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Analysis of Nitric Acid in the presence of Hydrofluoric Acid

A discussion started in 2002 but continuing through 2017


Q. I am trying to find methods of analysis of two Nitric Acid-Hydrofluoric Acid etching solutions (one of low HF consistency, i.e ~1 g/l HF in 200~g/l HNO3 and one of high HF consistency, i.e.~40 g/l HF in ~400 g/l HNO3). I have tried different methods (two stage titration with KOH in ethyl alcohol media and silica gel, titration with Thorium Nitrate, Ferrous Sulfate, NaOH) but none is really precise (repeatable) and accurate, even if I use freshly prepared STD solutions. The problem becomes bigger when I analyze used etching samples from the production etching baths.

Angela Nikolaou
aerospace industry - Athens, Greece


A. I use an ISE probe for HF%, using the concentration setting on our meter(2 mls of bath to 98 mls of 15%sodium acetate). I then titrate two mls of bath to 98 mls of DI, with Thymol blue ind.15 drops, with a stirrer. Please contact me if you need the calculations. I do subtract the HF from the Nitric in the Nitric calculation.

Bob Forrest
- Portland, Maine, USA


A. There is a titration specific for the nitric acid that I have used with great success in the lab, and which yields results that match the ISE measurement of fluoride/subtraction from total acid method. I do not have the method in front of me, but generally, it goes like this:

A 1 - 2 ml sample of the process bath is added slowly, with strong stirring, to 100 ml of ice cold concentrated sulfuric acid. The tip of the pipette must be immersed in the acid as the addition is made. Then, the mixture (in an ice water bath)is titrated with a solution of ferrous sulfate in sulfuric acid. The ferrous titrant must be added slowly, and such that it flows down the side of the beaker. Also, the temperature must not rise above 20 °C. As the ferrous is added, a swirl of red brown color appears that disappates as it mixes into the bulk of the solution. The endpoint is a red brown color throughout the mixture that persists for several minutes.

The chemistry is the same as that of the old "brown ring" test for nitrates I was taught to do in freshman chem. Each mole of nitric acid requires two moles of ferrous sulfate to reduce it to nitrous acid.You can calculate the factor with that information.

If you need more details, I'll look up the method and dig them out. Just post a request in this thread.

Hope this is of some help.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

simultaneous 2002

Q. Dear Bob and David,

Thank you very much for your response.

1. Bob, I will try your method. Please clarify: which titrant shall I use and of what normality? I suspect is 1N NaOH. Please send me the detailed calculation formulae.

2. David, I will try your method, which is approximately similar to one I tried in the past but it was not very repeatable. Is the end point sharp? When you say that the color persists for "several minutes" (relative unit of time), how many minutes approximately? Please send me the detailed method with the calculating formulae.

Thanks again.

Angela Nikolaou
aerospace industry - Tanagra, Greece


A. I think that I used a larger sample than Dave. I found this test to be a total pain and quite (very) time consuming. Reproducability was marginal to OK, depending on the standard applied. The ferrous sulfate has to be kept cool but not cold, in the dark and still needs regular titration with High quality nitric acid standard. I was very happy to have made the switch to the ISE.

James Watts
- Navarre, Florida


A. What we use is 49% HF, 70% HNO3, and DI. We clean parts with these baths. What I use the analysis for is process control only. The bath make ups are just volume percents, no real chemistry. So we may have a 20%HF/42%HNO3/38%DI. A 1:1:1, and a 3%HF/20%HNO3/77%DI. Along with some KOH baths. We are well aware of the hazards of HF and all who work with it have a tube of calcium glutamate at home and at the job. Any way, I was handed a calculation that did not work, so I modified it. For HF the procedure goes as follows: Standards, 25% and 50%(HF). 98mls 15%sodium acetate, 1.5mls DI, 0.5 mls 49%HF=25%HF standard. 98mls sodium acetate, 1 mlDI, 1 ml 49% HF=50% HF standard. We make up new standards every two weeks. A standard HF ISE probe and meter that has concentration setting will work. Turn meter on, set to concentration, calibrate. Record slope to be sure standards are in range. Sample analysis. Two mls bath to 98 mls 15% sodium acetate. Stir up slightly with probe and measure. This works sweet. It gives us an accurate reading that is usually within 10% of our theoretical %HF in the bath. For nitric. This is the part that I modified. I was having problems with the original calc and suspected that some how, for our process, we should be able to subtract the HF and get a reasonable % of HNO3. I made an HF curve, using the same parameters of the HNO3 titration. A five point curve using 1,5,20,35, and 50 % solutions. Ran a regression and put the curve into a spread sheet for subtraction. So the analysis goes as such. 2 mls sample to 98 mls DI. Add 15 drops of 0.04% Thymol Blue indicator. Put on stirrer. Titrate with 0.5N Sodium Hydroxide solution from a 50 ml buret. It will stay blue when complete. This is for mixed baths only, HNO3/HF. Now the calc. Don't tell any one about this it may be a secret. 6.49*((0.25*Titrant)-(0.35*(HFconc/100)))/mls used for HF curve at that %HF. So if you used say 45 ml and the HF was 29% it would go like this: 6.49*((0.25*45)-(0.35*0.29))/31.1

The original calc did not include the final subtration or the decimal percent of the HF.

