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, as notes below will explain.
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 and pay attention, but you really need to know what you’re doing and pay attention. Think of it like using a table saw, think through each move before you do it. 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, also see here, 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 usually the worst.
Unless otherwise specified, the deionized water described here is from the 18 MΩ deionized water system found in the ICP-MS prep hood (southwestern-most).
Starting the distillation
The place you should generally do HF and HCl 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 in one of the under-hood cabinets:
- 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.
To give yourself room to work, put the box on the side of the hood, with Teflon blocks under it to keep it off the hood counter. 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.
- Distilled 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, or on the upper part of the threads. 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 black hot plate so 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. Silicone is attacked by HF.
Put Teflon tape loops around the each end of the pad to form the pad into a hoop. This picture shows three tape loops. You need at least two, but more than four is probably excessive. You can make more out of any roll of plumber’s Teflon tape.
Put the pad with tape loops next to the full acid source bottle.
Push the full acid 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. Suit yourself.
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 somewhere in the lower part of the notch.
This is what it will look like once the bottles are inside. The electric wire will come over the top lip of the box.
Move the box back to the left side of the hood, back up on its Teflon blocks. Don’t cover the hole in the box bottom with one of the blocks.
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 is the best location, but suit yourself. Then put on the lid.
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.
Roll the caps up in the paper towel, folding in the edges as you go. This protects the caps from non-paper towel dust.
Look at the levels of acid every day, if possible. Distillation takes about a week.
Removing the clean acid
Distillation is finished when there is 50-150 ml of residual acid in the source bottle, meaning something like 700 ml in the distilled acid bottle. If you bump the still box, you should be able to see the sloshing meniscus of the acid on the source bottle side, through the gap in the heating pad. If you evaporate the source bottle to dryness, you need to distill the distilled acid again. Live and learn.
Turn the heating pad power off, and wait for the hot bottle to cool.
Get ready to remove the distilled acid (left) and residual acid in the source bottle (right). Orient the box to make this as easy for you as possible.
Lift the assembly out of the box.
Once out, slide off the heating pad by holding the distilled acid bottle, keeping its base on the counter, and pull off the heating pad by holding it and pushing against the source bottle bottom.
Rinse the distilled acid bottle cap in DI water. Then unscrew the distilled acid bottle from the elbow and put on the cap. This bottle can go back in its proper plastic acid bucket.
Take the source bottle cap and use it to cover the open end of the elbow. Bring the assembly over to the ICP-MS prep hood.
Dump the residual HF into the waste acid bottle, updating the bottle contents label as necessary. Rinse the elbow, bottle, and cap with DI water.
Repeat the steps above until you’ve made enough acid.
Cleaning up
The rinsed elbow should be completely wrapped in new paper towels to keep other dust and such out. Put it and the capped source bottle back in their drawer. Put the still box, heating pad, and heat controller back into its cabinet. That’s about it.
