Metal finishing Q&As since 1989

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Anomaly in Nitric/HF analysis

We have a nitric/HF titanium stripping tank. Last week we requested an addition of HF and although the concentration of the HF increased, so did the concentration of the nitric acid, however no nitric acid was added to the tank. What would account for this increase in concentration?

We are using a titration method that includes titrating with ferrous sulphate ferrous sulphate [affil links] (~0.3N). We are also curious to know how long ferrous sulphate ferrous sulphate [affil links] can last sitting in an amber bottle on the bench?

Jennifer Mackey
- Winnipeg, Manitoba, Canada
2003

First, can you verify that the tank was at the exact liquid level as the last time the analysis sample was taken. If it was, then your analysis is off. Your test method is subject to an operator influence, even when it is the same chemist or technician. Temperature is the largest offender in this analysis. Next, the titrant can be added too fast and you get very tiny red brown specks floating around that you have to wait for them to dissolve. This is one of the least precise volumetric analysis that I ever came across. Try it on three samples from the same original grab sample. I think that you will be shocked at the variance. I gave up and went to a selective ion electrode. Do a search at this site. It has been discussed several times. My feeling is the titrant should also be stored in a dark cabinet. I had to restandardize it every month to meet the customers requirements for QC.

James Watts
- Navarre, Florida
2003

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I think Mr. Watts thought that your question pertained to the fluoride titration. I use a different method for titrating nitric acid that I like much better. Since nitric acid is a stong acid and HF is a weak acid, it is possible to use sodium hydroxide to titrate nitric without titrating the HF. The trick is to end the titration at a pH that is low enough where no HF interferes. You will find that if you purchase ethyl violet indicator which has a very pretty endpoint at pH~2.4 (or just use a pH meter) that no HF interferes. At this pH, you have not yet neutralized all of the nitric, so you will need to make a set of standards and establish an experimental factor. Also spike some HF at various concentrations to prove to yourself that it does not interfere. Getting all of this done takes a little work, but I think that your results will be much better.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

2003

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Each to his own. The HF is the important portion of the analysis and the nitric is vastly less important. If you titrate the nitric and have a =/- 2% and titrate total acid (or a continued titration) with a =/- 2% , it is possible to equal most of the HF concentration if you miss in opposite directions. The specific ion electrode was repeatable to under 1% of the total HF, regardless of how I did it. Since this is the important component, I strongly preferred it to the standard procedure for analysis of 3 major turbine engine manufacturers.

Now back to the original question. Their analysis (the recommended one)used a weak procedure. HF is not a weak acid by the definition that I remember from chemistry. However,it very easily can have an inflection point at a different pH than the nitric and still be a strong acid. Your procedure is faster, as it would have to be to protect a pH electrode from the fluoride attack.

James Watts
- Navarre, Florida
2003

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Both respondents are correct but to answer the original question, I think you will find that the reason that the nitrate went up was because the titrant was not restandardised. ferrous sulphate ferrous sulphate [affil links] oxidises in air quite easily and requires regular standardisation. Suggest you standardise against a nitrate solution so that you standardise your analysis method at the same time as standardising your ferrous solution.

One technique I have used to minimise this oxidation is to store the titrant in a flexible polythene container and expel the air by squeezing the bottle to expel air and then putting on a tight cap. I don't think the brown bottle has any effect.

Then add a few bits of iron powder to reduce the oxidised iron-- but you have to standardise regularly

Geoffrey Whitelaw
- Port Melbourne, Australia
2003

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I was not disagreeing with Mr. Watts that HF should be determined by ISE, nor do I disagree that the HF is the most important component. I was offering an alternative analysis for nitric acid that seemed to me to be Jennifer's difficulty. As far as HF being a weak acid, it is weak by definition that it is not fully dissociated in water into H+ anf F-. It exists partially as molecular HF in equilibrium. The equilibrium constant for HF is 6.8x10-4. This puts it between formic acid and acetic acid [on eBay or Amazon] on a list of common weak acids. Nitric, on the other hand is a strong acid meaning that it is fully dissociated in water into H+ and NO3-. It is this property that would allow for titration of the strong acid, at least partially, before the weak acid interferes. Because you can only titrate it partially by itself without titrating the mix, I was recommending that she establish an experimental factor. It would not be nearly so simple a matter to account for the HF using caustic, so ISE would be the way to go as I share a dislike for the standard alizarin method. I hope this clarifies things rather than making them more muddy.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

2003

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I appreciate these answers, however at this point purchasing an ISE is out of the question, and I would like to learn more about the analysis requiring the ethyl violet indicator. I did fail to mention that we use 5mL of 5%v/v HNO3 in our blanks. I do obtain repeatability when titrating with the ferrous sulphate ferrous sulphate [affil links] when using the 5mL of 5%v/v HNO3 in the sulfuric acid solvent. So, repeatability is not an issue. In fact, I took Mr. Watts advice and did 3 different titrations from the same sample and received comparable results. However, I am still uncertain as to why the nitric acid increased without an addition of nitric acid.

Jennifer Mackey
- Canada
2003

Perhaps there is a simpler explanation to the anomaly of the levels. When the operator made the addition care was not taken in properly mixing the bath, so then upon collecting a sample the concentrations were for a lack of better terms out of whack after the analysis. Have the operator make another addition since the HF concentration has dropped by now, and emphasize the importance of mixing before collecting a sample. I'm certain this will provide you with a better idea of the true concentration of your nitric-HF bath.

Derrick Kasunic
- Winnipeg, Manitoba, Canada
2003

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Sampling is also a source of problems. I used a home built "colwissa" to sample the tanks from several different places and depths. I would not accept samples from the plating/cleaning folks because they were prone to NOT make sure that the tank was at the proper liquid level and stirred, as well as just dipping a beaker into the top 2 inches at the front of the tank. Congratulations on getting repeatable analyses. As to the original question, It is an absolute impossibility as stated. Not having a representative sample of the entire tank is the strongest probability. A properly mixed tank makes it a lot easier to get a representative sample.

James Watts
- Navarre, Florida
2003