It works fine for all except the 1:1:1 bath. I get high readings on the HNO3. All the other baths, the HNO3 is usually within 10% at make up. Any one know why the high readings? To much HF? Any out there that can tell me what the 6.49, 0.25, and 0.35 variables mean, I would be very thankful. And again this is for process control not exact analysis of Nitric acid in HF.

Bob Forrest
- Portland Maine


Q. Bob and James,

Thanks a lot for your interest and response.

1. Bob, I got your method and I will try to work it out. Your questions are also ours. You mention the 10% accuracy of the method. What about the repeatability?

2. James, the ISE is OK with the HF analysis but when it comes to the total acid analysis ( so that afterwards we will subtract to get the Nitric Acid), there we have the following problems, which I will describe in details: Wehave two baths: A: HNO3+HF= 200 g/l + 40 g/l B: HNO3+HF= 200 g/l + 1 g/l

For Bath A: When it is freshly prepared and we titrate with 1N NAOH, we get a curve with two end points (EP), the 1st at pH~4 and the 2nd at pH~8. When the same bath is being used in the production (where different salts are being formed) we get a titration curve with three (EP), the 1st at pH~4, the second at pH~8 and the third at pH~10. Why does this happen? Which EP in each of the two cases correspond to the total acid (HNO3+HF) content?

For Bath B: When it is freshly prepared and we titrate with 1N NaOH, we get a curve with one EP at pH~8. When the same bath is being used in the production, we get a titration curve with two or three or more EP, the 1st at pH~4, the second at pH~8 and the third at pH>10/. If we get more than three EPs, then these are spread over the whole pH range ( from ~3 to ~13 pH). Why does this happen? Which EP in each of the cases correspond to the total acid (HNO3+HF) content?

Angela Nikolaou
aerospace industry - Tanagra, Greece


A. See how this works for you.

Let A = g/L HF as determined by ISE

Let B = mL of 1.000N NaOH to titrate 10 mL of sample to pH of 4.0

[B*0.1-0.7801(A/20.01)]*63.02 = g/L HNO3

I don't like to titrate to the higher pH to get the total acid because, depending on the age of the bath, a significant amount of metals may be titrated along with the acid. I only titrate to the nitric endpoint. The contribution from HF (determined from the pKa of HF) is then subtracted.

Greg Conti
- Chino Hills, California, USA

July 31, 2008

Q. Hi Angela,

Did you resolve your problem on the Titration Method? I do have two type of methods but not very accurate, I will like to know if you did get good results on yours so you can give an advice. I appreciate your feedback.

Monica Bustamante
- Tijuana, BC., Mexico

July 13, 2010

Q. Hello Bob,

I am also having trouble finding a way to determine the HF/NHO3 concentrations.

Your method seems to work for you, but I was wondering where the 31.1 came from in the calculation. I am unclear on what the mls used for HF curve at the % HF means.

Also I was wondering if you would comment on your repeatability

Michael Ong
- Portland, Oregon, USA

July 30, 2010

A. I do this titration all the time, using a Mettler Toledo autotitrator. The weight %'s are roughly 5% HF and 40% nitric acid. The solvent is 70 mL of 70/30 acetone/water and titrant is 0.1 M NaOH. Sample size is 0.5-1.0 g.
I also use this same method to titrate mixtures of HF and HCl (both less than about 10 weight %). Both titration methods seem to be accurate to a few tenths of a percent.
There are two inflection points, a weak one and a strong one. The processor in the autotitrator can pick them both up and calculate endpoints. There's no reason this could not be done with a manual burette and graph paper, if need be.

Robert Rovito
- Quakertown, Pennsylvania, USA

October 4, 2011

Q. I determine hydrofluoric acid in hydrofluoric/nitric acid aluminum etching bath using Lanthanium Nitrate as tiffany. This method works as long as the solution is not contaminated by Aluminum. Have any idea how I could remove it from the solution and separate the free acids from it?

Mina Marouli
Chemist - Greece

October 6, 2011

A. There's a distillation method for fluoride. It's in "Standard Methods for the Examination of Water and Wastewater" [affil. link to book on Amazon].

dave wichern
Dave Wichern
Consultant - The Bronx, New York

April 25, 2012

Q. I am looking for a method to determine the concentration of nitric acid in arsenic acid product which is manufactured by the company I am working at. Thus far, I could only find a titration method using ferrous sulphate but then I could not find any suitable indicator...can anyone help me?

- Shah Alam, Bangladesh

May 10, 2012

A. Scanacon of Sweden makes acid analyzers that provide very accurate and fast results. The only downside is that the analyzers are rather expensive as are most analytical equipment. I have not found any "wet" methods that are reliable.

Pete Smith
- collegeville, Pennsylvania, usa

October 15, 2016

A. To Calculate concentration of HNO3 and HF mixed solution, rather than other complicated methods I think by observing pH we can find out % of both solution.
Make 0.2N Sodium hydroxide (NaOH) solution.
Dip probe of digital PH meter into mixed unknown solution.
Add 0.2N NaOH solution into it until pH comes to 3.2; this is your HNO3 %.
Go ahead with same up to 5.8 pH, this'll give HF %.

Any titration method will give you approx. value of solutions. So better is to follow electrode procedure.

chintan purohit
p ltd - gujarat, india

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