HCO2- as a Photo-dopant

[Kirk Keyes]

You can wonder, but you want to measure millivolts for pH or pAg.

I have a VOM that measure to 10 Amps. And I have to remember not to have it on that setting when I try to measure house VOLTAGE. I don't mind it tripping the 15 amp circuit breaker, it's the popping sound of the tip of the meter probe vaporizing off that scares me...

 

[Kirk Keyes]

PE - care to go into your pAg electrode design more? How are you doing the reference cell? The rest it not too tough, as I see it.

 

[Photo Engineer]

You know, I think a lot of digital meters from pre-2000 use simple point-to-point calibrations between each calibration point. That makes calibration much simpler. But it means you need to check your calibration with standards that fall between your standardization points to ensure accuracy.

The math is really simple when you do it that way, just simple slope calculations...

Well, VOM is as described by AgX.

Kirk, the calibration I meant is turning resistance or conductivity if you will into vAg or pAg. After all a value expressed in mV is not easily related to vAg. That is the complex calculation.

Old formulas gave values in ohms of resistance if they gave any measurement at all. The salt measured total conductivity including nitrates present and anything else. Current meters include the nitrate but the calibration software can get at concentration even in the presence of nitrate by cancelling it out.

There is a considerable difference between 1 M AgNO3 with and without 1 M of NaNO3 as a "side salt" produced by pptn.

PE

 

[Kirk Keyes]

Can you give an example of "an old formula" for this?

For vAg you would want to use the mV setting on the VOM, and then we are working directly in vAg, right? Forget pAg as you say. Graph it on semi-log paper, I beleive.

For conductivity, you use one of the ohm scales, as appropriate for the electrode you have. Then there are formulas for that, based on your conductivity probe. Are you using a commercial conductivity probe or making your own.

Maybe this talk should be in it's own thread?

 

[Photo Engineer]

Well, Kirk, I don't have an "old" formula that gives readings in resistance (or conductance) that I can publish but they exist.

As for current mV readings, at 40 C, 0.1 m/l NaBr should read a vAg of -50.11 mv with a pKsp of 11.61, pAg of 10.49, pX of 1.11, Activity Coefficient of 0.6616 nd Junction Potential of 1.78. This was in a previously deleted post about a year ago. Another APUG member disagreed with me on some of the "data" and the entire exchange was deleted by the moderators.

If this can be reconciled with your data, then by all means use it, but it depends a lot on your setup as we discovered at EK. If you want more information, then by all means call me. You have my #. This talk is getting to the limit of what I can put in print.

PE

 

[Kirk Keyes]

OK - by the way - Modern conductivity meters use a current that switches polarity - kind of like AC, but I think they use a fixed volatage. That is to keep the ions from all migrating to the electrodes. That means you need a little extra circuitry to go with your VOM :^)

 

[analogsnob]

So what do you do for the reference side? I assume the high flow junctions we used for e6 ph would complicate the readings?

 

[Photo Engineer]

You cannot use a high flow junction. High flow of anything will disturb the crystallization process, not to mention the vAg itself. And, the smaller the batch, the more critical this becomes.

PE

 

[Kirk Keyes]

Traditionally, calomel (mercury chloride) reference half-cells were used for pH, but Ag/AgCl reference half-cells are replacing them, mostly for environmental reasons. But since both of these cells use saturated potassium chloride as the filling solution, you probably want to use a double junction design for the reference cell. Like I said above, the silver electrode half-cell pretty much can be a wire of silver.

 

[Photo Engineer]

But, don't use Silver Sulfide. I know you mentioned this above Kirk and I forgot to say anything. It is not good.

A bare silver electrode is ok, but it takes time to equillibrate. Start with a plated electrode with the salt you are going to use in the PPTN.

To get good results, it has to be a very sophisticated junction.

PE

 

[Kirk Keyes]

Did I mention that getting it all to work is the hard part? Silver electrode analysis is a bit more tricky than pH measurements.

 

[Kirk Keyes]

But, don't use Silver Sulfide. I know you mentioned this above Kirk and I forgot to say anything. It is not good.

So a bromide coating is probably better than a chloride coating.

 

[Photo Engineer]

If you are precipitating AgCl and use an AgBr electrode, you will end up with either a bare silver electrode or an AgCl electrode with some AgBr in your emulsion.

PE

 

[totalamateur]

I hate to ask, but what does PPTN standfor, I googled it and all I got was pennsylvania public television network, which I assume we are not using a silver plated anode to monitor?

 

[Kirk Keyes]

PPTN = precipitation

 

[Photo Engineer]

DO NOT USE CALOMEL!!!!

I forgot this in the above post on Silver Sulfide. Sorry.

PE

 

[Kirk Keyes]

If you are precipitating AgCl and use an AgBr electrode, you will end up with either a bare silver electrode or an AgCl electrode with some AgBr in your emulsion.

Right, I was thinking about bromide as the main halide... and didn't consider a primarily chloride emulsion. It does get complicated.

That's perhaps why the more modern commercial silver ion electrodes don't have an exposed silver electrode. See http://www.thermo.com/com/cda/product/detail/0,1055,15071,00.html

They also can be bought with built in reference half-cells. But man, they are spendy! On the order of $600.

 

[Photo Engineer]

Well, with mixed halides it becomes even more difficult. That is why you do not measure pX or pBr or vX etc.. The silver ion is more representative as to what is in the kettle or going on than the halide is.

It is often best to use a bare billet and let it equillibrate. Our production billets were about one to two meters long and about 10 cm in diameter. They used a power lift to move it around. This would probably keep one of us in luxury for the rest of our life!

PE

 

[Kirk Keyes]

Our production billets were about one to two meters long and about 10 cm in diameter.

At two meters, I figure that's about 363 pounds of silver, or about $58,000!

I guess if you wanted to save some money, you could make it hollow...