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Determine % Nitric Acid. Titration? Conversion factor?

A discussion started in 2006 and continuing through 2017 . . .


Q. If I have a solution of unknown Nitric Acid, how can I test that solution to determine that percentage of Nitric Acid?

Kimberly NeVille
aerospace - Ogden, Utah, United States

simultaneous (2006)

A. Hi Kim,
If you work for an aerospace company and you use nitric acid, your company is supposed to have a chem lab or a qualified lab to do the testing.
When you say strength, you need to define it a bit more, as in percent of make up acid or percent of theoretical.
Also it can be measured in baumé or in specific gravity.
An old fashioned way is 10 ml of solution in 100 ml distilled water, a couple of drops of methyl orange indicator, titrated with 1N sodium hydroxide from a red to a yellow orange end point. Titrate slowly so you do not get a false reading from dissolved metals that precipitate out as hydroxides.
I like 1 ml sample in 50 ml distilled water and titrate with 0.1N sodium hydroxide. Uses less material and is less likely to get false readings from metal precipitation.
Tell us what units you want it in and someone will give you the fudge factor to multiply the mls of titrant by to give you the answer.

James Watts
- Navarre, Florida


A. Kimberly,

If nitric is your only acid you should be able to easily titrate the solution to neutrality using a strong base solution of known concentration and then calculate your original acid strength. Any general chemistry textbook can give you more info or you should be able to see the calculations on the web.

Terry Tomt
- Auburn, Washington


A. Titrate it against a known concentration of Sodium Hydroxide using a suitable indicator and calculate for the unknown.

Any indicator in the 4-8 pH range should be fine, the two chemicals should swing hard between the two pH extremes.

Jim Gorsich
Accurate Anodizing Inc.

Compton, California, USA


A. Jim, 8 is way too high a pH as metal hydroxides will form and not go back into solution in any reasonable time frame. Typically, pH's of 4 to 5 are used and have virtually no effect on the titration because pH is logarithmic.

James Watts
- Navarre, Florida


thumbs up signJames - Thanks for the correction. Anyone in doubt: follow his statement, not mine.

Sorry for the mis-information.

Jim Gorsich
Accurate Anodizing Inc.

Compton, California, USA


A. If you want to check that which kind of acid is there you have given , then you pl first confirm it acid by titration with caustic and find out the acidity. Then add barium chloride, silver nitrate, and lead acetate, in three different test tubes, and add that unknown and neutralized acid one by one. Check for any precipitation.if it gives white precipitates with barium chloride then that acid may be sulphuric acid, if it gives white precipitates with silver nitrate then that acid may be hydrochloric acid.

Rakesh [last name deleted for privacy by Editor]
- Vadodara, Gujarat, India


Q. James,

The solution of Nitric Acid I am measuring is simply Nitric Acid diluted with water. I am looking for a volume percentage of the solution. We currently test the acid tank and report it in oz/gal. However, somebody decided that we need to report it in percent. The dissolved metals will be titanium and stainless steel.

Kimberly NeVille [returning]
- Ogden, Utah, USA


A. Kim,
Simple if you have fluid oz per gal. Convert gals to fluid oz. Divide fl oz of acid by fluid oz of a gal and multiply by 100 to get %.

James Watts
- Navarre, Florida

May 6, 2016

Q. We titrate our nitric acid bath using the following process:
- pipette 5 ml sample into 250 mL flask
- dilute to 150 mL with distilled water
- add 2-3 drops of Phenolphthalein Indicator (Gardotest indicator 2)
- gradually add 1.0N Sodium Hydroxide (Gardotest Indicator 37) using burette while swirling until solution turns pink
- record the number of mL of 1.0N Sodium, Hydroxide used=V
- calculation is %=1.334xV

The discrepancy we have here is the interpretation of the end point ... different team members determine it at different points but we want to have a fail safe instruction/colour point. Our colournormally changes to a yellow-orange not pink as per above. Is the end point just when the clear solution changes to yellow-orange or you keep adding the Sodium Hydroxide until it is very dark and doesn't change anymore? Any advise would be appreciated.

Barb Hulse
aerospace - Orangeville, Ontario Canada

simultaneous May 7, 2016

A. First things first, I'm not a chemist, but here's what I'd do:

I would make a small amount of a known solution concentration (your ideal concentration), and titrate per your established procedure. When the known solution reaches the colormetric endpoint, corresponding with the known concentration of the solution, measure the PH. Use that Ph reading going forward as your endpoint during your titration, as opposed to the color change. This takes the variability of color perception out of the equation.

Marc Green
anodizer - Boise, Idaho

May 8, 2016

A. I concur with Mr. Watts. pH 8 is too high. At that pH you're going to see a lot of NaOH uptake from the hydrolysis of transition metal ions.

