Making Silver Halide Emulsions with a Silver Ion Specific Electrode

Back in the mid-aughts, there was a concern that all silver gelatin-based emulsion materials (i.e., film and paper) would disappear. I had that concern as well – and instead of lamenting the loss of these materials, I decided to learn how to make them.

My Emulsion Making Setup

With the help of Ron Mowrey, an ex-Kodak emulsion chemist, I began experimenting with ways to make silver gelatin emulsions at home. As an environmental chemist by profession, I immediately saw the potential to use equipment similar to what I used at my work.

Devices like silver-ion selective electrodes and peristaltic pumps were a couple of things I started using. I combined an Arduino with a Maxim Integrated DS18B20 one-wire temperature sensor to log temperature data to my computer.

I used a Corning PC-620 stirrer/hot plate with a Corning temperature controller to heat the emulsion during the precipitation steps. Denise Ross, an excellent emulsion maker, recommends using a silicone pad between the beaker and the top of the hotplate to make a more uniform solution temp.

I prefer to use tall-form or “Berzelius” beakers for emulsion making. It maximizes the stir-bar coverage on the bottom of the beaker. Some people use mechanical stirrers as the stir-bar can become disengaged from the stir plate magnet when preparing larger emulsion volumes. That’s because the viscosity and the volume of the solution will increase during precipitation.

APUG Silver Gelatin Post

I was recently asked to post a few photos that I used to have on the APUG Silver Gelatin forum from around 2007.

Peristaltic Pump

The pump I use is an ISCO Wiz Peristaltic Pump Diluter / Dispenser. They can be found on eBay for $75 and up. The pump works by placing a piece of peristaltic pump tubing between a “platen” and a set of rollers. The pump motor turns the rollers, which in turn pinch the tubes against the platten. When the rollers squeeze the tubing, it creates a vacuum that pulls the liquid through the tubes, and then pushes the li.

A digitally controlled, multiline peristaltic pump (with two lines in use – max of 4 possible.) Differing rates can be added by using different diameters of pump tubing.

Perhaps a nicer shot of the pump. Only one line is shown connected to the pump at this point.

The speed that the pump dispenses the liquid is controlled by two factors – the tubing diameter and the speed that the rollers turn in the pump. You can make variations to both to get the flow rate you need.

One issue with the Wiz pump is the clips that hold the pump platen can break, as they are made of plastic.

Here’s the setup in the dark. I have Rubylith red film over the screen of the laptop to prevent it from exposing the emulsion. The room lights are red LEDs and Rubylith-covered fluorescent ceiling darkroom lights.

Peristaltic vs. Syringe Pumps

An alternative to a peristaltic pump is a syringe pump. They can pump a precise amount of liquid. The reason I didn’t get one is they can only pump one solution at a time. Maybe two if you can rig two syringes into one pump.

The cool thing about peristaltic pumps is they are often multichannel, meaning they can pump more than one liquid at a time. You can also pump each channel at different rates by using different diameter tubing. The Tygon or Viton tubes are color–coded, so be sure to figure out what flow rates you want and then buy tubing to match.

The smallest pump tube I have is labeled “orange/black.” It can pump at 0.006 ml/min with a pump running at 0.5 RPM. That’s pretty slow to me! Purple/White will get you up to 22 ml/min at 90 RPM – so there’s a huge range of flow options available out there.

Dispensing Liquids

I used a small Teflon tube to dispense the liquids into the gelatin solution. www.smallparts.com sells Teflon (PTFE) tubing down to 44 gauge. That’s a 0.002-inch diameter! I bought some from them that are 32 gauge. Even at small flow rates, that diameter will help smoothly dispense the liquid being pumped.

I dispensed two solutions into the emulsion mix – one with silver nitrate and the other with my halide solution. Instead of allowing these solutions to drip down into the emulsion, as would happen when using a burette, I wanted the liquids to be added under the emulsion’s surface. So I inserted one of those lines into a small diameter stainless steel tube to give the tube rigidity. Then I secured the second tube to the outside of the stainless tube using a small section of Tygon tubing. See the photo below.

The ends of the silver nitrate and halide solutions are put below the emulsion surface when dispensing these solutions. Small diameter Teflon tubing allows a very fine jet of reagents to be added. Doing this disperses the two solutions very quickly into the emulsion.

Connecting the Tubing

You’re going to end up with many different diameters of tubing when using a setup like this. One lab trick to connect two tubes of different sizes is to simply press one end of the smaller tube into the end of the larger tube. Two pieces of Tygon tubing will stick together pretty well.

If the outside diameter of one tube is smaller than the second tube’s inside diameter, you can take a small length of a third Tygon tube and use it for mating the two other tubes. This third “adapter tube” should be a diameter that is smaller than the outside diameter of the larger of the two other tubes. Then press the smaller tube into the other end of the third tube. It’s like a step-down ring for a larger filter and a smaller lens thread.

Since Tygon stretches, you can press the larger tube into the end of this third “adapter” tube. It will hold the two other tubes in place as it squeezes onto them. Unless you have high pressure in the tubes, this will be more than enough to keep them connected.

If the third tube can’t make the connection, a fourth tube can connect the third one to one of the two smaller tubes. It’s like nesting dolls, but with tubes. Find sizes that allow you to nest all the tubes together, but in a linear fashion.

Silver-Ion Specific Electrode

Basically, it’s a piece of silver wire. I found some 12 gauge silver wire on eBay. That size is 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 a “pH” meter.

