What did I make [science fair experiment that dissolved drill bit]

Helping daughter with science fair project. She picked something simple - electrolysis of water. Simple enough, get distilled water, multimeter, DC power supply (plenty of old phone chargers), catalysts, jar, scale, wire for anode / cathode. We weren’t worried about capturing whatever gassed off, she wanted net weight loss after 24 hours. First few days were easy, tap water, softened tap water, pure distilled, sodium bicarbonate, sodium carbonate, pure 6% vinegar. I understood most off what happened even with using stainless steel wire which turned most of them reddish. The last one throws me though. I used pool chemicals as readily available and most importantly currently free. Our PH minus additive sodium bisulphate totally ruined the experiment. 1 tsp in distilled water with a total weight of 200 grams. Raised water temperature up to about 135F and dissolved the stainless steel cathode wire in about 1 hour. Anode just sat there bubbling eventually turning black. I then thought to self “Self bigger is better,” I grabbed a 1/4" carbide drill bit, connected it to the cathode power lead, shoved it in the jar and went to bed. When I awoke next morning there was no drill bit left below the water line, a lot of water was missing, and a black goo was on the bottom of the jar. When I filtered the black stuff out, the water had a grayish blue tint. What was it that dissolved my bit? Something spawned from the sulphate? I assume the black stuff was the carbon from the carbide bit. Where did the red iron go?

Once you added the electrolyte (the sodium bisulfate salt) you had an electrolysis cell and it became possible to oxidize the iron at one electrode (Fe(s) -> Fe+2(aq) + 2e-) and reduce it at the other (Fe+2(aq) + 2e- -> Fe(s)). This process is used all the time to electroplate materials, albeit usually with something more interesting than iron.

The black goo is going to be your iron I think, in some oxide/hydroxide/sulphate form or other.

I’m no chemist, but since sodium bisulphate is to lower pH, I’m guessing it’s a simple ‘acid dissolves steel’, with the electrolysis just a sideshow at that point (you could test that idea by running the same thing, except without connecting the battery).

It’s been ages since Chem but I think the reaction works this way

Fe + 2NaHSO[sub]4[/sub] --> Na[sub]2[/sub]SO[sub]4[/sub] + FeSO[sub]4[/sub] + H[sub]2[/sub]

The hydrogen from the sodium bisulphate is what drives the pH down so that leaves us to balance out the other elements. Iron Sulphate in solution tends to be blue so that would explain the colouring of your water.

Title edited to better indicate subject.

But acid should have dissolved the other stainless steel wire also unless there were a protective layer of something. Plus before I dumped it down the drain, I dumped in some baking soda to neutralize it. I had no reaction at that time.

In principle, strong acids can react with Fe, evolving H2, although for normal objects passivation by surface oxides (or any deliberate rustproofing) makes this slow. However HSO4- is not a strong acid.

It’s easy to electrolyze away iron, on the other hand, as anybody who works with steel in marine environments knows.

You only tested those on stainless steel.
You then tested bisulphate on stainless steel and then regular steel.

Your logic that the drill bit was more substantial than the stainless steel is faulty, the stainless steel is STAINLESS … the weak acid and weak hydroxide didn’t affect it.

Now test the regular steel with the other electrolytes… a drill bit won’t last long under electrolysis in the vinegar or bicarbonate.

It might go black in the sodium carbonate, as that is basic, and iron oxide in a basic environment is black. (not the only black iron compound …)
Carbon rods are a good cheap electrode for preventing reactions at one electrode… You can know that the carbon is not dissolving. you can plate it.