PDA

View Full Version : homebuilt vAg electrode?



Pages : [1] 2

rmazzullo
12-15-2007, 06:45 PM
Hello PE,

Has there been any progress on the idea of building a workable vAg electrode? Do you know of any references that may discuss how one would be constructed?

Thanks,

Bob M.

Photo Engineer
12-15-2007, 08:56 PM
Bob;

The starting point is a high accuracy volt meter with silver electrode(s). You will need more than that. A friend who has been helping me has been otherwise occupied. And, so have I.

For your use though, 0.1 molar NaBr at 20 deg C will have a vAg of -61.48 mv and a pAg of 11.463

That is, BTW, 10.289 g/l.

So, here is a calibration point for you.

PE

kb3lms
09-26-2012, 11:25 PM
While browsing the web at lunchtime, I found references to this article about constructing a reference electrode out of a used felt-tip pen. So I tracked down the article tonight and have posted it at the link below. The electrode described is for Ag/AgCl, but it seems like the basic design could be adapted by using KBr instead of KCl.

http://dl.dropbox.com/u/4212013/electrode.pdf

Any thoughts on it's usefulness for measuring vAg or at least pAg? The more I read I see how knowing vAg or pAg is important for making more advanced emulsions so it seems like finding an economical solution for the probes would be useful.

-- Jason

Photo Engineer
09-27-2012, 11:36 AM
Jason;

Thanks for the reference.

Technically, an Ag/AgCl electrode is used for all vAg measurements. The plating on the other electrode is what changes depending on the salt used. So, this reference electrode looks just fine.

PE

kb3lms
09-27-2012, 01:55 PM
Oh, I see. Well maybe that simplifies things.

So the other electrode could be a silver wire plated with AgBr, for example, if you were working with a AgBrI emulsion? Or would it be plated with AgBrI?

And the electrode should be solid silver, right?

Photo Engineer
09-27-2012, 03:12 PM
The silver electrode should be solid silver. I have a batch of them here that I made by hand. I used 12 gauge silver wire 99.99% pure. It should be plated with the salt which represents the major constituent of the emulsion. Use a reference electrode of Ag/AgCl in all cases. However, use of a bridge is advised due to Cl leakage from the electrode. The book contains photos and a description of the procedures. Much of that is posted here in less detail.

PE

kb3lms
09-28-2012, 08:37 AM
Thanks, PE. I will check that out when it arrives - hopefully today!

Kirk Keyes
10-01-2012, 02:24 AM
While browsing the web at lunchtime, I found references to this article about constructing a reference electrode out of a used felt-tip pen. So I tracked down the article tonight and have posted it at the link below. The electrode described is for Ag/AgCl, but it seems like the basic design could be adapted by using KBr instead of KCl.

http://dl.dropbox.com/u/4212013/electrode.pdf

Any thoughts on it's usefulness for measuring vAg or at least pAg? The more I read I see how knowing vAg or pAg is important for making more advanced emulsions so it seems like finding an economical solution for the probes would be useful.

-- Jason

Note that what's described in that article is for a reference electrode.

When making electrode measurements, you need to have both a reference electrode and what's called an ion specific or ion selective electrode.

For pH measurements, you use a what's typically called a "pH" electrode. What's really being said there is that you need a glass electrode (which can simply be thought of as a hydrogen ion specific electrode) and a reference electrode.

Most people never even mention the reference electrode when talking about pH measurements because most pH electrodes nowadays have the reverence electrode built right into the ion specific electrode. The reference electrodes are typically silver chloride electrodes or calomel electrodes. Calomel seems to have lost favor these days as they contain metallic mercury and mercury (I) chloride, both rather toxic compounds.

The silver chloride referece electrode consists of a piece of silver metal that is coated with silver chloride, and it is placed into a concentrated solution of potassium chloride to stabilize the silver chloride that coats the silver wire. The potential that is generated by the silver|silver chloride junction is the "reference" voltage that this type of electrode produces. The standard electrode potential that the silver reference electrode produces against the standard hydrogen electrode (SHE) is 0.230V 10mV.

The silver chloride reference electrode that is discussed in the article linked to above is this type of electrode, not an "silver" electrode that is used for making vAg measurements.

