View Full Version : Measuring vAg - preliminary results!

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Photo Engineer
12-15-2009, 02:34 PM
The measurement of vAg presented a lot of unexpected difficulties. This work compared to emulsion making is on the same order of unexpected difficulties in that every emulsion I make that is new, is different than I expect compared to what I might have gotten or seen made at EK. But, each of these tasks, vAg measurement and actual making present problems that, once solved, are solved and explainable. The same goes for coating.

I might add that those here that have claimed to be able to measure vAg have me puzzled, as they claim it to be easy. I knew how to do it, and still found it relatively hard to get to this point. Now, it will be easy for everyone else.


Photo Engineer
12-24-2009, 01:46 PM
I am nearing the end of the experiments to establish an easy and inexpensive method of measuring vAg during emulsion making.

I now have 3 Silver electrodes to be used for Chloride, Bromide and Iodide. They were all plated for 4 minutes using 2 AA batteries. They must be kept and used under reduced light. These electrodes, once plated, will keep for at least 1 week in distilled water. There is a tiny drift of about 2 mv during that time, but some have kept better than others.

I have 3 bridges with polarity and salt marked on them. They seem to keep as long as the ends are wetted with electrolyte gel.

Measurements seem to fall within +/- 4 mv of the correct values from 0.1 molar to 2.0 molar salt for all three halides. There is a slope value associated with this, so that at one end of the concentration scale, the error may be +4 mv and at the other end of the concentration scale it might be -4 mv. I am working on a slope control. I have also built a zero controller in a black box, so that the actual and calculated results can be made to coincide if there is an offset. This is similar to zeroing a pH meter.

I have devised a plater for short and long electrodes (1" - 6" or 2.5 cm to about 15 cm).

I have tested the electrode setup from 100 ml (about 3 cm) up through nearly the full length of the long electrodes along with full and partial plating and have gotten similar results. However, solution level must never go above the plating level of any electrode for stable results.

My next experiments involve electrode length (is 6" best or will 3" do?) and plating salt (do I need a halide or can I use Sulfide?).

In any event, when finished, this entire setup will cost under $150 USD. And, using the meter I suggest, it allows for data logging, but not control. I now have the data logger working perfectly with a 5 meter (~15 ft) cable. This allows the computer to be out of range of potential splashes.

I have verified this data with the Radio Shack meter mentioned above, with an el-cheapo analog RS meter, a Heath meter and a very very sensitive Keithly meter capable of very high accuracy. All of the data seems to be in agreement so I am ready to go on and finish this.

The one major problem remaining is that the plating seems uneven no matter what I do, and I think that this is part of the reason that my data is slightly off. Our electrodes at EK were very smooth and even, but mine here have spots of light and dark that develop during the plating operation.

If anyone is interested, I would be happy to make a custom Silver electrode for them. The cost would probably be in the range of $100 or less. This inclues a 6" or 3" Silver electrode sealed in glass with a 3' (1M) cable with a gold plated banana plug. The electrode is #12 Silver wire 99.99% pure. Usable length is about 5" or 1".

Pictures of this all will follow soon.


Ray Rogers
12-24-2009, 05:58 PM
I was thinking about your silver electrode and how you sealed it... are you using glass at both ends?
How do you seal the plastic coated cable end?


Photo Engineer
12-24-2009, 06:33 PM
The glass tube is sealed with epoxy with the cable to Silver junction inside of the tubing. That is the reason for the sealed glass, to protect the junction from corrosion and metal contamination. No moisture should enter the tube nor should it touch the Silver to wire junction. The other end is self sealed in the banana plug shround, and is 3' away from any contamination.


Kirk Keyes
12-24-2009, 06:36 PM
Ray - one the silver electrode I made for I took a couple inches of the 12 ga., 99.99% silver wire mentioned above and soldered it to the lead wire of a shielded video cable with a BNC connector on the other end. (The cable is from Radio Shack - I simple cut one end of the cable and set aside the unused half). I mounted it inside a 6mm diameter, 6 inch long pyrex glass tube by stripping back some of the shielding and the outer part of the video cable and slid the end with the silver wire soldered on to it into the pyrex tube.

I had hoped to have the shielding on the cable all they way to just a bit above the solder joint, but a 6 mm tube was too small in the inside diameter for it to fit. By removing some of the shielding, I was able to slide the silver wire and the teflon insulation around the lead wire into the glass tube.

