Stannous test must be acidic in order to work properly. With active nitric in solution, it can be tricky to see as nitric "fight back" and re-oxidize it.
Pd-DMG is best precipitated from acidic solutions, pH slightly above 0 is completely OK. It even work in non-neutralized dilute nitric solutions, tho the complex, as well as DMG itself are slowly decomposed by nitric. Active AR solutions are the worst to test with DMG, as active AR with nitric still present will decompose Pd-DMG complex relatively rapidly. Colour of the precipitate is bright yellow, it is very insoluble in water and it is very visible even in low concentrations.
When I test for Pd with DMG, I take sample of solution (few mL), treat it with DMG (as NaOH/DMG solution preferentially), and after formation of the possible Pd-DMG precipitate, I filter this sample through small Buchner funnel and wash the solids with dilute HCl. By washing with HCl, you will easily dissolve nearly everything except Pd-DMG (assuming you do not have silver in there). And then, it is very easy to see the actual colour of precipitate on the white filter paper.
Below there is a picture of how Pd-stannous positive test should look like. Deep green for Pd content of g per L of solution. Then gradually lighter green, when solution is more dilute. With concentrations past several grams per L, it is so deep green/nearly black, that you cannot see through it easily.
Also, second picture show filter cake of Pd-DMG complex dried on a hotplate. Notice very nice bright-yellow colour. This one is slightly contamined with some base metals and gold, so colour is somewhat off, but now you can clearly see how it should look like.
Third picture is of filter cake in the Buchner is shot when it was still wet. Notice yellow precipitate "crawling" onto the walls of the Buchner. This is that yellow colour. Pd-DMG is also quite hydrophobic and tend to smear on surfaces, from which it is only washed with great difficulty.
Pd-DMG is best precipitated from acidic solutions, pH slightly above 0 is completely OK. It even work in non-neutralized dilute nitric solutions, tho the complex, as well as DMG itself are slowly decomposed by nitric. Active AR solutions are the worst to test with DMG, as active AR with nitric still present will decompose Pd-DMG complex relatively rapidly. Colour of the precipitate is bright yellow, it is very insoluble in water and it is very visible even in low concentrations.
When I test for Pd with DMG, I take sample of solution (few mL), treat it with DMG (as NaOH/DMG solution preferentially), and after formation of the possible Pd-DMG precipitate, I filter this sample through small Buchner funnel and wash the solids with dilute HCl. By washing with HCl, you will easily dissolve nearly everything except Pd-DMG (assuming you do not have silver in there). And then, it is very easy to see the actual colour of precipitate on the white filter paper.
Below there is a picture of how Pd-stannous positive test should look like. Deep green for Pd content of g per L of solution. Then gradually lighter green, when solution is more dilute. With concentrations past several grams per L, it is so deep green/nearly black, that you cannot see through it easily.
Also, second picture show filter cake of Pd-DMG complex dried on a hotplate. Notice very nice bright-yellow colour. This one is slightly contamined with some base metals and gold, so colour is somewhat off, but now you can clearly see how it should look like.
Third picture is of filter cake in the Buchner is shot when it was still wet. Notice yellow precipitate "crawling" onto the walls of the Buchner. This is that yellow colour. Pd-DMG is also quite hydrophobic and tend to smear on surfaces, from which it is only washed with great difficulty.
Attachments
Last edited:
