Measuring cupric nitrate in solution in a silver nitrate/silver electolysis cell

[James Ball]

I understand that copper from cemented silver shot becomes an issue as cupric nitrate replaces silver nitrate in the electrolysis cell. I have read that at concentrations of >50 g/L cupric nitrate, copper begins to plate out. Short of using a spectrometer I would like to estimate the amount of cupric nitrate in my cell. How blue is too much blue? My plan: I am going to make up 6 vials with 0, 10, 20, 30, 40, and 50 g/L copper from cupric nitrate using analytical reagent grade cupric nitrate trihydrate. Then I will take a sample out of my cell and put it into a similar vial to keep the path length of the "measuring cell" aka the vial the same as for the standards. Comparing the cell vial to the others should show me where my cell is at with copper contaminination. I should be able to know when to change the electrolyte due to copper contamination. Has anyone else done somthing similar.? Are their alternative methods for measirng when to change out the electrolyte?

 

[FrugalRefiner]

That's exactly the technique I use. The only difference is that I dissolved a weighed sample of copper wire with nitric acid, then diluted samples down to the various concentrations. It's a quick and dirty method, but it has served me well. Yes, I use the same vial for my sample of electrolyte as the vials containing the standard solutions.

You can also detect when other metals contaminate the electrolyte, particularly palladium. It lends a brownish tint along with the blue. It's a nice contaminant to find.

Dave

 

[4metals]

Chris, GSP, used specific gravity on standards he made. Start reading here and he explains it. Reducing Silver nitrate to metallic silver

 

[James Ball]

That's exactly the technique I use. The only difference is that I dissolved a weighed sample of copper wire with nitric acid, then diluted samples down to the various concentrations. It's a quick and dirty method, but it has served me well. Yes, I use the same vial for my sample of electrolyte as the vials containing the standard solutions.

You can also detect when other metals contaminate the electrolyte, particularly palladium. It lends a brownish tint along with the blue. It's a nice contaminant to find.

Dave

I like the idea of using raw copper to make the solution. I am too impatient to wait for my cupric chloride to arrive. I have some copper pipe I will use tomorrow. Thanks for the comment.

 

[FrugalRefiner]

Copper pipe is not as pure as copper wire. I don't know by how much, but wire has to be extremely pure for conductivity.

Dave

 

[kurtak]

Keep in mind that just how much copper in the electrolyte it take for the copper to start co-depositing depends (somewhat) on the starting concentration of silver in the electrolyte

That is because when the copper co-depositing starts depends (somewhat) on when the "balance" between the copper ions in the electrolyte & silver ions in the electrolyte become "upset"

In other words - more silver ions in the electrolyte to start with allows for more copper ions (to corrupt the electrolyte) before copper starts to co-deposit

Example; - (using completely hypothetical numbers) I don't know the "actual" numbers but lets say co-depositing starts at 50/50 Cu/Ag ions in the electrolyte

That would mean that if you start with 10 grams per liter Ag in your electrolyte co-depositing would start once the electrolyte has been corrupted with 5 grams of copper (which means the silver has also been reduced to 5 grams) --- so - if you start with 100 grams Ag per liter you could run the cell to the point of 50 grams Cu per liter before co-depositing (in theory)

For this reason when I run anodes made of sterling I double the amount of Ag in my electrolyte compared to when I run anodes made of cement silver

I can allow my electrolyte to get much more blue before co-depositing starts by doubling the silver concentration of the electrolyte when running sterling

For what it is worth

Kurt

 

[James Ball]

I understand that copper from cemented silver shot becomes an issue as cupric nitrate replaces silver nitrate in the electrolysis cell. I have read that at concentrations of >50 g/L cupric nitrate, copper begins to plate out. Short of using a spectrometer I would like to estimate the amount of cupric nitrate in my cell. How blue is too much blue? My plan: I am going to make up 6 vials with 0, 10, 20, 30, 40, and 50 g/L copper from cupric nitrate using analytical reagent grade cupric nitrate trihydrate. Then I will take a sample out of my cell and put it into a similar vial to keep the path length of the "measuring cell" aka the vial the same as for the standards. Comparing the cell vial to the others should show me where my cell is at with copper contaminination. I should be able to know when to change the electrolyte due to copper contamination. Has anyone else done somthing similar.? Are their alternative methods for m easirng when to change out the electrolyte?

Hello all, I am somewhat puzzled by the spreadsheet calcuation by Gsracer (2020). I have a 7 L silver cell with 63.4 g silver/L electrolyte. With a threshold of 50 g/L copper I should have been able to only process 2.8 kg cement silver assuming a purity of 99% silver from my cement silver. So far I have processed >4 kg. The color of copper in the electrolyte is less than 10 g copper/L using vials of reference copper at 0, 10, 20, 30, 40, 50 and 67 g/L copper made from a known mass of copper dissolved in nitric acid. Something is not adding up here. I bought some cupric nitrate and will prepare standards using this reference material. I am wondering what I am doing wrong here??? Have I made some assumptions or used the wrong quanitity (e.g. g silver/L and should be using g silver nitrate/L)? I will take a photo of both standards solutions (made from copper and made from cupric nitrate) and show them here is another post.

