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pH and acid measurement in stainless steel pickling
We are looking pickle duplex stainless steel with a HN03 and HF mix.
I'm not a chemist! But, I am confused.
We know the acid is about 26% wt HN03 and 7% wt HF delivered to us. We then mix that 2 to 1 with water. This does not change the pH but does make the pickle less aggressive. I not sure I understand this as I thought pH is an H concentration factor.
How can we calculate the percentage of Nitric and hydrofluoric in our process. I suspect a simple titration method could be used.
Is it common practice to top up pickling baths with stronger liquors to keep the concentrations correct?
Any pointers would be very much appreciated.
Engineer - Wakefield, England
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First of three simultaneous responses -- February 2, 2010
To answer at least some of your questions:
pH is a log scale and therefore at the extremes of acidity and alkalinity even large changes in concentration result in slight changes in pH. I wouldn't expect diluting such a relatively high concentration to have a massive effect on pH.
It will be possible to check with titration methods however a mix of acids can be problematic. One more common method with be to perform a total acid titration and then perform a test for fluoride to check the HF concentration. You would find the nitric acid concentration by subtracting the HF from the total acid. You will need to look into ion selective electrodes for fluoride.
Finally, yes it has always been common practice wherever I have worked to maintain concentrations by adding more concentrated solutions.
aerospace - UK
Second of three simultaneous responses -- February 2, 2010
Scott,
pH measurements are really only appropriate for quite dilute acid solutions. Once you get up to the concentrations you are talking about pH becomes all but meaningless.
A standard acid-base titration would be the best control method for solution.
Use 1N sodium hydroxide and methyl orange indicator. You can find the method in most text books about plating such as the
Canning handbook and the
Electroplaters Engineering Handbook as well as books on analysis techniques (my favoured volume is Vogel's Textbook of Quantitative and Semi-Quantitative Analysis).
Aerospace - Yeovil, Somerset, UK
Third of three simultaneous responses -- February 2, 2010
pH is used to measure weakly acidic or alkaline solutions. Your etch is way too strong to use pH as a control. Apart from this the HF will dissolve the electrode and make it useless for any measurements. Whatever numbers the meter was giving, they are purely fictitious and the electrode is now useless.
Titration is the usual way to control the etch but it is not a simple one and you will need need a little training.
I am sure that you are aware that HF is particularly dangerous. It requires that the operators have training and specific first aid measures immediately available. You will also, of course, have a COSHH assessment in place that will have considered this.
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Geoff Smith Hampshire, England |
September 20, 2011
Titration equipment manufacturers have developed a method for analyzing HNO3, HF and Fe in stainless steel pickle liquors. Using thermometric titration, it is possible to differentiate endpoints for HNO3, HF and Fe. The method is fast, and is amenable to the full range of automation possibilities. The titrations are conducted in plastic vessels without special insulation.
Bets regards,
- Brisbane, Queensland, Australia