The vAg electrode that PE talks about for making vAg measurements, is simply a piece of high purity silver wire/rod that has been coated with the dominant halide for the emulsion that is to be measured. Like PE mentioned, somewhere here on APUG are a series of post where he and I discussed the construction of a home-made vAg electrode. Basically, it's the silver wire that PE mentioned, I think what I found was 12 or 14 guage - which is nice and sturdy and resists being bent from use. That silver wire was then soldered to a length of coax cable with a BNC connector so it could be directly connected to the "pH" meters that we both had. The silver wire/coax assembly was then inserted into some fire-polished glass tubing with bare silver wire outside the glass and the silver wire solder joint inside the glass and then 5-minute epoxy was then used to set the wire in place in one end of the glass tube and set the cable into the other end.

For use, this vAg electrode was cleaned with fine wet-dry sandpaper and then placed into a solution with the halide of choice, and a small current run through the wire and solution to drive the halides to the vAg electrode to plate it. (As they mention in the article, the vAg electrode can be connected to the positive terminal of a 3-volt battery and then current run through for 10 minutes using a second vAg electrode or a simple siver wire in circuit with the plating solution.) Once this had been done, the vAg electrode was ready for use. This treatment was good for several days if stored in solution and kept in the dark.

Now this vAg electrode would have to be used with a reference electrode - just like the one in the article linked to above, so that you had both an ion specific electrode and a reference electrode.

And actually, the idea of a felt-tip pen as the body of a reference electrode is pretty good, and great for the home lab, as silver chloride reference electrodes are expensive new and hard to find used. Note that the pen-based reference electrode has a "bridge" like PE mentioned above built into it - that's what the agar gel is doing. It's slowing down the mixing of the reference solution (the potassium chloride that surrounds the silver|silver chloride electrode). The felt tip is merely acting as the junction that allows the interior solutions to come into contact with the solution to be tested.

PE also suggests that you want to use a double bridge to minimize the migration of non-chloride halides from the test solution to the reference solution. If bromide or iodide get into the reference solution that surrounds the silver|silver chloride reference electrode, then it will be poisoned and it will no longer serve as a "reference" electrode.

kb3lms
10-01-2012, 09:20 AM
Kirk, thanks for the great explanation! I had understood that the article was describing the reference electrode and you needed a separate one for the salt but didn't quite understand the role of the Ag/AgCl inside the reference electrode. You have helped to clear this up.

Now the Ion-Specific Electrode (ISE) is the next part to investigate. Soldering silver wire to a piece of coax should be simple enough. What is the purpose of the glass? Sounds to me like it is to give a bit of stiffness and durability to the assembly plus protect the soldered connection and prevent it from contaminating the emulsion? I don't know anything about glass tubing but I suppose you can get it from a chemistry supplier or the big auction site.

I had found the other posts you mentioned and along with PE's book have started to fill in the shopping list for the two electrodes plus the salt bridge. This has all been very, very helpful.

-- Jason

Kirk Keyes
10-01-2012, 09:59 AM
Glass was chosen for several reasons - to give the electrode some rigidity, to keep the solder joint clean, to minimize the length of silver wire needed, to have something that you can insert into the solution that will not contaminate it, it's easy to clean, and can be supported in the solution using a ring stand clamp.

I think PE recommended around 2 cm of silver wire in the solution and I wanted all the wire under the liquid level so there's no possible changes as the solution level rises, so I wanted something that I could insert into the solution so I immediately thought of glass tubing. Commercial electrodes used to use glass, but now they often use epoxy for the electrode bodies. That was where I figured that 5-minute epoxy would be great for sealing the silver wire into the glass tubing. And it's much easier to use the epoxy and I think more water-tight than trying to seal the silver wire by blowing the glass to make a glass seal.

I wanted a technique that people without much glass-blowing skills could use. As it is, you'll need to practice cutting the glass, or get someone at a glass shop to do it. After cutting it, you'll probably want to fire polish the ends - Watch some online youtubes for basic glass-blowing techniques. None of this glass working is difficult and all you need is a glass triangle file to make a nick in the glass to "cut" it, and then a propane torch that you may have around the house already.

Oh, and when you connect the coax, just solder the signal wire of the coax and not the braided outer wire to the silver. Strip the braided part back a bit to it doesn't make contact with the center wire. It's going to take a fairly large piece of glass tubing to get the coax into the glass. Like 12 or 14 mm or so diameter tubing. Those are a bit hard to cut, so maybe a glass shop would be good if you've no experience cutting glass tubing.

