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.
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.
Originally Posted by kb3lms
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!
Originally Posted by kb3lms
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.
Originally Posted by kb3lms
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!
Here's a photo of my home-made vAg electrode as I discussed above:
And here it is in action in combination with a double junction Ag/AgCl reference electrode (in, I think, a potassium iodide solution):
Converting pAg to vAg
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.
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.
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.
Thank you, PE. I found them in my copy. Wasn't looking in the right place.
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.
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.
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.
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.
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!
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.