Analysis of nitric-hydrofluoric aluminum etching mixture

Hydrometer

ACRONYM:
TISAB = total ionic strength adjustment buffer

A. Hydrometer is a very poor method of control since as the acid is expended, it is replaced with aluminum, copper and a few other alloying metals which tend to be heavier than the basic solution, so a very false reading. A plain old acid titration where you add both acids in the correct percent to make up for the spent acid may be adequate and is certainly a lot better.

For a much better control, use a selective ion electrode (fluoride) and test for the fluoride. Do a total acid titration. Subtract the HF number from the total and you get the nitric value. It is not perfect, but it is good and normally affordable. The electrode requires a pH meter capable of 0.1 mV accuracy. These are not cheap compared to garden variety meters. I used an Orion 901 and their probe with very good luck on titanium etch. They provided me with a couple of papers that I modified for my particular use. It used an alternate TISAB, not the normal one for the swamping agent.

A. I have some paperwork which suggests a two stage acid base titration.

Titrate a sample using sodium hydroxide 1N [affil links] to pH 3.2 to get the Nitric acid. Then on the same sample continue to pH 5.8 to get the Hydrofluoric acid.

I have only used this method once but it seemed OK. Methyl orange ⇦this on eBay or Amazon [affil links] and mehyl red ⇦this on eBay & Amazon [affil links] indicators work at approximately the right endpoints if you want to do without pH meter.

You may need to change the concentration of Sodium Hydroxide for the second stage depending on the relative concentrations.

A. You can use an autotitrator to determine automatically the 2 equivalent points in a single sample. Multiple acids titration is quite common in the electronic chemical industry and most can be easily solved by autotitration. There is need for the use of colour indicators nor pH meters then. If you need further information, please feel free to write back.

A. We have recently taken over the running of a lab and have been looking for an improved method of testing HF/Nitric solutions used to etch titanium parts.

We have inherited 2 methods which we're examining at the moment:

In the first an aliquot is taken, KNO3 and ice added, and titrated against NaOH to first EP. The fluoride is estimated with an ISE. However, we find with this method the fluoride detected decreases with increased levels of Ti, presumably due to [TiF6]3- formation.

In the second an aliquot is taken, KF, (COOK)2 and CH3OH added, heated to 80 °C and titrated against NaOH to first EP. The nitrate is determined by IC.

Both methods seem ok, but not great. We've also tried fluoride by IC, and this result doesn't seem to be too affected by dissolved Ti.

I hope this may help some people ...

A. This is possible if using an autotitrator. The big trick here is that you must use dried acetone ⇦this on eBay or Amazon [affil links] Warning! highly Flammable! as the solvent, not water. In water, the nitric and HF react with NaOH at the same rate. In acetone, the dissociation constants shift. I am unsure how the indicators will work in acetone, so someone give it a shot and see. The nitric acid dissociates at a pH of approximately 4 and the HF dissociates at a pH of approximately 10 in acetone. I used a Mettler-Toledo T50M with a DGi116 solvent pH probe and 1.0N NaOH [1N NaOH on Amazon [affil links] . I repeatably found the nitric concentration (first inflection point to pH 4) to be 29.2% and the HF concentration to be 1.8% (second inflection point to pH of about 10). Good luck and let me know if you can use the indicators.

Ammonium Bifluoride
Aluminum Brightener

on eBay or

Amazon

(affil links)

A. Stephen,
I had to set up a known standard of the nitric and bifluoride solution, then titrate for both acid-base and bifluoride. Bifluorides are determined using a titration with Thorium Nitrate in Total Ionic Strength Adjustment Buffer. I'll go through my old notes and give you the constants. I know that using a 5 ml sample and 0.1N Thorium Nitrate, I multiplied mls of titre times 0.636 to determine oz/gal NH4HF2.

A. Thorium nitrate titration is a reliable analysis method but ... it is radioactive so most labs don't use it for years already.

A. Hi Chandrasekaran. A hydrometer measures the specific gravity of a solution. Nitric acid is heavier than water, so if you have a solution with just nitric acid and water (and nothing else), and you know the temperature (because water expands with heat, affecting the specific gravity), it is possible to correlate the hydrometer's specific gravity reading to the nitric acid concentration via a simple table. You will find that table in Perry's Chemical Engineers Handbook [adv: suggestions on Amazon, AbeBooks, or eBay affil links] or on-line at
handymath.com/cgi-bin/nitrictble2.cgi?submit=Entry

But this only works for initial makeup. A hydrometer will not work for ongoing analysis of etching solutions (or much else) because, as James Watts has explained, the specific gravity of the solution is also affected by the dissolved aluminum content and other dissolved materials, destroying the correlation. Please give the details of your situation. Thanks!