I was also going to try making one out of acrylic tubing as it can be cut with a hacksaw - I bought the tubing but I never got around to making the electrode. You might want to git that a try. Other than the siver wire/rod, nothing it really expensive here.

And the coax I used was from Radio Shack - just a 2 ft coax with BNC connectors on both ends. Clip on BNC off and then you have a nice 2 ft lead on your electrode.

kb3lms
10-01-2012, 10:31 AM
Thanks, Kirk. So add some glass tubing or similar to the shopping list. Coax with BNCs or other connectors I already have. In fact, I have a few broken oscilloscope probes here at work with a thinner coax that may be ideal.

Another question for you. If I have this correct, the reference elctrode sits in a beaker of solution and the salt bridge "connects" that beaker to the emulsion beaker and the silver ISE to complete the circuit. From what I can find, the bridge is made with a thickened electrolyte of KNO3 (potassium nitrate) and PE covers how to make that in his book. What sort of solution is in the beaker with the reference elctrode? I don't think I've seen that anywhere.

And is a pH meter better to use or more useful then a VOM/multimeter? I have use of several very good (Fluke) bench multimeters but do not have a pH meter to which I can connect an external probe. Have looked at used ones on the auction site but they almost never come with probes and new probes are quite expensive. The meters themselves are not too costly. However, I have thought about getting a better pH meter than what I have now.

Kirk Keyes
10-02-2012, 01:23 AM
Thanks, Kirk. So add some glass tubing or similar to the shopping list. Coax with BNCs or other connectors I already have. In fact, I have a few broken oscilloscope probes here at work with a thinner coax that may be ideal.

Old oscilloscope probe leads would be great. The foam insulation on the Radio Shack coax is still pretty thick, and it does make for a rather stiff lead.


Another question for you. If I have this correct, the reference elctrode sits in a beaker of solution and the salt bridge "connects" that beaker to the emulsion beaker and the silver ISE to complete the circuit. From what I can find, the bridge is made with a thickened electrolyte of KNO3 (potassium nitrate) and PE covers how to make that in his book. What sort of solution is in the beaker with the reference elctrode? I don't think I've seen that anywhere. .

Yes, you have it right. One beaker has the emulsion, the vAg electrode, and one end of a salt bridge in it. The salt bridge connects to a second beaker with some solution and the reference electrode. As I pointed out, the referece electrode mentioned above has a gel in it that helps act as a bridge by isolating the KCl solution in the reference electrode from the outside world. You do not want to use a solution with a halide in it for either the salt bridge to the emulsion or in the beaker with the reference electrode. A salt that will not affect the emulsion is needed. A saturated solution of potassium nitrate (KN03) would be a good choice here as you'll have both potassium and nitrate ions in the emulsion solution too. Get ACS or at least reagent grade so you know you have low halides - don't be temped with hydroponic KNO3!


And is a pH meter better to use or more useful then a VOM/multimeter? I have use of several very good (Fluke) bench multimeters but do not have a pH meter to which I can connect an external probe. Have looked at used ones on the auction site but they almost never come with probes and new probes are quite expensive. The meters themselves are not too costly. However, I have thought about getting a better pH meter than what I have now.

For vAg, there is nothing wrong with using your bench VOM. It will work just fine - so you may not even want to use BNC connectors on your vAg electrode - use the same type as your VOM. I was going for BNC as I have a couple pH meters that I bought off ebay. Depending on the age of commercial reference electrodes (for pH meters), they typically have a "pin-tip" or a "barrel" type connection to the meter, not a BNC.

I'm partial to Orion, Thermo, Mettler, or Beckman meters, as that's what I've used working in a chem lab and had good results with those brands. I found my meters by waiting along for ones that were tested and shown to at least turn on with the display working...

The advantage with pH meters over VOMs is when you actually put a pH electrode on it - one with a built in reference electrode and even a temp sensor. The pH meter is set up so these will all just plug right in and it will read directly off the readout and it can even correct for temperature that way. When doing vAg, you merely need to measure volts (although temp has a effect too, be we've been ignoring it...)

If you want to look for a commercial reference electrode - look for ones that are called "double junction" as they essentially have the second bridge built right into them, so I think it can reduce the need for the reference electrode being separated from the emulsion by the salt bridge. It's called an Ag/AgCl Double Junction Half-Cell if you want to get specific. These are usually pretty indestructable - a good cleaning and soaking in fresh filling solutions is often enough to revive "dead" ones!