 

[James Ball]

Hello all, I am somewhat puzzled by the spreadsheet calcuation by Gsracer (2020). I have a 7 L silver cell with 63.4 g silver/L electrolyte. With a threshold of 50 g/L copper I should have been able to only process 2.8 kg cement silver assuming a purity of 99% silver from my cement silver. So far I have processed >4 kg. The color of copper in the electrolyte is less than 10 g copper/L using vials of reference copper at 0, 10, 20, 30, 40, 50 and 67 g/L copper made from a known mass of copper dissolved in nitric acid. Something is not adding up here. I bought some cupric nitrate and will prepare standards using this reference material. I am wondering what I am doing wrong here??? Have I made some assumptions or used the wrong quanitity (e.g. g silver/L and should be using g silver nitrate/L)? I will take a photo of both standards solutions (made from copper and made from cupric nitrate) and show them here is another post.

My new solutions, using Cu(NO3)2 3H2O, are about the same as the solutions I made with copper metal. The photo (1420) with a vial with no numbers next to the 10 g Cu/L vials is the sample from my electrolysis cell - clearly much less than 10 g Cu/L. The vial on the far left of IMG_1420 is distilled water. The other vial colorations are somewhat confounded by the lighting but from my eye they are all consistent.

 

[FrugalRefiner]

Hello all, I am somewhat puzzled by the spreadsheet calcuation by Gsracer (2020). I have a 7 L silver cell with 63.4 g silver/L electrolyte. With a threshold of 50 g/L copper I should have been able to only process 2.8 kg cement silver assuming a purity of 99% silver from my cement silver. So far I have processed >4 kg. The color of copper in the electrolyte is less than 10 g copper/L using vials of reference copper at 0, 10, 20, 30, 40, 50 and 67 g/L copper made from a known mass of copper dissolved in nitric acid. Something is not adding up here. I bought some cupric nitrate and will prepare standards using this reference material. I am wondering what I am doing wrong here??? Have I made some assumptions or used the wrong quanitity (e.g. g silver/L and should be using g silver nitrate/L)? I will take a photo of both standards solutions (made from copper and made from cupric nitrate) and show them here is another post.

So, if you've processed 4 kg of ~99% cemented silver, you expect 1% copper. 1% of 4 kg is only 40 grams. In a 7 liter cell, that's less than 6 grams per liter.

Not sure why you expected to only be able to run 2.8 kg of 99% feedstock. Was that perhaps a figure for sterling?

Dave

 

[James Ball]

I was referring to the spreadsheet entitled "Silver Cell Electrolyte Worksheet" by Gsracer 2020. Your calculation makes a heap of sense and is consistent with what I observe using my standards. I am not sure why the spreadsheet calculation shows that I can process only 2765 g of 99% silver using 7 L and 444 g dissolved Ag quanitity with a 50 g Cu/L threshold for coprecipitation.

 

[James Ball]

I think I understand where I went wrong in my thinking about the threshold levels of dissolved copper before coprecipitation of copper begins. As I understand it now, the critical threshold level is the concentration of silver, which should be >50g/L. I thought the copper concentration had to be > or = to 50g/L to start coprecipitation of copper with silver. I think I have this straight now.

Copper in my silver shot displaces silver in my silver nitrate electrolyte solution. When the [Ag] is less than 50g/l trouble starts. So, now my question becomes the following. If I know how much copper I have in solution (based on blue color comparison with reference Cu(NO3)2 solutions) I can make more pure AgNO3 (from my silver cell product) and replenish the spent silver in the electrolyte solution. Theoretically, as long as I replenish the displaced silver in my silver electrolyte solution the cell can have a lot of Cu in solution. Is my thinking correct now? What is the upper limit of Cu(NO3)2 one can have in the cell (assuming the [Ag] > 50 g/L is maintained)? The solubility limit of Cu(NO3)2? I am sure this has been discussed ad nauseum I just do not know where to find it. Comments and/or affirmation would be appreciated.

 

[James Ball]

Here is an update to my method of measuring copper concentration in the electrolyte of my silver cell.

