Letter 17005

Ti interference in Nitric/HF bath  

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Hi,

I work in a semi parts cleaning facility. I am having a problem with our HF/HNO3/DI bath. It is not circulated. After making a fresh bath (1/1/1), with in two days the nitric concentration will go way up, to 40-50%. The major deposition stripped in this bath is Ti.

Question: Is the Ti effecting the nitric conc. if so is a matrix modifier needed for the titration (process control). If not what would make the concentration rise?

Thanks for your help.

Bob Forrest
- Portland, Maine, USA

My first thought is-How are you testing for the two acids? It is not simple and most methods are subject to operator technique which affects precision or repeatability and validity.

In stripping Ti, the active component is the flouride. Typically, 6 F ions for every Ti ion, so you see, it takes quite a bit of HF.

It was always my impression that the nitric was there only as a way to maintaing high acidity without excessive HF concentrations. I do remember articles talking about ratios of the two acids with pitting of The Ti caused by a low amount of nitric.

What percent HF are you starting with. If it is 70% HF, you are using an excessively high amount of HF and are really exposing the line operators to an un necessary hazzard. &05 is terrible to handle in itself, but a 33% solution of HF can cause burns many times worse tha a 10 or 15% solution. If your company is not aware of the seriousness of a HF burn vs a sulfuric or Nitric burn, I would strongly urge that you look into it closely. The *****stuff does not burn just the surface, it keeps going. Some never heal without surgery.

James Watts
- FL

Try etching in ammonium persulfate + sodium flouride, it works just fine and is readily available in pre-mixed dry compound that you just add d.i. water to. it's called "multi-etch".

Jeff Swayze
- Kelowna, B.C.

Another thought, are you titrating using phenopthalein as an indicator. If so, you are also measuring the amount of Ti that is being precipitated as a very fine white floc. Switch to Methyl Orange or something that changes colour in the 4-5 pH range to avoid the problem.

James Watts
- FL

Thanks for the info. I will try the Methyl Orange and pass along the different mixture to the gods above. Again thank for your input. Of course when I try the MO I will reply with results. Any idea what ratio to try on the different bath mixture. The present is 1:1:1, HF, HNO3, DI.

Bob Forrest
- Portland, ME

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