Titration of HF & HNO3 Mixture
Q. I'm working on aluminum acid etching using HF & HNO3. At the moment I'm using a hydrometer to check the concentration. However I'm looking for a more accurate titration technique to check concentrations of both HF and HNO3. Please help.Nicholas Cheah
TISAB = total ionic strength adjustment buffer
A. Hydrometer is a very poor method of control since as the acid is expended, it is replaced with aluminum, copper and a few other alloying metals which tend to be heavier than the basic solution, so a very false reading. A plain old acid titration where you add both acids in the correct percent to make up for the spent acid may be adequate and is certainly a lot better.
For a much better control, use a selective ion electrode (fluoride) and test for the fluoride. Do a total acid titration. Subtract the HF number from the total and you get the nitric value. It is not perfect, but it is good and normally affordable. The electrode requires a pH meter capable of 0.1 mV accuracy. These are not cheap compared to garden variety meters. I used an Orion 901 and their probe with very good luck on titanium etch. They provided me with a couple of papers that I modified for my particular use. It used an alternate TISAB, not the normal one for the swamping agent.James Watts
- Navarre, Florida
A. I have some paperwork which suggests a two stage acid base titration.
Titrate a sample using Sodium Hydroxide to pH 3.2 to get the Nitric acid. Then on the same sample continue to pH 5.8 to get the Hydrofluoric acid.
I have only used this method once but it seemed OK. Methyl Orange and Methyl Red indicators work at approximately the right endpoints if you want to do without pH meter.
You may need to change the concentration of Sodium Hydroxide for the second stage depending on the relative concentrations.Ciaron Murphy
- Great Britain
A. You can use an autotitrator to determine automatically the 2 equivalent points in a single sample. Multiple acids titration is quite common in the electronic chemical industry and most can be easily solved by autotitration. There is need for the use of colour indicators nor pH meters then. If you need further information, please feel free to write back.Lin Yau Yeng
Q. Does anyone have suggestions on how to measure the concentrations of nitric acid and hydrofluoric acid in a mix ratio of 20% to 3%? I used the pH method, measuring the amount of 0.2 N sodium hydroxide required to bring the pH up to 3.2 and 5.8 to determine the respective amounts. The results looked accurate until the nitric acid concentration dropped low. At that point, the hydrofluoric acid concentration appeared to spike up (even though HF wasn't added). Perhaps this method works only at high nitric acid concentrations? Does anyone have an accurate method of measuring for all acid concentrations?Keith Lew
- San Francisco, California
Q. Hello Keith:
Did someone have answer your question yet? I have kind of the same problems using HNO3/HF ranging concentrations and at this time it is difficult for me to detect fluorides at a minimum amount like 1%. Please advice me if you found something that helps us.Abraham Wall
A. We have recently taken over the running of a lab and have been looking for an improved method of testing HF/Nitric solutions used to etch titanium parts.
We have inherited 2 methods which we're examining at the moment:
In the first an aliquot is taken, KNO3 and ice added, and titrated against NaOH to first EP. The fluoride is estimated with an ISE. However, we find with this method the fluoride detected decreases with increased levels of Ti, presumably due to [TiF6]3- formation.
In the second an aliquot is taken, KF, (COOK)2 and CH3OH added, heated to 80 C and titrated against NaOH to first EP. The nitrate is determined by IC.
Both methods seem ok, but not great. We've also tried fluoride by IC, and this result doesn't seem to be too affected by dissolved Ti.
I hope this may help some people ...
