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Name: Carl
Status: student
Grade: 12
Location: MI
Country: USA
Date:  Fall 2011  

Question:
When borax is heated (naked flame) it turns into borax glass. What substance will dissolve borax glass? In other words, how can I get the borax glass back into solution? Is there an acid that will dissolve the borax glass?

Replies:
I think brass is a distinctly better idea than stainless, to be used in contact with large areas of copper. Brass is copper plus zinc, so you are adding only one new element to the whole dynamic decaying system. The zinc will try to corrode sacrificially in preference to the copper flashing, so I think the brass screws may corrode some in a few years.

But the iron in stainless will also try to protect the copper so the stainless steel's own corrosion will be accelerated much like the brass. As an abstract chemical system, it is more complex than copper+zinc, now you have copper+iron+nickel+chrome, in varying portions, variably oxidized and hydrated all water-streaked down the flank. This complexity often makes more ways to accelerate corrosion.

Oxidized zinc is the protective skin which tries to delay corrosion of brass, and oxidized chrome is the protective skin for stainless. Some would say chrome oxide is tougher than zinc oxide. That may or may not be what matters. Either may work a little better depending on the exact stainless and the exact environment. But it could be that the color of the resulting corrosion streaks blending gracefully with the copper sheet ends up being the difference that matters most to you. Iron-rust on copper is usually kind of ugly. I have some thought that maybe rust painted on copper will hurt the copper a little, accelerate its long-term corrosion. Certainly it will never grow clean green patina, should your flashing live that long. And brass corrosion-products will be easier to remove cleanly with acid and/or abrasion than rust and other decay products of stainless.

If you can find "silicon-bronze" in boating hardware stores, that would be even better; it's 98% or 99% copper, no zinc or tin, and the remainder is silicon which grows a glassy oxide which is very strong in normal environments. Then you have copper/glass fasteners against copper sheet, which creates no galvanic difference, and it should last very well indeed. It is expensive, but for small jobs that matter I use it. It does get black in something like 5 years, but it does not shed and streak much. It should have little effect on the appearance of the slowly corroding copper sheet around it.

Jim Swenson


Carl -

Bunch of thoughts. Phosphoric acid does a similar thing. Boil water out of it, its boiling point climbs, and it makes thick liquid that freezes into tough clear glassy stuff which is surprisingly slow to dissolve in acids. I think that the oxide is developing a partly-covalent bonding network, much like SiO2 has. Covalent bonds have high activation energies to make/break, so I heat is a key tool to dissolve it. Any strong acid _hot_ will dissolve your borax glass.

Temperature-cycling your glassy borax or giving it a lot of time at elevated-but-below-melting temperature might make it recrystallize and be faster to dissolve.

Consider that it may be dissolving steadily before you, but the slow rate gives the appearance of no progress. For example at about 1 micron per minute, it would take maybe a day to dissolve 1mm thickness. And the actual rate varies by substantial factors. Using long times and measuring progress drying and weighing milligrams would help.

Grinding it to fine powder first is another key tool, if it is not coated thinly on a valuable object.

Chloride ion and Fluoride ion tend to be catalytic for breaking these oxide networks.

HF + Nitric acid, with slight dilution, would probably be the fastest at a given temperature. But it is also one of the most dangerous substances and maybe you do not want to get involved in that. Cannot begin to use glass with this stuff.

4: HCl + 1:HNO3 + 4: H2O comes to mind. It will make brown fumes. Less Nitric, less brown, and maybe the nitric is not much needed. Fastest used warm in a deep large 50-100ml glass test-tube, cold at the top end and covered so the acids cannot evaporate away very fast. I stand mine in an Erlenmeyer flask (you know, conical, wide at bottom) 20% full of water to make an easy double-boiler and get 100degC.

Concentrated phosphoric acid is also special for tearing apart oxide networks. It only works when hot. 100degC is probably enough. Hotter than that is stronger.

And there is the idea of a hot concentrated solution of lye, sodium or potassium hydroxide. Most geologic rocks can be dissolved in molten NaOH+KOH in nickel beakers, perhaps with some Na or K nitrate. Your borax should be easier than that, so you can allow some water and use 100C instead of 200C+.

Consider not using a glass beaker; it will start dissolving a little too. The silica that goes into solution sometimes then slows down dissolution of your sample. What to use then is unclear, probably virgin (white) Teflon. Such fluorocarbons work best, but are expensive, and often take more effort to get the heat into. Polyethylene is chemically safe from the acids, but will melt if heated enough. Some metals might work, but their chemical vulnerability varies widely. Nickel, for example, corrodes faster when water is added to the NaOH. Some would try to use a microwave oven, but it is dangerous, and how to keep the temperature steady and right for a long time is a problem. I think you have to use Teflon in a double-boiler or a thick metal pocket-plate for heating.

It can be a mountain to climb, but your borax probably only needs any half of it. Welcome to the "I dissolve rocks" club.

Jim Swenson


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