Methyl orange is probably a better choice than phenolpthalein.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

May 8, 2016

A. Phenolphthalein is probably the worst indicator that you could use if you have any dissolved metal in it. You need an indicator that changes color in the 4-5 pH range. Methyl red and methyl orange are frequently used. Some like one better than the other.
Second, you need to add the hydroxide very slowly as it will drop out the metals as hydroxides and they do not go back into solution very easily. I adopted using a smaller sample (with calibrated pipet) and titrating with 0.1 N hydroxide with spin bar agitation.
To get everyone on the same page, why not use a good quality pH meter to establish a color change point that everyone will accept.

James Watts
- Navarre, Florida

May 9, 2016

A. Hi Barb,
Try Bromocresol green, that gives a more defined end point than phenolphthalein, so there should be less room for discussion as to where the end point is.
Best Regards

Mark Lees
Aerospace - Isle of Man British Isles

simultaneous May 10, 2016

A. Bromcresol green is a good indicator. Look it up and see what the color change points are. My foggy memory says that it had 2. If so, use the lower one.

James Watts
- Navarre, Florida

May 10, 2016

A. The way I remember learning it, when you start to see the indicator changing color as the drop falls in and then immediately vanishing as it mixes, it's time to slow the drops way the heck down. Then, when the color changes all the way without reversing due to the mixing, that's the titration end point.

Of course, just using a pH meter works too as long as you're confident of the calibration.

If dissolved metals are interfering as James Watts suggests, you may need to precipitate them out beforehand with something that will not disturb the acid content.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

May 10, 2016

A. Good day Barb.

I use phenolphthalein for my sulfuric anodize analysis, and it is tricky, as you must determine when there is no longer ANY colour change. I make up a known concentration of acid in a 100 ml volumetric flask and verify, it works for me.
For the nitric, try 1 ml in a 250 ml beaker, add 75 mls DI water, titrate with 1.0 N sodium hydroxide to pH 3.57, giving titre A. A stir bar on the magnetic hot plate gives good results, so titrate slowly. I found in my passivation solutions which do not contain iron or dichromate,pH values rise very slowly approaching 3.0, and then there is a quick spike.
You can use 0.1 N Na OH here, with the math.
Nitric acid in % BV = 6.45 X A
Nitric acid in g/l = 63 x A
Hope this helps.


Eric Bogner, Lab. Tech
Aerotek Mfg. Ltd. - Whitby, Ontario, Canada

June 17, 2016

A. I agree that Bromocresol Green is the most user-friendly indicator for Nitric baths.
Personally I'm less of a fan of the Methyl orange color shift, as it goes through quite a color range on its way to salmon red.
Phenolpthalein definitely has its place in Sulfuric bath titration (and others). But as was alluded to earlier, by the time you hit the PTH endpoint, you're in the precipitation range for many metal hydroxides, and can easily get a false high reading if you're not prepared for them. In an anodize bath you take the hydroxides out BEFORE starting the titration (KF50% at a ratio of about 2 parts KF to 1 part bath aliquot); the difference between a treated and untreated sample is your aluminum value but that's another thread :)
You might notice that the pH endpoints are all vastly different, with MO around 4, BCG around 5, and PTH over 8... But once the acid is used up, the pH rises so rapidly that a minimal amount of titrant will cause it to skyrocket.
I agree also that it is an excellent habit to validate your titrations by creating a known standard and testing it. I did that with EVERY TANK when I took over my current lab. It's also a good instructional tool for new techs.

Rachel Mackintosh
solutions control lab / industrial waste treatment - Brattleboro, Vermont, USA

October 12, 2016

Q. We titrate our nitric acid bath using the following process: (same as Barb Hulse above)
- pipette 5 ml sample into 250 mL flask
- dilute to 150 mL with distilled water
- add 2-3 drops of Phenolphthalein Indicator
- gradually add 1.0N Sodium Hydroxide using burette while swirling until solution turns pink
- record the number of mL of 1.0N Sodium, Hydroxide used=V
- calculation is %=1.334xV

Does anyone have a reference for the calculation? We do the same thing and have been asked for a reference to support our results. I am also going to look into changing our indicator as they suggested. The Phth endpoint is really tricky.

Thanks for your help.

Kati Owen
Aerospace - Niceville Florida USA

October 14, 2016

A. Hi Kate,
The indicator you should use is bromocresol green.
Also your multiplication factor looks like the one for sulphuric acid.
You should multiply your titre by 0.65. This works for a 10ml vat sample titrated with 1.0N sodium Hydroxide, and give you a result expressed as percent by volume of 70% (700g/l0 nitric acid. S.G. 1.41.
For 60% nitric acid use 0.76 as the factor.
I hope this helps, and good luck

Mark Lees
making stuff for aeroplanes - A rain soaked rock in the irish sea

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