Regards,

Hello cousin Tran. I can't help you technically, but as website curator I suggest ...

There has been an extensive discussion, suggesting specific titration procedures and analytical equipment -- please try to frame your question in terms of what has already been said, asking questions about what you didn't understand or explaining why you feel the suggestions may not be applicable to your situation. Otherwise it can look like you're asking people to do arithmetic for you, and they won't. Thanks!

Regards,

A. One can determine total acidity, then determine HNO3 conc. via titration. A 1.00 ml sample is added, with good stirring, to about 80 ml ice cold concentrated sulfuric acid. Then, one titrates, keeping the mixture cold with strong stirring, with freshly standardized 0.1N FeSO4 solution. The endpoint is a brown red color.

Or, if you like, you may determine F- via an ISE method, using the method of additions.

In either case, the concentration of each respective acid may be arrived at by arithmetic.

A. I have had success using a 0.1000 N cyclohexylamine in methanol [affil links]. I can determine both nitric and hydrofluoric acid simultaneously with an autotitrator. It is titrated in methanol with a pH electrode, a non-aqueous titration.

A. Hello Gabriel,
A great place to look is the tech sheet for a product called MacDermid-Enthone Metex Etch Salts, prod #13051.
It's an ammonium bifluoride product that is intended to be used with or without Nitric acid, depending on your application, and it has analysis instructions for both (though the calculation for 'Etch Salts" may or may not be EXACTLY equivalent to ammonium bifluoride content).
I have an ancient wrinkly fax in my files and trying to find a link online I'm discovering that you need an account/pw to view...
I have my own minor tweaks to the methods; this is not *quite* what theirs is, but here ya go:
Here's the one for the ammonium bifluoride, and take it with a grain of salt (no pun intended!):
Start with 1mL bath solution (let any particulate settle before taking your aliquot).
Add a drop of phenolphthalein, and a splash of deionized water, then very carefully raise the pH to JUST pink (I use NaOH 0.1N ⇦this on eBay or Amazon [affil links] for this so as not to overshoot) while swirling, stop, and put in enough drops of H2SO4 0.1N to JUST bring it back clear again.
Add some more DI water, about 75mL is plenty, and put on your stir plate.
Add 40mL Boric acid solution, saturated (I use 60g in 1L hot DI water, shake and cool, it keeps for a long time) and 40mL reagent alcohol (ethanol-methanol mix).
Add 8g Potassium chloride. Allow everything to dissolve and mix well.
Add methyl orange ⇦this on eBay or Amazon [affil links] indicator (my least favorite indicator ever!!) and titrate to full salmon-pink with H2SO4 0.1N.
mL H2SO4 0.1N x 4.5 =g/L ammonium bifluoride AS METEX ETCH SALTS... On the old MSDS, it's listed as 'Fluoride ion, 100% by weight' so you are absolutely going to want to make an analytical standard from fresh ammonium bifluoride, run a calibration curve, and confirm your multiplier.
Also, as the metals load up in the bath, it becomes wildly inaccurate... have fun! :)
There's also a titration listed for nitric acid in the presence of the Etch Salts that basically has a small correction [subtractive] for the interference of the salts themselves. Other than that, it's a normal Nitric test:
10mL bath sample, few drops methyl red ⇦this on eBay & Amazon [affil links] indicator (you could probably get away with phenolphthalein for this just fine), titrate with 1.0N NaOH to yellow if using MR or pink if using PTH.
Without the ammonium bifluoride correction, it's mL NaOH 1N x 7.06 = %vv as 42'Be or 67% Nitric acid. With the Etch Salts correction, it's (mL NaOH1N -(oz/gal etch salts x .131)) x 7.06= %vv as HNO3 67%.
We use the etch salts without Nitric to frost Aluminum (2xxx, 6xxx, 7xxx) prior to desmutting and then anodizing. I would actually recommend just getting a sample of MacDermid's product and trying that out, and then if it gives the results you are looking for, you can use their calculations with greater confidence.
Good luck!

 Hi Rachel,
Thank you very much for the detailed instructions, that was really appreciated.
Great dane ya got there, lovely dogs, had a pair of them, Guliver and Angelica, miss them a lot, take care.

? Hi Tosin. This discussion has been ongoing for 20 years now, with 23 previous responses offering many different methods. Please try to be explicit about exactly what you want in looking for "a better method" -- accurate to more decimal places, or whatever. Thanks.

Luck & Regards,