Sodium borohydride

[Lou]

This excellent reducing agent is clean, efficient, but slightly expensive. I think the hazards are over exaggerated in this msds, as the material is really quite tame when compared against other hydrides, that explode when contacting water. Borohydride is stable in alcoholic and even aqueous solutions for a period of hours. It is commonly used for reducing PGM salts to their ''black'' form.


MSDS courtesy of sciencelab.com

 

[zachy]

for me it is the best reducing agent, it is very useful for treating mother liquors with some PGM content, even though it acts on all metals, it is prepared in 20% NaOH, with 10% hydroboride finishes well, it must be added slowly because coming into contact with the acid medium of the mother liquors releases the hydrogen strongly.

 

[Lino1406]

Can you confirm this material precipitates even iron, or give pH limits to avoid that?

 

[razvanflorin]

This can precipitate gold from cyanide ?

 

[Lino1406]

Positive, it will

 

[Lou]

Yes, borohydride will precipitate gold (or other PGMs) from practically anything.

As for iron, it may be possible but I do not have experience supporting this.

 

[Lino1406]

We consider NaBH4 as anodic with redox potential around -1v. Hence the following divalent ions, with redox potential above -0.5v will be reduced by it: Fe++, Cd++, Co++, Ni++, Sn++, Pb++, Cu++,Pd++, Pt++

 

[zachy]

We consider NaBH4 as anodic with redox potential around 1v. Hence the following divalent ions, with redox potential above -0.5v will be reduced by it: Fe++, Cd++, Co++, Ni++, Sn++, Pb++, Cu++,Pd++, Pt++

But gold is like Au +, and in an alkaline medium, like borohydride, I don't see that I can precipitate it from alkaline media, verify your information, I thought it was only used in acidic media, so that borohydride can release hydrogen.

 

[Lino1406]

Borohydride gives electrons this way
BH4- + 3H2O = B(OH)3 + 7H+ + 8e-
E = from -0.481 to -1.24v, depending on pH. Hydrogen evolution can also reduce gold (not Fe), in a parallel path

 

[zachy]

In an alkaline environment, it does nothing and spreads gold from a bath of worn cyanide gold and does nothing, or it would have to wait a long time.

 

[Lino1406]

To be exact, best conditions are below pH=13, also avoiding excessive borohydride hydrolysis