I have been running a silver cell using cemented silver from sterling silver (see photo for amperage and voltage). I have wondered how to know when to change the electrolyte in the cell. According to this forum no more than 50 g/l of copper should be present in an electrolysis cell before copper co-precipitation begins. I have attached a spreadsheet that I downloaded from the forum FYI. My plan is to make standard reference solutions of copper nitrate at 0, 10, 20, 30, 40, and 50 g copper/L. I will make these standards using 4 mL vials from Amazon. If you look at my cell after 3 batches of silver (~4.7 kg) the solution looks pretty blue (picture attached). The problem with looking at the cell is that the pathlength is very long - like about 10 inches so the blue intensity is magnified. I made reference concentrations using either copper metal or cupric nitrate as described above. These are also shown in a photograph (only one reference sample from copper metal is shown). I took a sample of my cell and put it into a vial to keep the path length the same. When I compare it to the reference vials I have less than 10 g copper/L in my cell which means to me that I can still use this cell a lot before I need to replace the electrolyte. I would appreciate any thoughts or critic on this approach.

[Erratum - It looks like I have two 30 g copper/L vials - one should have been 20 g copper/l but you get the idea]

 

[orvi]

Just a tip.

I think visual detection compared to the standard is more than sufficient here, but more precise result (if desired) will be obtained, if you won´t use curved vials, but rectangular ones (like actual spectrophotometric cuvette). Also, filling the containers to precise height can give you more accurate reading for dilute solutions, when you then look from the top.

Always use exactly same vials, as glass can also add some hue of green and blue to the visual result, depending on lightsource and angle applied.

 

[4metals]

Do you titrate your solution to determine the silver concentration? That is done by what is known as a Volhard titration. I have an analytical method to determine the copper gravimetrically from the spent solution from the titration. It isn’t overly technical but not something a novice lab person may feel comfortable with. I have it written up as a PDF and I can post it if you feel you want to give it a shot.

 

[James Ball]

I do not titrate my silver nitrate solution. I rely on amps assuming that low amperage means low silver nitrate concentration. I can also calculate how much silver has been displaced by assuming a level of copper in the cell ( for example 8 g/L) using

Cu(s) + 2AgNO3(aq)→Cu(NO3)2(aq)+2Ag(s)

After I harvest my silver from the silver cell I typically dissolve some pure silver-cell product (hopefuly .9999 pure silver) in nitric acid and add it to the electrolyte after drying to minimize dilution.

On another note, I remember using a gravimetric method for chloride concentration as my first analytical analysis in my Anayltical Chemistry class as a sophmore in college.

I thought about buying a Spec 20 and generating a calbration curve using absorbance of cupric nitrate as a more quantitative method for measuring the [Cu]. But vials does the same thing just not as quantitatively. I would like to see your paper too though.

 

[4metals]

After I harvest my silver from the silver cell I typically dissolve some pure silver-cell product (hopefuly .9999 pure silver) in nitric acid and add it to the electrolyte after drying to minimize dilution.

The reason the silver concentration is measured is to allow you to add nitric acid directly to the cell to cause the extra boost in free nitric to raise the Silver concentration without you having to re-refine Silver you already harvested to accomplish the same thing. A lot of refiners titrate their cells daily and make the required additions. One indication that your copper is too high without actually analyzing copper is a point you reach where no matter how much nitric you add your Silver level does not climb. That is simply because the solution is holding too much dissolved metal and it's getting a little too crowded in there.

This is the volhard method and the copper analysis method as well. You will see, the left over solution from the volhard titration is used for the copper analysis, so they go together.

 

[James Ball]

Okay, really like the idea of adding nitric acid to the silver cell to replenish the silver concentration in the cell. All kinds of benefits of doing it this way. 1) you don't spend the time isolating pure crystals and washing them, etc, 2) you don't have to redissolve silver and make sure there is excess silver around (plenty of excess silver in the silver cell). The downside as I see it would be the formation of NOx but that can be handeled with a fan. I really like combining this with the silver titration method. These two methods would allow one to keep the [Ag] at the right amount. I think using cupric nitrate standard solutions is a better way to approximate the [Cu] in the silver cell. Any other downsides to this method of maintaining you silver concentration in the silver cell? What concentraton of silver do you use in a silver cell?

This is why I support this forum. In my opinion, this is an outstanding contribution to my project. Thank you very much,

Jim

ps time to buy some chemicals, buret, lab stand, pipets etc for the silver titration. what fun!

 

[James Ball]

Another question? How well does the gravimetric copper concentration method agree with a colorometric cupric nitrate method? They should be pretty close but it would be nice to see some data to verify this assumption.

 

[4metals]

What concentraton of silver do you use in a silver cell?

Cell manufacturers recommend 60 g/l as a target. But there is a sweet spot that is actually a range in which the cells work properly.

I have a spreadsheet which allows you to input how many grams per liter you want to increase the concentration and it tells you how much nitric to add. This is made for a cell of 275 liters but the cell volume can be tailored to your needs and all of the associated calculations are linked.

If you would like to try it out I will dig out the spreadsheet and post it.

 

[4metals]

How well does the gravimetric copper concentration method agree with a colorometric cupric nitrate method? They should be pretty close but it would be nice to see some data to verify this assumption.

I would like to see this comparison as well, I've only compared the gravimetric method to Atomic Absorption results.