Safety of Hydrofluoric acid vs. ammonium-bifluoride

A. For etching purposes, ABF is always a suitable substitute (with the correct ratio). I took the time to research and write a technical paper about ABF substitution for HF some years ago. I agree that the storage and handling is potentially safer and I always preferred to deal with ABF as you do. As far as chemical precipitation for treating the waste, it is typically done with a calcium compound being added to the treatment in order to precipitate calcium fluoride. It is a bit troublesome to get the balance right if your waste stream is variable and it does result in high sludge volume. In addition, in practice you will likely still be discharging perhaps 50 ppm Fluoride. It can be quite expensive in capital and operating costs. You will likely have some better results with a well controlled batch process for your concentrate dumps, just size your settling tanks and filter press to handle the relatively large sludge volume.

A. Clay-
As far as the ABF conversion goes, reach out to me at GOAD and I will send you the short paper that will include that information.

As far as the fluoride precipitation, the first question is do you need to remove it. You will need to check your wastewater discharge permit and perhaps also with your regulator. Fluoride is not a standard federal requirement to be removed from metal finishing wastewater, but I am starting to see that some local permits are adding it. Once you know your permit limit (if you have one), then you should get in touch with your treatment equipment supplier who can design an appropriate system and controls to remove the fluoride that will fit with your particular situation.

Hydrofluoric acid burns

Never use HF anywhere unless trained!
If you do, you MUST have the antidote gel on hand for instant use!

Calcium Gluconate
for HF acid burns

on eBay or

Amazon

(affil links)

A. Hi Jon,
Have not visited this site for years but have read to bottom going back to 2015. I am always fascinated when people say 1 part or % of acid etc. Are they thinking in HF's case, 35%w/w or 70%w/w or 33% HCl, 91% or 98%w/w H2SO4 etc. Are they talking % by vol or weight.
20% H2SO4 by vol is very different to 20% by weight, nearly twice as strong.
I also want to emphasize the need to have calcium gluconate on hand for burns as mentioned below, and the need for respirators for dealing with HF fumes.

Hi Geoffrey-
I remember your thoughtful posts from back in the day. You are right of course, it seems that there are many people who don't know how to do that math or even know that they need to. One of these days perhaps one of us will find the time to write a small paper on Basic Stoichiometry for Metal Finishers for the finishing.com technical library. I came up with the title, so I nominate you to write it. :)

Hi Jon,
Thank you for remembering this 83.7yo but don't think I am across all the fields these days.

A. Hi Michael. The relative safety of different acids isn't easily dispatched in a forum posting, but I DO NOT AGREE that ammonium bifluoride is necessarily safer than hydrochloric acid. Fluoride ions seem to present a biohazard that most other acids don't, as they apparently can penetrate the skin and attack the bone, and some people have reported an anesthetic effect such that a burn can go undetected until it is very severe. People have died from hydrofluoric acid burns, and I'm not sure whether that is the case with hydrochloric acid.

I'd say the general rule should be never use fluoride chemistry unless indispensable but, if you must, then ammonium bifluoride can be somewhat safer to have around and can perhaps be used more safely than hydrofluoric acid because its concentration is more limited.

As for etching a bathtub, remember that some are fiberglass, others acrylic, or porcelain, or cultured marble; acid will react with them all differently. Further, the acid splash and even the acid fumes will attack any metal in the drains, faucets, or elsewhere in the bathroom. Both from a personal safety viewpoint and a potential damage viewpoint, I wouldn't do it. Please either retain a professional bathtub surfacing company whose employees have received haz-mat training, or make do with a non-slip mat. Good luck.

Regards,

A. Ammonium Bifluoride, NH₄HF₂, is a weak acid salt of HF. If you add Sulfuric or Hydrochloric acid to the ABF you will have more HF in solution than with just the ABF. You will have to do some chemistry to figure the molar amount of acid needed to get the equivalent amount.

A. Hi,

I have used both ABF and HF a lot and know from first hand experience the effects of 70% acid on finger tips (A hole in glove). I have never had a burn with solid ABF, however (breaking lumps with bare hands). It does hurt when it gets in a cut or scratch but a quick rinse with water helps. This said, you should always wear eye and skin protection when mixing or using, and never handle ABF with your bare hands.

A. Even though they both produce a corrosive fluoride solution, ABF and HF are quite different.
HF is water-soluble in all proportions while ABF is soluble only up to ~28% at room temperature. ABF, in solution, yields ~ 1/3 as HF and ~2/3 as ammonium fluoride(AF) and, as such, provides a somewhat buffered solution (a 28% solution of ABF). In fact, semiconductor-chip manufacturers use different ratios of HF and ABF to produce various "Buffered Oxide Etches" that dissolve silica chips at various rates.

The use of (dry) ammonium bifluoride as a "safe" replacement for hydrofluoric acid is an unsafe practice. The uninitiated person who judges this as "safe" can quickly get into trouble since once the ammonium bifluoride (NH4F)HF is contacted with water; the HF content will be ~ 13% ... very corrosive, toxic and hazardous.

