Analysis of Nitric Acid in the presence of Hydrofluoric Acid

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.

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.

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 [pipettes on eBay or Amazon [affil links] 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 ⇦this on eBay or Amazon [affil links] in sulfuric acid. The ferrous titrant must be added slowly, and such that it flows down the side of the beaker [beakers on eBay or Amazon [affil links] . Also, the temperature must not rise above 20 °C. As the ferrous is added, a swirl of red brown color appears that disippates 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 ⇦this on eBay or Amazon [affil links] 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.

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). 98 mls 15%sodium acetate, 1.5 mls DI, 0.5 mls 49% HF=25% HF standard. 98 mls 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 ⇦this on eBay or Amazon [affil links] 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 anyone about this it may be a secret. 6.49*((0.25*Titrant)-(0.35*(HF conc/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 subtraction 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.

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.

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 ⇦this on eBay or Amazon [affil links] Warning! highly Flammable! /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.

A. There's a distillation method for fluoride. It's in 'Standard Methods ...' ⇨

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.