SV: [sdiy] Solder won't stick to old PCB

[David G. Dixon]

--- Warning! --- Pedantry alert! --- Warning! ---

> toothpaste is a very strong base. It is alkaline and, if it reacts
> with the oxides on metal, could leave salt (?) residues on the
> surface.

Alkaline things don't really react with oxides.  Oxides are, after all, just
dehydrated hydroxides, which are alkaline.  To wit: consider the dehydration
of lime, conducted in a calcining furnace (rotary kiln, fluidized bed, etc):

Ca(OH)2 --> CaO + H2O

Slaked lime (on the left) is a hydroxide, lime (on the right) is an oxide.
These two wouldn't react with each other, because they are essentially the
same thing.

To make a salt requires adding an acid and base together.  For example,
hydrochloric acid and caustic soda made table salt:

HCl + NaOH --> NaCl + H2O

> to remove oxides you need a reducing agent, such as an acid.

While there are some reducing acids (and some oxidizing acids), acids are
not, generally speaking, reducing.  Sometimes, people talk about the
reducing power of acids in corrosion of reactive metals such as titanium,
but in this case, the reduction is indirect.  The acid is actually eating
away the protective ("passive") oxide layer of the metal, and the metal
itself is then reducing water to form hydrogen.  One of my grad students
almost blew up a titanium autoclave this way, by heating up a sulfuric acid
solution to 220 deg C without having a suitable oxidant such as oxygen or
cupric ions present to protect the titanium (by maintaining the TiO2 layer).

A reducing agent is something which donates electrons.  An acid is something
which donates protons.  (Conversely, an oxidizing agent accepts electrons,
and a base accepts protons.)  Sodium borohydride (NaBH4) is a reducing
agent, because it accepts electrons and is thereby converted to an oxide.
In the presence of a suitable catalyst (or maybe not), NaBH4 will reduce
water to hydrogen gas (a reaction exploited in fuel cells):

NaBH4 + 2 H2O --> NaBO2 + 4 H2

In this case, the boron is being oxidized from the -5 valence state to the
+3 valence state, thereby donating 8 electrons to the proceedings (a very
generous reducing agent, indeed!).  Those 8 electrons combine with 8 protons
to form hydrogen gas, which can be burned.

---- End of pedantry alert --- It is not safe to go about your business ---


> The eraser idea is very good imo.. haven't tried it myself, but I
> heard it's good for reconditioning faders / pots
> 
> D.
> 
> On Sun, Sep 27, 2009 at 9:19 PM, John Alex Hvidlykke <john at hvidlykke.dk>
> wrote:
> > Well, one of the most effective cures for tarnished silver (but probably
> not
> > as good a The Dip) is regular toothpaste. Might as well try that on my
> > oxidised component leads ;-)
> >
> > \John
> >
> >> -----Oprindelig meddelelse-----
> >> Fra: synth-diy-bounces at dropmix.xs4all.nl
> >> [mailto:synth-diy-bounces at dropmix.xs4all.nl]Pa vegne af David G. Dixon
> >> Sendt: 27. september 2009 22:04
> >> Til: 'Graham Atkins'; 'John Alex Hvidlykke'
> >> Cc: 'Synth DIY'
> >> Emne: RE: SV: [sdiy] Solder won't stick to old PCB
> >>
> >>
> >> They used to sell this stuff for de-tarnishing silverware the easy way,
> >> where you just dip the silver into this solution and it comes out
> >> perfectly
> >> shiny.  I think it might have been called "Dip-It".  I think that
> >> infomercial guy who just died (Billy something-or-other) was hawking
> >> something similar on TV recently.  I don't know what this stuff
> >> is, but I'm
> >> guessing it's some sort of reducing agent (sodium borohydride,
> perhaps?).
> >> When I was a kid, I used it on bronze coins, and it worked perfectly.
>  I
> >> think one could brush this stuff on oxidized copper traces/pads
> >> to the same
> >> effect.
> >>
> >> > Before now with badly corroded joints I've used a mildly abrasive
> >> > paste like
> >> > T Cut (For car paintwork) applied on a cotton bud, but clean well
> >> > afterwards.
> >>
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