Very interesting: pH elektrodes!

[archeonist]

Hi all,

Today I did some work on a few pH electrodes I got from my workplace. These electrodes are build to last a few years and after that period they are thrown away. I got my hand on twenty pH electrodes, and a few redox electrodes like a silver electrode and a gold!!!!! electrode. The gold electrode has a massive golden tip, not plated but really massive!! And was also ready to be thrown away! Ill show you this electrode later.
Today I broke all the pH electrodes down and I found thick plated pins on the inside. I also found silver coils and possibly very thin platinum wires.
For now I have removed the gold plating on the pins and as expected it was very thick. I chose to dissolve the base metals in nitric, a very easy and quick way to deplate the pins. When doing this I noticed a white precipitation. This is possibly due to the fact that the inside of the pins are not copper but silver. As some of you know the inside of a pH electrode contains KCl. I did not rinse the pins enough so some of the chloride reacted with silver ions to form the insoluble AgCl(s). At least this is what I think happened. And I did not mention the little bit of AR I created in this way :evil: To separate the foils from the solution and the precipitate I had to use a small sieve instead of a filter, so I lost tiny bits of gold. I will work on the precipitate later.

 

[archeonist]

...

 

[gold4mike]

They kept their shape very well - should be nice thick foils.

 

[niteliteone]

When doing this I noticed a white precipitation. This is possibly due to the fact that the inside of the pins are not copper but silver.

To verify that is silver chloride and help separate and gold foils you can dissolve the silver chloride in ammonia, run it through a filter collecting any undissolved metals and then re-precipitating as silver chloride by adding a little HCl.
I don't remember the steps involved off hand but, I think Butcher posted a detailed description a little while back.

 

[archeonist]

When doing this I noticed a white precipitation. This is possibly due to the fact that the inside of the pins are not copper but silver.

To verify that is silver chloride and help separate and gold foils you can dissolve the silver chloride in ammonia, run it through a filter collecting any undissolved metals and then re-precipitating as silver chloride by adding a little HCl.
I don't remember the steps involved off hand but, I think Butcher posted a detailed description a little while back.

Interesting! Thanks for the tip! I already found that the bleu filtrate I got contains a large amount of silver. When I put in a copper wire, silver crystals start to cement out.

 

[johnny309]

So...if the base metal is cooper ....normally the barrier which prevent gold to migrate to cooper will be an alloy of Ag and Pd (70/30)....so test your solution after Pd also...

 

[Harold_V]

So...if the base metal is cooper ....normally the barrier which prevent gold to migrate to cooper will be an alloy of Ag and Pd (70/30)....so test your solution after Pd also...

In general, the barrier layer is nickel, although that may not be true for the items in question. However, the presence of palladium will color the solution green, a color that's obviously lacking in this case. That the solution appears to be blue indicates there is no palladium present----but a test with DMG can be conclusive. It would also indicate the presence of nickel, assuming ammonium hydroxide was introduced to the test before applying the DMG. A drop of the solution in a spot plate, and an added drop of ammonium hydroxide would yield a vivid blue color, then upon adding a drop of DMG, if there's nickel present you'd witness a pink reaction. The testing procedure is in Hoke's book if you have any questions.

Harold

 

[johnny309]

From the colour....the barrier should be nickel because is too shiny ..............the fact that Ag is involved.....in 2 weeks I will gather 20 electrodes and do some checking........
P.S.: it is no so rare to find "barrier" of an alloy of Ag/Pd alloy in industrial usage......when "GOOD" products are needed....the price is not an option.

 

[archeonist]

Thanks for the reply Harold and Johnny,

I will do the test for Ni. But so far I am pretty sure there is Ag in the solution because I saw crystals growing on a copper wire I held for a while in the bleu solution.

 

[Harold_V]

Not understanding how the electrodes work, I can only guess---but if you've found they contain silver, my money says they are made of silver, with a nickel barrier to prevent the gold from migrating to the silver. Whether palladium is or can be used for a barrier I don't question---I'm simply saying there's no visible evidence it exists with the pieces in question. Palladium in solution is VERY dark, and would influence the color of the resulting solution. It can't contain palladium and remain blue. If the material was fully dissolved, so nothing remained to cement any palladium that might be present, there is no palladium.

Silver is a better conductor of electricity than copper. That may explain why there's silver present. I'm still tripping over the idea that the barrier layer is a silver/palladium alloy. I'm not trying to be controversial----just trying to have others understand that the presence of palladium is generally quite obvious.

Harold

 

[chlaurite]

Usually we see a barrier layer to keep the gold from migrating into the copper.

Does silver by itself have that problem? If not, why would they have a "barrier" layer at all, instead of just plating gold right on the silver electrodes? Thus, no need for palladium, either.

 

[Harold_V]

Gold readily migrates to silver, so if one is plated to the other, a barrier is required. Without such a barrier, in time, evidence of plating would be gone.

The migration of atoms can occur simply through intimate contact. The metals need not be plated to one another.

Harold

 

[Geo]

all metals within the "transition metals" will alloy. they contain an incomplete sub shell or lend to cations that have an incomplete sub shell. some metals alloy so readily that intimate contact needs not occur but only close proximity. a pure gold bar and a pure silver bar placed side by side in close proximity and in a vacuum will eventually become an alloy of both metals given enough time. gold and silver will seek each other atom by atom.

 

[chlaurite]

Ah, thank you, Harold and Geo. Funny, how many truly useful concepts "textbook" learning can leave out - No sarcasm intended!

 

[nickvc]

The beauty of refining and recovery is that there is always something to learn as the subject is just so vast, not even our top pros know it all, most maybe but there's always those little things that can make a difference for us all to discover even after tears of study.

 

[butcher]

Well said nickvc, after many tears of study, and quite a few years of studying I still seem to learn something new every day.

 

[johnny309]

Maybe some more pointers in the followink link : http://www.sensorland.com/HowPage037.html

 

[johnny309]

I study this particular PH probe http://www.endress.com/eh/home.nsf/#product/CPS41 which I have on hand....The probe wire is Ag,the solution is KCl....so...when is process with HNO3 ,without the initial incineration ,some AgCl come in the solution.
Hope this clear some aspects about.....
P.S: study the datasheet of the PH electrode.....some of them have thermal sensor built in(maybe some extra Pt..)