Analysis of Pickling Bath

A. A guess on the nitric acid is that you are using a slightly weaker makeup acid than was intended. If you look in a scientific supply catalogue, you will see that there are many different concentrations other than red fuming which very few people would use. Commercial nitric in the USA is always a couple of baumé weaker than analytic grade acid. Test a sample of your raw acid in the lab. Bet that you find it weaker than expected.

A. The method I use for analyzing nitric acid concentration is as follows:

1 mL of sample
Approx. 50-100 mL DI water
Add a few drops of phenolphthalein [on eBay & Amazon]
Titrate with 0.5 N sodium hydroxide [on eBay or Amazon] to bright pink color

mLs x 2.258 = % nitric

I hope this helps.

A. Personal opinion, phenolphthalein changes color at too high a pH and is not significantly higher than a pH4 indicator or meter in the titration. The higher pH end point is going to allow precipitation of metals which will indicate that the solution is much stronger than actual.

A. The answer to this is to be practical.
If you want to analyse and control the "Free" nitric then find a pH indicator that changes somewhere down in the 3-3.5 pH range. Normally you would suggest using a pH meter but as you have acidic fluoride you would wreck your glass probe. If you use a high pH indicator like phenolphthalein you will titrate all your acids including the acetic.
Control analysis of processes like this are only guidelines and don't need to be absolutely accurate.
Make up your process with the amount of x% nitric and if it works well, analyse it by your method and then maintain it at that concentration.ie if you make it up at 20% nitric and this analyses with a titre of 25ml and it goes down to 20ml after use, you have used up 20% of what you put in in the first place and that needs to be replaced. When you add this of course it will never be the full amount because of dilution.

A. Try using Thymol Blue indicator. pH 1.2 Red to pH 2.8 Yellow. Nitradd test from the manufacturer is unreliable. It is only reliable for the initial makeup. Once the solution is introduced to metals (Ti, Al, etc.), the fluoride ions form complexities and detected through ISE procedures (which the manufacturer will recommend). Fluoride complexities are useless in the pickling, descaling of metals.

Let me know if the indicator helps.

Dear Mr. Whitelaw,
We prepared std solution of Nitric and Nitrad and we found out that the Nitric Acid is given at a pH of 3.2. We will use this pH point for the determination of Nitric in the production solutions, even though the last ones will have different matrix than the std.due to metals diluted, and other probable impurities. Do you think the difference will be important? Thanks a lot for your suggestions.

Dear Mr Yang,
We tried the Thymol Blue indicator on a std solution of Nitric and Nitrad, bu we do not have repeatable results due to non-clear end point. Working with pH 3.2 as end point gives much more repeatable and accurate results. Thanks a lot for your suggestions.

C'mon now, cousin Prakesh ...

Please take the time to frame your question in terms of what has already been offered, rather than starting over. Thanks!

Regards,