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Passivation with nitric acid. How to check pH level?

We do passivation of metal parts in house and would like to know if there is a chart and or formula to determine the pH level of our solution. We mix nitric acid and distilled water at 50/50, and 25/75. We do not do large quantities but want to know exactly when to replenish or discard.

Keith Schneider
Mfg Metal Shop - Morrisville, Pennsylvania
2004

Forget pH. This requires a simple acid base titration. methyl orange ⇦ this on eBay or Amazon [affil links] or methyl red ⇦ this on eBay & Amazon [affil links] are suitable indicators.

James Watts
- Navarre, Florida
2004

You shouldn't look at pH, but at density. I don't know what concentration you want, but 37 % nitric acid have a density 1.227 g/ml

Bo Kønig
Food industry - Odense, Denmark
2004

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I agree, pH is NOT useful for this task. You should rather check density with a hydrometer (Go to a lab equipment vendor, such as ColeParmer) and/or acid-base titration. Go to Chemical Engineers Handbook [adv: suggestions on Amazon, AbeBooks, or eBay affil links] to check what's the correct density for the nitric acid solutions.

Manuel Sández
- Hermosillo, Sonora, Mexico
2004

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Measuring pH with this strength of nitric acid is a fruitless task. The pH is so low that it is quantitatively meaningless. The best way to determine the nitric acid content is by titration, although density (specific gravity) is a good indicator. The problem with density is that metal contamination can also increase the density, so you can get a false acid valuation

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2004

We use a nitric acid test kit that consist of 3 different chemicals. Its an easy process and we get it from our chemical distributor. What should the PH be at? We passivate at 20-25%. How can we figure out when its time to change our nitric solution?

Ryan Roy
- Bristol, Connecticut, USA
December 9, 2010

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That would make your solution about 4 Normal concentration. If I remember correctly that is very near pH of zero. pH meters do not function well at pH's below 1. The cheap ones do not work well below pH2.
Your vendor's test kit will probably include a very elementary
titration of free acid, probably using an organic indicator that changes color around pH4.
You can make additions based on free acidity if you have a correct titration procedure. You change it out when it does not work well. Normally, it will have turned a fairly dark brown and will start to smell.
A few companies may have specific procedures for testing and for change out of the tank. I am not aware of who that might be.

James Watts
- Navarre, Florida
December 10, 2010

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A 50/50 mixture of nitric acid has normality between 4-5. to check your normality just take 1 ml into a flask, add a few drops of fenolftaleine and about 25 ml demi water. Titrate this with NaOH 0,1 N until change of colour. Divide your outcome with 10 and you have your normality. As long as it remains between 4 and 5, your mixture should perform well.

You may also want to check on your metal contamination which also has an effect on your mixture.

Harald Machiels
- Genk, Limburg, Belgium
January 12, 2011

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January 21, 2011

Time to change your nitric? Type ll at 20 g/l iron, type Vl at 40 g/l iron or as your spec dictates.
pH of nitric? Try 1 ml in 250 ml, add 50 ml water, titrate with
1.0 N Sodium Hydroxide to pH 3.57 --- using pH meter calibrated from pH 1 to pH 4---- giving you titre A.
Nitric acid in % B.V .= 6.45(A). As with any new analysis, it's helpful to make up a known standard, say 25% nitric and compare results. Problem overshooting? Try 0.5, 0.25 N Sodium Hydroxide and or inter change where needed. Nothing is written in stone. This analysis can be as accurate as you need it to be.

Eric Bogner
Lab Tech - Toronto Ont Canada