Hello All,
It is well known that using stannous chloride as a test for pm's is a standard practice. Would not using a concentrated solution of smb give the same indications. Both actually drop pm's out of solution and you see it as a dark stain indication but the stannous (tin II chloride) does have a shelf life of 4-6 weeks, liquid smb does not have a shelf life and a batch should be fresh for a long time.
Am I wrong in my thinking?
Cheers
You don´t think about it in the wrong way. All you said is true.
But as it goes many times, it need deeper insight. Because from your assumption, it would all work well... But there are several facts you did not considered - which are determinal to this idea.
Most important omitted fact is what happen to gold when it contacts SMB/SO2 solution and stannous chloride. Gold has very high oxidation potential and could be dropped with thousands of substances which are available. Formaldehyde, oxalic acid, SMB, hydrazine, hydroxylamine, glucose, stannous chloride, iron (II) compounds, zinc, copper, iron, borohydrides, oxidizable phenols etc...
All of these "drops" are unique in the way the gold particles form. Think about that particular step. Gold is in the form of ions in solution, isolated one from another in the liquid. When you add the reducing agent (SMB or stannous), that
SINGLE separated ions turn into monoatomic particles of gold (
keep it just as simplification, it is more complicated). But if it stopped there, you cannot be able to ever recover any gold from solution, because we simply cannot filter single atoms from liquid.
What happen next is
agglomeration. These persumed monoatomic gold particles start to clump together to form larger agglomerates of particles, firstly few nanometers, then larger. And this is the point.
All of these reducing agents act differently in the terms of producing aglomerated particles or nanoparticles. Different physical processes on the phase interfaces changes polarities of the particles, differing the way they agglomerate together to nanoclusters.
With SMB, the solution has different properties for agglomeration, giving the particles that are prone to clump together due to specific action of this reducing agent.
With stannous chloride, the "clumping" effect is not so strong, so the gold form ultrafine/nanoparticles suspension which scatter and block the light from coming through the solution - and you see it as violet or black colour (depending on the size of particles formed).
That is why testing the gold presence with SMB is not the best idea.
Few larger particles cannot be detected with your eyesight, because light scattering is minute. On the other hand,
billions and billions of small particles of gold formed with stannous chloride are clearly visible, because they scatter the light effectively, and you can clearly see the colour change.
This is by the way also influenced by concentration of the gold in solution. You can notice that if diluted gold solution is tested with stannous, you get faint violet colouration. When it is strong, colour is more black and you can after some time see the black sludge forming on the bottom of the plastic spoon, if you are testing by dropping stannous to the gold solution (not on the paper, which is more common practice).
Also, when you drop gold with SMB from diluted solution, in most cases very fine particles are formed, because in their close proximity, there is far less other particles which they could clump with. From more concentrated solutions you often get dense and well packed precipitates - because of the same phenomena -
more available particles in proximity=bigger agglomerates=easy decantation and filtration.
Also when solution is dirty and full of other junk ions such as iron, copper and awful lot of chlorides...
Forming particles have hard time to "travel" through all the mess. Also, ionic particles of dissolved copper and other metals in solution adsorb on the surface of the particles of gold, blocking the effective aglomeration due to ionic repulsion. Not in all cases, but that´s what happens most of the times