Kirk Keyes
10-02-2012, 01:54 AM
Here's a photo of my home-made vAg electrode as I discussed above:
57950

And here it is in action in combination with a double junction Ag/AgCl reference electrode (in, I think, a potassium iodide solution):
57951

kb3lms
10-03-2012, 04:15 PM
Today, during the lunchtime browsing, I found the following equation to convert pAg to vAg. This was from a master's thesis at RIT. I was wondering if anyone would like to comment on it. Other than finding it in the paper, I don't know much else about it. There is also a short BASIC program for use in a calculator.

vAg=602-0.1*(T-298)-0.1984*T*pAg

where T is in degees Kelvin, which equals degrees C + 273.

Plugging in the numbers from post #2, above, in a spreadsheet I get:

NaBr at 20C and pAg of 11.463 = vAg of -64.36 mV. So, close to the given figure of -61.48 mV but not exact. (Maybe this is within margin of error, IDK)

Another paper give a BASIC (yes that old!) program for calculating a number of different parameters. Will pass that along if it seems to have any validity. I do not have a BASIC interpreter any more so I will have to convert it to something else to try it out.

-- Jason

Photo Engineer
10-03-2012, 04:39 PM
Jason;

Extended equations are in the book and posted here. The original code at EK was Fortran converted to BASIC and then C++. It was produced as a portable DLL for use throughout Kodak Park. A simple call got back basic as the result.

The fundamental patent with refinements on this method are in a set of patents in the book.

PE

kb3lms
10-03-2012, 10:26 PM
Thank you, PE. I found them in my copy. Wasn't looking in the right place.

kb3lms
10-10-2012, 10:43 PM
OK, here's the felt tip pen reference electrode and the silver electrode with the bridge. The left glass contains a 100 ml 0.1M KNO3 and the right one contains 100 ml 0.1M KBr. The bridge contains a 0.1M KNO3 solution gelled with ky.

58332

The other photo is the meter setup with a high impedance interface buffer box attached. The interface is battery powered and uses a very high input impedance AD824 opamp as a unity gain buffer. The buffer box is not needed, however, as I get the same reading with and without it.

58333

My initial reading is -74.8 mV. From an old post I found here, the reading should be around -51 mV. The reading stays rock-steady. Earlier, I tried the 0.17M KBr solution used for plating the silver electrode and that gave a reading of -96.5 mV. So possibly I will need a POT to calibrate or find the source of the error in the reading.

-- Jason

Photo Engineer
10-10-2012, 11:15 PM
Is the electrode plated properly and is the VOM high impedence? I gave some plating and VOM information here on APUG and also in the book. My tests come within 6 mv of those calculated.

PE

kb3lms
10-10-2012, 11:25 PM
I think it's plated correctly, but this was a first attempt. I think I will clean it and try again to see what happens. The meter does have a high impedance input. I borrowed it from the lab at work and it was just calibrated so it ought to be correct. Besides, the opamps in the buffer box use j-fet inputs IIRC and have in input impedance on the order of 10^8 ohms! I get the same reading with and without the buffer.

How much would concentration of any of the solutions have to be off to make that sort of error in the reading? I am guessing pretty far off for the sort of error I am seeing.

Probably would be a good idea to check out the homemade reference electrode as well, once I figure out how to do that. Open to suggestions!

Kirk Keyes
10-11-2012, 10:28 AM
My first suggestion it that your beakers are upside down in the first photo - stop working on the cieling! All the solution will fall out! ;^)

Don't worry about the interface box - a modern VOM should have high enough impedence that it will work fine. Keep things simple. And don't put a pot in there - it's not needed.

Stir the test solution so that fresh solution is being brought to the electrode continuously.

Also, try replating the electrode - I think it took me a few times to get mine seasoned so that it worked better.

And most important - remember that reference electrodes are half the equation. If your reference electrode doesn't match everyone elses reference electrode, then you will have a fixed bias, positive or negative, from everyone else's results.

Make a series of solutions with a log series of concs, like say (and just pulling numbers from my head) 0.03M, 0.1M, 0.3M, 1.0M, 3M and then measure those. Plot them on some Gran's plot (semilog) paper and see if they are linear. If so, then you have a functioning elelctrode. If there is a bias, then it could be the vAg electrode or the reference electrode. If there is no linearity on the Gran's plot paper, then something is not working as it should.