Side reactions while dissolving silver

[ldanielrosa]

TL;DR

poor man's nitric acid and 'corrosion inhibitors' in drain opener

I have some assorted silver scrap that I committed to refining/recycling. I decided I prefer to save money and avoid bureaucratic headache by making "poor man's nitric acid" with sulfuric acid drain opener and calcium nitrate.

The first stage goes well enough. I use gravity filtration on the mud and run about 25% of the liquid volume of water through it, twice, to scavenge what I reasonably can. The rest goes with the chicken manure and ashes to be used in the garden later.

The product is somewhat less than 8 molar nitric acid, with impurities. I am aware that there will be some dissolved calcium nitrate and possibly a slight excess of sulfuric acid, both of which will generate silver sulfate as a side product. This is not the first item I'm asking about.

After stirring with heat, I allow the liquor to sit still then decant the clear portion into another container. Here I add copper wire to precipitate the silver. There is inconsistency in precipitate color between batches, sometimes grey and sometimes lavender. After washing and drying the precipitate, the conductivity is not good. I have never seen this when using better quality nitric acid. Is the corrosion inhibitor coating the surface and possibly entrapping come of the copper ions?

I am not sure what more to say or ask. Does anybody here have experience with this?

 

[.Scott]

I am hoping that you have consulted an MSDS for the materials you are dealing with. The two concerns I notice are proper PPE (glove, glasses, whatever) and the potential for dealing with hazardous waste. In the US, the setting (your home) and the kind of small quantities you're likely working with would be considered "Household Hazardous Waste" - which is more lightly controlled than regular industrial hazardous. I am hoping that the "garden" you mentioned is not a vegetable garden. You're not going to accidently ingest an LD50's worth of these salts, but there's a certain yuck factor to having veggies with silver. Even if it wouldn't kill you, it could discolor you.

 

[.Scott]

About 20 years ago, I organized a few State-funded annual Household Hazardous waste sites (1 day events) in New Hampshire. There were several visitors with stories like "grampa left this in the garage years before passing on - we're not sure what it is". The hazardous waste site cannot take it unless we can identify it. In one case, I identified it as powdered aluminum - which is a problem because DOT regulations will not allow it to be transported with most other hazardous materials. But the local fire department volunteered to take it for their use in flammable metals training.
In addition to managing the operations on the day of the event, I also did the planning, budget, vendor selection, etc. Just so you know, this was not an efficient way of spending public funds.

 

[Mayhem]

Impossible to say without knowing what corrosion inhibitor is in the product.

Am I correct in assuming that 'lavender' refers to pale violet/purple or is the color intense?

Also please elaborate on your procedure, step-by-step.

 

[ldanielrosa]

Thanks for taking interest.

MSDS for Black Swan ZAP, and for Rooto drain openers show 93% sulfuric acid. Corrosion inhibitors are not specified, but product bottle mentions their presence.

Very pale lavender color. I had to hold two different samples side by side to see it.


I will elaborate on the procedure.

All reactions are performed outside with southwestern wind as my fume hood.

I weigh a full bottle of drain opener (1883g), and tare out the empty bottle (60g). Per MSDS for rooto and for black swan (two brands I use) the concentration is 93%. Available acid comes to (1883-60)*.93=1695.39g. Molar mass is 98.079, so I have 1695.39/98.079=17.286 moles, or 34.572 normals.

I weigh out 4082 grams of calcium nitrate (Ca(NO3)2.4H2O, 236.15, purchased as fertilizer) and dissolve in the minimum water at room temperature in a HDPE bucket. Into this I slowly drip the drain opener, stirring occasionally with a nylon paddle. I then transfer a bucket modified to be gravity filter with cloth over the holes, nested into another bucket with plastic bowl inside to keep slightly elevated. Wait. Smooth over cracks, add slightly less than one liter of water to wash acid out of mud. Repeat one more time.

Filter bucket now contains mud which is mostly calcium sulfate, with possible excess of calcium nitrate or possible excess of sulfuric acid. Also contains portion of nitric acid which I could not reasonably reclaim. Any acids present will react with available wood ash to make potassium, sodium, magnesium, or calcium nitrate salts. All of these qualify as fertilizers. There is no silver in my garden.

