drinking water to contain some dissolved substances ("impurities"). Unless you are in very poor living conditions, a combination of public water treatment and a home filter will be more than enough to ensure a supply of safe drinking water from the tap. Techniques such as reverse osmosis are only necessary in exceptional cases, like making seawater or stagnant surface water drinkable. Feb 22 '15 at 16:16