You can see the silver-ion electrode I made here. It’s a length of 12 gauge 99.99% pure silver wire that’s been soldered to the center lead in a length of coax cable with a BNC connector. The end of the silver wire was then inserted into a glass tube and epoxied in place. Before use, the silver portion of the electrode must be plated with the salt representing the principle halide constituent of the emulsion you’re making.

Here it is in action in combination with a double junction Ag/AgCl reference electrode (in, I think, a potassium iodide solution).

The silver-ion electrode must be used in conjunction with an Ag/AgCl/Saturated KCl reference electrode. They are then both connected to a pH meter that can read millivolts (mV). You can see the reading on the pH meter display while verifying my home-made silver-ion electrode’s function. An ion bridge must be used to prevent contamination of the emulsion from the reference electrode. I used a potassium nitrate solution in the bridge.

Reference Electrodes

Most people never 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 electrodes have lost favor these days, as they contain metallic mercury and mercury (I) chloride, both rather toxic compounds.

The silver chloride reference electrode consists of a piece of silver metal coated with silver chloride. It is placed into a concentrated potassium chloride solution 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 often discussed in the articles on pH measurement is this type of electrode. It is not a “silver” electrode that is used for making vAg measurements.

The vAg electrode that Ron Mowrey often discusses on APUG/PhoTrio.com 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.

Coax It Into Shape!

The coax I used was from Radio Shack (RIP). It’s just a 2 ft coax with BNC connectors on both ends. Clip one BNC connector off, and then you have a nice 2 ft lead for your electrode.

When connecting the silver wire to the coax, solder it to the coax’s signal wire and not the braided outer wire to the silver. Strip the braided part back a bit, so it doesn’t make contact with the center wire. It will 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 could be a good place to get it cut if you’ve no experience cutting glass tubing.

Glass Tubing Housing

The silver wire/coax assembly was then inserted into some fire-polished glass tubing. Adjust it with bare silver wire outside the glass and the silver wire solder joint inside the glass. Then 5-minute epoxy was used to seal both ends of the glass tube. Completely fill the ends of the glass tube with epoxy.

Glass tubing was chosen for several reasons. It gives the electrode rigidity, keeps the solder joint clean, minimizes the length of silver wire needed, and is something that you can insert into the emulsion solution and not contaminate it. It’s easy to clean and can be supported in the solution using a ring stand clamp.

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.

Keep it Simple

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 videos on the youtubes for basic glass-blowing techniques. None of this glass working is difficult. 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.

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 give that a try. Other than the silver wire/rod, nothing is really expensive here.

Preparing the Silver-Ion Electrode for Use

Before 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 was run through the wire and solution to drive the halides to the vAg electrode to plate it.

To plate the vAg electrode, connected to the positive terminal of a 3-volt battery. Then run some current through it for 10 minutes using a second vAg electrode or just a silver wire in circuit with the plating solution. Once this had been done, the vAg electrode was ready for use. This treatment was sufficient for several days if stored in a halide-containing solution and kept in the dark.

Now, this vAg electrode has to be used with a reference electrode – so that you had both an ion-specific electrode and a reference electrode.

A Bridge Not Too Far

Ron Mowrey suggests that you want to use a double bridge. That minimizes the migration of non-chloride halides from the test solution to the reference solution. If bromide or iodide gets into the reference solution that surrounds the silver|silver chloride reference electrode, then it will be poisoned. It will no longer serve as 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 slows down mixing with the reference solution (the potassium chloride surrounding the silver|silver chloride electrode). The felt tip is merely acting as the junction. It allows the interior solutions to come into contact with the liquid being tested.

Setting up the Bridges

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. The reference electrode mentioned above has a gel inside that helps act as a bridge. It isolates 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 grade or at least Reagent Grade, so you know you have low halides. Don’t be tempted to use hydroponic quality KNO3!

Silver Ion Meters

For vAg, there is nothing wrong with using a benchtop Volt-Ohm-Meter (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 of 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. That’s what I’ve used working in the chem lab, and I’ve had good results with those brands. I found my meters by waiting along for ones that had been tested and shown to at least turn on with the display working…

The advantage with pH meters over VOMs is when you want to use a pH electrode with a built-in reference electrode and a temp sensor. The pH meter has the proper connectors so that a pH electrode will plug right in. It will also read directly off the readout – it can even correct pH for temperature that way. When doing vAg, you merely need to measure volts (although temp affects it too, I’ve been ignoring it…)

Commercial Silver-Ion Electrodes

If you want to look for a commercial reference electrode – look for ones that are called “double junction.” They have the second bridge built right in. So that reduces 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 indestructible – a good cleaning followed by soaking in fresh filling solutions is often enough to revive “dead” silver-ion electrodes!

Testing Your Silver Emulsion Electrode

Make a series of solutions with a log series of concentrations. Try 0.03M, 0.1M, 0.3M, 1.0M, 3M, and then measure those. Plot them on some Gran’s plot (semi-log) paper and see if they are linear. If so, then you have a functioning electrode. But if there is a bias, then it could be the vAg electrode or the reference electrode. If there is no linearity on Gran’s plot paper, then something is not working.

Don’t expect your measured results to be textbook perfect. Silver-ion electrodes can be hard to get working. Keep trying. Re-plating it with halide may take a few tries to get a good coating.

“Photographic Emulsion Making, Coating and Testing” Book By Ron Mowrey

If you’re interested in making your own emulsions, I highly recommend Ron Mowrey’s book, “Photographic Emulsion Making, Coating and Testing.” It was not inexpensive, but it’s packed with excellent information.