The liquor at this point is clear. Poured into a 1 gallon glass jar on hot plate magnetic stirrer with an excess of broken silver jewelry, brown vapor is emitted and liquid gradually turns light blue. Reaction slows as nitric acid is consumed, and color darkens as dissolved silver nitrate attacks available copper on surface of metal pieces. More acid is added until debris is mostly insoluble pieces (stones, paint, glue). Jar is removed and precipitate is allowed to settle.

Clear blue liquid is decanted into another gallon glass jar on HPMS, and copper wire is added until displacement reaction stops. Jar is removed and allowed to settle. Dark blue liquid is transferred for storage and later reclamation of nitric acid and of copper as shown in .

Precipitate is collected, washed, and dried. Different batches are not identical in color, and reflectivity is less than when done with higher grade nitric acid. Multimeter probes are inserted into a jar full of the dried precipitate, and conductivity is very poor. Similar test done with better sample shows resistance lower than the probe leads.

 

[Mayhem]

Your descriptions a little hard to follow. Gardening is irrelevant to the chemistry.

What I'm seeing is

Step 1:

H₂SO4₂ (aq) + Ca(NO₃)₂ (aq) → CaSO₄ (s) + 2 HNO₃ (aq)

A CaSO₄ (s) ⇌ CaSO₄ (aq) is present, which is shifted to the LHS in an excess of SO₄^2-.
Gravity filtration separates the CaSO₄ "mud" from the mixture and the filtrate should predominantly be HNO₃ with some H₂SO₄ and Ca(NO₃)₂ inevitably present. Avoid rinsing the "mud" with distilled water. It will carry over more impurities, so sacrifice yield in favor of purity.

One thing you should consider for your own edification, but certainly not attempt under any circumstances whatsoever, is whether or not distillation would be advantageous to this step. H₂SO₄ is high boiling and Ca(NO₃)₂ is a salt. HNO₃ solutions are however low boiling. Either way, it would definitely be a way to get rid of Ca²⁺ ions. Again: Don't do it.

From here, I understand you want to dissolve old silvery jewelry. So your starting material is mainly Ag (s) and possibly various silver oxides, which would be black/gray. Dissolving Ag (s) in your fairly concentrated "poor man's nitric acid" with heat, you see brown gas evolution. This is bad. You are probably driving the reaction

Step 2:

Ag (s) + 2 HNO₃ (aq) → Ag(NO₃) (aq) + H₂O (l) + NO₂ (g).

NO₂ (g) is a brown gas and also forms when NO (g) reacts with air. Both of these are toxic gases. Anyway.

In this step, if any solid precipitate forms, positively filter the mixture and make sure the filtrate is perfectly clear and wash whatever you filter off with cold distilled water. You won't carry over significant AgSO₄, its aqueous solubility is much lower than CaSO₄.

Now you have a choice: isolate your hopefully bountiful amounts of dissolved AgNO₃, or work with the filtrate as is.

Adding copper wire, i.e. Cu (s), drives the spontaneous redox reaction

Step 3:

2 Ag⁺ (aq) + Cu (s) → 2 Ag (s) + Cu²⁺ (aq) (E^o = 0.46 V)

If the additives to your H₂SO₄ drain cleaner have made it this far, then the chemistry is unpredictable. The lavender color you describe is most likely a reaction between Cu²⁺ and the additive, which form some sort of insoluble complex (speculative). It is not a Ag(I) complex, or it would precipitate before the introduction of the copper wire.

Poor conductivity (what did you measure, how did you measure it?) of the supposed Ag (s) precipitate is a bit beyond what I'm familiar with. If the precipitate is truly Ag (s) then melting it in a crucible would surely yield a highly conductive material?

Your descriptions are hard for me to follow so I did my best. The video you included demonstrates several steps you do not detail so I'm not going to comment on it.

 

[Borek]

Note: silver you dissolve is already not pure (hallmark - if present - can suggest purity, but typical jewellery is made of silver alloys with up to 10% of other metals), so it is an additional source of things that can interfere.