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Ion Specific Electrode


I am looking for an ion specific electrode that measures dissolved titanium in an HF etching solution. Any suggestions as to where I can find one (or if it even exists).

Shari Taylor
- Valley Forge, Pennsylvania, USA


To the best of my knowledge, there is no such probe. If you're not prepared to do some fairly involved chemistry on samples from the bath, or make a serious investment in instrumentation, your best bet is to send samples out to an outside lab for titanium analysis by ICP.

If you have a lab with a good fume hood, and have a colorimeter, I believe that the 'fairly involved chemistry' could be done as follows: Dilute a 1.00 ml sample of the bath to 500 ml with distilled water. Take 10.00 ml of this solution, combine it with about 50 ml distilled water, and carefully add 10.0 ml of sulfuric acid. Mix, and heat this solution to a boil in your fume hood. Continue heating until the water is driven off, and white sulfur trioxide fumes begin to be evolved. Keep the sample preparation at this temperature for several minutes, remove it from the heat, and allow it to cool.

Carefully add the cooled sample preparation to about 75 ml of water, allow this solution to cool, then add water to bring the volume to 100 ml. Now, add 30 ml of 3% hydrogen peroxide and mix. A yellow color will form, the intensity of which is proportional to the titanium present. Measure the absorbance of this in your colorimeter at 410 nm.

Repeat the above procedure, using 10.00 ml portions of 100, 200, 300, and 400 ppm titanium standards in place of the diluted sample solution. Make a graph of the concentration versus the absorbance of these standards at 410 nm, and use it to find the titanium concentration of your diluted sample preparation. If the absorbance that you got from the sample is greater than that of the 40 ppm titanium standard, redo the procedure using a higher sample dilution. Finally, divide this value by 2 to obtain the result, in units of grams per liter. It is only necessary to run these standards occasionally, not each time the analysis is done.

Or, as I said above, call your friendly local certified testing lab and have them run the analysis for you.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


A crude, but simple, estimate can be done by neutralizing the acid slowly to the point that the white precipitate just begins to form, transfer to a cone bottom centrifuge tube, add 5 ml of 1N NaOH, mix well and centrifuge. Volumetrically compare this to the amount of standards precipitate. Standards only have to be done once. I would make up standards with 1 to 15 grams per liter of Ti,in 1 gram increments. It is decently repeatable. Centrifuge RPM and time do make a difference in how well it packs, so keep that a constant. The main problem would be accounting for tramp metals, mainly aluminum, (which also affect the etch rate).

James Watts
- Navarre, Florida

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