Q. But I would also appreciate if anyone can shed light on why the reagents are added, or if anyone's got any experience using ISEs or ICs to measure fluoride in similar solutions.Chris nattrass
- nottm, notts, uk
July 1, 2009
A. This is possible if using an autotitrator. The big trick here is that you must use dried acetone as the solvent, not water. In water, the nitric and HF react with NaOH at the same rate. In acetone, the dissociation constants shift. I am unsure how the indicators will work in acetone, so someone give it a shot and see. The nitric acid dissociates at a pH of approximately 4 and the HF dissociates at a pH of approximately 10 in acetone. I used a Mettler-Toledo T50M with a DGi116 solvent pH probe and 1.0 N NaOH. I repeatably found the nitric concentration (first inflection point to pH 4) to be 29.2% and the HF concentration to be 1.8% (second inflection point to pH of about 10). Good luck and let me know if you can use the indicators.Matt Eby
- Columbus, Ohio
December 17, 2010
Q. How to check concentration of nitric acid with the help of hydrometer? Please give the help.chandrasekaran.v sekar
- CHENNAI, India
A. Hi Chandrasekaran. A hydrometer measures the specific gravity of a solution. Nitric acid is heavier than water, so if you have a solution with just nitric acid and water (and nothing else), and you know the temperature (because water expands with heat, affecting the specific gravity), it is possible to correlate the hydrometer's specific gravity reading to the nitric acid concentration via a simple table. You will find that table in Perry's "Chemical Engineers Handbook" [link is to info about book at Amazon] or on-line at
But this only works for initial makeup. A hydrometer will not work for ongoing analysis of etching solutions (or much else) because, as James Watts has explained, the specific gravity of the solution is also affected by the dissolved aluminum content and other dissolved materials, destroying the correlation -- so please explain your situation. Thanks!
Ted Mooney, P.E.
Pine Beach, New Jersey
October 25, 2013
Q. I'm working on aluminum acid etching using HF & HNO3 with Deionized water at ratio:
HF:HNO3:H2O = 3:20:77
Please advise how to determine the concentration for these acids above.
October 30, 2013
Hello cousin Tran. I can't help you technically on that, but as website operator perhaps I can make a suggestion that may help you get helpful answers from others ...
There has already been an extensive discussion on this topic, suggesting very specific titration procedures and analytical equipment -- so please try your best to frame your question in terms of what has already been said, asking questions about what you didn't understand or explaining why you feel the suggestions may not be applicable to your situation rather than starting over. Thanks!
Ted Mooney, P.E.
Pine Beach, New Jersey
Determining nitrate concentration in HNO3/HF acid bathSeptember 30, 2015 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. I am trying to determine the nitrate concentration in a HNO3/HF acid bath. I am using an Ion Chromatograph. I also use a titration with NaOH to determine nitric acid concentration. The bath must be held to 10-15 oz/gal HNO3. We use 42 degrees Baume HNO3 in the tank and 49% HF. I need to develop a standard curve using 1000 ppm NO3 standard solution. I need assistance to determine the best curve to cover the range of concentration in the tank. Any assistance would be appreciated.Todd Roberts
Lab Tech - Tullahoma, Tennessee USA
October 2, 2015
One can determine total acidity, then determine HNO3 conc. via titration. A 1.00 ml sample is added, with good stirring, to about 80 ml ice cold concentrated sulfuric acid. Then, one titrates, keeping the mixture cold with strong stirring, with freshly standardized 0.1 N FeSO4 solution. The endpoint is a brown red color.
Or, if you like, you may determine F- via an ISE method, using the method of additions.
In either case, the concentration of each respective acid may be arrived at by arithmetic.
Consultant - The Bronx, New York
February 26, 2016
I am using a Nitric / Ammonium Bifluoride mixture ~(20%/3%) and for pickling Aluminum 5052. The intent is to increase the surface roughness of the base aluminum as much as possible and maintain weight loss between 0.5% and 1%.
1. Does anyone have a specific procedure for measuring the HNO3 and HF concentrations? I have seen a lot of information regarding this but would like a more specific procedure if available.
2. How much faster does the HF react than the HNO3? I know the fluorides attack the silicon and I believe deposit on the surface of the Aluminum. Trying to figure out what the ratio of the make-up solution should be for replenishment.
- Chicago, Illinois, USA
October 19, 2016
A. I have had success using a 0.1000 N cyclohexylamine in methanol. I can determine both nitric and hydrofluoric acid simultaneously with an autotitrator. It is titrated in methanol with a pH electrode, a non-aqueous titration.David PARKINSON
Hydrite Chemical - Milwaukee, Wisconsin USA
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