I plan on getting some larger glass tubing - either 8 or 9 mm diameter so I can get the BNC video cable inside without having to remove the shielding, or perhaps 12 mm diameter so it's the same diameter as a pH meter so it fits into a standard pH electrode stand. Also, instead of cutting the 6 foot BNC cable in half, I'll simply snip on BNC connector off so I can use the full 6 ft of lead on the cable so the pH meter is not so close to the emulsion setup.

Note that I use a pH meter in millivolt mode rather than a standard VOM volt meter like PE so I prefer the BNC connector over a bannana connector, but adaptors can be used to mate up one with the other if needed.

With the silver wire sticking out one end of the glass tube, and the lead wire out the other end of the glass tube, I took some 5 minute epoxy that I prepared and used a pipette to squirt some of the epoxy into both ends of the glass tube. This sealed both ends. I wiped off any epoxy that was on the exposed end of the silver wire. There was about 1.5 inches of silver outside the glass tube, and about 0.5 inches inside.

Ray Rogers
12-24-2009, 08:01 PM
By removing some of the shielding, I was able to slide the silver wire and the teflon insulation around the lead wire into the glass tube. [snip]

Note that I use a pH meter in millivolt mode... so I prefer the BNC connector over a bannana connector, but adaptors can be used to mate up one with the other if needed. [snip]

I took some 5 minute epoxy that I prepared....

Thanks Kirk for that very good answer.
I think I am using the adaptor...
Two ?s went through my head though...

Teflon insulation on Radio Shack cable?

5min=5 min to set ?
You're using a double solution epoxy?
Does it ruin your pipette? (or are you using throw-a-ways?)

Ray Rogers
12-24-2009, 08:12 PM
The glass tube is sealed with epoxy....

I see. Somehow I thought you were heat sealing it... I was not too successfull with that approach. :(

Kirk Keyes
12-25-2009, 11:14 AM
It's "5 minute epoxy" which two part epoxy that sets in 5 minutes.

Yeah, I thought about heat-sealing the silver into the glass, and that's just too complicated. And a lot of pH electrodes have epoxy bodies and I figured why not try the 5 minute type for sealing the tube. It seems to work and it doesn't appear to corrode the silver wire - at least the stuff I tried.

Maybe the inner dielectric insulator is polyethylene instead of teflon. I'd have to look at it again and maybe burn some to see if it gives off really acidic fumes - it's a really easy way to tell polyethylene from teflon.

I used little 3 ml polyethylene disposable pipettes to transfer the epoxy. I didn't want to use glass as I didn't want to break the tip of the glass pipette into the epoxy.

03-04-2010, 10:17 PM
I am also saving my 120 paper backing. I have a drawer full of them.


I did that for a while! Finally had to recycle them. One has a limited amount of space. :-(


Photo Engineer
03-04-2010, 10:55 PM
I did that for a while! Finally had to recycle them. One has a limited amount of space. :-(


Yeah Ed. See the other thread on 120!

I still have my drawer of paper backing.


Kirk Keyes
03-05-2010, 12:15 PM
Awww, man! I was all excited seeing there was some action in this thread about vAg and it turns out it's just banter about 120 paper...

02-11-2011, 06:01 PM
I am now at the point of being able to replicate results to within about +/- 5 mv, but with a slope adjustment as the solution becomes more dilute. I am working on that. It may be due to having immersed too little of the billet in the electrolyte.

Here are photos of the reference electrode, the billet, the bridge and a trial setup of a beaker ready for precipitation of a real emulsion. The reference electrode is immersed in its conditioning/storage solution, and the bridge is also immersed in its own gel/salt storage solution. The bridge is marked as to polarity and the salt that it is to be used with. These are two critical items in this measurement.

The making beaker in the 4th picture contains 2 inlet lines for salt and Silver, a temperature probe, the plated Silver billet, and the bridge. The bridge is a mockup in this but not in the other picture. The bridge is connected to the salt solution beaker on the far right which contains the reference electrode.

The pump is partially seen to the far left.

Both electrodes are connected to the VOM. It is then connected to a PC with a cable and data logging can be carried out as a function of time.



I'm slightly confused as how the bridge attaches to the meter,


Photo Engineer
02-11-2011, 06:16 PM

The bridge does not attach to the meter. The two electrodes attach to the meter and the bridge "bridges" the circuit between two beakers. One is the beaker in which you are making the emulsion and the other is the beaker that you have one of the electrodes in and which contains a buffer solution.

One electrode, in the making beaker, is made of silver metal and the other, in the buffer is a Silver, Silver Chloride double junction reference with the part # given elsewhere in this thread or forum.

The bridge is tubing which contains thickened salt.