We distill our own high-purity acids, and HF and HCl are distilled in a genuine 2-bottle still that we made in 1989. It uses two PFA Nalgene 1-liter bottles, an elbow from Savillex, and two threaded adapters that we had made at the Union College machine shop. Most of the description below will reference HF. HCl is a little different, and a notes below will explain that.
First of all, no students are allowed to do any of this, faculty only. It’s not especially difficult or dangerous if you know what you’re doing, but you really need to know what you’re doing. Second, wear all necessary safety equipment, including long pants, closed-toe shoes, lab coat, apron, goggles, face shield, and HF-resistant gloves. No fooling around here. Third, be sure you know about the hazards of HF, what to do if you get some on you, and check to make sure you know where the HF spill and emergency materials are in the room.
For HF we usually distill reagent grade 50% solutions, and that is good enough for most of our purposes. If not, try starting with trace metal grade. For HCl you should only distill trace metal grade ~37% solutions, otherwise there will be too many volatile chlorides that carry over during distillation. Tin is the worst, usually.
Starting the distillation
The place you should generally do distillation is the ICP-MS digestion hood (southeastern most).
Move things to the right to give yourself room, and wipe down the whole area to clean up dust and other messes. A clean workspace more likely results in clean distilled acid.
Find these parts:
- Polyethylene foam arch.
- Still box lid.
- Still box.
- Some Teflon blocks.
Sponge off and rinse the parts with building deionized water in the sink. Don’t bother drying them.
Set up the still box along the side of the hood, with Teflon blocks under it to keep it off the hood counter. Don’t let one of the blocks block the little hole on the box bottom. The big square hole should be on the right.
Lay down some paper towels and gather the still parts:
- Source bottle, holds the low-purity acid.
- Clean acid bottle, receives the distilled acid, usually an empty high-purity acid bottle from the plastic acid storage bucket.
- Connecting elbow.
Rinse the parts outside and inside, caps too, and shake out excess water. Don’t dry them.
The low-purity acid stock bottle will probably have a lip, like this. When pouring, rest the groove on the source bottle lip. That will keep everything steady and help avoid spills. Pour slowly.
Pour acid from the low-purity stock bottle into the (rinsed) source bottle. Fill the source bottle 75 to 85% full.
DON’T OVERTIGHTEN OR THE THREADS WILL STRIP!
Screw the empty clean acid bottle onto one end of the elbow, and screw the acid-containing source bottle onto the other elbow end. Twist the bottle, don’t spin the elbow plus empty bottle.
Put the empty bottle bottom against the hot plate to ensure the assembly doesn’t fall over.
Gather up the heater parts. These include the controller, heating pad, and Teflon tape loops to keep the pad pressed against the source bottle wall.
The silicone pads last several years, but consider a Teflon-coated pad next time. They will last much longer.
Put Teflon tape loops around the upper and lower ends of the pad to form the pad into a hoop, and put it next to the source bottle with acid in it.
Push the source bottle down into the heating pad hoop. Adjust the Teflon tape Loops and the pad so that the pad is mostly in contact with the bottle, and that the top of the pad is about at the top shoulder of the bottle.
I fancy that the best place to orient the heating pad gap is toward the empty bottle, which is down when the still is working.
This is tricky, so think about this before doing it. Arrange the still box and still assembly as you wish to make this step easier.
Pick up the still assembly, especially holding the top of the heavy source bottle with your elbow up. Gently lift the assembly into the still box, to rest the elbow in the central notch. The bottle bottoms will rest on the box bottom. The black heater wire should rest in the lower part of the notch somewhere.
Once in place, the power wire will loop over the top of the still box.
Thread the power wire through the big square hole so it doesn’t interfere with the still box lid.
Put the foam arch on top of the elbow, on the hot side of the box. I fancy that this is the best location, but suit yourself.
The acid meniscus should visible somewhere around here. Too high and you risk sloshing the impure acid into the high-purity side, too low and you have to do more distillation procedures to make enough acid.
Plug the white cord into the back of the power supply, and turn on the power to a value of 4 for 50% HF.
For HCl, turn the power on to a value of 1. Each subsequent day, raise the setting by one of the minor increments until the setting is at 2. Leave it there until the distillation is done.
Put the caps open side together on a piece of paper towel abut 40 cm long.
Roll the caps up in the paper towel, folding in the edges as you go. This protects the caps from non-paper towel dust.
