Analyzing HF in Titanium pickle

A. You can analyze for HF with an ion selective electrode. It is not exactly what you call a wet analysis, but probably the only way.

Q. Now I have a question for you, what do you do for the waste treatment of this solution? Do you have a proven method for its destruction? Sara Michaeli Tel-Aviv-Yafo, Israel 2002 A. We pay for a disposal company to remove it.

A. There is a thorium nitrate titration for HF described in various metal finishing handbooks that used to be standard. I never performed it because of the problem of disposing of the thorium waste, which is quite toxic and mildly radioactive.

Sara is quite right in saying that the ion selective electrode method is the most practical method by far. You will find colorimetric methods (all based on the inhibition by fluoride of the formation of colored "lakes" resulting from the reaction of various dyes with a metal, generally zirconium) in the literature, but these are all plagued by interferences and are very unwieldy.

The way I used to do this test is by using the fluoride electrode as an endpoint detector in a lanthanum nitrate titration. The procedure is given in the manual that comes with the fluoride electrode manufactured by Orion.

As for treating it after it's spent, I would dilute it by a factor of 3 - 5 with water, depending on its strength, and add hydrated lime [affil links] to a pH of 9.5 - 10. It is best to add the lime as a powder until the pH reaches 4- 5, then start adding it as a water slurry, slowing the rate of addition greatly towards the end. Otherwise, you will wind up with an even larger volume of sludge than necessary, from undissolved lime particles. Even when the treatment is performed perfectly, expect very large sludge volumes. The remaining dissolved fluoride in the free liquid and filter press return water generally runs from 5 - 10 ppm.

If you do not have a discharge limit for fluoride, the waste can be neutralized with sodium hydroxide instead. This greatly cuts the solids generation, but leaves the dissolved fluoride in the 500 - 1000 ppm range.

Be careful with the initial lime additions, because the neutralization generates a great deal of heat, and NOx and HF fumes are released. The treatment needs to be performed in a very well ventilated area, and the operator should wear an air purifying respirator fitted with acid gas cartridges. The treatment tank needs to be made of a material that will stand up to heat as well.

A. Sara is correct by saying you can determine HF concentration of a solution using a fluoride probe. However, in my experience, this only works with the initial setup of a nitric/HF bath. After parts (titanium) have been introduced into the processing bath the fluoride ions will begin to form complexes in solution. The fluoride probe with give you a measurement of free fluoride, but not complexed fluoride. So if you continue to use the probe for HF determination you will see a decrease in fluoride concentration, but only because the concentration of free fluoride in solution has changed, not the total fluoride. Testing the HF in this etching bath is a bit tricky, but there are several ways to make this determination. The best method I have found, and the one that I use, is to run an etch rate test on a titanium coupon. If you continue to monitor the etch rate of the bath you will see etch rate decreases over time and thus indications to add some HF to your solution. Depending on what specification your working off of, it should include a range for your etch rate test. Keep your rate in this level and your HF concentration should be up to par.

February 11, 2009 Steven If your etching solution is for example 4% HF and 18% HNO3 and the rate decreases, how do you correct it? Suppose 100 lts of solution. If you add HF 70% how much is enough? Thanks Yohands Rey - Chiahuahua, Mexico

A. This analysis method can be found in the Metal Finishing Guidebook, Vol. 96, No. 1, 1988, I've been told...I've been meaning to look it up myself.

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Thanks Douglas. The MFG contains a 44-page "Chemical Analysis of Plating Solutions" chapter by Charles Rosenstein and Stanley Hirsch which includes a "Table XI Test Methods for Acid Dips and Electropolishing Solutions". Here's one line of that table; hopefully it's the one you are thinking of:

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 2014

A. Take 5 ml solution in a round bottle, make a slurry with cab-o-seal; titrate with 1N KOH until red to phenolphthalein [on eBay & Amazon] . Add 40 ml of water, boil and continue titration again.
Calculation:
HNO3 70% =(A-B/2)*12.8
HF 70%= B*6.8
A=first titration
B=second titration
This procedure gives you free nitric acid and free hydrofluoric acid; with the above procedure you get the ion concentration.
Of course, from practical point of view get a reliable system of etch rate depending on your alloy and thickness you want to remove.
I use only 30 seconds for etch rate with air mixing of the solution.

Hi Kyle. Yes, the subject has been covered, and we've appended your inquiry to a previous thread on the topic. But everyone's situation is a little different, and you have presented yours in an interesting fashion such that readers may wish to revisit it and offer their comments. Thanks.

Regards,

A. Hi Kyle,

A simple way to control etch rate is to do sample panels. Take a panel of titanium of known alloy (so you know the density of the material), weigh it (you will need an analytical balance accurate to 4 decimal places), clean it and immerse in your etch solution for a set time (say 10 minutes, but really can be any time that will give a reasonable weight loss), dry the panel and re-weigh it. The difference in weight can then be directly related to the thickness loss (weight loss divided by (surface area times the density) equals thickness loss for time x where x is the immersion time). Make sure that everything you use for the calculation is in the same units, so weight in grams, surface area in cm2 and density in g/cm3.

If you do this at the beginning of the shift you will know what your starting etch rate is.

Hi Israr. That has already been the subject of the dozen postings before yours. Is there a good reason you don't trust the ion selective electrode that was strongly recommended by three readers? Are you willing and able, or unable, to do the thorium nitrate titration? What about the method from the Metal Finishing Guidebook, which I posted, or Moshe's titration?

Please try to express your problem in terms of the answers already provided to keep the dialog moving forward. Thanks.

Regards,