Some folks have used ammonium bifluoride as a substitute for HF and created (what they refer to as) an "acid free" wheel cleaner -- it's a dangerous and deceptive practice.

Bottom line here is that the ABF solution will still produce an extremely hazardous liquid.

? What is the function of the caster sugar ?

A. The function of sugar is make it less liquid. It is easier then to apply (honey consistency).

A. Hi Laurie. This is an industrial chemical that won't be found in a supermarket, but depending on volume and purity required I'm sure it's offered by both "laboratory supply" services and "commodity chemical" companies. And glass etching creams which contain ABF, and possibly HF, are widely available. Please explain your situation and additional help may be available. Good luck.

Regards,

A. Craft stores often carry glass etching cream that contains ammonium/sodium bifluorides.

A. Hi, I've just been given this recipe, I haven't tried it yet:

2.25 ltr Distilled water
2 Kg Ammonium Bifluoride
2 Kg Caster sugar
6 Kg Barium sulphate

Good luck!

A. Clean the substrate before etching; if white patches are found try to wash in diluted HCl that will be water diluted HCl and water.

A. They are not totally interchangeable. It is common to use ABF in an acid solution as a source of fluoride ions. It will work very nicely on many, but not all applications.

----
Ed. note: We have many threads on line here regarding those specific applications. Please try the search engine, adding terms related to the different potential applications like "aluminum", "stainless steel", "glass", etc.

A. Hi, George. Mike was saying the solubility of ABF is 28%. I'm quite sure he wasn't suggesting a formulation for an aluminum brightener, which might include many different ingredients and additives; I think he was just explaining areas of equivalence and non-equivalence between HF and ABF.

What exactly are you trying to do? ABF is dangerous; if you don't know about its reaction heat, fumes, etc., are you sure you have received the necessary haz-mat training? Thanks.

Regards,

A. Hi again, George. One reason I asked is that fluorides are very hazardous, so it is imperative that proper protective equipment including goggles and gloves be worn. The cleaner would probably have some other ingredients like a mild abrasive like feldspar and perhaps a wetting agent like sodium lauryl sulphate. If you are a consumer, there are number of companies already offering this product commercially, and you should use a packaged solution rather than attempt to custom formulate from raw chemicals.

If you are a manufacturer intending to market a fluoride-based cleaner like this, I think your plan must first be examined by a knowledgable consultant ... you can't possibly market such a product without your company intimately understanding the chemistry. Plus, you need to carefully study patents and do reverse engineering, or you'll be starting from zero 50 years behind your competitors :-)

Good luck.

Regards,

A. Hi All, Looking through the responses to this letter and the amounts people are using made me wonder if the solutions being suggested are of the right percentage.
The percentage of ammonium bifluoride in a solution should be no more than 20 %. this equates to 20 g to every 100 mls water.

That 20% solution is the MAXIMUM amount found in proprietary brands and "home brewers" should start with lesser amounts finding what works for them.

I would be interested in hearing how previous responders have gotten on with the solutions they have posted here in the last couple of years.

A. Ammonium bifluoride has a molecular weight of 57.0432 and hydrogen fluoride (HF) has a molecular weight of 20.006. Ammonium bifluoride can be viewed as a mixture of ammonium fluoride and hydrogen fluoride. Ammonium bifluoride is therefore 35.072% HF. So if you desire a solution that contains 5% hydrogen fluoride you need 2.85 times as much or 14.25% ammonium bifluoride. If you desire a solution that contains HF equivalent to 5% of the commercial product (typically, but not always, 48% HF) you will want 48% of this or 6.84% ammonium bifluoride. However, the ammonium fluoride will have a buffering effect on the solution and solutions prepared as described above will not be as active as their HF concentration would indicate. The desired strength will have to be determined experimentally. These calculations, though, will give you an idea of where to start your experiments.

A. I read your many questions and answers page, but I think two chemicals use another solution. I have a business of glass etching near about 7 to 8 years. I use HF and ABF another -- another solution for 2 another glass etching, ABF with solution use for frosting design and HF solution.

For deep etching, when necessary of chemical etching on glass, when necessary of deep etching, I use another way: sand blasting for frosted and also deep etching process. Nowadays, I think chemical etching is hazardous so, can we use blasting process? It is better for tomorrow, for own safety.

A. To complete David Beck's answer, which gives by far the best and safest advice --

One should do such work under chemical fume hood dedicated for work with fluoric acid. They're made from plastic only, and the ventilation is equipped with a water trap. Otherwise, vapors will corrode the ventilation system, and it can be pretty bad in case of extensive use.

Ammonium Bifluoride

A. Hi Myrick. Wikipedia explains how it can be done, but calls it "a once contemplated route" rather than a practical process. Simply mixing other chemicals with ABF will not convert it into HF, but mixing other acids with it can free up additional F ^- ions, making it more aggressive.

Please tell us your situation. Thanks.

Regards,