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HOW TO TITRATE ETCHING BATH FOR TITANIUM TYPE1 COMP C

M name is Jeff, I am a level 2 ndt in liquid penetrant. and we are looking for welding titanium, and we are in the process of making a bath for etching titanium, and we have the solution that it consist of nitric acid and ammonium bifluoride and water, but my question is how can we titrate that solution to add the correct amount of solution,if we are below the etch rate that is .004/.005 inch/side/hour. the solution is 7.5 liter of water 7.5 liter of nitric acid and .270 gr of ammonium bifluoride.and to control it we do etch rate and it tell us if the solution is strong or weak,we did research on multiple spec. on different industries.and we come up empty handed,so do we add by experimentation by verifying every day etch rate and add small amount.but what is the right amount to add,there must be a way of titrate nitric acid and ammonium bifluoride

Jeff Gervais
lab technician - QUEBEC CANADA
August 25, 2009

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August 27, 2009

There are two ways:

1) Get a fluoride ISE, and measure the fluoride concentration of the bath. It is best to use the method of additions. Then, do a simple acid/base titration with NaOH solution...subtract the acidity from the HF from the total acidity to get the nitric conc.

2) Do a titration of the nitric acid via titration with standard FeSO4 solution in ice cold H2SO4. Do a simple acid/base titration, as above, then do the arithmetic, this time subtracting the nitric acidity from the total to get the HF conc.

I favor (1).

Dave Wichern
Consultant - The Bronx, New York

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First of two simultaneous responses --

Dave's method 2 is very erratic, so when you are subtracting an erratic (large) from total acid, it massively affects the small value of the HF.

Method 1 requires the use of an appropriate swamping agent
It requires a good probe and a very good pH meter. I preferred to make a set of standards and to plot the readings on a graph. I then took the reading of the unknown and referred to the graph for the HF value. Note that things are done with dilute solutions and you need appropriate nalgene volumetric flasks for solution prep.

Personally, I used Orion products which is now a Thermo company. Note that I had a really good regional rep and had to ask a number of questions from tech services, sometimes more than once, to get correct answers. Thermo can provide you with a number of reports on the subject to get you started.
Done right, the repeatability is excellent.

James Watts
- Navarre, Florida
August 28, 2009

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Second of two simultaneous responses --

Daily checks of etch rate with small additions of ammonium bifluoride are best.

Terry Tomt
- Auburn, Washington
August 28, 2009

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August 31, 2009

Terry's method is good if great care is taken to make sure that the Ti is precisely the same. Since it oxidizes, I would wet sand the panel first,dry it weigh, etch at the same temp and agitation, possibly with a sample taken to the lab, rinse,dry and weigh.

A problem with this method is you need to keep the fluoride to nitric ratio above 1:10 or you run into the probability of pitting.
Also, as the dissolved Ti builds up in the solution, you need more and more F in the solution to maintain a rate.

With an analytical method, you need to look for available F and not total since the Ti forms a Ti:F complex with a typical 6 F's per 1 Ti.

James Watts
- Navarre, Florida